Oral Embryology and Microscopic Anatomy Figure I.— A photomicrograph of a ground section cut faciolmgually through a human maxillao' hnt premolar tooth The enamel has a deep fissure in the central desclopmental groove. At the entrance to the fissure, on the triangular ridge of the buccal cusp, there is a barely visible dark- area which is probably the beginning of a carious lesion. There is no cNidentt of caries at the bottom of the fissure. In the enamel on the buccal and on the lingual surfaces of the crown there may be faintly seen near the dentinoenamcl junction the narrow light and daiL areas which are the bands of Ilunter-Schregcr Notice the curvature of the dentinal tubules in the crown of the tooth. Notice the con* figuration of the pulp chamber: the pulp bom in the buccal cusp extends much farther occlus- ally than the pulp horn in the lingual cusp The pulp cavity here is empty because the pulp tissue was destroyed in the preparation of the section Notice the relatively narrow rtwc canals. It is difficult to distinguish the thickness of the cementum on this section at this magnification (As seen under very low power of the mietoscopc.) Figure 1 A MANUAL OF ORAL EMBRYOLOGY AND MICROSCOPIC ANATOMY A Textbook for Students in Dental Hygiene By DOROTHY PERMAR, B.S.. At.S. Assccialt Fro/essor of Vtnttstry, {Oral HuMoiy and Dental Anatomy), College of Dentistry, The Ohio State Vntvenity, Columbus, Ohio Third Edition 96 ILLUSTRATIONS LEA & FEBIGER PHILADELPHIA 1965 Thtrd Edttton Copyright © 1963. by Lra & Febiger All Rights Reserved Firit Edition, 1955 Steond Edition, 1959 Libtar)' of Congress CjtaloK Card Kumbef 63-12339 rtinced in 1 he I'nited States of America Preface to the Third Edition In the hope of partially compensating for the lack of opportunity in most Dental Hygiene Schools for students to study sections of oral tissues u'ith a microscope, a number of photographs have been added to this edition. I want particularly to thank Dr. Duncan McConnell for instruction in photography which enabled me to make these additional pictures. I wish to express appreciation also to Dr. Paul Kitchin for doing some critical reading, to Dr. Joseph Hunter and Mr, Ralph Ulbrich for technical assistance, to Airs. Helen Fox for assistance in typing, and to the several teachers of Dental Hygiene students who offered helpful suggestions for this edition. Dorothy pERirAR Columbus, Ohio ( 7 ) Preface to the First Edition The Dental Hygiene student must arciutre a basic knowledge of the origin and structure of the tissues of the oral cavity. This is important for several reasons. First, it gives the student a basis for exercising judg- ment in the execution of an oral prophylaxis. An ability to visualize the structure of the oral tissues below their surface may prevent incorrect or careless performance. Second, acquaintance with normal tissue will help the student to recognize abnormal conditions which will inevitably be encountered, and will emphasize the importance of informing the dentist of these conditions. Third, a knowledge of the oral tissues will assist the Dental Hygienist in instructing patients clearly and correctly in the pro- cedures for oral care. Fourth, an elementary knowledge concerning the embrj'onic development of the face and the oral cavity may give some factual information on which to base answers to patients’ questions about anomalies. If it does nothing else, it wll create an awareness that there is much to be learned in this field. It may even develop the prudence, too often found only associated with maturity and great wisdom, to say "I don’t know” at the proper time. And fifth, but not of least importance, increased knowledge in any field of work means increased interest and enjoyment for the worker. Dental Hygiene curriculums var>' in different schools. This manual presupposes a course in general biology or in zoology. It is written with the assumption that the student understands the meaning of cells and of cell division, and that the vocabulary of biologic science is not entirely unfamiliar. It docs not assume previous instruction in embryoIog>' or histology. The manual is intended for use in conjunction with lectures or discussion periods. The order of study of the various phases of oral histology and embry- ology is somewhat a matter of personal preference. In teaching this subject I have found it necessary' for the student to know something of the microscopic anatomy of tooth tissues Ijefore trying to visualize their embryonic development. For this reason, tooth embryology follows tooth histology'. And an understanding of the development of the face and of the meaning of the three primary embryonic layers needs to precede the study of tooth development. Rather than insert the chapter on the embryology' of the face and oral cavity' between tooth histology and tooth embryology, this chapter is placed at the beginning. 10 PRIIFACE TO TllE FIRST EDITION The microscopic structures of enamel, dentin, pulp, cementum, perio- dontal membrane, bone, oral mucosa, epithelial attachment, and sali\'ar>’ glands are tlescrilK’d as simply as possible. An effort has been made to avoid elaboration of details which arc iinnccessarv' for the efficient and intelligent practice of Dental Hygiene. A few of the statements made as facts may be somewhat controversial, but it seemed advisable to avoid confusing conflicts of ideas, particularly in matters not of immediate concern to the Dental Hygienist. I am greatly indebtal to Dr. Paul C. Kitchln for kindly permitting me to use his large collection of microscope slides of oral tissues Many of the illustrations were drawn directly from these slides Some are composite pictures of several of the slides. .■Ml of the drawings are diagrammatic and not detailed. This type of illustration was used in the IxjHcf that the beginning student often understands a simplified diagram better than a photomicrograph. I wish to express sincere thanks to Dr. Kitchin, Dr. Hamilton Robinson, and Dr. William Lcfkowitz for reading the manuscript and offering many valuable suggestions. Without their assistance, this little book would contain many more errors than it perhaps does contain. Gratitude is also due to Mrs. Marice Kersey Musgrove for special assistance with the manuscript, and to Mr. Jack Gottschalk for help in the preparation of the manuscript. Dorothy Permar CoLtutius, Ohio Contents Chaitfr I’age 1. EMBRYONIC DEVELOPMENT OF THE FACE AND ORAL CAVITY The Human Body . 15 The Beginning of a Human Body 15 Si7e of a Human Fetus 16 The Face and Oral Cavity 16 Early Development 16 The Development of the Face 18 The Development of the Palate 21 The Development of the Tongue 22 Anomalies . 23 2 INTRODUCTION TO HISTOLOGY General Histology 25 The Nature of Histology 25 How Tissues are Studied 25 Components of a Tissue 27 CiassiScation of Tissues 29 Epithelial Tissue 30 Connective Tissue 3-1 Nervous Tissue 36 Muscle Tissue 37 Oral Histology 39 Location of Oral Tissues 39 Tissues of a Tooth 39 3. TOOTH ENAMEL Location 44 Composition 44 Macroscopic Structure of Enamel 44 Microscopic Structure of Enamel 46 Permanence of Enamel Structure 53 Clinical Importance of Srn/cru/c of Enamel 53 4. DENTIN Location . Composition Structure of Dentin Clinical Importance of Structure of Dentin 5. TOOTH PULP Location . . .... Composition Structures in the Pulp . Functions of the Pulp ... Age Changes m the Pulp Qinlcal Importance of the Pulp 60 60 60 66 68 68 68 72 ( 11 ) 12 CONTENTS Chapter Page 6. CEMLNTUM Location 74 Composition . . 75 Structure of Cementum 75 Hypercementosis and Cementicles 78 Clinical Importance of Cementum 78 7 PERIODONIAL LIGAMEM lA>catton 82 Structure of the PenoJontal Ligament 82 Pnncipal Fibers of the 1‘eiiodontal Ligament 85 Structures in the Periodontal Ligament 87 Functions and Clinical Importance of the Periodontal Ligament 88 8 HONE AND TilE ALVEOLAR PROCESS Hone 90 Gross Structure of a Hone (an organ) 90 Microscopic Structure of Hone (a tissue) 90 Periosteum and Endosteum 95 Growth of Done 95 The Alveolar Process 99 Clinical Aspects of Bone Reaction in the Alveolar Process 102 9 THE ORAL MUCOUS MEMBRANE AND THE SALIVARY GLANDS Mucous Membranes 104 Definition 104 Histologic Structure of Mucous Membranes lOI ITe Oral Mucous Membrane 105 Histologic Strunure of Oral Mucous Membrane 105 The Sahvar) Glands 110 Hmolog>' and Function of Salivary Glands 110 Distribution of Sain ary Glands 1 1 1 10 'IHE GINGIVA Location 1 15 Qinical Appearance 115 Histologic Structure 111 The Gingival Sulcus Ilv The Epithelial Attachment 117 Clinical Considerations 1 19 11 TOOTH DEVELOPMENT The Beginning of Tooth Development 125 The Tooth Bud 125 Dentin Formation 127 Enamel Formation 150 Root Formation 151 Formation of the Periodontal Ligament 155 Anomalies . . . 155 CONTENTS 13 Chaiter Pace 12. TOOTH ERUITION AND THE SHEDDING OF THE PRIMARY TEETH Tooth Eruption 139 Eruptive Movements 139 The Mechanism of Tooth Eruption 141 Cen’ical Exposure without Occlusal Movement 141 Mesial Drift 141 Shedding of Primary Teeth 142 Primar)' and Permanent Dentitions 142 The Process of Shedding 143 Chronologj' of the Human Dentition 145 1 Embryonic Development of the Face and Oral Cavity THE HUMAN BODY The Beginning of a Human Body The life of an individual human body begins at the moment the sperm unites tvith the ovum and forms a fertilized egg. The egg soon divides into two cells, and then each of these again divides, making four cells. Continuing cel) divisions produce a minute mass of undifferentiated cells which soon undergoes a marked change in shape and comes to resemble a thick-walled tube. Essentially the pattern of an adult human body is that of a large tube with a small tube running through it. The outside body wall is the large tube, and the digestive tract is the small inside tube. The mouth is the cephalad (head) opening to the inside tube, and the anus is the caudad (tail) opening. The region between the two tubes— that is, between the body wall and the wall of the digestive tract— is occupied by the internal organs such as the liver, the heart, the lungs. The region within the smaller tube— that is, inside the oral cavity, the esophagus, the stomach, and the intestines— is not actually inside the body cavity: it is merely the space within the tube which runs through the liody. This pattern for the human body is established ver>' early in embr>'onic life when the minute embryo changes its shape from a mass of undifferenti- ated cells to a thick-walled tube. The inside of this tube Is the primitive digestive tract. At this early stage the mouth and the anus have not formed; the tube is closed at both ends. The thick wall of this embryonic tube is composed of three primary embryonic layers: the outer layer is the ectoderm (ccto * outside; derm = skin); the inner layer, which lines the primitive digestive tract, is the endoderm (endo = inside) ; and the layer i>etu‘een the ectoderm and endoderm is the mesoderm (meso =» middle). The entire body develops as a result of the multiplication and differenti- ation of the cells of the three primary embryonic layers. Cells derived from the three layers differentiate into specialized types and develop into the various tissues which form all of the organs of the body. Such wdely different structures as the epithelium of the skin which covers the body, and the brain and spinal cord, arc derived from the ectoderm as the cells from this layer multiply and become specialized in different ways. The endoderm gives rise to the epithelial lining of the digestive tract (excepting , the oral epithelium) and the lining of the lungs, and to the liver, and to ^ (IS) 16 EMimvONtC DEVELOPMENT OF THE FACE AND OIUL CWilT' parts of the urogenital system. Cells derived from the mesoderm produce the skeleton, the muscular system, the blood system, and parts of the urogenital system. The epithelium of the ora! cavdty is derived from ectoderm. It rests upon connective tissue which is derived from mesodenn. A tooth is derived from these two primary embryonic layers: the enamel from ectoderm and the dentin, pulp, and cementum from mesoderm. SiZF. or A Human Fetus At the end of the third week after fertilization the enibr>’o is a tube about 3 millimeters long (approximately j inch). By the end of the second month it has acquired a form recognizable as a human being. In later stages of development the size of the human fetus usually is given in terms of the crown-rump length. At the end of three months the fetus has a crown- rump length of about 5fi millimeters (about inches); and at the end of four months it has a crown-rump length of about tl2 millimeters (over 4 inches). (Sec Fig. 2. A and B.) FtcuRE 2, .-/and A human emhryoiS mm Ians— aliotir 8 «^ecks old. The head w large, making neatly half of the liodv length Compared to the swe of the brain, the face appears smalt. The e> es are Htdespread, theears are close to the neck Thenose isnearl> flat. Notice the small size of the mandible Inside of the mouth of an emhno of this age the lateral pala- tine processes are in a sertical position and ihe tongue is tall and narroiv and nearly touches the Itmer liorder of the nasal septum (Fig 6.^ (Csiuriesy I3r Kud> Melfi) THE FACE AND ORAL CA\'IT^' EaRLV Dl.VELOrMI.NT The human face starts to develop during the tlunl week in utero when the embryo is a i«l)c maile up of cctotlemt, mcsoilcrm. ami endwlcnn am! is closcti at lioth ends. Development of the face begins with the establish- ment of the oral ca\ ity. The formation of the early oral cavity, or primi* PLATE I DE\Tto^M^_vr of tin. Human Fact A ami B. Iliiinan tmliotj, thinl w«rL: MulIUiy procc*s<’^ ha%c (lir»rIopetl from JIk‘ Cr't lirinchbl arch Stotnotlciim his fornuiL Frontal process (which here w hint) is utiJi'kldi Second anti tliml branchial arcliw arc visible. Doprevsion iin top of heat! Iv the noimptwc C Fourth wirk: Nasal nils liavc rlivided the frontal iirotTss into llic nicdlin nai.il proersj process Lateral nasal process Ma xillnr process cr Mandibi arch D. Fiftli week: Globular processes hate appeared and are fused tvitlj the maxillary processes. E Sixth week: Due to differential growtli, med/on nasal process is now relatively narrow. E>es arc on lateral border of face. F. Scvcntli week: Median nasal process fa nlatitely narrow. Eyes are now anterior in position. C and II. Eiglitli week: Lidless eyes are on anterior of face. Tlie mandible fa smalL Xosc protrudes sbghtly. Ears are low on side of head. I and J. Tviclflli week; E>Tlids closed. Ears appear higher. Mandible furtlier detelopcd. K. Adult face: DerivaUtes of median nasal process are blue; of lateral nasal processes arc pinkj of maxillary processes arc green; of mandibular processes arc yellow. (Orban’s Orel Ilistohey and Embryc^ogy, courtesy of the C. V. Mosby Co.) EMIIRYONIC DKVELOl’MENT OF TIIE FACE AND OILVL CAVITV 17 tivc mouth, starts as an invagination of the ectoderm at the cephalic end (head end) of the closed embrj'onic tube. The ectoderm in this area dips in until it meets and unites wnth the endodenn of the primitive digestive tract (Fig. 3). The intervening mesoderm in this area disappears. The cavity fomied by the invagination of the ectoderm is the priniiltve mouth* The primitive mouth is separated from the primitive digestive tract by a membrane composed of the uniter! ectodenn and endodenn. This is the ^nccopharyngcal membrane. It lies approximately in the region that u'll be occupied by the palatine tonsils. The l(M-ntion of the Inircopharj'ngeal membrane makes it evident that the primitive oral cavity is lme< w’lth ectoderm, although the lining of the primitive digesliyi* tract caudnd to the buccopharyngeal membrane is derived from endoderm. During the fourth tvcck in utcro the tiiir«>phiiryngcul " ‘urcs, establishing communication bcUveen llie |.rnnil.ve M.onth nn( t K pnnutivc digestive tract. Further clevelopnient of the f.U'e tenters aliout the mouth. . , , , Above the opening of the primitive mouth a large bulge is ptoi utti ly the gro«h of t'hc^orahraTn (Figs. 3 an.l -1). Th.. rCo.lerm .nn.l uu-soderm 18 EMBRYONIC DEX'ELOPMENT OF THE FACE AND ORAL CAVITY which cover the forebrain (le\xIop into an cmbr>-onic structure called the frontal process. The frontal process gives rise to the structures of the upper part of the face. Below the opening of the primitive mouth in the region of the future neck, five paired branchial arches are formed. These are ordinarily designated as branchial arches I, II, III, IV, and V (Fig. 4). They are homologous to the gill arches in fish. In the development of the human face and oral cavity only the first three branchial arches play a part ficuRi; 4 —A drawing of a lateral »i«wr of a pig cmbr>-o. This i< similar to human develop- ment at tins stage. Branchial arches IV and V are not visible here Branchial arch I is destined to develop into the mandible and a large part of the ma.xilla. Branchial arches 11 and III join the first branchial arch in the devclopn^ent of the tongue. By the end of the fourth week in iitcro the primitive mouth is establishetl and the buccopharj'ngeal membrane lias ruptureti. Above the primiti'c mouth is the frontal process and liclow the primitive mouth is the first branchial arch (Fig. 3). The entire face and all of the structures of the oral cavity with the exception of the fKjstcrior part of the longue arc now going to(Ieveloi> from these two primordui fl)cginnings): the frontal process and the first branchial arch. Tin: Development of the Face As soon as the primitixe oral cavity is established anti the frontal procc*:'; UMDRYONIC DEVELOPMENT OF TIIE FACE AND ORAL CAVITY 19 and the first branchial arch become distinguishable, small buds develop on the upper border of the right and left ends of the first branchial arch (Figs. 4 and 5). These buds arc the maxillary processes. The original lower portions of the first branchial arch arc now called the right and left maiulibular processes. The mandibular processes will form the mandible and the lower part of the sides of the face. The maxillar>’ processes are destined to give rise to the upper part of the cheeks, the sides of the upper lip, and a largo part of the palate and maxillary arch. (See Plate I). After the mandibular processes and the maxillarj' processes have formed, growth of the lower part of the face is retarded and the frontal process FlouRh S.— A phoiograpli of tlie face of a pig embryo of about 12 mm. At this stage of development the pig face and the human face arc very much alike starts a rapid development. On its lower border the frontal process de- velops a pair of invaginations, the right and left olfactory pits, which are the future opening.s into the nose fJ'ig. 5 and Plate I). The olfactory pits divide the lower part of the frontal process into three parts: a center por- tion called the median nasal process and two lateral portions called the lateral nasal processes. The lateral nasal processes become the sides of the nose. The median nasal process fonns the center and tip of the nose. Later, an ingrowth from the center of the nose fonns the nasal septum (the division between the right and left nasal chambers). On its lower border the median nasal process develops a pair of bulges, called the globular processes (Plate 1, D). The globular processes arc not separated, but remain as a single imtlian structure which grows downward so that it extends below the olfactory' pits and lies l>ctwcen the maxillary’ processes. It forms tlic center of the upper lip (thcphiltrum}. The maxillary processes font! the sides of the upper lip. During the second month in utcro the globul.ar processes fuse %rith the right and left iiiaxtllary' processes, the lines of fusion being beneath the nasal openings (Ftg. 7, .\). Thus the three Vjf.VRT 6 — Fn>nta5 sections throwRh tht head «f 3 pie* showinR pTOBr«M>f stapr^ 5n vclopment of the palate ’I'liis i* siirilar to human palatal clc'eJopnicnt //. T he ' ertical posi- tion of the lateral palatine processed and the piisition of the tonsue Hhich neatly fowchci the lower border of the naul septiim ate similar to the condition in an 8-«cek-order of the nasal septum. The premaxilla grows inward from the oral side of the globular processes and becomes a small triangular area in the anterior part of the roof of the mouth and that part of the maxillary arch which usually liears the incisor teeth (Fig. 8). At the l)oginning of the third month in utero there is a considerable growth of the mandible, a structure which to this point has lagged in development. This mandibular growth makes room for the tongue to drop down from its position l)ct%\'cen the lateral palatine processes (Fig. 6B). With the tongue out of the way the lateral palatine processes assume a horirontal position and meet in the center line of the roof of the mouth where tJicy fuse with each other and with the nasal septum (Fig. OC). At their anterior l)orders the lateral |Kilatinc processes fuse wnth the pre- maxilla (Fig. S). 22 EMBRYONIC DEVELOPMENT OF THE FACE AND OR.\L CWm* The fusion of the lateral palatine processes with each other and with the premaxilla results 5n a Y-shaped pattern of fusion in the roof of the mouth (Fig. 8). The palatal fusions are normally completed by the end of the third month in utero. These palatal fusions are fusions of soft tissue, not of bone. The first evidence of bone formation in the area of the forming palate is seen during the eighth week in utero when the lateral palatine processes arc still in a vertical position. At this time specialized bonc-fonning cells (osteoblasts) become differentiated in the mesenchyme. By the cntl of the thirtl month, Flct'KE 8 —Drawing of ihe oral surface of a human maxilla The Y-shapeJ lines of embryonic fusion are indicated when the soft palatal structures have cotnpletetl their fusion, there is con- siderable I>one formed in the palate, but the Iwnes of the riglit and left sides of the palate arc not together at the midline. A separation of the bones of the right and left sides of the p.i!atc still exists at the end of the fourth month in utero. The cmbrx'onic derivations of the structures of the oral cavity may be briefly summari/ed : The onil structures arc ilerivctl mostly from the first branchial arch; only the prema.\illa develops from the frontal proces.s. The rest of the hard i>alate and all of the soft palate develop from the maxillaiy processes, which are buds from the first branchial arch. The mandible develops from the first branchial arch. The DnvELOPME.NT of the Tonoue During the second month in utero the first, second, and third branchial arches together give rise to the tongue. The tongue develops on the ventral EMBRYONIC DEVELOPMENT OF THE FACE AND ORAL CAVITY 23 wall of the upper part of the throat- It has been described as a sac of mucous membrane into which a mass of muscle has grown. As it develops the tongue ascends into the mouth and is directed anteriorly toward the opening of the mouth. I3y the banning of the third month in utero the tongue has acquired a recognizable form. Summary A summary of the derivations of the structures of the face and oral cavity is presented in the following outline: 1. P'rom the Frontal Process are derived 1. Median nasal process, which gives rise to a. Center and tip of nose b. Nasal septum c. Globular processes, which give rise to (1) Philtrum of upper lip (2) Premaxilla 2. Lateral nasal processes, which form a. Sides of nose b. Infraorbital areas H. From Branchial Arch I are derived 1. Mandibular processes, which give rise to a. Mandible b. Lower parts of face c. Anterior part of tongue 2. Maxillary processes, which give rise to a. Lateral palatine processes (t.e., all of palate and maxillary ah’eolar arch excepting premaxilla) b. Upper part of cheeks c. Sides of upper Up III. From Branchial Arches II and ///arc derived portions of the posterior part of the tongue ANOMALIES An anomaly is a marked deviation from the normal standard. Anomalies of the face and oral cavity sometimes result from failure of fusion of, or from arrested development of some of the parts. Cleft lip is the result of failure of proper fusion between the center portion of the upper lip and one or both sides of the lip— that is, between the globular processes and one or both of the maxillary processes (Fig. 7, A). This defect may occur either unilaterally or bilaterally, with or without accompanying cleft palate. If cleft lip occurs, it will be evident before the end of the second month in utero bwause by this time the fusion of the structures concerned has normally taken place. Cleft of the palate may occur at any place along the lines of fusion of the hard or soft palate. Normally fusion occurs in the roof of the mouth be- tween the right and left palatine processes and between the palatine processes and the premaxilla (Ftg. 8). This makes a Y-shaped pattern of 24 EMBRYONIC DEVELOPMENT OF THE FACE AND ORAL CVVITY fusion in the roof of the mouth. Failure of fusion may he of any (Icgrce of severity, ranging from a scarcely noticeable bifurcation (division into two I)ranchcs) of the uvula to a complete lack of fusion of all stnicturcs In- volved. Any severe cleft of the palate results in direct communication between the oral cavity and the nasal cavity. Where failure of fusion of the lateral palatine processes and the preinavilla results in a cleft of the alveolar ridge, the maxillary’ lateral incisor tooth is affc'ctecl. The lateral incisor may be medial to the cleft, it may be distal to the cleft, it may be missing entirely, or there may 1» two lateral incisors, one on cither side of the cleft. Fusion of the intraoral structures is normally completed by the end of the third month in utrro. Therefore, if cleft palate occurs it will bo evident by the end of the third month of intrauterine development. Research conducted in Pennsylvania in 1942 intlicated that 1 chiUt in evety SOO bom in that state had some type of cleft palate, cleft lip, or both. Another defect which may occur is an oblique facial cleft. This cleft extends from the eye to the lower comer of the nose. Its cause is some- times said to be trauma to the face of the fetus. A condition called macroslomta (large mouth) may result fmm insuf- ficient fusion of the maxillaiy processes ami the mantlibular processes at the corners of the embiyonic mouth. A cleft of the chin may occur in the center line ns a result of a cleft be- tween the right anti left mandibular processes (Fig 7, B). 2 Introduction to Histology GENERAL HISTOLOGY The Nature of Histology is the science of tissues (histo = tissue; olog>' *= thescienceof). Botanists study plant tissues; zoologists study animal tissues. Students of human histology study the tissues of the human body ; and those having a particular interest in Dentistry* study especially the tissues of the oral ca\nty. A tissue is sometimes defined as a group of more or less similar cells with intercellular substance and tissue fluid, combined in a characteristic manner and performing a particular function. Some examples of human tissues arc muscle, bone, blood, epithelium of the skin and of (he mucous membrane, connective tissue of the skin and of the mucous membrane, and the pulp of a tooth. The hard components of a tooth, enamel, dentin, and cementum, are also tissues, although enamel and dentin have excep- tional characteristics. Tissues vary greatly in appearance and in structure. Some are hard fbone) ; some are soft (muscle). Some are sturdy and withstand wear and injury’ (surface layer of the skin); and some are delicate and ser\'e as linings Oining of the respiratory' tract). Some are secretory in function (salivary' gland tissue); and some are nutritive in function (blood). How Tj.ssues are Studied Microscopic examination of the structure of tissues started over 100 years ago, and study has been c.vtended and developed as improvements have been made in the construction of microscopes and in the techniques of tissue preparation. Usually tissues must be stained in some manner in Order that their different components may be seen clearly with a micro- scope. Sometimes dyes arc injected into the blood stream of a living animal, and the tissues which have taken up the stain from the blood are subsequently removed from the animal and cut into thin sections. Or tissues may be grown in artificial nutrient medium in a glass tube, and their growth and development observed from hour to hour or from day to day. The most usual method of tissue preparation is probably that of removing a small piece of tissue from the body, embedding it in paraffin, sectioning it, and staining it. Let us suppose, for c.\ainplc, that a dentist wants to examine the micro- scopic structure of the soft tissue surrounding the teeth. A small piece ( 25 ) 26 INTRODUCTION TO HISTOLOGY of this gingival tissue is carefully removed from the mouth tnth a sharp instrument and is immediately placed in a Iwttle containing 10 i->cr cent formalin. In this solution it is sent to a microtechnique laborator>‘. When the specimen is received in the lalwratorj', it is dchyclratc’ pink. Stains other than hematoxylin and eosin are used to bring out different tissue structures. After they arc stained the tissue sections are covertxl with a small, very thin cover glass. In this way the slides are permanently preserved- Specimens w'hich contain calcificUCTIOX TO HISTOLOGY a diameter of about 7/25,000 inch. The structure of human red blood cells is unusual in that these cells have lost their nuclei. White blood cells, on the other hand, contain one or several nuclei (Fip. 11). Muscle cells are distinctive because their cytoplasnt has the power of strtmg con- traction fFigs. 13, £ and F). Fat cells contain large amounts of fat, which V/hite blood cells Red blood cells Ftcune 1 1 — Dcavsinpof some cells ot human blood as se« under the oil immersion objective of the inicroscope. f'lr.LRt 12.— Drawing of fat cells. (High power nugnificaeion ) pusli the nuclei away from the center of the cells and against the cell uTills (Fig. 12). I'rom this brief discussion it i» clear that there are many kinds of cells, and later it will be seen that these difTerent kintls of cells along A\ilh var>nng kinds and amounts of inlercellular sulwtance ami difTerent amounts of tissue fluid determine the nature of the various kinds of tissues. INTRODUCTION TO HISTOLOGY 29 Intercellular substance is a product of living cells and is distributed among the cells in all of the tissues of the body. It holds the cells together and provides a medium for the passing of nutrients and of waste materials from capillaries to cells and from cells to capillaries. The amount and kind of intercellular substance differ in different tissues. In some human tissues, such as bone, intercellular substance is the predominent element. In other human tissues, such as the epithelium which makes up the surface of the skin, intercellular substance is small in amount and the cellular elements predominate. Intercellular substance occurs in twm forms; one form xz fibrous in nature, and the other is amorphous in nature (a « without ; morphous = form). Tissue fluid is that part of the blood plasma which can diffuse through the walls of capillaries. In the tissue fluid nutrients are carried out through capillary walls to the surrounding intercellular substance and thence to the cells; and waste products of the cells are returned in the same manner from the intercellular substance to the capillaries. Tissue fluid may be present in a tissue in relatively small proportions, as in the epithelium of the surface of the skin; or it may form a large proportion of the tissue, as in blood. The tissue fluid of epithelium, of bone, and of other tissues is, of course, derived from blood. Classification of Tissues In their gross and microscopic appearance, and in their function, tissues vary so widely that the beginning student may well feel that there is little or no relationship among them. This seeming confusion grows less, how- ever, when it is discovered that for purposes of easier study and better understanding histologists have developed a classification for tissues. Tissues are alike in that they arc made up of cells, intercellular substance, and tissue fluid. Tissues differ in the form and number of cells, in the type and amount of intercellular substance, and in the amount of tissue fluid. It is on the similarities and differences of tissues that the histologist has based his classification. Human tissues have been assorted into four primary groups: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. The tissues of each of these primary groups have certain major, fundamental character- istics in common. But there are also differences within the groups; and so each of the four primar>' groups has been subdivided. The subdivisions are based on structural differences which clearly distinguish one tissue from another. A brief classification of human tissues is presented in Table 1. Tissues arc combined in characteristic ways to form organs, such as heart, lungs, liver, bones, skin, tongue; and organs arc combined in characteristic ways to form organisms, such as the human Ijody. I-et us consider the tongue as an organ. The tongue Is made up of a combination of epithelial liMUc, connective tissue, muscle tLssue, and nervous tissue, all of which are interdependent in the functioning of the organ (Fig. J4). Epithelial tissue comprises the surface of the tongue and set^’cs as a protective covering. Beneath the epithelium is connective 30 ISTROnUCTlOS TO HISTOLOGY tissue which supplies both support and nourishment; and beneath this connective tissue layer are strong muscles which produce movement of the organ. Nerves of the tongue supply both motor and sensor>' functions. Another organ of the human b^y is the skin. Skin is made up of a combination of epithelial tissue, connective tissue, and ner\'es. The skin covers and protects other body oigans, such as bones and muscles. Bones support the body, and muscles move the Ixjnes and other organs under the rlirection of the nerv’ous system which supplies the Impulses. The entire organism is nourished by blood pumped through the pipe line of blood vessels by the cardiac muscle, which is the chief tissue of the heart. T.\l«LE 1 — Classwicwio'. OV Tl^SlIVS 1 Surface cells of coteneg and of lining Epithelial Tissue owmbtanes (as of sLin and of mucous ^ membranes) 2 Glandular tissue Connectixe Tissue 1. Fibrous us in sti» i«nentli the epithelium) 2. fyoose areol.ar (as in thm membranes betu-een vAnous laj«r$ of tisaucl .1 Adipose (fat) 4 Hemopoietic (» blood'forining Iwne marron. lymphalic tissue) 5 CartilAKC 6 Done Nervous Tissue I Tissue of cvntml nervous sisient 2. Tissue of pcnpheral nersous ij'steni [ I Smooth imoliintar} (as in H-all of in(estinei) ^tusclc Tissue ‘ 2. Stnaieil soluntao (as in sketei.sl muscle) 3 Scnateil involuntary (heart muscle) Ei'mir.LiAL Tissue Epithelial tissue is distributed widely throughout the liody and has many different kimls of structure. It occurs as a covering or a lining tissue making up l>oth the surface l.tyur of the t>kin and the surf.nce layer of the muctnis membranes which line Ivotly caNities such as the mouth, the stomach, and the intestines. Epithelial tissue alsr> gives rise to several organs during cmbrj’onic development. Of epithelial origin in the tlcvclop* iiig embryo are the highly Sf>ccia1izcd cells of such organsns the pancreas, the liver, the thyroid gland, anil the salivarj' glands. Also in certain locations in the upper and lower jaws epithelial tissue Ix-comcs differentiated into structures callal the enamel organs which. located ilecp in the jaw. pnxluce the enamel on the crmvns of developing teeth (Chapter 11). The \arioiis kinds of epithelial tissues are similar to one .mother in that they have a proportionally large iiumlwr of cells nrul very little inter- cellular substance. They an* dissimilar in many wiys, their stnictim* m different locations in the ImxIv being fortunately ailapted to the functions wliich they jx-rform. I'or purposes of description histologists have classified epithelial li<.sii(*s into two major groni» and se\*cral sul^ronps; INTRODUCTION TO HISTOLOGY 31 1. Surjace cells oj cowrinf and lining tmmhranes ( Squamous Cuboidal Columnar b Pseudostratiffed columnar Squamous Cubrndal Columnar [ Transational 2. Glandular Tissue а. Endocrine glands, e.g., tfiyrotd б. Exocrine glands, e g., salivary glands c Mixed endocrine and exocrine, e g , liver, pancreas All types of epithelium will not l)o discussed here. The enamel organ, which is neither a covering or a lining incinbranc nor a gland, will he con- sidered in the chapter dealing with the development of teeth. The salivary' glands will be described in the chapter on the oral mucous membrane and the salivary glands. The endocrine glands while important physiologically to the development and health of the structures of the oral ca^'ity are of little importance to the dental hygienist so far as their histologic composi- tion is concerned. These glands wnll not. therefore. l>e discussed in this text. The types of cpitheJiuni which make up the surface cells of covering and lining membranes will be considered brielly. Epithelial cells of covering and lining membranes are not all alike in shape and arrangement. The s//apc of epithelial colls is described by the words squamous, cuboidal, and columnar; and the arrangement of epithelial ccllsisdescribcd by the words simple, stratified, and pseudostratified. The word squamous means scalc-liko, or flat; and iguomowi epithelial cells are flat cells. Cuboidal epithelial cells are roughly cube-shaped; and columnar epithelial cells are tall and narrow. When the arrangement of epithelial cells is in n single layer the tissue is said to be simple epithelium. When epithelial cells arc arranged in several layers the tissue is called stratified epithelium (stratified = in layers). The word pseudostratified is applied to an arrangement of columnar epithelial cells in which the cells appear to be stratified but are actually in a single layer (pseudo = false). All simple epithelium is vcr>' delicate in structure. Simple epithelium is found only in those areas of the body which are subjected to little or no friction In functional use. Simple squamous epilhelium (a single layer of flat epithelial cells) is found lining the inside of the walls of blood vessels (Fig. 13, A). Simple cuboidal epithelium (a single layer of cuboidal epithelial cells) is found in the covering epithelium of the ovar>- (Fig. 13, B). Simple columnar epithelium (a single layer of columnar epithelial colls) lines the ccm.v of the uterus (Fig. 13, C). PseudostratificdcolumnarepitheliumiFi^. 13, D) makes up the epithelial part of the mucous membrane which lines the upper respiratory tract : the m.i\j)lar>’ sinuses, the nasal cavity, and the trachea. This epithelium is actually composctl of a single layer of columnar epithelial cells; but because in some of the cells the nucleus Is located near the base of the cell, wliilo 32 INTRODUCTION TO HISTOLOGY in other cells the nucleus is located nearer the outer end, this epithelium has the appearance of being made up of two or three layers of cells. In the upper respirator}’ tract the pscudostratified columnar epithelium is supplied with cilia and with gohUi cells (Fig. 13. D). Cilia are minute, hair-like projections which co\xr the surface ends of the columnar cells and act as dust catchers, or filters, for the air breathed in through the nose. Goblet cells arc modified epithelial cells which arc interspersed acnong the other columnar cells and which secrete substances that keep the tissue surface of the nose and sinuses moist. Ficirf 13 — I)iagrammancdniwinR*ofson»edifrwnr t>pcsofcelI» J cpiiLdiwm (S Sj £.) Tfstina on conntctne tissue tC.T.i li. Simple c\iIwkUI epithcluini {.S’. Cub. £.) rrsttnj; on conneahe iksur iC.T.) C. Simple i-oliimnar epnhflnirn (.S' Cu'. r) r«tlnc on connecti'e tissue (C 7*) D I’seuclastratiSnl colutnnir epitlitliom (I'jtudo. C- £) resting on conncctisc tissue (C.r.). £..\*n>ortfhmu$clcfiher f A striatci! muscle fii’ct. INTRODUCTION TO HISTOLOGV 33 Stratified epithelmm consists of cells which are similar in shape to cells of simple epithelium, but which are arranged so that they are from 2 or 3 to many layers deep. Stratified ctiboidal epiiiielium and stratified columnar epilkeliitm occur as the lining of some of the larger ducts of glands, such as the large ducts of the major salh'ary glands. The stratified epithelial cells which line the urinary' bladder change m shape from round to flat, Ficurf U — DiigramiTiatic draning of a section of tissue taken front the under side of the human tongue. This section illustrates the way in which tissues are combined (High power magnification.) depending on whether the bladder is empty oi full, and this epithelium is therefore descrilicd as stratified transitional epithelium. Stratified epithelium is more resistant to hard use than the simple typos of epithelium. By far the most sturdy of all kinds of cpithelia is stratified squatnous epithelium which is found covering all surfaces of the body which are routinely sul)- jected to considerable wear and tear. Stratified squamous epithelium (Fig. 1+) makes up both the surface of the skin and the surface of the mucous membrane of the oral cavity. In both of these locations the epithelium is composed of many layers of 3 34 INTROPUCTIOX TO HISTOLOGY cells. The deepest layer of these epithelial cells rests on a fine structure called the basement membrane which separates the epithelial tissue from its underlying connective tissue. The epithelial cells of the deej^cst layer, called basal cells, are not Hat in shape but arc somewhat cuboidal and often show cell division. As these l)asal cells multiply, some cells are gradually moved toward the outside surface, becoming /latter in shape as they ap- proach the surface and 6naIIy becoming dead cells. On some areas of the oral cavity the flat, dead epithelial cells on the surface are sloughed off as they are replaced from Mow. On other areas of the oral cavity, and on the skin, these deatl surface cells arc not quickly sloughed off. Instead they lose their nuclei and their ccil boundaries and iiccome converted into a very’ tough, resistant surface layer which is called the keratinized layer (Fig. 66). This keratinized layer, of course, gradually wears off and is replaced from beneath as a result of continued cell division in the bas.i! cel! layer. The most heaidly keratimVeil epithelium of the body is found on the palms of the hands and on the soles of the feet, particularly if these areas have been suhjectcil to hard use and are callousetl. More alwut nonkeratinized and keratinizcil epithelium will be learned in the study of the oral mucosa. Covering and lining epithelial tissue docs not contain blood vessels; it receives its nourishment via the blood vessels contained in the connective tissue which surrounds or underlies it. Connective Tissue The tissues which have l>ccn classified together as connective tissue dilTer from epithelial tissues basically in that they are made up of a rela- tively larger amount of intercellular sulistance and relatively fewer cells. In spite of this characteristic which connective tissues have in common they differ so greatly in form and in function that at first glance they some- times appear to be unrelated. Connective tissue (fibrous) underlies the epithelium of the skin ami thcepitheliuin of the oral mucosa and also makes up tendons and ligaments. Connective tissue (areolar) attaches skin to muscle. Connective tissue (fat) stores foinl. Connective tissue (hemopoie- tic) forms blood. Connective tissue (cartilage) gives support and permits skeletal growth. Connective tissue (Ixine) supports the Mly. Fii/Feivs iiasut vs Itaww'i tViTWigWat tVn; VAvdv. Iv, vVs v.’ASt dense form it makes up tendons and ligaments. In a less dense fonn it is the connective tissue which underlies the epithelial pari of skin and of mucous membranes (Figs. 14 and 66). Like other tissues, fibrous connec- tive tissue is made up of cells and inlcrccllular substance, and the inter- cellular substance is of two kinds; fibrillar and nmorfihous. The fibrill.ir conqvonctvt of the intercellular sul«tancc is the prwlominent element. The fillers of fibrous connective tissue nrv probably prtxluced by tlw cells of the tissue uhich are callcil fibroblasts (blast •=* germ, builder). The individual fillers an? m.idc up of minute fibrils. Sjioclal staining techniques used on histologic sections reveal that the fillers arc not all alike, some being collagenous fibers .and ollicrs elastic fibers. These two types of fillers arc distributed in different proportions in different kinds of fibrous connective tissue. INTRODUCTION TO HISTOLOGY 35 Suspended along with the fibers in the all-enconipassing amorphous sub- stance are the various kinds of cells of the fibrous connective tissue. Proportionally the most numerous type of cell is the fibroblast. Special tissue preparation will show also the presence of other types of cells, some of which are capable of becoming defense cells when the tissue suffers injury or bacterial infection. Areolar conneclh'e tissue is seen during any gross dissection of a mammal where skin is attached to muscles, muscles to muscles, or where internal organs are held together by membranes of connective tissue. These tissue membranes, called fascia, are made up of areolar connective tissue and fat tissue. The areolar connective tissue is composed of a relatively small number of cells and of a ver>' loose and thin network of fibrous inter- cellular substance, all held t<^elher by a large amount of amorphous intercellular substance. Fat tissue is distributed throughout the body among the soft tissues and in the marrow cavities of bones, and is found more or less generously concentrated in certain parts of the body beneath the skin. It is composed of specialized connective tissue cells, called fat cells (Fig. 12), which are held together by fibrous Intercellular substance. Fat cells are capable of storing fat. They may become so filled with fat that the cytoplasm is pressed into a thin layer around the periphery of the cell, and the nucleus is crowded against the side of the cell. Hemopoietic tissue {hQmo » blood ; poictic = to make) not only produces blood cells which arc added to the circulating blood, but removes worn out blood cells from the blood stream. In the adult, hemopoietic tissue occurs in two different forms: as red bone morrow and as lymphatic tissue. In the human fetus, red bone marrow is found in nearly all of the bones; but in the adult, the marrow in many of the bones has become transformed into so-called yellow bone marrow (Fig. 65) which is largely fat. Certain bones, however, such as the vault of the skull, the ribs, the sternum, the bodies of vertebra’, retain the red bone marrow throughout adult life. Yellow bone marrow in other bones can be converted into hemopoietic red marrow in c/rcumstanccs of emergency. Red bone marrow consists of a fibrillar meshwork of intercellular sub- stance throughout which are scattered many cells which have the potenti- ality of differentiating into several kinds of blood cells. Red bone marrow produces red blood cells and also certain kinds of w’hite blood cells called gra«ius tissue in these tAvo divisions are very different. The tissue of the central ncr\ous system is extremely soft due to the absence of connective tissue as a supporting tissue for the nen'o cells. In the central nervous sj-stem the nen-e cells arc stip- (Kjrted by a delicate and vcr>' fragile tissue called neuroglia (literally. ner\-e glue). The brain and spinal cord aft* made up of two distinct typ« of this soft ncrs’ous tis.suc callwl gray matter ami icliile matter which differ greatly in histologic structure. In contrast to the nervous tissue of the centra! nervous systern the ncrx'cs of the peripheral nervous sy'Stciii arc tough. This finn quality is INTRODUCTION TO HISTOLOGY 37 derived from the considerable amount of connective tissue which covers and supports the bundles of nerve fibers. A microscopic examination of a cross section of a nerve shows it to be circular in shape and coi'ered by a connective tissue wrapping, or sheath. Inside this sheath are several bundles of nerve fibers, each bundle being surrounded by another connec- tive tissue sheath. Each ncrv’c fiber inside the bundles is encased in its individual connective tissue sheath, and often between this sheath and the individual fiber is a layer of substance of a fatty nature called a myelin sheath. Each nerve fiber in this neiwe is actually a long, thin, stretched-out part of the cytoplasm of a neuron. The liody of the neuron, which con- tains the nucleus, lies deep in the body of the individual. Ner\’e cells do not multiply with the growth of the human body. A person is bom with all the neurons he will ever possess. However, if a fiber in the peripheral neivous system is cut, repair may eventually take place. The end of the fiber which has been cut off from the body of the neuron will degenerate, and the end of the fiber still attached to the neuron may after a time grow out to the length of the original fiber. In the central nervous system, on the other hand, damage is permanent. There is no regeneration following damage to the brain or spinal cord. Muscle Tissue Muscle tissue is composed of muscle cells which are supported by con- nective tissue. Muscle cells are in all cases much longer than they are wide, and for this reason an individual muscle cell is called a muscle fiber. Muscle fibers ha%'c to a greater degree than other cells the property of contraction. Microscopic e.\anunat{on of muscle tissue from \'arious parts of the body shows a difference in the appearance of the cytoplasm of muscle fibers. In muscle tissue from some areas, for example from the intestine, the cytoplasm of the muscle fibers appears relatively clear. In muscle tissue from other areas, such as from the arm or the heart, the cytoplasm of the muscle fibers has conspicuous cross striations. This difference in the appearance of the cytoplasm of different muscle fil)ers led histologists to classify muscle tissue as smooth muscle tissue (where the cytoplasm of the fibers is not cross striated), and striated muscle tissue (where the cyto- plasm of the fibers is cross striated). (See Figures 13, E and 13, 7^.) This division of muscle tissue into two types was not quite adequate, however. Smooth muscle is involuntary witjc/c— that is, the contraction of its fibers is not under the control of the %vill of the indiv'idual. For instance, the peristaltic movements of the intestine, which contains smooth muscle in its walls, are not willfully controlled. On the other hand, striated muscle is for the most part voluntary muscle because the contractions of the muscle fibers may be consciously controlled by the individual. One moves an arm or a leg .as one ^rishes. However, there is an exception. The fibers of heart muscle, although striated like the fibers of the muscles of the ann, are not consciously controlled by the individual. Therefore cardiac muscle has been describcrl as Ixjlonging to a class by itself and is called striated involuntary muscle in contrast to skeletal muscle which is referred to as striated voluntary muscle. 58 I.N'TRODUCnOS TO HISTOLOGV Thus we arrive at a classification for muscle tissue consistinjf of three ratagorics: smoo//j muscle, striated voJttnlary viu^clc, and striated iuvoUiiitary muscle. Smooth muscle tissue is founti in such plaos as in the waits ti{ the intes- tines, in the walls of blood vessels (big. 15), and at the roots of hairs where its contraction jjroduocs an erection of the hair. 'I’he individu.al muscle fi!>ers (cells) of smooth muscle tissue are shapetl somewhat like a cigar and have a centrally located nucleus (Fig, 13, E). In length they may \aiy from 1/1000 mm. in the small blood vessels to ^ mm. in the pregnant uterus. The libers are supportetl by associated connective tissue. Striated voluntary muscle is known also as skeletal muscle. As well as the obvious inclusion in this class of such muscles as those which move the arms and legs, skeletal muscles also include those in and alx)ut the oral cavity; muscles of the tongue, of the lips and cheeks, of the soft palate. I'he muscle fibers of skeletal muscle s from those -Connective tissue 4 —Smooth muscle -Sndothelial lining I 1 I'lCiURB IS.—Oiaiifam of a cross section of a Mood sesse) power masnificatton k of smooth muscle. Most conspicuous of course Is the difference in the appearance of the cytoplasm, which shows microscopic crriss stri.itions (Fig. 13, 1'). Also, the nuclei in skeletal muscle libers arc pushed to one side of the fibers instead of lying in the center, and then' is more than one nucleus in a fiber. Skeletal imiscle fillers are relatively long. var>’iiig from 1 to 40 mm., and they arc supported by connective tissue which not only covers individual fibers, but binds the filwrs into bundles. This supporting connective tissue is well supplietl with blood vessels and ner\cs. Striated involuntary muscle is confincrl to the heart and is knoivn also as cardiac muscle. In microscopic .appearance it resembles skeletal imiscle in that the cytoplasm of the muscle fillers has microscopic cross slrialions. Its appearance differs from that of skeletal muscle in the arnmgcinont of the fibers. Whereas skeletal muscle fibers exist as indivKiuat fillers with several nuclei, cardiac muscle fillers branch and come together so th.it they form a sort of network which comprises the heart mu.«sflc. This net' work i.s supportctl by surrounding connectii’c tissue which contains nerifs and a rich supply of blootl vessels. Muscles are the organs resjionsible for liolh the voluntary and the involuntary movement of all of the various parts of the liody. They act in response to impulses received from the neri'ous system. They are surrounded by. supported by. and nourishcti by the tliffercnt kinds of connective tissue: the fibrous, areolar, and ailiposc; the Ixine ami cartil.ige; and the blood. INTRODUCTION TO HISTOLOGY 39 ORAL HISTOLOGY Of particular importance in Dentistry arc the tissues of the oral cavity*. The entire practice of Dentistry and Dental Hygiene is based on a knowl- edge of the structure, arrangement, and reactions of oral tissues. Instruc- tions given to a patient by the dentist and the dental hygienist concerning the correct method of brushing the teeth are based on a knowledge of oral histology. A dentist prepares a tooth for a filling with careful attention to the nature and arrangement of the tissues comprising the tooth ; and he constructs an artificial denture with regard for the structure of the tissues of the palate, of the alveolar ridge, and of the oral ^'estibule. The pathosis which occurs in the tissues around a tooth when calculus is present can be understood only in the light of a knowledge of tissue structure and tissue reaction; and the tissue repair which follott-s remov*al of the calculus is explainable in the same terms. Other diseases of the soft tissues of the mouth are recognized and treated only as normal tissue structure Is known. The nearly universal disease of the hard tooth tissues, dental caries, must be described in terms of the microscopic structure of the calcified tissues of a tooth. Oral histology is the study of the tissues of the ora! ca%ity: the tissues lining the mouth and the tissues beneath the lining, the tissues of the tongue, the periodontium, the tooth, and the tissues from which a tooth develops. This text will include also a study of the process of tooth development. Location of Oral Tissues The relative locations of some of the oral tissues may be seen in the diagram of a section through a human mandible in the region of the first prcmolar tooth (Fig, 55). The tooth is attached by fibers of the periodontal Uganicjil to the lamina dura which comprises the tooth socket. The out- side surface of the mandible consists of cortical bone inside of which is cancellous bone and bone marrow. The lining of the inside of the cheek, the gingiva, and the coi'ering of the longue are mucous membrane. Under the tongue arc located the salivary glands and several vinschs^ Tissues of a Tooth The tissues which make up a tooth are the enamel, the dentin, the comentum, and the pulp. I'igurcs 16, 17, 18 and 19 arc diagrams of teeth which have been cut approximately through the middle in a facio- lingual direction. Tooth enamel comprises the surface of the crown of the tooth and rc»ie«hoints of reference in descriptions of a tooth. Figcrf t8.— DuRrammauc of a longitudmil faciolmcua! »frtion of a maxiltaf) first prtftiolar tooth. See FiRiite I for a photoi;ni|>h of a similar tooth and compare thr structure* indicated in the JuRrammatic ilrawins with the same structure* seen m the pltotograph of the actual tooth section INTRODUCTION TO HISTOLOGY 43 Figure 19. — Diagrammatic dramng of a longitudinal faclolingual section of a maxillary first molar tooth. Dental caries has destroyed the enamel in the area around a developmental groove on the occlusal surface. The caries has spread horizontally at the dentinoenamel iunction, with a resultant undermining of enamel. Caries has spread pulpward in the dentinal tubules to about 3 the thickness of the dentin The dentin close to the pulp is sclerotic dentin; the tubules are filled with mineral salts. On the pulpal wall beneath the carious lesion a small amount of secondary dentin has formed. The secondary dentin on the floor of the pulp chamber (next to the root) was not caused by caries; secondary dentin in this location is not unusual. (See Figure 36), Location 3 Tooth Enamel Tooth enamel makes up the outside layer of the anatomic crown of a Uwth (.Figs. 1 and 16). Composition Enamel is composed of both inorganic (mineral) and organic substances. Mature human enamel is about 96 per cent inon?an1c. The remaining 4 per cent of its substance is an organic nnitriv (framew'ork) and \cntcr. Enamel is the hanlest tissue of the l>ody, its mineral content far exceeding the mineral content of dentin (70 per cent), of cementum (50 per cent), or of bone (SO per cent). Macroscopic Structcke of Enamel Examine the teeth of several persons who maintain reasonably good oral hygiene. The crown surfaces are composed of hard, shiny enamel. In young individuals who arc free from dental caries, no other tooth tissues arc e.xposed in the oral cavity. On some of the teeth of older individuals, due to a normal aging process, the gingiva may have receded to such an e.xtcnt that onnentuin is visible. In |X“rsons of any age wliost' tcetli have unrepaired carious lesions, dentin will l>c cxtxiscd in the areas where the disease has destroyed the enamel. On the labial stirfaccs of the maxillary central incisors of a young iwrw>n you usually see a numlicr of fine horizontal lines on the enamel. In the cendcal part of the crown these lines arc close together; inrisally on the crown they arc farther .ap.irt (Fig. 20). These lines art* c.alleci pcrikyrtifttn. Their presence is not confined to the ma\iHar>’ ccntr.i! incisors, but they arc easier to see here than on les.s accessible surfaces. In older iK'n^ms l>erikyniata usually an? not visible. Examination of the teeth of persons of iHfTenml ages will show that generally in older persons the enamel apiwars darker in color than m younger persons TIte reason for this darkening is not t learl> uuclerstrxxl. Despite its hardness tooth enamel is subject to attrition ^ that is, wear- ing o/T under the friction of use. Examine the teeth of sc\er.il iniddle-agrndan drntm ((. haptcr 4) M seen m the pulp horns Considering the magnification of this pktiire, it is clear that the fissure i* too nattov' topeimit the instition of an% dental insttamei't The enamel at the hate of the nisure IS thin. It shoof nosignofcanec. TOOTH EiVAXJEL 4y crystals fill the loose organic matrix. Their size is appreciated when w’e learn that they arc measured in terms of millionths of a niillimeter. Microscopic examination of a thin ground tooth section sho\s's other structures in enamel: bands of Hunler-Schreger, stripes of Retzius, enamel lamellee, enamel tufts, and enamel spindles. Some of these structures are of no known clinical importance, while others are of great importance. Dentin Dcntinoenamel junction Enamel Stripes nr Ectzius Figure 24. —A photomicrograph of a small area of tooth crowm cut lonRitudinally. Stripes of Rct7ius in the enamel arc clearly visible. The direction of the dentinal tubules can also be Seen. (Low power magnification ) Bands of Uunter-Schreger arc seen when a longitudinal ground section of a tooth crown i.s examined by reflected light under the low poux*r of the microscope (Kig. 23). The}' arc alternating broad light and dark bands which c.xtcncl perpcnrlicularly from the dcntinoenamel junction to the tooth surface. Their manifestation is due to the cur\'ature of the enamel rotls. The stripes of Retzius arc a different kind of bands, orlinc-s, in the enamel. In longitudinal ground sections of the tooth crown they are seen under the low power of the nnVroscoiie as narrow brownish lines c.xtcncHng diagonally outward from the dcntinoenamel junction toward the occlusal, or incisal, part of the crown (Figs. 17, IS, 23, and 24), ^'bcse structures are fonuerl 4 so TOOTH FNAMEL Figure 25.— Schematic ciiagraiit of a small area of a cross section of a tooth crii«n tn the region of an enamel lamella. Mi pmcHon K'sfc — F.canwl tpltullc Fihurf 26 — A photomicTocraph of a small area «f a ttwirh i-ro«n cut honrontalh The enamel tufts are clearlv sWiblc Among tW tufts, m some places, ate seen the muih smillrt enamel spmdtei. (As seen under low power macnifieattun ) TOOTH ENAMEL 51 during the development of the enamel matrix as a result of the layer-upon- laycr pattern of enamel matrix formation. They are due to variations in structure and calcification along the lines corresponding to the formation pattern. On most of the crown the stripes of Ret 2 ius end on the crown surface, and their termination on the surface sometimes is marked by a series of shallow depressions. The ridges between the depressions are called the penkymala. >sear the incisal or occlusal part of the crown the stripes of Retzius do not reach the enamel surface and therefore there are no perikymata at the incisal edge or cusp tips (Pig. 18). Perikymata often may be seen in clinical e.xaniination. To the dental hygienist the stripes of Hetzius arc of interest chiefly because of their association with the perik}-- mata. (Fig. 20). Ficurf 27.— DinKrammaticdra^ ini' of details of area a m FiRurc 17 shouinR enamel spindles. (Ver)’ hiRh pindlejt .ire ends of dentinal filters which project fntm the tlentm TOOni ENAMEL 53 across the dentinoenamel junction into the enamel (Figs. 18, 26, 27, 28, 29). These structures will be understood when you study the structure of dentin. The deniinoename! junction in many teeth is not a straight line, but rather a scalloped line in which smalt cur\'et} projections of enamel fit into small concavities of the dentin (Fig. 38). Enamel contains no cells; it is a product of special enamel-forming cells. It has no circulation in the form of !>lorxl vessels or other structures, but that it is permeable to some substances has been demonstrated in studies using dyes and solutions of radioactive substances. Any clinical signifi- cance of this permeability is not at present understood. Ficunc 29,— Photomicrograph of an enamel spindle as seen s\ith the oil immersion objec- tive of the microscope. Enamel ts at left, dentin is at right of the picture. It is possible to see that the spindle was a continuation of the dentinal fiber Notice the branching of the dentinal tubules at the dentinoenamel junction (Chapter 4), Permanence of Enamel Structure Once tooth enamel is formctl the calcification is never decreased by any physiological process within the tooth. All present evidence indicates that mineral substance is not withdrawn from enamel once it has l)een deposited there. The notion that pregnancy produces a withdrawal of calcium from the teeth of the mother is not supported by factual evidence. Enamel has no possibility of anatomic self-repair following damage by injury or by caries. In the study of tooth eruption it will be seen that the cells which form the enamel in the developing tooth are lost when the tooth emei^es into the mouth, making sulisequent enamel formation imj)ossible. Clinical I.mj*ortanci; or the Structure or Enamel The structure of enamel is important clinically for several reasons: its hardness makes it resistant to the friction of onlinar>' use; the curvatures of the enamel rods probably increase the strength of the enamel ; the pres- ence of pits and fissures influences theocciirrence of tIentaJ caries; and the 54 TOOTII EN'AMEL arrangement of the enamel rods and the presence of areas of less niitifnili- 7ation influence the pattern and spml of progress of dental caries. Let us consider ho\e the structure of enamel may afToct the clinical con- dition of an individual. The high mineral content of enamel makes it a \er\’ hanl substance which is resistant to. hut no proof against, attrition (Kigs l(i, 23, 33, and 63). On the indsiil edges of anterior teeth and on the cusp tips of posterior teeth, where the teeth of opposing arches nieet forcefully in occlusion, at- trition may be sufficient to expose the umlerlying dentin. The penkymaia which usually mark the enamel on all surfaces of all teeth are ol no clinical importance. The bauds of Hunler-Schreger are visible in ground sections of teeth lie- cause of the cur\'aturcs in the enamel rods, 'rhese cur\'aturcs ]>robahly make the enamel stronger b}' rcriucing the chances of cleavage (splitting) of the enamel along the rod length. Enamel lamella: are belies'ed by some investigators to lie areas which arc particularly susceptible to dental caries. Other investigators disagree with this idea. The stripes of Retzius, being areas of slightly more organic material, tend to facilitate the lateral sprcalh is dissolved by acid and the then e\|>osed organic substance is tlestrojed by pmtcolysi-s. The acul is produced by the kinds of onil b.acteria which, in the process of their nietaljolism. convert carlHihydratcs, esiKX'ially sug.nrs, into acids. Caries-susceptible individuals h.ave in their saliva and in their dental placjues large nurnhersof these acidogcnic (arid-prorliicing) b.icteri.a. TJic bacteria which cause the danmgc arc the ones which arc located In the dental plaques. A drutal plague is a dense nccumul.ilion of micro- organisms which adheres finnly to the surface of a trxith. In mouths kept oniinarily clean, plaques are found chiefly in prtrlecled areas- around contact areas, and in pits and fissures. (Mg. 30.) A known setiucnce of events follows the ora! intake of sug.ir li> a caries- susceptible person: FckkI taken into the mouth isrctainetl in thcarc.iof the plariuci the acidogcnic bacteria of the plaque rwlure the sugars to acids; the acids in the plariue .nre in contact with the enamel surface to which the plaque is nlt.ached; and since tcKith enamel is soluble in acid, the enarm'l iwneath the plaque is slightly dissolvcil. This is the U-ginning of .ac.'irics li-sion. keiK-.itei! eating of sugar results in repcatinl ix'riods of en.'unci dissolutinn. TOOTH ENAMEL 55 _ Ficurf. 30.~Groun(^ section cut buccohnguaUy through a mandibular first premolar tooth. 'I he developmental groove on the occlusal surface of the tooth is not deep, and there is no fissure at its base. No caries is present in this groove. A plaque is retained in the groove. If the plaque is mmeraltzed, it is called calculus. Notice the direction of the dentinal tubules which extend from the dentinoenamel junction to the pulp chamber (Chapter -1). (Low power magnification.) Fjcure 3L— Ground section of a molar tooth. The developmental groove on the occlusal surface does nor hat'e a fissure at its base. No dental caries is present in the groove. Notice the direction of the dentinal tubules. The pulp bom extends far into the cusp (Chapter S), 56 TtWTU ENAMEL Now . how (Uh'S the stnicliirc ol cnamc) innucnce the ocx'urixrice anecU'd to fonji here (Figs. 32 and 33). In a rarics- susccptible person acid is pnxluccrl by acldogetiic harteri.i in the plat{ue and this acid damages the enamel walls of the fissure. This is the start of fissure caries. Id fissure caries, or in caries tKCurring around contact arc.as, or in caries in any other Ux-ation on the Irntth crown, the pattern of arrangement of the enamel rods in the alTected are.! dciennines (he direction of {senetration of the lesion. This is true Ijocause caries prrjgresbes more raphlly in arr.ts where there is Ie«s iiiiner.-tl substance to 1 k" »!is.soIvifl away -that Is, in areas of lower minendization ; and the rod she.iths and interrw! siihst.anrc are less mineralized th.an the rorls. In inicrt>s:opic study of sections of carious enamel it can K* stvn that the lesion pedetratc-s along the direction of the rod length. TOOTfl ENAMEL 57 On the smooth facial, lingual, and proximal surfaces of a tooth the enamel rotJs He nearly perpendicular to the straight or broadly conv’ex dentinoenanjel junction (Fig. 21), even though thej' show characteristic curv’atures (seen in the bands of Huntcr-Schroger). Therefore, a lesion oc- curring around a contact area, for instance, penetrates more or less in a straight line toward the dentinocnanjcl junction. Figure 33.— Ground section of a molar tootli- The fissure in the occlusal des elopmentai Rroove clearly is affected by dental canes. Notice the radiating pattern of the lesion in the enamel near the bottom of the fissure. This is due to the fanning out of the enamel rods (See Figure 21.) The dentin beneath the bottom of the fissure is destroyed near the dent inoenamcl junction (C), leaving the enamel unsupported. There is little evidence of sclerosis of the dentin beneath this lesion. 'ITie bacteria here have probably penetrated well into the dentinal tubules (Chapter 4). The plaque is indistinctly seen In this ground section. Often plaques are destroyed in the process of grinding the section. Notice attrition on the cusp tip. In the base of fissures the enamel rods still He nearly perpendicular to the dcntinocnantcl junction, but the sharp concavity of the junction in this location results in a radial pattern — a fanning out— of the rods (Fig. 21). The caries lesion starting at the base of the fissure and penetrating in the direction of the rod length, radiates as it deepens (Fig. 33). The resulting clinicn! picture may be only a small carious area visible in a developmental 58 TOOTII ENAAfEL groove; but at the dentinoenamcl junction, invisible without x-ray pictures, the broadening of the lesion has left the surface enamel unsupported (Fig. 36). We may therefore conclude that a fissure offers an ideal site for the for- mation of a dental plat|uc; and if the individual is caries-susceptilde, caries will probably develop in the fissure, and the radiating arrangement of the cnjimel rods in the area will Ciiuse a sul>surface spreading of the lesion. Now let us consider the effect of the presence of ettamel tufts and enamel spindles on the pattern of a caries terion. Dental caries always begins on F teuse J4.— Ground section of a molar tooth. In thi« tooth the caries started in the enamel at the bottom of the fissure and spread along the deniinoenamel junction. In this area the lesion has visibly penetrated the dentin, and the effect of caries is seen in the dentin o\er a broad area at the dentlnoenamel junction and pulpward. The dark region in the dentin be- neath the fissure, broad near the dentinoenamel junction and more narrow nearer the pulp, ir carious dentin. (See Chapter 4} Caries will spread doivnward, destroying the unafiecced dentin beneath it and around it. Destruction oF dentin will leave the superficial enamel unsupported. the outside surface of a tooth, never, so far as ux? know, on the inside. But as the surface enamel beneath a plaque is destroyed by caries, the lesion deepens and eventually reaches the area of tufts and spindles near the dentinoenamcl junction. As we hate seen, caries spreads more rapidly in areas of low mineralization than in areas of high mineralization, and tufts and spindles are unniineralized, or hyponiincralizcd, structures. Therefore, there is a horizontal spread of the lesion in the enamel near the dentino- enamel junction. The enamel which is superficial to the deep spreading lesion often shows no injury c.\cept at the point of initial damage under the plaque; but for an estendetl area around this point of entry, the intact superficial enamel is undermined bv caries and is therefore unsupported (Fig. 36). With these several structural characteristics of enamel tending to pro- duce a spread of caries beneath the enamel surface, it is easy to sec how one day an individual may bite on such an area an^l part of the surface of a TOOTH ENAMEL 59 seemingly (to the patient) good tooth will cave in. An understanding of the subsurface spread of dental caries* motivates the dental hygienist to instruct patients of the importance of regular dental examinations and of the necessity for prompt attention to ex'cn seemingly small cavities. Figure 3S.— Ground section of a rnandibuhr incisor tooth cut mcsiodistally. The intact ti»th had mamelons that 3ppe.ired unusually pronounced. The section shows deep pits in the depressions betiveen the mamelons, with a scry thin layer of enamel at the base of the pits. In making ground sections of a number of teeth that had unusually pronounced manielons, it was found that such pits were generally present. Notice the curvature of the dentinal tubules in the tooth crown. In the root the tubules are nearly straight and arc di- rected slightly apically from the cemeniodentinal junction. *A word about the term Jentat cams: Canes is the correct form of the word. You may say that a person has caries; or that he has a carious tooth; or that he has a casity in a tooth. Or you may say that caries is (singular verb) present in a mouth. But )ou never say a person has “a carie”*— any more than you say a person has a measle or a mump. 4 Dentin Location Dentin is located in both the crown and the root of a tooth, making up the bulk of the tooth. In an intact tooth the dentin is not visible because in the crown it is covered by enamel and in the root it is covered by ceinentum (Figs. 19 and 36), COMPOSTION Dentin is a calcified tissue, and like all calcified tissues of the body it is composed of both organic and inorgamc (mineral) substances. Although not nearly so hard as tooth enamel, dentin is harder than bone. Mature dentin is about 70 per cent inorganic substance and about 30 per cent organic material and water. (Compare these figures with those given for enamel in Chapter 3.) Structurc of Dentin Dentin is made up of a calcified matrix which is perforateii by dcjil/nnf tubules. The dentinal tubules contain The dentin matriv is an organic framework composed of verj' small fibrils surrounded and held together by a structureless cementing substance. During the development of dentin, the organic matrix is formctl first and then minerals in solution arc deposited in its cementing substance. These minerals then cry’stalize out of solution and the cry’stals form on and around the minute fibrils. Thus the organic matrix of the dentin becomes hardened and the dentin is a calcificti tissue. The mineral substance of dentin is similar to the mineral substances of enamel, cementum, and lione. Dentin is perforated by innumerable holes called dentinal tubules which contain dentinal fibers (not to be confused with the fibrils of the dentin matrix). The dentinal tubules He close together and extend from the tooth pulp to the dentinoenamel junction in the crown of the tooth and to the dentinocemental junction in the root (Figs 1, 16, 23). At their outer ends the dentinal tubules are divided into a number of branches (Figs. 28 and 29). Much smaller branches connecting adjacent dentinal tubules are often found along the length of the tubules. In diameter the dentinal tubules measure about 4 microns (4/25,000 inch) at the pulpal ends and somewhat less at the outer ends, fn the cusp tips in the tooth crown (Fig. 34) and in the apical half of the root (Figs. 35 and 36) the dentinal tubules are nearly straight and aix* arranged nearly perpendicular to the dentinoenamel or dentinocemental junction, fn the facial, lingual, ( 60 ) DENTIN 61 62 DENTIN mesial, and distal areas of the croum (Figs. 30 and 35) and in the cer^■^caI portion of the roots (Fig. 36) the dentinal tubules are S-shaped. The outer tubules tubules arc always occlusal to the pulpal ends of the A dentinal fiber (also called a Tomes' fiber) occupies each dentinal tubule Figure 37 A diagrammatic illustration of a small portion of tooth pulp showing the structure of the pulp and the relation of the odontoblast cells of the pulp to the dentin and name . ooth pulp as seen under high power magniRcation B. Diagrammatic Illustration of the small enclosed area shown in J Tlie odontoblast cells of the pulp ha'C ^toplasmic process«. the dentmal fibers, which extend m the dentinal tubules to the . 1 , j The fibers are brandied at their peripheral ends The portion of !pfndk'°^ dentinoenamel junction into the enamel is an enamel ^^I’P' ^ is the cytoplasm of a pulp cell. Those ce s of the pulp which lie next to the dentin ha\ e the somewhat surprising distinction of being cells of the dentin also. They are named odontoblasts. 1 he nucleus of the odontoblast cell remains in the pulp surrounded by part ol the cell s cytoplasm. The remainder of the cytoplasm of the odontoblast cell IS stretched out like a long, thin tail and enters a dentinal tubule (big. 43). This cytoplasmic e.\tension of the odontoblast is called a DENTIN 63 dentinal fiber (Tomes’ fiber). Each dentinal fiber extends through the tubule to the dentinoenamel or dentmocemental junction. At their peri- pheral ends near the dentinoenamel and dentinocemental junctions the dentinal fibers are branched just as the dentinal tubules are branched (Fig. 37). In some places in the crowi of a tooth the peripheral ends of some dentinal fibers penetrate the dentinoenamel junction and protrude into the enamel. Here they appear as short, slightly thickened structures. These ends of dentinal fibers in the enamel are enamel spindles (Figs. 27, 28. 29 and 37). Fiourf 38. — i’hotomicro;rr3ph of part of a pround section of a tooth cro'%Ti. The enamel ts dbided from the dentin by the scalloped dentinoenamel junction. Curvatures in the enamel rods arc evident and in some areas cross-stnations of the rods can be seen. In the dentin at the bottom of the picture are irregular areas (dark) of interglobular dentin. (Medium power magnification.) _ In the crowns of some teeth the dentin has in it spots which are uncal- cificd or are hypocalcified (hypo = under, less than ordinary). These uncalcified spots, irregular in shape, usually occur in a layer a short dis- tance inside the dentinoenamel junction (Figs. 16, 17, 18, 38 and 46). In this location such areas of uncalcific«l by the dental histologist Sir John Tomes (1815-1895) who thought that this region 64 DENTIN of the dentin had a granular appearance. Later it was found that this area is made up of verj’ small unralcificci spots of dentin. Tomes’ granular layer has considerable clinical importance which will be discussed later. In the root dentin of some teeth a short distance beneath Tomes' granular layer there may be also a layer of interglobular dentin similar in size and configuration to the interglobular dentin found in the crowm of the tooth (Fig. 39). A modified type of dentin known as secondary denlin is usually found in older teeth along the pulpal wall of the dentin. Aside from the fact that in varying degrees it has fewer and less regular dentinal tubules than the first dentin produced, secondary dentin is similar to the earlier dentin. Secondary dentin may be formed throughout the life of the tooth as long FicuRfc 39.— Photomicroffraph of part of a ground section of tooth root. The cemcntinn the relatively narrow hqht bjnJ at the left of the picture; the dentin is the wide area at tne right of the picture In the dentin close to the cementum is the band of closely packed small areas of uncalcified dentin called Tomes’ granular layer. Deeper in the dentin are larger, ir- regular areas of interglobular dentin (dark ateas) Notice that the dentinal tubules are straight, and are not ijuite at ritht angles to the cementodentinal junction. In the cementiim ate small lines, perpendicular to the cementuin surface, which are spaces once occupied by Sharpej’s fibers (Chapter 6). (High power magnification.) DENTIN 65 as the tooth pulp is intact. In posterior teeth it is formed most frequently in the pulp homs—that is, in the part of the pulp extending toward the cusp tips— and in the floor of the pulp chamber (Figs. 18, 23. and 44). In anterior teeth it is formed most frequently beneath the incisal edge when there has been considerable attrition (wearing off) (Fig. 40). Secondary dentin may occur also on the pulpai U'all of dentin in areas where dental caries has started to penetrate the dentin at the dentinocnamel junction (Figs. 19 and 45). The formation of secondary dentin is often a result of the reaction of the tooth pulp to the irritation of attrition or of the caries process. Secondary dentin Fjoure -10.— Photomicrograph of a ground section (cut fadolingually) of a canine tooth Extensive attrition has resulted in the loss of the enamel and part of the dentin of the cusp The formation of secondary dentin in the incisal part of the pulp cavity has protected the pulp from exposure. (Low power magnification.) Sclerotic dentin is a modified dentin found in some areas of most old teeth and sometimes in young teeth. Dentin is said to be sclerotic when the dentinal fibers have degenerated and the dentinal tubules have become filled with calcium salts. Sclerotic dentin is often found beneath worn enamel such as occurs in the incisal area of anterior teeth, and beneath slowly progressing dental caries in locations where sccond.ary dentin is lieing produced on the pulpal wall (Fig. 19). Sclerotic dentin is also found l>cncath 'I'omcs' granular layer in the ccrx’ical area of older teeth where the cen'Ical ceincntum has become cxixised to the oral ca\-ity as a result of recession of the gingiva. (Chapter 10). In the mandibular incisor tooth shown in figure 41 the dentinal fibers have degenerated licneath the worn incisal edge, but the dentinal tubules have not become filled with calcium salts. Such an area has been referred to as a dead tract. The pulp beneath the dead tract in this tooth is protcctwl by the presence of secondary dentin. 66 DCrJTIN At the cen’i\ on the facial side of this tooth there is a change in the dentin beneath the cerv’ical abrasion. The white area, indicated by the pointer, is probably a dead tract. Outside of the dead tract, nearer the sur- face of the tooth, the dentin appears to be sclerotic— i.e., the tubules arc filled, or are becoming filled, wth calcium salts. Dead tract Dead tract and Sclerotic dentin FjfiiiRE 41 —Photomicrograph of a ground section of a mandibubt mnsoi tooth cut faciolingually Beneath the uorn incisal edge is a dead tract in the dentin Deposition of secondary dentin beneath the dead tract has protected the pulp from damage. On the faiial side of the tooth in the cersical area there is abrasion thecemenium is pone and some of the dentin has been worn away. (See Chapter lO, Figure 77 ) Beneath this abraded surface there IS an alteration in the dentin T'lear the pulp is an area of dead tract, which appears white Superficial (nearer the surface) to this dead tract the dentin seems to be sclerotic. On the pulp wall in this region secondary dentin has been produced; it protrudes into the pulp cavitv in- side the dead tract Notice the curvature of the dentinal tubules in the cen ic.tl area (Low power magnification ) CuNictL Importance or the Structure or Dentin From the point of view of the tlental hygienist the structure of dentin is important because (1) it influences Iwth the pattern of a carious lesion and the speed with whicli dental caries destroys a tooth ; and (2) it accounts for the sensitivity frequently c.\perienced by patients during the perform- ance of an oral prophylaxis or during the eating of hot or cold foods. In the preceding chapter it was explained that dental caries is a disease of the hard tissues of the tooth: acWogenic bacteria convert sugars into DENTIN 67 acids which dissolve the minerals out of the hard tooth tissues; and pro- teolytic (protein destroying) bacteria destroy the organic component of the hard tooth tissue in the same area. When at any point the caries process has penetrated the enamel to the depth of the dentinoenamel junction, the caries-producing bacteria will also have reached this depth and will come in contact with the peri- pheral ends of the dentinal tubules. Since the bacteria are smaller than the diameter of the dentinal tubules they enter the tubules. The dentinal fibers which occupy the tubules are destroyed. The Ixacteria travel pulp- ward in the opened tubules and the dentin is slowly destroyed. Because the bacteria follow the course of the dentinal tubules, a carious lesion originating around a contact area or in the cervical area of a tooth extends in an apical direction as it approaches the pulp. Xotice the direction of the dentinal tubules In Figures 19. 36, and 41). The progress of dental caries through the dentin is often retarded, but not stopped, by defensive reactions that take place in the pulp. One such reaction is the production of sclerotic dentin. From the pulp tissue calcium salts arc deposited in the dentinal tubules causing them to become filled with mineral substances, and so the progress of bacterial invasion is slowed do^\'n. Another defensive reaction against caries is the formation by the pulp of additional dentin, called secondary denlin, at the location where the bacteria-filled tubules reach the pulp. This increases the thick- ness of the dentin wall and helps to protect the tooth pulp, for a time at least, against the invasion of the 'toplasni of the odontoblast remains surrounding the nucleus in the usual manner of cells, the remainder of it is stretched out in a long, thin tail which enters a dentinal tubule and extends in the tubule to the dentinoenamel or the dentinoccmental junction fPigs. 37 and 43). This cytoplasmic tail of the odontoblast is called a dcntiml fiber. When a dentinal fil>er crosses the dentinoenamel junction into the enamel, the portion of it that is in the enamel is called an euamel spindle (Figs. 28. 29 and 37). fflaiiii Hiiii 70 TOOTH PULI^ Korff's fibers are minute, rope-Hke, corkscrew-shaped structures which lie among the orlontohlasts. They are produced by the merging of the fibrils of the fibrous intercellular substance of tlic pulp. Korff’s fibers may be made visible for niicn)scoptc examination by special staining techniques applied to thin sections of young pulp tissue. They seem to have an important function in the fonnation of the (ientin matrix. (Sec Chapter 11). Figure 43 A section through the pulp and predentin of a young hufnJn tooth. The odonto- bl.-ists are dark-stained columnar cells. TTie nuclei and part of the cytoplasm of the odonto- blasts aie m the pulp. Long fibers of cytoplasm extend into the dentinal tubules. These are the dtiittnal fibers, or Tomes’ fibers. Blood vessels are plentiful in young pulp (Fig. 42). Small branches from the superior or the inferior alveolar artery enter the tooth through the apical foramen. They pass through the root canal to the pulp chamber and divide into capillaries. The circulating blood Is collected into veins which pass out from the pulp through the apical foramen Lytnphalic vessels also have been demonstrated in the pulp. Along with the blood vessels nerves enter the tooth pulp through the apical foramen giving it a rich ner\e supply. These arc branches of the second or third division of the fifth (trigeminal) cranial ncr\-e. Just TOOtH PULP n beneath the layer of odontoblast cells around the edge of the pulp the nerves m the pulp form a network wdth some nerve fibers having endings on the odontoblasts. This arrangement helps to account for the sensiti^-itj’ of the dentin, since the odontoblasts have part of their cytoplasm in the dentmaJ tubules. Whether or not nerves extend Into the dentinal tubules is a subject of controversy. pevRE 44.— A longitudinal section of a human molar tooth This «'as an old tooth. The soft pulp tissue has been shrunken and damaRed in the preparation of the tissue, but the den- ticles (pulp stones) are clearly seen. Secondary dentin is seen in the pulp horns. This section was cut at such an angle that it does not pass through the narrow toot canal, but of course a toot canal was present. The lo« er left corner of the picture Is the area of the root furcation. Denticles are calcified structures of an irregularly rounded shape com- monly found in tooth pulp (Figs. 19 and 44). They may lie free in the soft tissue or they may be attached to the dentin wall. They vary in shape and size, the sire increasing with the age of the tooth. Generally they are regarded as of Uttle clinical importance e.\cepting as they may interfere with endodontic treatment. They arc never a source of infection. Disuse colcifications are small, thin scatterings of catcifieci material 72 TOOTH POLP frequently found in the pulps of older teeth, usually in the root canals (Figs. 19 and 45). Clinically they are usually unimportant. Functions or the Pulp Tooth pulp has several functions. For purposes of description these functions may be listed as follows: (1) formative, (2) sensorj’, (3) nutritive, (4) dcfensi^ e. The formative function. 7ooth pwlp forms the dentin of the tooth, as will be explained in the discussion of tooth dev'clopment to be found in Chapter 11. We will see that KorfPs filers, which are composed of the fibrillar material of the intercellular substance, gh'c rise to the fibrils of the dentin matrix. The pulp also produces the amorphous cementing substance of the dentin matrix. And the odontoblast cells of the pulp ha\e their cytoplasm stretched out into thin, thread-like extensions which are the dentinal fibers that occupy the dentinal tubules. The sensory function. Tooth pulp is very sensitive to external stimuli. The nerves in the pulp are responsible for the sensation experienced by an individual when a stimuUts is appUetl to the tooth. It is an interesting (act that the sensation educed by stimuli received by a tooth pulp is a sensation of pain. A person cannot differentiate between e,xtrcmes of heat (hot coffee) and of cold (icecream) applied to the tooth. If sensation is experienced it is mert'ly pain in both cases. Slight pressures on a tooth will produce a sensation of pressure or touch. Most of this sensation is due to pressure on the periodontal ligament. The iiitlrtlive function. Since tooth pulp is a living tissue nirh a blood supply, it receives nutrients from the blood stream. It mny be suppos^ that nutrients enter the dentinal tubules either by vvay of the cytoplasmic dentinal fillers or around the outswle of the dentinal fibers. Such nutrients may be carried in this way as far as the dentinoenamel and dentino- cemental junctions. There is a good deal yet to be learned about this subject. Since unwarranted deductions are sometimes made from a set of briefiy presented facts such as those given above, it should be made clear that whatever the manner of nutrition of the dentin this is a matter entirely apart from the question of dental caries. We cannot associate the nutritive function of the pulp and the general nutrition of the indiv idual with the presence or aljsence of tlcntal caries activity. Such association should not be attempted from the above discussion. Dental caries is a disease which starts on the outside surface of the tooth and is a process of an entirely different nature. The defensive function. Defense reactions of the pulp arc e.xprcssed in several ways, pulp may show an inflammatory' reaction ; pulp may' change the character of existing dentin; pulp may produce additional dentin (secondary dentin). In case of pulp damage the pulp shows an inflaminatory reaction. Cells appear which arc commonly found at any site of inflammation. Some of these defense cells arc derix'ed from histiocytes and undifferentiated mesenchymal cells of the pulp; some arc carried into the pulp by' tlie blood TOOTH PULr 73 stream from their points of origin in the bone marrow and lymph nodes. As the defense cells become effective in controlling the damaging process, the pulp may produce sclerosis of the existing dentin and may also lay do\vn secondary dentin along the pulpal wall. Sclerosis (sclero = hard) of the dentin involves the filling in of the dentinal tubules, usually in a restricted area, with calcium salts so that the dentin in this area is a solid calcified tissue instead of a tissue perforated with tubules which contain dentinal fibers (Fig. If)). Sclerotic dentin usually occurs beneath a carious lesion and its presence tends to retard the progress of the destruction of the tooth tissue. The stimulus to the pulp which causes the production of sclerosis is received through the dentinal tubules. Pulpal to the sclerotic dentin the pulp may produce, as a defensive re- action, varying amounts of secondary dentin which gives the pulp addi- tional protection against external irritation. The formation of secondar>' dentin and sclerotic dentin occur in aging teeth, where infection is not a factor, as a result of the stimulation produced by attrition (Fig. 41). Ace Changes in the Pulp Just as age brings about changes in other parts of the body.it brings about changes in the pulps of teeth. These changes are universal and normal and are not to bo regarded as pathologic. The continued formation of secondary dentin with increasing age causes the pulp chamber to become smaller and the root canals to become narrower. In some old teeth which show heavy attrition or dental caries of long standing, the pulp chamber may be entirely filled by the deposition of secondary* dentin. The cells of the pulp, very numerous in young teeth, decrease in number with age, and the fibrous intercellular substance is relatively increased. Old tooth pulps are composed mostly of fibrous intercellular substance. The blood supply of the pulp decreases with age. Denticles are larger and more numerous in old teeth and diffuse calcifications are increased. These changes of the pulp do not alter the function of the tooth. Clinical Importance of the Pulp A fully developed tooth may function for many years after its pulp has l>ecn removed and the pulp canal filled. Although the enamel of the tooth becomes more brittle, its function is not affected by the loss of the pulp. The cementum is not alTcctcd, nor is the process of continued cementum formation. A tooth without a pulp cannot, however, produce secondary* dentin or sclerotic dentin. Loss of a tooth pulp usually comes about as a result of caries or of tooth fracture, w'ith accompanying pulp infection. Careful treatment by the dentist is essential in either case to prevent the infection from travelling through the root canal and apical foramen into the tissues surrounding the tooth, w'ith a consequent Joss of the tooth. Cementum Location Cementum is the thin layer of calcified tissue, often about as thick as a coat of paint, which makes up the surface of the root of a tooth fFig. 16, and 46). It overlies and is attached to the root dentin In the area of the ccinentoenamel junction the cementum may ha\e anj’ one of three rela- tionships with the enamel of the tooth crown; it may evactly meet the enamel; it may not quite meet the enamel, leaving a little dentin exposed, or it may slightly overlap the enamel fFigs. 16. 18. 45, 46, 47 and 47/1). This last arrangement is the most common. FicuRF 45. — U'lagrainmatic tlrawingof a longitudinal faciolingual section of a madllary first premolar tooth. The tip of the lingual root shons a vert’ large amount of cementum, which is sometimes referred to as hypercementosts. For a photomicrograph of such thick Cementum sec Figures 48 and 50. CEMENTOil 75 Composition Like enamel, dentin, and bone, cementum is made up of organic matrix which contains crystallized mineral substances. Cementum may have cells, calle'tre is called Cfllnlar cementum, and cemen- tum which has no ccmcntocytes is called acellular cematlum {Figs. 16, 17, 18, 45, 48, and 49). 78 CEMENTUM The outer surface of ceiuentum, next to the penodonta) ligament, re- mains less calcified than the rest of the cementum and is called cementoid. Visible in the cementum by microscopic examination are stnictures called Sharpey' s fibers (Fig. 39). These are the ends of bundles of fibers of the periodontal ligament which have become embction of the bone of the tooth soctet, hut also in a localized resorp- tion of the tooth root. Underlying dentin, as well as cementum, may be rcsorbeti in some cases. When the cause of the resorption is removed, if the damage has not been too e,\tcns{ve, new cementum may be laid down over the danjaged area, replacing both the l(»t cementum and the lost dentin. 80 CEMENTUM The presence of cementoid on the outer surface of the root is an im- portant factor in the clinical success of orthodontic treatment. Cementoid, because it is only slightly calciSed, undergoes resorption less readily than bone. When the orthodontist establishes continued light lateral pressure on a tooth by the use of orthodontic appliances, the pressure is transmitted to the periodontal ligament and to the bone of the tooth socket, and the bone, not the tooth root, is resorbed. On the opposite side of the tooth Figure 50.— A photomicrograph (low power) of a ground section of a mandibular Incisor tooth cut mesiodistally Before this tooth v%as cot the apical half of the root looked like a round ball on the end of the tooth. This is hypercementosis. Notice, also, the conspicuous mametons, and the pvt between the left and center mamelon. socket, where the pericxlontal ligament is under tension (pull) due to this lateral movement, addition of bone takes place, and there is thereby an actual change in the location of the tooth s«:kct and in the position of the tooth (Chapter S). The dilTcrence betw'ccn the pressure used by the or- thodontist and the pressure which causes damage to the tooth root lies chiefly in the difference in the sewrity of the pressure. Cementum is sometimes formed in excessive amounts. Excessive cc- mentum is variously called hypercemenlosis, excementosis, or cementnni hyperplasia. It may occur on all oron only a few of the teeth in any mouth; and it may occur over the entire tooth root or only in localized areas (Fig* CEJfENTCm 81 45). The cause of excessive cementum formation is not fully knowai. A large amount of cementum sometimes is useful in that it may furnish addi- tional attachment for periodontal ligament fibers. Again, it may be a handi- cap. If it occurs as a spicule protruding from the side of the root and inter- locking in a resorbed area of the lamina dura (the bone of the tooth socket), or if it occurs as excessive cementum at the root apex producing a ball- shaped root end (Fig. 50). it may create a problem in e.vtrdctlon if for any reason the tooth must be removed. CemciUicles are small bodies of cementum which are sometimes found in the periotlontal ligament. They are usually regarded as of no clinical importance. 7 Periodontal Ligament (Periodontal Membrane) Location The pcrioclnntaniganient (peri = around; odontos = tooth) is a layer ol connectixe tissue about j^-o inch in width which surrounds the root of a tooth, occupying the space between the tooth root and the bone of the tooth socket (Figs. 51, 55, 65) The periodontal ligament is also called pertodontal membrane. Structure of the Periopontal Lioamcnt The periodontal ligament is made up of cells and of amorphous and fibrous intercellular substance The most outstanding constituent is the fibrous intercellular substance which makes up the fibers of the periodontal ligament. Among these fil>crs are located fibroblast cells, blood vessels, lymph vessels, nenxs, and in some areas small groups or strings of epithelial cells, and sometimes cementicles In addition to these stnictures there arc often speciallitcd cells, which function in the formation of cementum (ccinentoblasts) and of bone (osteoblasts) ; and sometimes there are special- ized cells assfjciated with the resorption of cementum (ccmcntoclasts) and of bone (osteoclasts). In width the periodontal ligament has l>een found to range from 0 12 to 0.53 mm. The width varies on different teeth and in different areas around the same tooth. Decreased (unction of the tooth seems to be ac- companied by decreased width of the periodontal ligament. Buntllcs of periodontal ligament fibers are attached at one side of the periodontal ligament to the cementum covering the tooth root; and with the exception of certain fibers around the cerx’bc of the tooth they arc at- tached at the other side of the periodontal ligament to the bone of the tooth socket (Fig. 51). This attochincnt takes place when the cementum and the bone are fonning* ends of bundles of the periodontal ligament fibers become entrapped in the forming hard tissue. This attachment sen cs to hold the tooth firmly in the jaw. These attached heavy fiber I)undlcs of the periodontal ligament arc referred to collcctix-ely as Sharpey's fibers (Figs. 52. 53). While large bundles of hirers are attached to both cementum and Irone, it does not thereby follow that each fiber of each bundle extends unin- tcrrupterl from cementum to bone. As a matter of fact, the individual filrers extend from the cementum or from the bone towanl the center of the periodontal ligament where probably their ends are intenvoven wi(h ( 82 ) PERIODONTAL LlGAilENT 83 Mandibular central Incisor Interdental papilla Transseptal fibers Horizontal fibers Area X (See Pig. 52) Oblique fibers Apical fibers Marginal gingiva Free gingival fibers Alveolar crest fibers Alveolar process Oblique fibers Figure 51.— A diagrammatic illustration of the arrancement of the periodontal ligament fibers around the tooth roots of the mandibular incisors. The Nsidth oF the periodontal ligament is exaggerated in order to show the direction of the fibers. .1. A facial view. B. I’rosimal view*. Area X In Figure J is shonn in detail in Figure 52. (Adapted from Noyes. Schour, Noj-es.) PERIODONTAL UGAAIENT ends of fibers from the opposite direction. This area of Intenvoven fibere is called the intermediate plexus. The fact that the bundles are spliced in the center of the periodontal liginent enables a readjustment of these fibers as a tooth moves occlusaily in eruption. Around a nonfunctioning tooth (one \\'hich is not in occlusion with the teeth in the opposing arch) the periodontal ligament fibers are relaxed and & -a. Sharn^v’B rih^rc •' Y'. ~~ — Dentin *• Cementiim . ^ • ) - Lamina rfura . * ‘ 1."'. - Figure 52.— Diasrammatic dtawin*' of a small area of periodontal UEimenl shooi ing Sharpey': fibers Enlargement of area X m figure SLf FiGCRt 53 —A photamiccogtaph of an area of bone in vshich numerous bundles of peno- clontal ligamenc fibers are embedded. The embedded ends of these fiber bundles are known as Skarpty’s fibers. As seen here the strength of this attachment is impressive. (IliRb ptn'Cr magnification ) PERIODOXTAL LKUSIEN'T 85 wavy, with no definite orientation (I'ig, fil); but around a tooth which is in heav'y function the larger bundles arc stretched straight and have a characteristic orientation on different areas of the tooth root. These large, well-oriented bundles of fibere arc referred to as the principal fibers of the periodontal ligament. pRiNcii’AL Fibers of the Periodontal Ligament The principal fibers of the periodontal ligament around a heavily functioning tooth hnve such a clear and consistant arrangement that they hav'e been described as being composed of six groups of fibers, each group being named according to its location and orientation : (I) the free gingival fibers, (2) the transseptal fibers, (3) the alveolar crest fibers, (4) the horizontal fibers, (5) the oblique fibers, anr! (6) the apical fibers. Stud>’ the position of these groups of fibers in Figures 5 lA and B as you read their description in the text. 1. Free gingival fibers arc located around the cervical part of the root. Bundles of these fibers are embedded at one end in the cementum. The fibers extend from the cementum out into the gingiva which surrounds the neck of the tooth. In the gingiva the hea \7 bundles of fibers separate into individual fibers and intermingle with the connective tissue fibers of the gingiva. E.\amine Figures 51/1 and 73 for the location and arrange- ment of the free gingival fibers. N'oticc that a pressure applied on the incisal (or occlusal) part of the tooth would cause the free gingival fibers to be stretched taunt with the result that this group of fibers would func- tion to hold the gingiva firmly to the tooth surface. 2. Transseptal fibers arc located just apical to the gingival fiber group and are on the mesial and distal sides of the tooth only. Bundles of these fibers arc embedded at one end in the cementum of one tooth and at the other end in the cementum of the adjacent tooth. A\’lth the e.\ceptlon of the maxillary and mandibular central incisors, where the mesial sides of the right and left centrals are connected by these fibers, the transseptal fibers extend from the cementum on the mesial side of one tooth to the cementum on the distal side of the adjacent tooth. Notice in Figure 5iA how they cross over the top of the bone between the teeth. These fibers help to maintain the teeth in their proper relationship to one another. 3. Alveolar crest fibers are located at the level of the alveolar crest (the margin of the Iwne winch surrounds the tooth root). These fiber bundles arc embedded on one side of the periodontal ligament in the cementum of the tooth root and on the other side of the periodontal ligament in the alveolar crest. They are of course found all around the tooth. Examine I'igurcs 51/1 and B and notice how the arrangement of this group of fibers helps to resist horizontal movements of the tooth. 4. Horizontal fibers arc located apical to the alveolar crest fillers. They are embedded in the cementum of the tooth root and in the bone of the tooth socket. They lie in a horizontal position relative to the jaw bone. They are of course all around the tooth root. Examine the arrangement of these fil>crs in Figure 51/1 and you will see how they function to resist horizontal pressures appUetl to the tooth crown. 86 PERIODOJJTAL UGAMEST lionc of looOi socVel Haversian systems Acellular cemcnliii-' Penodontal ligament Dentin Area A Done resorption and repair Area B Bone resorption and no repair Fiquke 54.— a photomicrograph of a small area of tooth root with the associated perio* dontal ligament and lamina dura The direction of the periodontal ligament libers tells us that the crown of the tooth was at the bottom of the pictured area Notice the resorption of the bone of the alveolar crest at ereo B. At arta A there has been bone resorption as far as the end of the pointer line, and then there was bone apposition (addition) on the surface of the resorbed area (repair). 5. Oblique fibers are located immediately apical to the horizontal group of fibers. They arc attached to the cementum and to the bone of the tooth socket and they nm in an oblique, or diagonal, direction. Look at the orientation of these fibers in Figures 5i>l and B. the end attached to the bone is always more toward the tooth crown than is the end attached to the cementum. Imagine a pressure applied vertically to the incisal (or occlusal) surface of the tooth. Such a pressure would stretch the oblique fibers taunt and the tooth would be literally suspended in its socket (Fig. 54). The result of such vertical pressure is a pull on, rather than a pressure on, both the cementum of the tooth root and the bone of the alveolus (tooth socket). This pull (tension) is fortunate, Ivecause con- tinued pressure on bone ordinarily results in bone resorption. This group of strong oblique fiber bundles prev-ents the apex of the root from being jammed against the bottom of the socket. At the transition between the oblique fibers and flic radiating apica) fibers there ts a small region in which the fil«:rs again exteml in a horizontal plane. Although these fibers are usually not named In the arbitrarj’ classification which has Ixren given to the periodontal ligament filwrs, TERIODONTAL LIGAMJvNT 87 they function with the previously described horizontal fibers in stabilizing the tooth, (>. Apical fibers radiate around the apcK of the tooth. At approximately right angles to their attachment in the ccrnentum they cvtend to their attachment in the bone at the base of the alveolus. As you can sec by examining Figures 51A and 3 these apical fibers resist any force tending to lift the tooth from the socket, and function with the fibers of other groups to stabilize the tooth against forces tending to produce a tilting movement. Scattered among the principal fibers of the periodontal ligament arc other smaller fibers which have no distinct orientation. Structures in the Periodontal Ligament Blood vessels of the periodontal ligament are branches of the superior or the inferior alveolar arier>' and vein. They enter the periodontal ligament at various locations. (1) at the fundus (bottom) of the alveolus, along with x’cssels which supply the tooth pulp: (2) through openings in the bone of the sides of the alveolus, coming from the bone marrow spaces; and (3) from the deeper branches of gingival blood ^'esscIs which pass over the alveolar crest, Lymplialie vessels follow the path of the blood vessels. Irenes of the periodontal ligament generally follow the blood vessels. They are sensory nen'es from the second or third division of the fifth (trigeminal) cranial ncr\'e. They proxiclo a sense of touch— that is, they enable an individual to l>e aware of a touch or a tap given to the tooth. Hesls of Malesses are small groups of epithelial cells which are seen in microscopic examination of the periodontal ligament (Figs. S2, 74 and 76). They arc sometimes called epithelial rests. .At times such epithelial cells are seen microscopically as strings of colls rather than as round groups of cells in the periodontal ligament, in which case they are referred to as remains of Herlu'ig's epithelial root sheath (Fig. 745). Whatever name is applied to these epithelial cells found in the periodontal ligament, they have come to be rccognizctl as cells derived from the enamel organ which produced the tooth enamel at the time the tooth was forming (Chapter 11). Their presence may lie important pathologically in the formation of certain tumors and cyst linings. Ccmenlicles are minute calcifietl IkkHcs sometimes seen in microscopic examination of the periodontal ligament of older individuals. Cemcn- ticles may be attach^ to the cementurii, or they may be entirely separate from the tooth root. Their size vnries but their shape is oixlinarily spherical. Usually they arc not consider'd to be of clinical importance. Osteoblasts (ostco = *bonc; blast = germ) arc specialized connective tissue cells which arc found at the surface of bone in locations where bone formation is occurring. They may be seen In the periotlontal ligament at the surface of the bone of the tooth socket in locations where bone is Iwing laid doum. Osleodasls (clast « break) are specialized connective tissue colls which border Ixmc which is 1>eing resorbed. They may occur in the jx-riodontal ligament next to the Ixinc of the tooth siKkct in loca- 88 PERIODONTAL LIGAMENT tions where bone resorption is taking place. Similarly, ccmenloblasls arc specialized connective tissue cells which accompany cementum forma- tion, and cemenloclasts are specialized connective tissue cells which accompany cementum resorption. These cells occur in the periodontal ligatnent at the surface of the ccinentuni where cententum formation or cementum resorption is taking place. Functions and Clinical Importance of the Periodontal Ligament The functions of the periodontal ligament may be described under five headings- (1) supportive, (2) formative, (3) resorptivc, (4) sensor)-, and (5) nutritive The supportive function of the pcriociontal liganvent results from the ingenious arrangement of its principal fibers. As you hav'e seen, the fibers are so arranged that functional pressure on the tooth crown from any direction produces a tension (pulling) of certain fiber groups. Con- sequently, pressure on the tooth crown is transmitted to the bone of the tooth socket and to the cementum as a pull. Due to this fiber arrangement which suspends the tooth in its socket the tooth is not pressed against tlie bone of the socket wall during the process of biting and chewing. These sturdy principal fibers of the periodontal ligament which separate the cementum of the tooth root from the w'all of the socket by about 1/100 inch are able to withstand the tremendous force produced by the powerful jawniusclos in closing the jaw's. It has been determined that the maximum biting force between the molar teeth in a group of 100 adult American men varied around ISO pounds. In a group of Eskimos on their original native diet the force was much greater. A sudden excessive pressure applied to a tooth crown, such as in case of an accidental blow, may damage the jwriodontal ligament sufficiently to loosen the tooth. 7'hc formative fundion of the periodontal ligament is seen in both the developing tooth and in the adult functioning tooth. During tooth development the periodontal ligament provinces both the cementum of the tooth root and the bone of the tooth socket. In the functioning tooth the periodontal ligament is able to produce cementum at any time during the life of the tooth: and it maintains the bone of the tooth socket by producing new bone following bone resorption (Fig. 54). The resorptive function of the periodontal ligament accompanies the fomintivc function. WTiereas tension (pull) on the periodontal ligament fibers tends to stimulate cementum and Ixme formation, pressure stimu- lates l)one resorption. Severe pressure proviuccs rapid lione resorption, and sometimes may cause resorption of the more resistant cementum. If sufficiently severe, pressure may destroy areas of the periodontal ligament. More will be learned about the processes of bone formation and bone resorption in the discussion of tionc and the alveolar process (Chapter 8). The sensory fundion of the periodontal Hganieiit is seen in the ability of an individual to estimate the amount of pressure in mastication and to identify which of several teeth receives a slight tap with an instrument. PERIODONTAL LIGAMENT 89 The nutritive function is sen-ed by the presence of i)lood vessels in the periodontal ligament. It is evident that without the periodontal ligament a tooth cannot be retained in its socket. Localized destruction of the periodontal ligament may be repaired by the formation of new tissue when the cause of the destruction is removed. Localized detachment from the cementum of the principal fibers of the periodontal ligament may be followed by fiber reattachment if removal of irritating factors permits the formation of new cementum. Extensive destruction of the periodontal ligament may result in the necessity for the removal of the tooth. 8 Bone and the Alveolar Process BONE The word bone is used to designate lx)th a tissue and an organ. Bout, a tissue, is one of the connective tissues. It is made up of (1) an organic matrix which calcifies and (2) osteocytes (bone cells). A bone, an organ, such as the mandible, for exaniptc, is composed of bone tissue: it contains in its center bone marrow, and it has closely associated with it blood vessels and ner\-cs. Gross Structurh or a Bone (an oi^an) Bones are solkMooking organs, but they are not solid structures through* out. The bone tissue of which bones are composed may be described in two classes: (1) compact bone anecula? and bone marrow in its center is much lighter in weight than would be the same organ composed of solid bone tissue throughout. AIso,^ the presence of the bone marrow makes available to the bone tissue nutrition from blood vessels which are located inside the organ as well as from blood vessels w’hich lie on the outside surface. The outside surface of all Ijones is coverctl by a thin connecti\e tissue membrane called the periosteum (peri = around; ostcum = bone). The inside surfaces of bones are co\cred with a much more delicate connective tissue membrane called the endosteum (cmlo = within) (Fig. 55). Microscopic Structure of Bone (a tissue) Bone tissue consists of l>one cells and a bone matri.v which is made up of two kinds of intercellular substance, fibrous and amoriihous. The inter- ( 90 ) 92 nONE AND THE ALVEOLAU PROCESS the bone matrix which is occnpietl by an ostcocyte is calletl a henna (little space) (Figs. 57 and 58). Lacunae are connected with one another by a system of canaliculi (little canals). These canaliculi extend not only from one lacuna to another, but some of them open into the various canals of bone where capillaries are located. Tissue fluids pass from the capil- laries to the canaliculi anti from one lacuna to another throughout the bone tissue. Ficuni S6 — PliotomiCTi^raph of a groitn THE ALVEOUVR PROCESS - Volkmantrs canal Circurafcrential lamellae r Ha\ersian system Volkroann's canal Haversian lamellae I H!i\t»tsian canal fi^re sy-Thh pi,o>omicEoj,Eph of ” STonlts'e'p 1" of the area maded « in fisut' mfercnMl lamellar and enter, an " Volltmann's canal cuts thtnuEh the „„,;nues (tmiard bottom nff'«.":f> canal slightly at left of center. The bone lamc11.T around the canal), fhe to a second lIa^e^sia^ canal (note the ^ picture). By such a route blood vessels mann's canal then tnnts tlsht (across bottom ol ptetute). nerves supply hone tissue. In lamellar bone the InniL-ll.-n “f' Lamellar bone makes up the ou (Figs. 56, 57). In this following the surface, or given the additional names of location the lamellar bone is f”" r Al..y^subi>criosteal » Ixmcath the cirrHui/crrHlm/ hone, or suhperios ^ m-ikcs up the surfaces of the periosteum). I.amcllar bone also often makes u 94 DONH AND THE ALVEOLAR PROCESS trabecultc of trabecular bone; and in this location it may be callet! SHfc- endosteal bone (beneath the endcsteum)- Both patterns of arrangement of lamellae, i e., Haversian system bone and lamellar bone, are found In all mature bone tissue. Regardless of the arrangement of bone lamellm. all bone tissue contains osteocytes which He in lacxina; and connect throvigh canaliculi. This system of connected bone cells (Fig. 58) is the means by which nutrients are distributed throughout the Iwne tissue. In an Haversian system some of the canaliculi open into the MaxTirsian canal, providing a pathway by which nutrients from the blood vessels contained in the canal may reach the osteocytes of the Haversian s>'stcm. Bone is a very vascular tissue; it contains many blood vessels. Arteries and veins enter and leave a bone in various places both from the outside surface and from the bone marrow cavity. The canals in a bone through Fiourb S8 — DiaBfammatic d^a\^mg of a sector of an Haversian system cut in cross section. (As seen under high power macnification ) which blood vessels pass into the bone tissue from the outside of the organ or from the bone marrow caAJty are called Voiknmnns canals (Figs. 56 and 57). Branches of the blood vessels contained in Volkmann’s canals enter the smaller Haversian canals It seems remarkable that blood vessels should enter bones and be dis- tributed throughout bones m the way that they arc; but actually, in the embryonic development of the body, the larger blood vessels arc formed and are in place before bone formation begins. As lionc is formed it simply surrounds and encloses any blood vessels hxrated in the area. Therefore we find blood vessels entering and leaving bones at various points. Bone marrov.', which occxipies the centers of bones in the spaces around the trabecula*, is a soft tissue (Figs. 55 and 65). There arc two types of bone marrow: (1) red marrow, which is found in most of the bones of young individuals and has the function of protlucing red and white blood cells; and (2) yellmv marrow (fat marrow), which tlocs not have a blood- forming function. In adults most red marrow becomes converted into yellow marrow. Only certain locations in the adult skeleton retain the red type of marrow which continue to i)crform the function of hemopoiesis (hemo = blood; poiesis = creation). nONE AND THE ALVEOLAR PROCESS Pfriosteum and Endosteu^i 0„thcoutsidesurraceaboneUc„«redWa- live tissue Ldmteum, covers the inner surface connective tissue in the bone marrorv cavity, and lines of compact bone and the . „ni5. These two membranes, '''TreTriXnta‘l™rg. 3 t^ separates it from ° formation and resorption of the bone com- ;^lT^hrto“o%“roct?rndttheother^^^^ tion of the cemcntum covering the tooth root. Growth of Bone Bone growth includes both uns'l^cTaUzcd connective Bonefomahon is the . the subsequent calcification of tissue into bone matrix and ‘ two kinds of intercellular the hone matrix. Bone matrix is ns a result of a chemical substance: hbrous and »™rP’’“f fibrou "and amorphous intercellular change which takes place in t tjs-ne The bone cells are certain substances of unspccializcd “""“"'' V ? „„ entrapped in the forming cells of this same connective t.ssue winch ^e entr^W rb, bone matrix. In a growing bone c°nn« be, depem - new bone is the pcriosteunl, or njjed to the outside or to the inside ing on whether the new bone is bei B ^ the periosteum of the organ. The intercellular Jurface'^U chemieally changed (or endosteum) which lies op'nsi periosteum (or endosteum) .into hone matrix. Some of the cell^ the Pe next to the bone surface become ‘ the new bone matrix (osteo - hone; blast = germ) J surrounded by it and so is formed, some of these osteoblas Because the osteoblasts m become cells of the bone tissue, cells hut have the periosteum (or endosteum) arc not co"'P'c“ J ; jbe osteocytes their cytoplasm connected '’V "“™®™“,VnW?tSSm connected by the likewise arc not isolated .ccHf b"* have matrix occupied Slime thin projections (Fig. aS). . P‘ j occupied by the numerous by the osteocytes are the lacuna:; and the spaces occup cytopl.asmic connections are the cana *e^- ^bc formation of bone In the embryonic development bojies resemb es tissue is preceded by the formation of a c bich serves as a pattern for in slmpc the bone tliat is to be r bone calcifies and then the future bone. This cartilage prcdec jjicificd cartilage is rcsorbed. is gradually removed by resorption. which arise in this way. wi_ iKine tissue is formed to replace it. tissue, arc said a cartilage structure preceding Examples of bones formed to be formed by endochondral bonefon^o • ^ in this manner are the long bones of the a I ^Cementum on tooUi root FiGURf photomlcfognph of an area of tooth root, periodontal ligament, and al- veolar bone (bmina dura) This was a tooth not m function: notice the lad of orientation of periodontal ligament fibers There is bone resorption on the bone surface ne« to the perio- dontal ligament Notice the osteoclasts. On the other surface of the bone (to the right) the presence of ostcobhsts indicates hone fonnation (^tedium poner magnification.) bone ”tagni(ication) of osteoclasts in the area “f Bonedeselopment of ciurse includes bone resorpt^ sorption. P^?**"** of osteoclasts indicates that this ii — iiirmir.r,™ - j t “006 oes ctopmcnt ot coufsc includes DOPc sorption. Bone is at thf '0« Supports the tooth roots on the facial and mesial and distal it extends between the teeth, separating _-,ots of multirootcd teeth sides; and it extends into the furcation o 1 - nroccss, located near (Figs. 55. 65). The occlusal border of the aU^lar prc«^ the cetA’ix of the tooth, is ferred “ f ‘b", “tr^ompos'ed of the/a-D'-u The alveolar process may be descnl^ _ Ij^ne of the ^a ‘irfffl and the supporting bone. The lamina ‘ cortical of the tooth socket. The supporting ‘ . the ma.\illa-' and (2) the bone, which is the outside wall of the "mandible anu trabecular bone which is located bettveen the ^ alveolar Ixine in many areas (Figs. 55, 63, 64, 6o, )’ 100 DONE AND THE ALVEOLAR PROCESS I Maxillar) third molar Figuke 64. — A hutnan maxilla and mandible with the facia! pans of the alveolar procesi removed excepting near the cervical border. In the mandible some trabecular bone remains among the tooth roots Notice the location and position of the developing mandibular and maxillary third molars. (Chapters U and 12). The crown of the mandibular third molar is nearly* completed in the ramus of the mandible. Its occlusal surface is directed mesial]/ and occlusallv. It is surrounded by a bony erj-pt. The maxillary thircl molar is directed buccally and distally. Figurf 65. — Diagrammatic drawing of a section of a human mandible bearing an isolated first molar tooth, cut mesiodistally I. Pulp horn. 2. Gingival fibers of periodontal liga- ment. 3. Bone of interradicular septum (between the roots) 4. Cortical bone on ridge of mandible. 5. Lamina dura bone near apex of distal root. 6. Cross section smalt blood vessel 7. Fat marrow. 8. Trabecular bone scattered throughout bone marrow* cavity. 9. 1.ongiru- dinal section blood vessel. 10. Inferior alveolar nene. 11. Cortical Iwine on inferior surface of mandible. A, B, C, and D indicate different areas of the periodontal ligament. A, the penodontal ligament on the mesial side of the mesial root: and B, the periodontal ligament on the mesial side of the distal root. At both A and B the principal fibers have a wavy appearance and there is evidence of resorption of the lamina dura bone C. the periodontal ligament on the distal side of the mesial root; and D, the periodontal ligament on the distal side of the distal root At C and D the principal fibers are stteiched and there is no evidence of resorption of the lamina dura. It may he inferred that this tooth was undergoing a slight movement in a mesial direction: the slight pressure on the mesial side of each root resulted in relaxed periodontal ligament fibers and in bone resorption in the lamina dura On the distal side of each root the fibers are straight and there is no evidence of bone resorption. Figure 65 . — Ltgend on oppoute pat 102 nONC AND THE AEVEOIJVR PROCESS process is thin and contains Httle or no trabecular bone: the lamina dura and the cortical plate are fused. This kind of thin alveolar process is found, among other places, on the facial surfaces of mandibular anterior teeth. CuNio\L Aspects of Bone Re.\ction in the Alveolar Process The trabecular bone and the lamina dura which support a tooth react in different ways to changes in tooth function. When a tooth is in strong masticatory' function, the trabecular bone in the alveolar process and the trabecular bone beneath the alveolus of the tooth arccomposerl of numerous heav-y bone trabecula;. If the tooth is removed from function by loss of of opposing teeth, these supporting trabecula; become less numerous and smaller. If the tooth in question is again placed in occlusion by the replacement of the lost teeth in the opposing arch, more trabcfular bone udll again form around the tooth which has resumed masticatory’ function. The lamina dura, which conjprises the tooth socket, does not respond by resorption to loss of masticatory function of the tooth. However, the lamina dura may respond by resorption to various stimuli such ns trauma due to faulty occlusion, or periodontal disease, or pressures produced during orthodontic treatment, or pressures produced by mesial drift (See Chapter 12 ). Also. ade done to the periotlontal ligament and to the lamina dura that remoi’al of the tooth becomes necessary. Good oral hygiene and prompt dental care will pre\ent such serious invohement because of the folloiving consequential reactions: (1) Removal of the calculus may be e.xpccted to result in the elimination of the inflam- mation and swelling. (2) Elimination of the inflammation and swelling permits repair of the periodontal ligament. (3) At the same time there will be an arrest of the resorption of the bone of the alveolar crest. f4) Additional cementum may be produced on the tooth root in the affected area with an accompanying reattachment of the prc\ iously damagc' bladder, the uterus. Histologic Structurl: of Mucous Mcmdranes Histologically a mucous membrane is a modified skin. It is made up of two layers: (1) a surface layer of epilhelial tissue and (2) an underlying layer of connective tissue. Mucous membranes are structurally less thick and tough than the skin; an' moist by secretions from mucous glands, serous glands, or other secrctor>' cells which empty onto the surface. The epithelium of a mucous membrane is protective in function, and in some areas it is also secretory and al)Sorptivc in function. The conneeth e tissue of a mucous membrane underlies the epithelium and contains blood vessels, nerves, and sometimes glands. Mucous membranes in difTercnt parts of the body differ in a number of ways, and in each location their structure seems to be excellently suited to perform the functions required. In places where the mucous membrane •V=, 'ittn’urMii ’^nrifxix Tati, \\ 'wrj vV/n. 'i/tcL xHV/wA'i.. while in areas which are subjected to functional friction it is thicker and resistant to injury. Protected cavities such as the nasal cavity, the stomach, and the intes- tines are lined by delicate types of mucous membranes. /« the nasal cavity the epithelial part of the mucous membrane is composed of pseudo- stratified columnar cells (Fig. i3D). Many of these columnar cells have «7ta— tiny hair-like projections— on their e.xposed ends, while others, called goblet cells, secrete tnucus vv’hich keeps the surface of the mucosa moist. The connective tissue part of the nasal mucous membrane is likewise thin and delicate, and contains nerv’es and blood vessels, andmucousand serous glands vv’hich open onto the surface of the mucosa. This moist, ciliatc’ function. The under- lying connective tissue contains many glands. In many respects resembling the stomach mucosa, the mucosa of the small inleslive has an epithelium composed of simple columnar cells some of Avhich are goblet cells that secrete mucus and others of which have the function of absorbing food materials from the intestine. The connec- tive tissue contains many glands. Contrasted with these protected, delicate mucous membranes of the nasal cavity, the stomach, and the intestines, is the more sturdy mucosa which lines ihe oral cavity. 7'ho lining of the mouth is constantly subjected to rubbing and scraping, not only l)y the process of mastication of food, but also by the presence of the teeth in the mouth; and here again the construction of the mucous membrane is suited to its usage. THE ORAL MUCOUS MExMBRANE Histologic Structure of Oral Mucous Membrane The mucous membrane lining the mouth is heavier anti more resistant to injury than the mucous membranes of more protected cavities. Its histologic structure enables the oral mucosa to withstand the wear and tear of ordinary oral function and to resist bacterial infection. Like all mucous membranes, the oral mucosa is composed of a combination of epithelial tissue and connective tissue. The epithelial portion of the oral mucosa is stratified squamous in char- acter—that is, the epithelial cells arc mostly flat in shape and are several layers deep. As in all stratified squamous epithelium, the basal layer of epithelial cells, which rests upon the connective tissue, is composed of cuboidal rather than flat cells; and it is in this basal layer that most of the cell division takes place. As new epithelial cells are produced by mitosis in the basal layer, some of the basal cells and the cells superficial to them are forced outward and eventually they reach the surface. DifTcrent things happen to these surface epithelial cells in different parts of tJie mouth. In areas of the mouth where the mucosa is relatively pro- tected, such as on the inside of the checks and lips and on the under side of the tongue, the surface epithelial cells arc sloughwl off into the saliva as new epithelial cells are produced in the basal layer (Fig. 14). A scraping from the inside of the checks spread onto a glass slide and evamlned under the microscope \rill l>e scon to contain squamous epithelial cells. In some other parts of the mouth where the oral mucosa is subject to considerable wear and tear, such as the hard palate and the gingiva, the surface epithelial cells arc not sloughed off (Fig. 66). Instead, they loose their nuclei and their cell boundaries and form a noncellular, tough, pro- tective laj’cr on the surface of the stratified squamous epithelial cells. This tough layer is called the keratin layer., and epithelium on which such a layer occurs is calfeel keratinized epithelium. The keratin layer wears 106 TEIE OR.a MUCOUS MEMORAKE AND l-Jin SALlVARV GLANDS with use, of course, but it is continuously replaced by the aging cells beneath it. The connective tissue portion of the oral mucosa is composed chiefly of fibrous connective tissue in which are blood vessels and nen'es. It is separated from the overlying stratified sf|uamous epithelium by a thin basement membrane. In most places the junction between the connective tissue and the epithelium is an irregular boundary uith projections of con- nective tissue e-vtending like fingers up into the epithelium, but not reaching the surface. In some areas of the oral mucosa these projections r IGURF 66 — A pliotomicrt^rapli of a scitinn of human Rinch a. (As seen under low power magnification ) are characteristically much longer and more numerous than in other areas (Figs. 14 and 66). This irregularity of contact surfaces between the two tissues setves to increase the area from which the epithelium can receive nourishment from the underlying connective tissue. The connective tissue of the oral mucosa varies in thickness in different parts of the mouth; and it also varies in the nature of its attachment to the underlying tissue. In some areas of the mouth the connective tissue of the orarmucosa is attached directly to underlying bone, ns in the part of the gingiva which is attached to the i^riostcum of the alveolar process. In other areas of the mouth the connective ti^ue of the oral mucosa rests upon a looser type of connective tissue calleecn made to classify the different areas of oral mucosa. The classification divides the oral mucosa into three categories; (1) the maslicalory mucosa, (2) the lining mucosa, and (3) the specialised mucosa. If you will carefully c.xamine someone’s mouth and obser\'e the mucosa in different places, such a division will seem logical. Masticatory mucosa Gingiva llaril palate Lining mucosa Lips anti cheeLs Floor of mouth Untler side of tongue Soft paUte .Alveolar niurusa Specialized mucosa Dorsum of tongue The maslicalory mucosa is the name given to the mucous membrane of the gingiva and of the hard palate. These are the areas of the oral mucosa most used during the mastication of food. The epithelium of the mastica- tory mucosa is usually keratinized. [.aaoking into a person's mouth you will see that the necks of the teeth and the bone in which the teeth arc set arc covered with a firm mucous membrane which fits clo.se around the teeth and is tightly attached to the bone. This part of the onil mucosa is called the gingiva fFig. 71 The gingiva is usually keratinized, is firm, and has a stippled appearance. Excepting for a narrow zone around the necks of the teeth the gingival iVfficosa h attached hnnly to tite WKlcrlyinp: tissttc, ll you ^ill cxamiac the facial side of the upper and lower jaws, you will notice that several millimeters rootward from the margin of the gingiva there is a scalloped line which divides the gingi>al mucosa from the ah colar mucosa (Fig. 71). The alveolar mucosa may be distinguishetl from the gingiva because it is redder, shiny, and loose-fitting. The mucosa covering the hard palate is usually well keratinized. It is attachcfl directly to the bone of the roof of the mouth only in the area of the palatine raphe, which is the anteroposterior cIcNTition in the center of the hard palate that you can feel with your tongue. On cither side of the palatine raphe the palatine miicos.i has a submucosa between it and the bone. In the anterior part the submucosa in the hard palate contains much fat tissue, anenings are not visible to the naked eye. Notwithstanding the presence of the fat ti.ssuc and gland tissue in the submucosn of the hard palate, the oral mucosa in this area is firmly 108 THE ORAL ^fUCOUS MESIBRAKE AND THE SALtVARV CLAXnS attached to the underlying bone structure. This is accomplished by strands of connective tissue w'hich extend from the mucosa through the submucosa and attach firmly to the bone of the roof of the mouth. Lining mucosa is located In those areas of the oral cavity where the mucous membrane might logically be regarded as functioning as a lining organ rather than as a masticatory oi^an. Such nonmasticatory areas are the inside of the checks and lips, the floor of the mouth and the under side of the tongue, the soft palate, and the alveolar mucosa. In none of these areas is the mucosa firmly attached to the underlying bone, and ordinarily the epithelium of those areas is not keratinized. In the checks, lips, and soft palate there is a thick submucosa which contains fat tissue and numerous salivary' glands whose minute duct openings arc scattered over the surface of the mucosa. In the cheek or Up mucosa you may occa- sionally see what appeare to be a pen-sized bubble just beneath the surface. Ficure 67.— Drawing of a section of human toncue duough fungiform and filiform papillx. This fungiform papilla mMsured about I mm. in diameter. (As seen under high power magnification.) This condition is due to a stoppage in one of these small salivary ducts which has resulted in an accumulation at this point of tlie secretion from a salivarj’ gland. This bubble is callctl a mucocele. If you will examine the cheek nnwosa inside the corners of the mouth of several individuals you will often find an area which has a few, or numerous, small yellowish spots which are callotl Fordyce's spots. These spots are the openings onto the oral mucosa of sebaceous glands. Of course sebaceous glands arc usually thought of as being restricted to the skin on the outside of the body. Their occurrence in this location of the oral cavity is not unusual, however, and is of noclinical significance. They probably occur here as a result of the narrowing of the wide embr>’onir mouth during the early rEMBRANC AND THE SALIVARV GLANDS Due to their thinner epithelium they appear redder in color than the fili- form papilla*. They may be clearly seen by careful examination of the tongue. Foliate papilla are located along the lateral borders of the posterior part of the tongue. In the human tongue these papilla; are not well developed. Circumvallale papilla can be seen well back on the tongue, located be- tween the body and the base of the tongue (Fig. 6S). They are large, conspicuous structures, S to 10 in number, arranged in a V-shaped line with the point of the \' directerl toward the throat. The fungiform, foliate, and circumvallate jjapilla; contain organs of taste. If a longitudinal section of a circunuTillatc papilla is exaniinerl with a microscope, there may be seen groups of specialized epithelial cells, which are called taste buds, located along the lateral surfaces of the papilla (Fig. 68). Figure 69 is a drawing of such taste buds seen in a section of rabbit tongue, where the details arc more easily studietl than in the human tongue. These arc similar to human t.aste buds. At the bottom of the trench surrounding the hvjman circumvallate papilla there arc openings of ducts leading from salivary* glands (\‘on Elmer’s glands) which arc located deep in the tongue. I'he saliva from these glands floods the trench around the circumvallate papilla and scr\'cs as a solvent for food substances. \Mien these food substances in solution come in contact with the taste buds the individual experiences a sensation of taste. THE SALIVARY GLANDS Histology and Function of Svlivary Glands Salivary glands produce a colorless, slightly sticky fluid called saliia which is discharged into the oral cavity through ducts that open onto the surface of the oral mucosa. ticcRF 70.— A section oF a human sublinRual gbnd Medium power macnificaiion. Most of thepland cells are of the mucous type. Someserwis cells (darker stain) form a cap around a group of mucous cells (Area X). TIIK ORAL MUCOUS MUMURANC AND THU SALIVARY GLANDS 111 In cml)r>'onic development the salh'ar>' glands are formed from the epithelium that lines the early oral cavity. In certain areas cells of the cmbiy'onic oral epithelium grow downward into the underlying connective tissue, and as they multiply the epithelial cells of this downgrowth become modified and very specialized and form the salivaiy glands. Some of the cells develop into the secretory cells of the glands, and others develop into the ducts of the glands. The secretory cells secrete the saliva. There are two types of salivary secretory cells; serous cells and mucous cells (Fig. 70). Salivaiy glands are made up of one. or of the other, or of a combination of the two kinds of secretory cells plus the ducts which con- nect them. The secreted product of serous cells contains an enzyme called amylase (ptyalin) which contributes to the breakdown of carbo- hydrates. The secreted product of the mucous cells is mucin which acts as a lubricant to the oral cavity. Besides these substances the salivaiy glands secrete other materials among which are proteins and salts which act as buffers and prevent the saliva from becoming suddenly acid or alkaline. Also, saliva contains some antibacterial factor which inhibits the growth of some bacteria. In addition to these products of the salivary glands saliva which has been lying in the mouth contains various sorts of debris such ns epithelial cells sloughed off the oral mucosa, degenerating M’hite blood cells, and bacteria. The total amount of saliva producerl by the salivary' glands varies greatly in different indiNnduals, but an approximate amount is about 3 pints a day. The functions of saliva may be cnumeratetl as follows: (1) it assists in the mastication of food; (2) it scrx'es as a solvent; (3) it contributes to the digestion of carbohydrates; (4) it lubricates food and oral tissues; (5j it acts as a buffer; (6) it cleanses the mouth by flushing out debris; (7) it acts to inhibit the growth of some bacteria. Distributio.v or Salivary Glasos The salivary' glands may be descrilied in two groups: (A) the vtajor salivary ^lauds, and (B) the minor salivary glands. The major salivary glands arc (1) the parotid, (2) the submandibular {submaxillary), and (3) the sublinsual. The minor salivary glands vary- in size and are widely’ distributed beneath the oral mucous membrane. The major salivary glands arc, of course, paired structures. The parotid glands are flattened organs located under the skin of the face in front of and below each car. They are the largest of the salivary’ glands, and in adult human beings their secretory' cells are all serous in type. The parotid gland is the gland involved in epidemic parotitis, commonly known as mumps. The main duct of each parotid gland, the parotid duct {Sten- sen's duct), opens into the oral caxnty on the wall of either chock opposite the second maxillary molar tooth. If the tongue is passetl over this area a small prominence may be felt at the point where the duct opens into the mouth. The submandibular glands arc located Ix!ncath the posterior part of the longue. They lie in a ilcprcssion on the inner surface of cither side of the 112 THE OR,\L MUCOUS MEMPRAKE AKD THE SALIVARY GLANDS mandible just anterior to the angle of the jaw. The secrctor>' cells which make up these glands are a combination of serous and mucous cells, the serous being the more numerous. The secreted product of these glands is a mixture of serous and mucous substances. The main ducts which carr>’ the products of the submandibular glands to the oral cavity open into the mouth beneath the tongue, one on cither side of the center line. You can see the openings easily as two small prominences in this area. These ducts are known as the submandibular ducts {Wharton's ducts). The sublingual glands are located beneath the mucosa in the floor of the mouth anterior to the submandibular glands. They are composed of a mixture of serous and mucous types of cells, but are predominantly mucous in character (Fig. 70). The main ducts which carry the products of this pair of glands are several in number, and all open beneath the tongue Some open along the folds of tissue which are seen on either side of the floor of the mouth, while others join the submandibular ducts and share the same openings. Rx-amine carefully the area under the tongue in a human mouth. The minor salivary glands may be described according to their location in the oral cavity. Beneath the mucous membrane of the checks and lips are numerous small glands which arc a mixture of mucous and serous secretory cells, the mucous cells being the more numerous. The minute duct openings of these smalf glands are scattered over the surface of the check and Up mucosa. Larger saUvaty’ glands made up entirely of mucous cells lie beneath the mucous membrane of the roof of the mouth and have minute duct openings distributed over the entire surface of the hard and soft palate. The tongue has glands in its anterior part which are mixed in character (contain both serous and mucous cells). In the posterior part of the tongue are the pure serous Von Ebner's glands whose ducts empty into the trenches of the circumvallalc papillie. There arc also pure mucous glands in the root of the tongue. 10 The Gingiva Location The gingiva is that part of the oral mucosa that is firmly attached to the alveolar process and to the ccr\'ical parts of the teeth and which surrounds the cenlces of the teeth. The gingiva on the facial side of the maxillie and mandible in the pre- molar and nioJar regions is called the buccal gingiva, and in the incisor and canine regions the labial gingiva. The gingiva on the inside of the mandibular arch is called the lingual gingiva, and on the inside of the maxillar>’ arch, the palatal gingiva. InterUenlal papillt cf gmgna Gingiva AK polar mneova Firure 7l. — Clinical photograph of mandibular anterior teeth with assoaated gingiva and alveolar mucosa. The subject was twenty j'ears old. Notice the difference in color of the gingiva and the alveolar mucosa; the stippled texture of the gingisa; the interdental papillie. CUNTGtL ApPEAILVNCE To Study the clinical appearance of the gingiva stand before a mirror or use a classmate for a subject. Pull out ami clown on the lower lip. Below the crowns tif the mandibular anterior teeth you will see that the ora! mucous membrane is firm and has a stippletl or finely pittetl appearance. This is gingiva (Fig. 71). In iwople who have fair skins the color of the gingiva is a slightly grayish pink. In dark comple.xioned individuals the gingiva frequently is cither spotted with brown or is fairly even grayish brown all over due to a pigment which occurs in some of the cells of the 8 (113) 114 TIIC GINGIVA epithelium. Four or Hve millimeters below its cer%'iral margin the gingiva ends In a scalloped line and the oral mucosa below this line is red, shiny, and loosely attached to the underlying tissue. E.vainine also the gingiva al)ove the maxillary anterior teeth, and the gingiva on the buccal side of the mandibular and maxillarj* ix)sterior teeth. The color and surface te.xturc here are the same as in the anterior region, and a scalloped line marks the apical border. Now examine the lingual side of the mandibular arch. The gingiva is firmly attached to the underlying hard tissue, while the oral mucosa be- neath the tongue is loose, shiny, and redder in color than the gingiva. PicURE 72.— Clinical photograph of maxillar)’ anJ mamlibular anterior teeth with associ- ated gingiva. The subject was sixty years old. Nonce the stippled pingiva; the position of the interdental papillx; tlie exposed but umlamaced cementum on the left maxillaiy lateral incisor and on the right maxillary central incisor Notice the exposed and deepl> cut cemen- tum on the mandibular anterior teeth. This is cervscal abrasion Examine the palatal side of the maMlIar^’ arch. In the maxilla the palatal gingiva blends xvithout a distinct line into the oral mucosa of the hani palate. Look again at the facial surfaci's of the mandibular and niaxi!lar>' arches. The shape of the incis.il for occlusal) border of the gingiva depends largely on the age of the individual. In .t x'cry young person the gingiva c.xtcnds between the teeth in a triangular papilla, reaching as far occlusally or incisaily Iwtwccn the teeth ns the contact aa*as: and on the facial and lingual surfaces of the teeth the gingiva xvill cover the cervical part of the enamel of the tooth crown. In this young person the clinical crou'u of the tooth (the part axposed in the oral cax-ity) is smaller than the anatomic crovjn of the tooth (the part that has an enamel surface). In a person who is near thirty years of age the points of the interdental papilla? do not extend to the contact areas, and most or all of the tooth THE GINGIVA 115 enamel probably is uncovered; the clinical crown and the anatomic crowm are often about the same. Now examine a person fifty or sixty years old and compare what you see here with what you saw in the verj- young person and in the younger adult. In the middle aged or older person you will probably find that not only do the interdental papillae fail to fill the spaces that exist between the teeth cervical to the contact areas (the interproximal spaces), but on some or all surfaces of the teeth cementum can be seen around the cerv’ix (Fig. 72). The gingiva! margin in this older person is located so far rootward that some of the root cementum is exposed in the ora! cavity, and the clinical crown of the tooth is larger than the anatomic crowm. This age change in the position of the gingiva on the tooth is a condition that is to be expected, just as it is expected that with age hair will become gray and skin will become wrinkled. Of course, diseases of the tissues around the teeth may produce patholt^ic changes with gingi\’al recession at any age. What we are discussing here are usual and expected age changes. Histologic Structure Like all other mucous mcmf>ranes the gingiva is composed of connectixe tissue and epithelial tissue. The connecli\c tissue is of the fibrous type, while the epithelial tissue, which is of course on the surface, Is stratified squamous in charactorand is usually keratinized (Fig. 66). This keratin- ization causes the color of the gingiva to be grayish pink rather than red if it is not pigmented, or grayish brown rather than reddish brown if pigmentation is present; and the finger-shaped projections of the connec- tive tissue into the epithelium probably produce the pitted condition of the surface of the epithelium in some places. The gingiva does not have a submucosa, but rests directly on the underlying hard tissue; and ordinar- ily the gingiva contains no salivary' glands. The Gingival Sulcus The part of the gingiva tliat covers the ah'colar process is firmly attached to that bone structure. Occlusal to the crest of the alveolar process the gingiva for a short space is firmly attached to the tooth surface by the gingival fibers of the periodontal ligament (Fig. 73). Occlusal to the portion of the gingiva which is attached to the tooth surface there is a border of gingiva surrounding the tooth which is not attached to the tooth. This unattached part of the gingiva is calied the free gingiva. Often the free gingiva fits so snugly to the surface of the tooth that there is only a potential space, rather than an actual space, between the tooth surface and the free gingi^'a. Reganlless of whether the space between the tooth and the free gingiva Is actual or only potential, it is called the gingival sulcus. The gingival sulcus could be roughly described as a ditch, or a potential ditch, e-xtending all around the tooth between the tooth surface and the cemcal Irordcr of the gingiva. The structure of the tissue associaterl with the gingival sulcus is impor- tant. To simplify description, let us suppose that a small instrument is 116 THE GINGIVA inserted into the gingival sulcus so that an actual space is created between the tooth and the free gingiva (Obsene Fig. 73). The inner wall of this space is of course the surface of the tooth. The outer wall of tlie space is the free gingiva, the inner surface of which is composed of straUfied squamous epithelium. This epithelium is called the epithelium of the gingival sulcus, and it is continuous over the gingival margin with the stratified squamous epithelium which makes up the outside surface of the gingiva. Ficvre 73.— DiagraiTimatic drawing of a section oftlie buccal cervical area of a mandibular molar tooth cut buccolinguallj’. THE GINGIVA 117 To what depth our instrument may be inserted between the tooth and the gingiva without tearing loose gingival tissue %vhich is actually attached to the tooth surface is a matter ateut which there is a surprising difference of opinion. There exists a question among oral histologists whether, around the partially erupted tooth of a young person, the gingival tissue which covers the cendcal part of the tooth crown is merely fitted snugly to the enamel surface or whether it is in some way attached to the enamel surface. If the gingival tissue is not attached to the enamel, the bottom of the gingi\'al sulcus Is ahrays at least as deep as the line of the cemento- enaniel junction. If, on the other hand, in a partiallj' erupted tooth the gingival tissue which covers the ccrv'icat part of the crown is attached to the enamel which underlies it, the gingival sulcus may be ver>' shallow. A number of studies have been made on this problem but the findings have been contradictor^'. Undoubte) The length of the root dentin is complete, and the crown has mosed farther into the oral cavity- T he epithelial attachment is still entirclr on the enamel The apical foramen is narrower F, G) fhe epithelial attachment grow* onto the cementum at its apical border and separates from the tooth surface at its cervical border. Cementum becomes exposed (If, f) Increased cementum exposure and improper use of an abnsive deniifnce have resulted in abrasion of cementum and dentin in the cervical area. (H8) TUB GINGIVA 119 Exposure of cementum in older indiv'iduals Is to he regarded as the usual and expected condition. Such ccmcntuni exposure is not infrequently seen to a slight degree on the maxillary canines, and sometimes on other teeth of persons less than thirty years old. After the age of thirty cemen- tum exposure increases in frequency and extent. By forty years of age many individuals have cementum exposed on some areas of most of their teeth. By the age of sixty the amount of cementum exposure in some areas may often amount to 3 or 4 mm. (Fig. 72). It is unusual to find an in- dividual of middle age with no cementum exposed on any teeth. Clinical Consideiutions The intactness of the epithelium of the gingival sulcus and epithelial attachment is important to good periodontal conditions. Since the gingival sulcus, despite the snug fit of the free gingiva to the tooth surface, is exposed to the saliva and bacteria of the oral cavity any damage to the epithelium of the gingival sulcus can result in damage to the underlying connective tissue. Connective tissue is not a covering tissue as is epithelial tissue, and it does not resist injury and bacterial invasion the way epithelial tissue does. Therefore, if damage to the epithelium of the gingival sulcus is accompanied by damage to the underlying connective tissue, the ensuing effects on the tissues in this area may be inflammation, swelling, damage to the periotlontal ligament fibers, resorption of the bone of the alveolar process, loosening of the tooth, and perhaps finally the necessity for tooth removal. The presence of calculus around the cerx ix of a tootli is often an impor- tant factor in periodontal disease (Fig 46). The calculus damages the epithelium of the gingival sulcus and of thcopitholial attachment. .A scries of consequential events follows; (1) There is inflammation of the con- nective tissue of the gingiva. (2) There is swelling. (3) There is damage to the periodontal ligament filK^rs, particularly the gingii’aJ, the trans- scptal, and the alv’colar crest fibers. (4) There Is resorption of the l>ono of the alveolar crest. (S) There is an increasing amount of cementum exposed in the cervdea! area of the tooth. Figure 75 is a drawng of the cerxdcal area of the lingual side of a maxil- lary molar tooth. The periodontium is in gorxl condition: there is little or no inflammation in the gingiva; the epithelial attachment fits against the tooth; the periodontal ligament fibers are well oriented; and the alveolar crest shows no signs of rcsorjition. Figure 76 Is a drawing of the cervical area of the buccal side of the same tooth. Here the presence of a mass of calculus has resultcfl in the forma- tion of a gingival pocket. There has followed injury to the epithelium of the gingival sulcus, inflammation in the connective tissue, damage to the periodontal ligament fibers, and resorption of the alveolar crest and of the lamina dura. Continued irritation by' the calculus wilt be accom- panied by continued inflammation of the gingiva and continuctl l>one resorption. Such pathosis snlTiciently cxlcndixl results in the loosening of the tooth. Another important clinical problem is creatwl by the presence in the mouth of exposed rcn’ical cementum. When the extent of axposure of 120 THC CINOIVA cen-ical ceincntum exceeds 1 mm. there frequently is found a condition know-n as cervical abrasion. Cervical abrasion is a wedge-shaped cut in the cervical cementum of a tooth. It is produced when the person uses an abrasive dentifrice and brushes his teeth with a cross-brushing stroke. The tooth enamel. ' abrasive dentifrice can result in such an amount of abrasion that a niaxillao' canine tewth measuring 7 mm. THE GIKGIV’A 121 at the cen'ix may be cut half through in 7.6 years; that is, in that length of time the wedge of cervical abrasion can extend half way through the 7 mm. cerv'ix. Such cer\'ical abrasion is illustrated on the facial side of a mandibular incisor tooth in Figure 747. Figure 72 is a clinical phntograjih showing cervical abrasion. Figure 77 is an extracted maxillary canine tooth showing deep cervical abrasion. Tigvrs 76 — A Jiaerammacicdraninpiorthefacial sideof the tooth illustrated in Figure 75 A large mass of calculus at the tooth ceivi\ i$ responsible for the pathologic condition in this area. Here there is inflammation, destruction of the epithelial attachment, damaged peno* dontal ligament (periodontal membrane), and resorption of the alseolar crest and of the lamina dura. Such information concerning ccr\4cal abrasion makes clear the import- ance of using proper tooth-brushing techniques and a dentifrice which docs not contain an excessive amount of abrasive material. This is particularly important for those individuals in whom there has lK«n con- siderable gingival recession leaving cxposetl to the oral cavity a relatively broad area of cenientum. 122 THE GINGIVA Figure 77 —Mesial surface of a maxillary canine tooth. There is deep .xbrnxinn of the rer^ teal cementum on the facia! surface. *1 he enamel is not affected Sclerotic dentin and secondan' dentin (Figure 41) ordinarily form beneath such an area of abrasion 11 Tooth Development 13i:roRK thy human embr>'o is three weeks old the oral entity is estab- lished. At the anterior end of the cmbiyo the ectoderm has invaginated to meet the endodenn and has formed the primitive mouth and the bucco- pharyngeal membrane (Kig. 3). This membrane is located in approxi- mately the position the palatine tonsils will later occupy. The primitive mouth is lined with ectoderm, beneath which is mesenchyme.* The ectoderm gives rise to the oral cpithdium, and the mesenchyme gives rise to the underlying connective tissue. The Beginning or Tooth OEVELoMfc.sT Not all teeth start development at the same time. 'Fhe earliest sign of tooth development in the human embr>’o »s found in the anterior mandib- ular region when the cmboT> is five to six weeks old. Soon after this, evidence ol tooth development appears in the anterior ma.xillarj’ region, and the process progresses posteriorly in Iwth jaws. The development of the teeth l>egins wdth the development of the dental lamina. The dental lamina is a narrow liand of thickened oral epithelium (ectoderm) which extends along the occlusal borders of the mandible and maxilhv on a line where the teeth will later appear. This dental lamina grows from the surface downward into the underlying mesenchyme. Concurrently with the development of the dental lamina, at ten places in the mandibular arch and at ten places in the maxillao’ arch some cells of the dental lamina multiply at a rate faster than the surrounding cells, and ten little knobs of epithelial cells arc formed on the ’onic tissues: the part w'hich develops from the dental lamina is derived from cctodenu, and the remaining parts arc dcrivetl from the mesenchyme which underlies this ectoderm. A tooth bud is made uj) of three parts; (1) an enamel organ," which develops from the knob-like growth on the dental lamina (which is, of course, derived from ectoderm); (2) a dental papilla, which develops from the underlying mc.scnchynic (the early connective tissue) ; and (3) a denial •.Mcm-nchyme is an embryonic connectlxe tissue dtrived from mesoderm. fAnothcr name for enttme! ifrgan is denial itrgaii. ( 12 }} 124 TOOTH DEVELOPMENT sac, which also develops from the underljdng mesenchyme (I'ig, 80). The tissues of the tooth bud produce the tissues of the tooth: the enamel organ produces the tooth enamel; the dental papilla produces the dentin and the pulp ; the dental sac produces the periodontal ligament; and the periodontal ligament produces the cementum and the alveolar bone. The enamel organ is the first part of the tooth bud to form. It develops from the dental lamina as a growth of ora! epithelium into the underlying connective tissue (Fig. 78). As it enlarges, the enamel organ acquires the shape of a cap (Fig. 79). The connective tissue which is inside the cap Figure 78.— Photomicrograph of the beKinning of tooth development ns seen in a section from a fetal pig. The otal epithelium appears to be gtovvin;; into the undetlying mesenchvme. This is similar to human tooth development. (As seen under high power magnification ) Orn) epithelium Dental I imina Early enamel organ Connretiv e U«Hc which will liecume dent.il p.ipllld Figure 79. — Pliotomlcrograph of aix early cap-shaped enamel organ in s section from a fetal pig. The dental papilla is becoming disceroable. This is elightly more advanced development than that in Figure 78. {As seen under high power magnification.) TOOTH DEVEU)PMENT 125 undergoes a change and becomes the dental papilla. The connective tissue which is beneath the dental papilla becomes fibrous, the fibers encircling the papilla and part of the enamel oi^an. These encircling fibers are the dental sac (Fig. 80). The enamel organ for a time remains connected with the oral epithelium by the dental lamina (Figs. 78, 79, 80). Fjcure 80 — Djagrammatic dran ing of a faciolmguai section through a developing mandible and tooth bud of a fetal calf (As seen under low power magnification ) The Tooth Bud As the tooth bud grows, the cap-shaped enamel organ changes form and Ixjcomes somewhat bell-shapcil, and four layers are distinguishable (Figs. 80, 81, 82): The outer euamel epithelium is the outside layer of the enamel organ, and is cotnposed of low ruboidal cells. The stellate reticulum is the layer immediately inside the outer enamel epithelium and is composetl of a very loose network of epithelial cells. The stratum intermedium is a layer of closely packed, flat epithelial cells cells inside the stellate reticulum. The inner enamel epithelium lines the inside of the bell-shaped enamel 126 TOOTir DEVELOPJinNT organ. It is a single layer of cuboidal cells. This layer of cells is separated from the dental papilla by a basement membrane. The growing tooth bud changes form rapidly at first, but by the time the four layers of the enamel organ are W'clt defined the shape of the base- ment membrane has become fixed. When the final shape of the basement membrane is established it marks the line which will become the clcntino- enaniel junction of the finished tooth. Further cell differentiation in the tooth bud occurs along either side of the basement membrane. First the cuboidal cells wiiich make up the Stratnin Int^rnKHhum flRURr 81 — A phofrtmicroRraph of the tootit bud of a mandibular posterior tooth from a fetal pig The enamel and dentmha>enot jet started to form, but the shape of thedentino- enamel junction can be seen from the configuration of the dental papilla. On the lingual side of this primary tooth bud the bud of the permanent tooth which will succeed it has already started to form (As seen under low power magnification ) inner enamel cpillieVmm elongate into colimmar cells called ameioblasis (Figs. 80, SI and 82). Formation of the aincloblasts is followed by a change in the peripheral cells of the dental papilla which also take on a columnar form and liccoinc odontoblasts (Fig. 82). The ameloblast and odontoblast layers arc scparatetl from each other by the basement mem- brane. In embrv’onic development many things arc occurring, of course, at the same time. As the early tooth buds arc tlcvcloping they become sur- rounded by islands of bone which e\cntuaU> coalesce and form the man- tliblc (or niavillae). Figure 80 is a dniw'ing of a fnciollngual .section of a forming calf mandible cut through the center of a primary' tooth bud. Blood vessels, nerves, and the tooth bud, ha\lng foriiicrl earlier, arc becoming enclosed in tite Irane of the mandible w-Iiich is ' tooth bud the dental lamina is giving rise to the Ijcginning of the enamel organ of the succeeding permanent tooth bud (Ftgs. 80 and 81). The pennanent tooth bud vnll develop slowly as the primary' tooth develops and goes into function. Also there occurs disintegration of the dental lamina connecting both enamel organs with the oral epithelium. The cells of the dental lamina break apart into small groups and either gradually disappear or remain as small groups of epithelial cells. When amcloblasts and odontoblasts have differentiated along either side of the basement membrane in the tooth bud, formation of the hard tooth tissues Ijegins. The earliest formation of the hard tooth tissues in the human embryo occurs during the fifth month in the primar>' incisors. Dentin formation starts just a little licforc enamel formation begins. De.STIN FoIl.^fATIO^^ Dentin is composed of (1) a fibrillar matrix which calcifies, and (2) dentinal fibers which remain uncalcified. Dentin is formed by the dental papilla. Recent and current studies on dentin arc raising tiuestions about details of dentin formation. L'ntil the answers to some of these questions become known, wc wiW present the process as it has long been accepted, recognizing at tlie same time that here ns elsewhere our understanding may lie altered as it is increased Cytoplasmic processes of the odontobasts fonn the dentinal fibers. Korflf's fibers form the fibrils of the dentin matrix; and cells of the dental papilla produce the cementing substance which surrounds the fibrils of AmHohlasu Enamel matrix fonnstlon Dentin lonnaHon OdnnlobLixtf Dental papiIU AnicInbUtls (innrr rnaim Stratum Inlrrmrditnn Slrlltte rpticuinra Oiili-r rnaiiwl rptlfwiliiim . rpUhrlium Ficure 82. — A photomicrograph of the tooth btid of a mandihular anterior tooth of a fetal pit. A small amount of enamel matrW is seen at the tip of the incisal edge. Beneath the enamel matrit, and extending farther ccrvically. is a laver of dentin. These tissues wiH become thicker and will be extended ccrsicall). Notice the columnar amelohlast cells and the columnar odontoWasts. The tissue of the dental papilla here has setj’ much the same appearance as does the pulp tissue of a j oung, fully formed tooth. 128 TOOTH DEVELOPMENT the matrix. The first dentin is formed at the incisal edge or cusp tip of a tooth, and formation progresses in a rootn'ard direction. It may be accomplished somewhat in this manner: The odontobasts, which have differentiated from the fibroblasts of the dental papilla, form a single layer of columnar cells at the line of the dentinoenaniel junction (Fig. 82). They start moving inward— that is, they back up toward the center of the pulp. As the cells pull back, they beha\-e as if several spots of their cytoplasm were attached to the base- ment membrane, causing the cytoplasm to stretch out into several narrow Conncctnc tissue Enamel matnv Dentin Denttnoenainel junction I’rrilenhn nio(xl vessel in pulp Tooth pulp Figure 83.— A photomicrograph of one cusp of a developing molar tooth of a monlev. The thickness of the enamel matrix appears to be nearly completed The outer enamel epithe- lium and the stellate reticulum of the enamel organ are clearly seen; the amelohlast layer is still distinct In this section the stratum intermedium is not e.isily distinguishable. The narrow, pale part of the dentin ne« to the pulp is less calcified than the outer pan and is called predenttn Notice the bone near the cusp tip. This bone is being tesorbed. and its disappearancewill permit the tooth to move occkisatlv. extensions (Fig. 84). .As the pulpwartl migration of the body of the odontoblasts progresses, the sc\’eral cv'toplasmic extensions of each cell join to make a single dentinal fil)cr. The part of the txJontoblasts con- taining the nucleus comes to lie some distance pulpward of the basement membrane (now the dentinoenamel junction), but the cells remain con- nectcti with the dentinoenamel junction by the cytoplasmic extensions which are branched at their peripheral ends. Now when the odontoblasts ' corkscrcw-shajwtl fibers called Korff’s fibers. These were producctl by the aggregation of numerous fine fibrils In the dental papilla (see Chapter 5). When dentin TOOTH DEVELOPMENT 129 formation begins with the odontoblasts moving inward, Korff's fibers remain in place (Fig. 85). With the bulk>' part of the odontoblast cells out of the way the thick Korff’s fillers spread out, somewhat in the manner of a piece of rope becoming unwound and frayed. In this way Korffs fibers separate into tiny fibrils which surround the cytoplasmic e.\ten- sion of the odontoblasts. These are the fibrils of the dentin matrix. FfCl/RE 84.— Schematic diagrant of the po$sible manner of formation of a dentinal fiber. W A pulp cell called a fibroblast wiH differentiate into a columnar odontoblast, {b) The odontoblast has moved asray from the dentinoenamel junction, but part of its c>toplasm remaitis stretched beliind in two places, (r) The odontoblast has mo\ed farther inward, and its cytoplasmic processes have united, (d) Hie odontoblast has moved still farther inward, and a long dentinal fiber has been forni^. Cytoplasmic extension of an odontoblast Odontoblast Korff's fiber Fibrillar matrix of dentin (splayed Korf/’.s fiber) Area of inner surface of predentin pulp Fiwre Si. — Schematic dfagram of odontohfasts and KorfiT’s fibers. Verj' much enlarged. 0 130 TOOTK DCVEU)PMCN’T They are surrounded by an amorphous substance called the ground stance of the dentin matrix, which is produced by other pulp cells, cytoplasmic extensions of the odontoblasts are the dentinal fibers. \ the dentin matrix is formed and calcifies around the dentinal fibers dentin is perforated by dentinal tubules which are. of course, filled the dentinal fibers. Dentin matrix calcifies progressively as it is fornied. The inner layer of dentin matrix (next to the pulp) is the most recently formed, in developing teeth it is not calcified until a successive layer has formed. This newest, noncalcified dentin is called predentin or denli (Fig. 83). Dentin may be produced along the pulpal wall in a tooth of any aj long as the pulp is intact. Dentin formed in older teeth in respons attrition or caries is called secondary dentin. The tul)ules in secom dentin are fewer and less regular in arrangement than those of the ea dentin because, due to age changes in the pulps of older teeth, there fewer exlontoblasts, and so fewer dentin.al fibers (Figs. 16, IS, 19. and When dentin formation begins, the fonning organ is calletl the di papilla. After some amount of dentin has been proeluced the name ol dental papilla Is changed an third of its roots resorbed would have s nrobahly would not have thi would be an older tooth), and the root ends probam> ^ square shape. * dtown in Figure 89. This picture Figure 90 is a tooth similar to the oue sho t n m t,,e was taken looking up into the open en dentin walk . large site of the root canals and thinness oUhe a Root length is not complete until one ^ „ short root emerges into the oral cavity. hcconie older and the root Icng and a ver>' large apical opening. AS tt Figure 89 — Huccal surface of a left mandibular 3rd molar. The roots have attained about two thirds of their length The openings at the ends of the incompletely formed roots are large. In this tooth the enamel ettends between the roots and meets the enamel on the lingual surface of the tooth. (m) TOOTH DEVTSLOPMEKT ■„ completed, additional -f inucs » the PuM child necessary. Formation or Periodontai. Ligament and Cements, As dentin is forming pcriodontaliigam cireuiariy arranged fibers of ^ , ,• „ent produces the cenientum around the tooth root. The p nroduces the iamina dura of the rvhich covers the root dentrrr. ^ J “^^d^ra are being produced tooth socket. As the iigament i.ecome en- *X".r "Sf. j;: s — £,x socket (Fig. S3). „^^ln^otlv it carries with it the reduced As the tooth crown moves enamel epithelium which covers It. strands which enamel epithelium some of ce periodontal ligament around the remain stretched iike " forming root. This net« ork is ca (unction, a few fragments of Hcrt- when the tooth is fully formed a microscopic evamination wig's epithelial sheath be of cells are called the ot the periodontal ligament. TheK small g 1 mis d Malasscz. (Sec Chapter 7 and Figs. oA Anomalies . ...yg from the Development of the '"^L'n'^hirnoriTial number of tooth buds usual standard. Sometimes Ic {hypodontia), or perhaps no develop and an individual has t number of tooth buds teeth {anodoniia). Sometmies duplication of certain teeth, sue ^ develop and the individual may ■ called supcnui^'^^y ns two left maxillary laterals. Thc^ ^number). Somctimescertam teeth (super = additional; numcrar> may be peg-shaped; the have a peculiar shape: the having a mesiodistal dimcn- maxillary central titay be scrcwdnve - • cer\ ical portion, sion which is smaller in the mcisal t ‘ hard tooth tissues. Again, there may be a faulty P j. the cnamel_ is lost from Enamel development may l>e so ii This condition is hereditary . the surface of the tootji soon hypoplasia (hypo = more frequent condition is ioraW cn.T.ne >^1 disturliance during plasin = tomintion), which is the r»ult seen in the torn lo the time the tooth crowns are fonning. penn.Tnent incisors, the tip a pitted line across the facial ° in the same mouth, t of the canines, and the cusps of t process of formation at aflects the areas oi the teeth that were m the pr 136 TOOni l>E^•n.OP>IE^'T Fiourc 9I.->DistaI surface of a maitllaf}’ right third molar. On the root trunk is an enamel pearl. The tooth was ground to a thin section along the vertical line. See Figure 92 Figure 92.— A ground wtion of the tooth shown in figure 91. 'I he enamel pearl i< in the upper right. The roots are not present here liccaiise the section was madeat the root furcation Notice that besides hav ing a formation of enamel m this unusual location, there is a v anatuvn in the dentin formation which has resulted maprotursionorjentin beneath the enamel pearl TOOTH DEVELOPMENT 137 time the s>-steniic cause of the hypoplasia occurred. If the systemic cause occurred before the child was ten months old, the maxillary laterals probably will not be affected, because of their normally later development. Still another abnormality is an interference with the calcification of the enamel matrix which produces hypocalcified areas in the enamel. This condition is seen in individuals who have lived during the first eight years Vicvnr 97.— A maxlllao' ncht third molar with a \eo' large enamel pearl on the distal surface of the lingual root. The tooth was ground to a thin section along the horizontal line See Figure 94. Figure 94.— A gtouml secrion of the tooth shown in Figure 93. The enamel pearl in the lower left. In this tooth there is not only a protrusion of the dentin beneath the enamel pearl, but there is also an extrusion of the pulp cavity in the location of the pearl, JO 138 TOQTH DEVXLOPMEST of their lives in regions where the drinking water contained over 2 parts per million of fluorine. Fluoride in the drinking water causes enamel hypocalcificntion only when it is consumed during the periotl of tooth fonnation; and such hypocalcificntion is clinically visible only when the fluoritle is in amounts in excess of approximately 2 ppm. The hypocalcified areas of enamel soon become stained brown when exposed to the oral cavity, so that there are unsightly brown spots on the teeth. This condi- tion is called mottled enamel. An interesting anomaly which is sometimes seen in the process of examining a collection of extractctl teeth is the enamel pearl. This is a spot of enamel, usually in the shape of half a sphere, which is found on the roots of teeth. The most frequent location of enamel pearls is said to be on the distal surface of third molars (Figs. 91 and 93), although the\ arc sometimes found on the l)ua*al surface of a molar at the root furcation. Their formation is the result of a small group of cells of Ilertwag’s epithelial root sheath adhering to the surface of the newly formed root dentin instead of becoming separated from the dentin and moving out into the periodontal ligament. These epithelial cells adhering to the root dentin difTerentiate into ameloblasts just as the cells of the inner enamel epi- thelium in the crown area differentiate into ameloblasts, and a drop of enamel (enamel pearl) is then produccrl on the root. That the protiuction of an cnanwl pearl is more than merely a suuple matter of the cells of Hertwig’s sheath adhering to the root dentin is indicated by the fact that the dentin l)cncath the enamel pearl in many, if not all, cases protrudes into the pearl. That is, the oxtcmal dentin surface is not flat, as on the rest of the root, there is a protrusion of dentin which is cot’ered by the enamel i»earl (Figs. 92 and 94). In some teeth the configuration of the pulp canal is also altcretl. In the cross section (Fig. 94) of the tooth shown in Figure 93, there is an e.xtrusion of the wall of the pulp canal beneath the enamel pearl. Another variation in enamel formation found in molars, chieilv man- dibular molars, is an extension of the enamel at its ccn’ical bonier Ijctwecn the roots of the teeth (Fig. 89). Perhaps this pattern of enamel distribu- tion should not be called an anomaly, as it has been reported to occur in 90 per cent of the mamlibular first molars in j)crsons of Mongoloid mci.al stock. It is imusu.al in teeth of jicrsons of Caucasian racial stock. 12 Tooth Eruption and the Shedding of the Primary Teeth TOOTH ERUPTION Tooth eruption is the combination of bodily movements of the tooth, both before and after the eniergence of its crown into the oral cavity, which serv'cs to bring it and maintain it in occlusion with the teeth of the opposing arch. Tooth eruption begins at the time the root starts to form and continues throughout the life of the tooth. Eruptive Movements Tooth eruption Ijcgins when the crown of the tooth has been formed and the formation of the root dentin starts. In its simplest form tooth eruption may lie picture' tooth has emerged into the oral cavity the development of the permanent tooth is well ad\'anccd. In the anterior region the permanent teeth continue de\'cIopment lingual to their primar>’ predecessors, but in the region of the premolars a change in relative position occurs. The prcmolars replace the primary molars. By the time the primary molars have come into occlusion, the crowns of the developing premolars occupy a position not lingual to, but between the roots of the prlmar>’ molars (Fig. 95)- This change in relative positions occurs as a result of a vertical movement of the primarx' teeth and a hori- zontal movement of the developing permanent teeth. ( 139 j 140 TOOTH KRUPTION AND THE SHEDDING OF THE PRIMARY TEETH The permanent molar teeth do not have predecessors. The enamel organs of the tooth buds of the permanent molars develop from an exten- sion of the dental lamina distal to the position of the primary’ molars. The first permanent molars develop in approvimatcly the position they will hold upon emen^ence into the oral cavity. Ilut the crowns of both the second and third molars form in a vcr>' different position, and must undergo complicated motions of rotation and forward movement in order to emerge into correct relation to other teeth. At the time the second and third molars Iregin to develop neither the maxilla nor the mandible is large enough to accommodate them. The mandibular second and thinl molars de\'eIop in the ramus of the mandible with their occlusal surfaces directed nicsially. The second molar usually emerges into the oral cavity in its correct position clistal to the first molar. But inadequate jaw development and a failure of sufficient rotating move- ment in the early stiiges of eruption sometimes cause the crown of the mandibular third molar to press against the roots of the adjacent second molar. The result of such a positional relationship is an impacted third molar. In Figure (A is seen a mamlilmlar third molar cn>wn forming in the usual position in the ramus of the mandible. In the maxilla the second and thir' third molar with its occlusal surface directed iHstally and buccally. The change in position of developing teeth in the jaws is correlated with the growth of the teeth, the growth of the alveolar process, and the growth of the jaws (Fig. 64). Incldently, in the course of the total process of tooth eruption, the tooth emerges into the oral cavity. Figure 74 is a diagram of some events in- volved in this emergence. In Figure 74/1 the tooth crown is nearly com- plete but root formation has not yet licgun 'Fhc reduced enamel epithe- lium covering the crown is scparatcti from the epithelium lining the oral cavity by an area of connectiie tissue. In Figure 74B root fonnation is in progress and the crown has moved occlusally. With the occlusal move- ment of the tooth crouTi the rcducceen mentioned: (I) the easily recognized occlusal movement of the erupting tooth; (2) the hori- zontal movement which puts the dex-efoping cro^^'ns of the permanent premolars between the roots of the primaiy' molars; and (3) the complicated 142 TOOTH ERUPTION AND THE SHEDDINO Of THE PRIMARY TCnTII combination of rotating and lateral movements which brings second and third molars into their final position. In addition to these tooth movements there is a type of tooth movement known as mesial drift, ^festal drift is the lateral bodily movement of the teeth on both sides of the mouth toward the midline of the arch. The con- ditions leading to mesial drift may be understood if we picture the teeth in function. Since teeth are suspended in their sockets by the fibers of the periodontal ligament, they are not rigid in the jaws but undergo con- siderable movement during the process of mastication. This functional movement produces a rubbing of the contact areas. In newly cmci^ed teeth the contact with adjacent teeth is on a very small area of spherical surface. After years of function the spherical contact areas are worn to flattened surfaces because the teeth have remained in contact. This maintenance of contact is a result of the movement of the teeth toward the midline of the arch— that is, mesial drift. Mesial drift is possible because of the adaptability of bone tissue. Pres- sure on the periodontal ligament fillers results in the resorption of the lamina dura, while pull on the fibers results in bone apfiosition (fonnation). As the contact areas of the crowns wear, the teeth tend to move mcsially, maintaining contact. The slight pressure thus produced on the mesial side of the socket results in slow resorption of the lamina dura. The accompanying tension of the periodontal ligament filiere on the distal side of the root induces apposition of lamina dum bone in this area. As a consequence of these bone changes there is an actual shift in the position of the tooth socket (Fig. 6S). It should be understood that the process of mesial drift is a ver>' slow one, taking place over a period of many years as the mesial and distal contact areas of the tooth crowns wear off. SHEDDING OF THE PRIMARY TEETH Primary and Persianent Dentitions The human primary deiililtott is made up of 1 central incisor, 1 lateral incisor, 1 canine, and 2 molar teeth in each quadrant of the mouth. The first teeth to Inx-ome visible in the oral caHty are usually the primarj- mandibular central incisors, which emerge when the child is nlxmt si\ months old. The last priniar>' teeth to appear arc the maxillary second molars, which appear about the end of the second year. Each tooth of the primar>’ dentition is ci’cntually lost and is replaced by a tooth of the permanent dentition. The permavenl detitiliou consists of 1 centnil incisor, I l.ntcral incisor. 1 canine. 2 prcmolars, and 3 molars in each quadrant of the mouth. The first permanent tooth to appear is usually a first molar, which cnieiycs just behind the second primary molar when the child is aliout six years old. The last priinar>’ tooth to remain in the mouth is usually the second primary' molar. This is replaced by the permanent second prcmolar in about the twelfth year. The permanent molars have no predecessors. Since the first permanent molars appear in the mouth during the sixth year, TOOTH ERUPTION AND THE SHEDDIN'O OF THE PRI.MARV TEETH 143 when the primary dentition is sometimes still intact, it is important that these teeth be recognized as teeth of the permanent dentition and not be regarded as primary teeth soon to be lost. In Table 2 will be found the time of emergence into the oral cavity of the teeth of the primary and permanent dentitions, as well as the time of the beginning of hard tissue formation in each of the teeth (See page 145). The Process of Shedding The shedding of primary teeth is the result of the gradual resorption of their roots with the consequent loss of periodontal ligament attach- Pnniary oniiK tooth Prfmary posterior teeth Pcrmanetil IxMtenof teeOt Permanent anterior tooth Figure 95.— A photomicrograph of a longitudinal section nf the mandihle of a litten The primar)' canine tooth and posterior teeth are in functional position lleneath the root of the canine tooth and between the roots of the posterior teeth are the developing rroivTjs of the teeth of the pennanent Jenmhn This same kind of srrangonent is seen in human teeth. A kitten mandible is used here for illustration because of the difficult)’ m obtaining suitable human material. ment. The developing permanent successor, located lingual to, or beneath the root of the functioning primary tooth, creates sufficient pressure by its increase in size to produce resorption of the primary’ tooth root and of the bone surrounding the root (Figs. 95 and 96). As the root resorbs the tooth loosens. Eventu.ally all periodontal ligament attachment is lost and the rootless crorni of the primary' tooth literally falls off of the jaw. An interesting phenomenon frcr|Uontly obscrvctl in children is the alternate loosening and tightening of a primary' tooth l^efore it is finally shed. One day the child reports a loose tooth, and sc\'eral tlays later the tooth seems to Itc firmly attached. This rcattachment is due to the fact that when resorption of the primary' tooth root causes the tor;th to become loose, not only Is pressure rclicx’Cfl but slight tension seems to be iniluced on the adjacent connective tissue. This tension stimuhtes the connective 14-t TOOTH ERl?PTION AND TIIK MIEDDING OF TIIK TRIMARV TEETH tissue around the rcsorbed root end to fonn new cernentum on the remain- ing root end and new bone around the root. This results in the attach- ment of new periodontal ligament fibers and the tooth tightens in the jaw. But further development of the permanent tooth bud soon causes more resorption of both bone and root. Loosening and rcattachment may alternate several times, but as the pcniiancnt tooth continues to develop the pressure brought about by Us growth will cause sufFicient resorption of the primary tooth root to bring about the shedding of the tooth. Figure 96.— A diaRrammatic drawinpofa mc'ioJmal stetion throufth a portion of Litttn mandible. ThN drawing was made from the same jpecirticn .as tliconeilliistrated m ti8Ufe9j. but from a different section Mere there K root resorption on the primarj tooth ai a remit of the growih of the permanent tooth ftwii. Notice also the resorption of the bone between the roots of the primarj* tooth ijetsseen the Jeselopmg roots of the permanent tooth bone growth (bone apposition) is taling place. TOOTH ERUPTION AND THE SHEDDING OF THE PRIMARY TEETH 145 Occasionally the relative positions of the primary tooth and its perma- nent successor are such that the primary' tooth root is not subjected to the pressure which would cause its resorption. In this case the permanent tooth may emerge into the oral ca\dty lingual to the primary' tooth which it is supposed to replace. This condition is seen most often in the region of the mandibular incisors. In cases where the permanent tooth bud has failed to develop, the roots of the primary tooth may r7. 58 ^o^«l>ccs spots. lOS Fusion(s). facial, 19, 21. 10$ failure of (clefts), 2^21 freiiuenci of, 24 oral. 20. 21. 22. lOS Ectoderm, 15, 16, 17, 123 Embrx'o, IS, 16. 123, 127 Enamel, 39. 44-59, 73. 76. 117, J20, 124, 144 cr>'srals, 48 cuticle, 4G, 50, 130-131 formation of, 51, 124, 127, 130-131 hypocalcification of. 137 hypoplasia, of, 135 intettod substance, 48 lamella, 49, SO. 52, 54 matnx of, 46, 4$. 69, 128, 130-131 mineral content of. 44, 54 mottled, 138 pearl. 136-138 permanence of, 53 rods, 46. 47. 51, 52, S3, 54. 5$, 63. 130, 131 sheath of, 48 i; 'cisciSA. 39, 83. lOS. 106. 107. 113-122 attached, 115-U7 definition of, 113 free. 83, MS-117 recession of, 115, 117-118, 140, 141 f.ingival margin. 83, 114, MS. 116 Gingival sulcus, 102. 115-117, 119, 121, 131 Glands, salisan, 31. 39. 91. 104, 107. 10«. 109. II0-I12. IM minor. Ml, 112 parotid, ill sublingual, Ml). Ml. 112 siibiiundibiilir, Ml. M2 Von hbner’s, 109. HO. M2 n ilASFRsiAN canal(s), 86. 92, 93. 94, 93 Haversian sjstemfs), 92. 93. 94, 97 Hertwig’s epithelial sheath, 87, MS. 135. 138, 140 llistologv. general, 25-38 Ilypercementosis, 74, SO ll^pudontia. 135 INDEX 149 I N' 1nvi.amm\tion, 68, 72, 91, 102-103, 117, 119-120, 121 Inner enamel epitViel'mm, 125, 126, 127, 128 Intercellular substance, 27, 29, 68, 73, 82, 91, 93 Interdental papilla, 83, 84, 113, 114, 115 Intermediate plexus, 84 Interradicular septum, 100 Interred substance, 48, 52, 56, 130 Keratin layer, 34. 105, 106, 107, 108, 109, 115, 116, 120 KorfTs fibers, 68, 69. 70, 127, 128-129 'NaSai septum, 16, 20, 21, 23 NcrveCsJ. 33, 63, 6S, 70, 71. 82, ST, 90. 100. ' 104. 106, 107, 125, 126 I Nervous tissue, 36-37 [Nose, development of, 16, 19, 23 O iOdovtoblast, 62. 68-70, 71, 126-130 'Olfactory pit, 19 Oral clefts {See Fusion, failure of) Osteoblast(s). 22, 82. 87. 95, 96. 98, 125 ,Osteoclast(s), 82. 87. 97, 98 ^ Osteocj'tefs), 36, 90-103 Outer enamel epithelium, 125, 126, 127, 128, 131 L Lacuna(e). of bone, 36, 92, 94, 95 of cartilage, 36 of cementum, 76, 77, 78. 79 I.ameltafe). circumferential, 92, 93 Haversian sj stem, 92, 93, 94 subendostea!, 94 subperiosteal, 93 Lamina dura, 39, 81, 84, 86, 91, 99. 100. 102, 103. II6, 119. 121, 124. 135. 142 Lymphatics, 70, 87 M Macsostomza, 24 Mamelonfs), 45, 59, SO Mandible, 16, 21, 23, 90, 91, 99. 100, 101, 117, 123, 126, 140 Maxilla, 18, 99, 123, 126, 140 MecteFs cartilage, 20, 123 Mental foramen, 91 Mesenchyme, 22, 123, 124 Mesial drift, 102, 14W42 Mesoderm, IS, 16, 17, 123 Mucin, 111 Mucocele, JOS Mucosa {S/e Mucous membrane) Mucous membrane, 34, 104, 105 alveolar, 107, 108. 113, 116 classification of, 107 definition of, 101 lining, 107, 108 mastjcatorr, 107 oral, 16, 31, 10M12 palatal. 105, 107 specialized, 107, 108 Musclc(s), 39, 91 tissue. 33, 37-58 r PAtATE, clefts of (Srr Fusion, failure oO development of, 19, 20*22 fusions of (Srr Fusion, oral) ' mucosa of (Srr Mucous membrane, palatal) Palatine raphe, 107 PapiUa(e), of tongue {See T ongue, papillae of) , Penkymata, 44, 45, 51, 54 ’ I Periodontal ligament, 39, 68, 76, 78, 80, 81, 82-89. 91. 98, 100, 102-103, 120, 121. 124, 140, 142, 143 , fibers of, 82, 83, 86-87, 100, 115,1 19, 135. 142 I formation of, 135 I functions of, 76, 88-89 ) pressure on, 99, 103, 144 5, 1 tension on, 99, 103, 143 I width of, 82 jPeriodontalmembrane {See Periodontal liga- ment). I Periosteum, 90, 91, 92, 95, 106 Philtruro, 19, 23 Pitfs), 43, 46, S3, 54, 56, 59 Plaque, 54, SS, 56, 57, 58 Predentin, 62, 69, 128, 129 IPremaxilla. 21, 22, 23 ' Primary embr> onic lav ers, 1 5 • Primitive mouth, 16, 17, 18, 19, 20, 123 Processes, frontal, 16, 21, 22, 23 globular, 19, 21. 23 lateral nasal, 19, 20, 21 lateral palatine, 16, 19, 21, 22, 23 mandibular, 18, 19, 21, 23 maxillary’, 18, 19, 21, 22, 23 median nasal, 19, 21, 23 Pulp, 39. 62, 67, 68-73, 128, 129, 130, 133 age changes of, 73 chamber, 39, 43, 65, 71, 73, 131 148 INTIEX Cortical plate, 91. 99 Crown, anatomic, 40, 114, 115, 141 clinical, 114, 115, 141 Dead Tract, 65, 66 I Dental caries, 43, +4, 48, 53, 54-S9, 65, 66-67, 1 72. 73. 74 Dental lamina, 123-127, 139 i Dental papilla, 123-128. 130 Dental sac. 123, 124, 125, 126, 135 . Denticles, 42. 43, 68,71, 73, 74 ) Dentin, 39, 40-43, 43. 49, 50, 51. 57, 58. ■ 60-67, 69, 71, 74, 75, 76. 78, 84. 120. 133, 135, 144 formation of, 124, 127-130. 131 imerglobular, 40^2, 63, 64, 74, 75 mineral content of, 60 \ sclerosis of, 43, 65, 66, 67, 72, 73, 122 i secondary, 40, 42, 43, 48, 6-1, 65, 67, 71, ' 72. 73, 74. 122, 130 i Dentinal fibers, SI, 52,60,62, 63, 67, 69, 70, 127-130 ' Dentinal tubules, 40-43, 51, 52, 53.55. 59.60, t 62, 63, 64. 66, 67, 69, 71. 72. 75. U8. 150 Dentinocemental junction, 41, 42, 58, 60, 63. | 64, 69, 75. 78 Dentlnoenamel junaion, 40-13, 48, 49, 50. SI, 53. 57. 58. 63. 65, 126. 12S. 129, 130 ' Denttnoid Ptedentin) Dentitions, pnmaf)' and permanent, 142 j Developmental grooves. 43, 45, 55, 56 Diffuse calcifications, 42, 43, 68, 71, 73, 74 Duct(s), of salivarv glands 107, 108, 109. 110,111,112 Stenscn’s, 111 Wharton’s, 112 Enamel, spindle. 42, 43. 49, 50, 51, 52-53, 54, SR, 62, 63, 69 tWeVness of. 46 tuft, 43, 49, SO. 52, 54. 58 Enamel organ, 30.97. 123-127, 130-131,139, 140 Endodemi, 15. 16, 17. 123 Endosteum, 90, 91. 95 Epithelial attachment, 91, 116, 117-119, 120, 111 140, 141 rootuard rntgration of. (See Ginfiua, recession of) I E,pithcltal rests. (See Ke'its of Malasscr) 'Epithelium, 15,30-34, 104. 123 classification of, 31 E’ccemcntosis, KO FACt, development of, 16, 18-21 Fetus, site of, 16 Fibrils, of bone matn’t, 90. 95 ofcementiim matrix. 75 of dentin matrix, 60, 70, 127, 129 of enamel matrix, 48 rmuic(s), 42, 45. 46, 47. 48. S3, 54, 55, 56, 57, 58 Fordyce's spots, 108 riision(s), facial, 19, 21. lOS failure of (clefts), 23-21 ftequenev of, 24 oral. 20, 21. 22. 108 Ectodeiim, is, 16, 17, 123 ' Embtyo. 15, 16. 121, 127 Enamel. 39. 41-59, 73, 76. 117, 120. 124, 144 crystals, 48 cuticle. 46. 50, 130-131 formation of, 51. 124, 127, 130-131 i hjpocalcification of, I37 hypoplasia, of. 135 interrod substance, 4S i lamella, 49, 50. 52. 54 matrix of. 46, 48, 69, 128. 130-131 mineral content of, 44. 54 mottled. 138 peatl, 136-138 permanence of, 53 i rods. 46, 47. 51. 52. S3, 54, 58. 63. 130, j Givciva. 39, 85. 105, 106, 107, 113-122 afcached, 115-117 definition of, 113 free, 83. U5-117 recession of, lli. 117-118, 140, I4I Gingival inarRin, 83, 114, 115, 116 GiciRtval sulcus, 102, 115-117, 119, 121, 131 Glands, salivary. 51. 39. 91. IW. 107. 10«. 109. 1 10-112. 115 minor. 111, 112 parotid. 111 sublincual. MO. Ill, 112 submandibular. 111, 112 Von Ebner’i. 109. nO. 112 IJAvmsivv canal(x}, 86, 92. 93. 94, 95 Ilavmian system(s), 92, 93, 94, 97 llertwiR’s epuhelial sheath, 87, HR, 135. Il**, 1 Ihtologv , general. 25-38 Jlvpercrmemosis. 74, fiO ilypodonti^i 155 INDEX 149 1 N Inflammation, 68 , 72, 91, 102-103, U7, 119-120, 121 Inner enamel epithelium, 125, 126, 127, 128 Intercellular substance, 27, 29, eg, 73 , gi, 91, 95 : Nasal septum, 16, 20, 21, 23 !Ncrve(s), 33, 62, 68 . 70, 71, 82. 87, 90, 100. ! 101, 106, 107, 125, 126 I Nervous tissue, 36-37 I Nose, development of, 16, 19 , 23 Interdental papilla, 83, 8-1, 113, 114 , |15 Intermediate plexus, S4 Interradicular septum, 100 Interred substance, 48, 52, 56, 130 K Kfratin layer, 34, 105, 106, l07, lOK, 115, 116, 120 KorfPs fibers, 68 , 69, 70, 127, 128-129 I 0 I 1 Odontoblast, 62, 68-70, 71, 126-130 l’/92^rrVi547*^/^, JP [Oral clefts (S/e Fusion, failure of) IOsteoblast{s), 22, 82, 87. 95. 96, 98, 125 IOst«ii:last(s). 82,87,97,98 [Osteocyte(s), 36, 90-103 tOuter enamel epithelium, 125, 126, 127, !2S, ' 131 Lacusa(e), of bone, 36, 92, 94. 93 of cartilaRc, 36 of cementum, 76, 77, 78, 7? bamellafe), circumferential, 92, 93 Haversian system, 92, 93, 94 subendosteal, 94 subperiosteal, 93 Lamina duta, 39, 81, 84, 86 , 91, 99 , 100 , lOl, 102, 103, 116, 119, 121, 124, 1 J 5 , h 2 Lymphatics, 70, 87 M Macrostomia, 24 Maniclon(s), 45, 59, 60 Mandible, 16,21, 23, 90, 91, 99. lOO, 101. 116, II7, 123, 126, 140 Maxilla, 18, 99, 123, 126, HO Mental foramen, 91 Mesenchyme, 22, 123, 124 Mesial drift, 102,141-142 Mesoderm. IS, 16. 17, 123 Mucin, 111 Mucocele, 108 Mucosa (Srf Mucous membrane) Mucous membrane, 34, 104, IO 5 alveolar, 107, 108, lU, IJ 6 classification of, 107 definition of, 104 linins, 107, lOS masticator)', 107 oral, 16, 31,161-112 palatal, 105, 107 specialized, 107, 108 Musclcfs), 39, 91 tissue, 33, 37-38 PaLatl, clefts of (S/r Fusion, failure of) development of, 19, 20-22 fusions of (S/r Fusion, oral) mucosa of ($// Mucous membrane, palatal) Palatine raphe, 107 Papilla(e), of tongue (iVr Tongue, papillae of) Penkymata. 44, 45, 51, 54 Periodontal ligament, 39, 6 S, 76, 78, 80, 81, 82-89, 91, 95, 100, 102-103, 120, 121, 124. 140, 142, 143 fibers of. 82, 83, 85-87, 100, 1 1 5, 119, 1 35. 142 formation of, 135 functions of, 76, 88-89 pressure on, 99, 103, 144 tension on, 99, 103, 143 width of, 82 Periodontal membrane (S// Periodontal liga- Pcrkctcom, 90, 91, 92, 95, 106 Philcrum, 19, 23 Pills), 45, 46, S3, 54, 56, 59 Plaque, 54, 35, 56, 57, 58 Predentin, 62, 69. 128. 129 Premaxilla, 21, 22, 23 Primary embryonic layers, 15 Primitive mouth, 16, 17, 18, 19, 20, 123 Processes, frontal, 18, 31, 22, 33 globular, 19, 31, 33 lateral nasal, 19, 20, 31 lateral palatine, 16, 19, 21, 22, 23 mandibular, 18, 19, 21, 23 maxillary, 18, 19,21,22,23 median nasal, 19, 21. 23 Pulp, 39, 62, 67, 68-73. 128. 129, 130, 133 age changes of, 73 chamber. 39,43,65,71, 73, 131 150 INDEX Pulp, Tunctions of, 72-73 rtactton to caries, 65, 72-73 stones (Ste Denticle). R Reduced enamel epitlieVtum, 118, 131, 135, 140 Rests of Malassez, 82. 84. 87, 118, 121, 135 Root. 82, 99 anatomic. 40 canal, 39, 40-43, 71. 73, 131, 133-135 formation of, 81, 118, 131-135, 139, 141 resorption of, 133, 143, 144, 145 S Saliva, 105, 110, 111, 119 Salivary glands (Srf Glands). Sharpej’'s fibers, 64, 78, 82, 84 Stellate reticulum, 125, 126, 127, 128, 131 Stomodeum, 17 Stratum intermedium. 125, 126, 127, 131 Stripes of Rcrius. 41, 42, 48. 49, 51. 54 Submucosa, 106, 107, 108, 115 T Taste bud(i}, 109, 110 115806(8), classification of, 29-30 definition of, 25 derivation of (embryomcally), 15, 16 Tissue(s), fluid, 27, 29. 92 ptepatation of, for examination, 25-27 Tomes’ fiben (S'f Dentinal fibers). Tomes’ granular layer, 40-43, 63, 64, 65, 67, 74. 75, 78 Tongue. 39, 112, 125 development of, 16, 18, 19, 21, 22*23 mucosa of (See Mucous membrane, specialized), 33, 107. 108, 114 papillae of. lOS-llO, III, Il2 Tooth (teeth), absence of, 135 bud, 123-127. 131, 139, 140, 143 development of, 20, 100, 12^138 eruption of. 100, 117, 131, 135, 139-I4G mechanism of, 141 < Linds of, 143 I shedding of. 142-146 I socket of (.9ee Alieolus and Lamina I dura) I sujsemumerary, 135 tissues of, 39, 40, 41, 42, 43 Irabecubr (See Rone, trabecular) V Volkmann’s canal, 92, 93, 94, 95 Von Khneris glands (See Salivary glands) w I Whartos’s duct, (See Duct).