Oral Anatomy ORAL EDMUND
and Embryology AND EMBRYOLOGY. D.D.s.,*
of Jaws and Teeth
E HAVE studied the facial primordia and tooth germs in the embryo and fetus. We can now go on to study postnatal jaw development and its Many workers have investigated the synchronization with tooth development. Their findings sometimes conflict, at problem from different points of view. least in certain details, but the main sequential story is becoming increasingly clear. The alternating growth of the ramus of the mandible and the clinical eruption of teeth into the intermaxillary space thus created is responsible for the development of the dental height according to Diamond. Broadbent used an arbitrary plane (Bolton plane) for the superimposition and comparison of xrays (cephalograms) of individuals from birth to adult stage. The assumption in using such a fixed plane is that all head structures above the plane grow upward, while those below it grow downward, during development from birth to adult stage. Brash showed the changes in t,he form of the pig’s mandible by experimental madder-feeding during growth. He observed that the main increase in the height of the body of the mandible takes place at the alveolar and not at the lower border. He also obtained some idea of the extent t,o which the teeth move upward and forward during their eruption. His work should be compared with Broadbent’s t,o understand that as increase in length of the ramus occurs in relation to the mandibular fossa of the temporal hone, t,he mandible is steadily lowered. Hoffman and Schour gave multiple intraperitoneal injections of 2 per cent alizarin to rats at various intervals. They used the cementoenamel junction on the rat molar as a fixed point, and determined the rate at which this point changes in relation to fixed alizarin lines in t,he alveolar bone. They found that the rate of apposition of alveolar bone is subservient to the physiologic migration of the tooth. The rate of eruption equals the sum of the rate of root elongation and of fun&c bone apposition. Diamond and Applebaum showed that Hertwig’s epithelial sheath is not present until the crown of a tooth is virtually completed. At an early stage of tooth development as presented in Fig. 10 (Part I), there is no evidence of an epithelial sheath. The region generally designated as Hertwig’s epithelial sheath (C in Fig. 10) is, instead, the cervical terminal point of the enamel York
Presented Institute *4ssociate
at the of Clinical Frofessor
second session of the Oral Pathology. of Dental Anatomy, 1185
organ and the eventual c~mentoenamel junction of the tooth. In later stages of crown formation, the crown increases in size in all directions including the fundic area (Fig. 12, Z?) . The evidence of bone resorption in the fnndie area of this developing crown supports this view. On t,he other hand, there is bone deposition in the fundus of the deciduous molar (Fig. 12, -4 ). where the prawn is completed and the root is forming.
In the study of the enamel organ, we pointed out that epithelium is the formative tissue of the human tooth. The formative process takes place in two stages, one for t,he crown and the other f’or the root. The enamel organ completes forrnation of the hard tissues of the crown up to the cervical line. Then the epithelial root sheath serves to outline> the form of the root and to organize the connective tissue ot’ the l)nlp for the formation of odontoblasts and root dentine.
Fig. 1X-Photomicrograph of a mesiodistal section of a human of age. A shows the deciduous second molar with crown completed B shows the permanent first molar crown in process of formation. was taken from a specimen in the collection of Dr. Isaac Schour).
mandible at 9 months and enamel calcifled. (This photomicrograph
The life cycle of the epithclial root sheath cannot be observed in ally Om stage of development of a tooth. It,s histogenesis from the enamel organ may be observed only when the enamel matrix is still in the process of formation (Fig. 12, B) in the cervical regions of the tooth. At a later stage in development when root dentine and cementum are in the process of formation (Fig. 12, A), the organic connection between the epithelial sheath and the inner and outer layers of the enamel organ can no longer be seen. Fig. 13 shows the cervical area of the permanent first molar in Fig. 12, B. The enamel organ is still functioning here but narrows down at the true cervical ameloblasts may be seen at A. loop enclosed within a square at 0. Functioning Predentine is seen at P, but the ameloblasts in this region are not as yet functioning to produce enamel. The outer enamel epithelial cells gradually come closer to the ameloblasts and merge with them in the region of the square at C.
The area of contact at the cervical loop region is seen under higher magnification in Fig. 14. Beyond the loop at L, the stellate reticulum and the stratum intermedium of the enamel organ are absent,. The ont,er epithelial cells lie adjacent to the inner epithelial cells, in the position formerly held by the stratum
Fig. 13.-Higher magnification of cervical region of developing D, dentine; A, functioning ameloblasts Fig. 12. &I, Enamel matrix; odontoblasts ; ST, stratum intermedium ; 1’, predentine ; SR, stellate enamel epithelium : A’, undifferentiwtecl ameloblasts ; (‘. cervical loop
magnification blasts : F, tooth
of cervicnl loop C shown in follicle ; A’, epithelial shc;tth
permanent molar B in in enamel organ: 0, reticulum : 0, outer enclosed in a square.
Fig. 13. 9, Pulp tissue: : I,. cwv~cal loop.
intermedium cells of the enamel organ. The epithelial root sheath begins just beyond the cervical loop. Y is the pulp; A, ameloblasts; F, tooth follicle ; A’, epithelial sheath; L, cervical loop.
A little later in tooth development, during t,he functional stage of the epithelial root sheath, the organic connection between the ameloblasts and the inner layer of the epithelial root11 sheath is disrupted. Fig. 12. A is a low-power view of a deciduous molar with root dentine forming. Thr cnamcl of the crown is completely formed and nearly completely calcified. Fig. 15 shows the cervical area of the deciduous molar seen in Fig. 12, d, under slightly higher magnification. The cervical enamel is still in a matrix state terminating at the cervical line, CL, where it joins a thin layer of cementum, C. The enamel organ, R, is atrophied and the epithelial sheath is seen at EP. The cervical area of this molar, enclosed in a square, is seen under high magnification in Fig. 36.
Fig. 15.-Higher 12. R, Reduced or cementoenamel
magnification of cervical region of cells of enamel organ; M, enamel junction ; C, cementurn ; EP, epithelial
developing matrix; sheath
deciduous dentine: odontoblasts
molar A in CL, cervical ; E’, pulp.
Fig. 16 shows the ameloblasts, A, as large oval cells in contact with the These cells are seen to terminate at the cervical periphery of the enamel matrix Adjacent to the ameloblasts at ZZ line where the enamel matrix terminates. are several layers of flattened, elongated cells. These layers appear widely separated in region L, as they extend beyond the cervical line in the direction of the root. Several rounded cells (DE) are seen lying between the layer of cementum (C) and the elongated flattened epithelial cells of the outer enamel epithelium. CB arc cementohlasts which have differentiated from the surrounding connective tissue t.ooth follicle, and passed through the t,wo layers of the epithelial root sheath. These cells have deposited a layer of cementum (C) on the dentine surface of the root.
A continuous epithelial sheath is to be seen only at the beginning of root formation. As development progresses and the root elongates, the connection between the loop of the root sheath and the enamel organ is severed. Epithelial rests (zzr) remain in the periodontal membrane between the enamel organ and the loop of t,he root sheath as the root elongates (Fig. 17). But in addition to elongation of the root there is actual tooth movement during eruption, as evidenced by deposition of bone in the fundus of the alveolus (Figs. 12, A and 18).
Fig. 16.-Higher magnilkation of cementoenamel junction (CL) shown in Fig. 15. R, Reduced cells of enamel organ ; A, ameloblasts ; L, loose strands of reduced outer enamel epithelium : CL, cementoenamel junction where ameloblasts terminate : DE, degenerating cells of inner layer of epithelial sheath: CB, cementoblasts: C, cementum.
Fig. 17 is a diagram illustrating the development of the epithelial rests during growth of the root. We have modified the original diagram by Orban, to include the changing position of the tooth at different stages of development and during eruption. In other words, not only are the epithelial rests (xzz) being carried along with the growing root but with the whole tooth, which is moving during eruption. At 1 we see a tooth germ ; at 2 we see the completed crown with ,the loop of the epithelial sheath located deeper in the expanding
jaw. As the root elongates in 3 and 4, t,he cpithelial rests (xxr) move up with it. In 5 and (i, the whole t,ooth has shifted its position during clinical crklption to occlusion, carrying the epithelial rests into a new position in the lower jaw.
Fig. l$.-Diagram epithelial root during root
of stages in tooth development and eruption sheath is severed from enamel organ and broken These rests are carried along with development.
(mollified frOrn up into eplthelial the developing
Orban). rests tooth.
of the Tooth
The tooth follicle is the vascular connective tissue sac surrounding the developing tooth. It serves to form cementum on the root, and the periodontal membrane which attaches the root to the surrounding alveolar bone. Specialized connective tissue cells, cementoblasts, of the tooth follicle form cementurn on the root, after penetrating the epithelial sheath, which disintegrates. Other cells, fibroblasts, differentiate to form fibers of the periodontal membrane, and still ot,hers, osteoblasts, deposit layers of bone as the tooth erupts (Figs. 12, 16, and 18). The tooth follicle consists of cells and a fluid ground substance in which are found fibers forming wide meshes. This loose, unattached connective tissue gra:!ually changes to form the relatively dense periodont,al membrane. Sicher suggested that the tooth follicle forms a cushioned ligament, rich in fluid, acting to correlate root growth and bone growth durin, w active crnption of t.he tooth. Fig. 18 is a low-power photomicrograph of an erupting incisor of an g-yearold child. The level of the alveolar crest, C, is above the cement,oenamel junction, E. Movement of the tooth is indicated by new bone, N, at the apex and on the lingual surface of the tooth. This bone is oriented in the direction of eruption and so is the periodontal membrane. Fig. 19 is a higher magnification of E in Fig. 18 showing that during this stage of eruption the fibers of the periodontal membrane are not oriented for functional stress, but are more like the fibers of the tooth follicle, which formerly encircled the tooth. Note the
Fig. l’hl 4 level the tooth X, Extent
Fig. the flbers on tooth:
18.-Low-power of’ the alveolar is indicated by of the epithelial
photomicrograph of an erupting incisor of an 8.year-old C, is above the cenwntotmamel junction, E. luovement crest, new bone, N, at the apex and on the lingual surface of this attachment on the enamel.
lg.-Higher magnification of the periodontal membrane, B, bone of alveolus.
of E in Fig. 18 showing F, are parallel to the
that during this stage of eruption long axis of the tooth. C. Cementum
Fig. 20.-Low-power old. The level
photomicrograph of an erupted, of the alveolar crest, C. has rrctvle(l
Fig. 21.-Higher magnification function the flbers of the periodontal flbers run directly across from the
functional pant the
cuspid of an adult 47 years rrmentoenamel iunction, 3~‘.
of C in Fig. 20 showing that during membrane, F, are oriented to support cementurn, C, to the bone, B.
this stageT;; the tooth.
wavy periodontal fibers, F, parallel to the long axis of the erupting tooth and the thin layer of cementum, C, and the alveolar bone, B. In this periodontal membrane, cells are more numerous than fibers. Fig. 20 is a low-power photomicrograph of an erupted and functional cuspid of an adult 47 years old. The level of the alveolar crest, C, has receded of area past the cementoenamel junction, E. Fig. 21 is a higher magnification C in Fig. 20 showing that during this stage of function the fibers of the periodontal membrane are oriented to support the tooth. These fibers are no longer parallel to the tooth but almost at right angles to it. In Fig. 21 we see the fibers, F, running directly across from the cementum, C, to the bone, B. There is a high content of cells in the periodontal membrane, spindle-shaped fibroblasts, osteblasts, and cementoblasts. Embedded in loose connective tissue spaces between the bundles of white fibers are blood vessels and nerves. The blood supply of the periodontal membrane is derived from several sources: openings in the wall of the alveolus and from the gingival area, and also from the apical region in common with the blood supply of the pulp. Generally nerves follow the path of the blood vessels. In inflammation, the engorged blood vessels literally lift the tooth out of its socket and it causes pain during chewing. Groups of epithelial cells, rests of Malassez, previously described as remnants of the epithelial root sheath, may give rise to cysts, tumors, granulomas, or even cementicles, under pathologic conditions.
Fig. 22.-Photograph gastric ulcer. There due to pressure from
of models of the is excessive stress on the upper molar-traumatic
jaws of a man the distal surface occlusion.
49 years of the
due to molar
The thickness and structure of the periodontal membrane are influenced by functional conditions. With lack of function the periodontal membrane becomes thinner and atrophies ; with increase of function it becomes wider and hypertrophies. Similarly the alveolar and supporting bone around a tooth are influenced by varying functional conditions. Thus the spongy bone around the alveolus of a nonfunctioning tooth shows marked rarefaction, although the alveolar bone proper (lamina dura) is generally well preserved. In sections through human teeth with known functional and occlusal relations, it is possible to interpret the influence of these conditions on the periodontal tissues. This is illustrated by Fig. 22, a photograph of models of the jaws of a man 49 years old. Many teeth have been extracted as a result of which the lower left second molar has no antagonist in the upper jaw. However, the upper third molar strikes on the distal surface of the lower second molar. This is
traumatic occlusion and the lower molar is drifting toward the median line. h’ote extrusion of bot,h npp,er and lower molars. iI11 this can bc observed “clinically, ” and by sect,ioning these teeth anti csaminaiion rrntlcr thr microscope we can observe changes in the tr~~riodontal t issrrcs drtc~ to this trarrmat iv occlusion.
E’itg. 23 is a low-power photomic~og~al)h of a dccaicificd section of the lower left molar seen in Fig. 22. Note the regular distribution of alveolar bone (lamina dura) around both roots, and the strpportin, 0’ 1,one between the roots. in the nest t,wo figures. we shall SW some But under higher magnificatiou, changes in the alveolar~ crests mcsially anal distally as a result. 01’ Ihc traumatic occlusion. Fig. Z-1 is a higher magnification 01’ il1~1mesial crest of the alvcolu.~ seen in Fig. 23. This shows bone rcsorpt,ion which is the result of t,he pressure on the bone. Fig. 25 is a higher magnification of the dist,al crest of t,he alveolus
seen in Fig. 23. This shows hone deposition, which is the result of tension the bone. These changes in the bone arc adaptations to t,he excessive stress this lower molar, allowing a mesial shift of this tooth.
the mesial of bone on
The Epithelial- Attachment Gottlieb and Orban described the development of the gingival area, the relation of the gingival soft, tissues to the teeth. They showed that a reduction of the epithelial layers of the enamel organ takes place before the eruption of a tooth Figs. 12, A and 16. The stellate reticulum disappears first. Then the ameloblasts and the stratum intermedium go. The primary organic cuticle is the last formation of the ameloblasts and is continuous with the enamel rod sheaths. It may become calcified like the rest of the enamel, Finally the calcified enamel crown is covered with the remnants of the enamel organ, forming a stratified epithelium. These cells remain orga,nically attached to the enamel cuticle, which in turn, is connected with the enamel. AS a tooth approaches eruption into the mouth, the reduced enamel epithelium, epithelial attachment, unites with the oral epithelium. At the time when the summit of the crown erupts into the oral cavity (Fig. Is), the root end is often incompletely formed; even the enamel may still be uncalci6ed in the
: SK-Photomicrograph I2 is thP mamel organic cuticle of the gingiva
of tbe distal crest on of “bundle bone”
of a decalcified which has been dissolved attached to the enamel (G ). There is no gingival
the alveolus seen siile of tension.
section of & molar by the acid used and is continuous crevice in this
of a kangaroo. in preparing the with the stratum ideal specimen.
the en ;
cervical region. An opening into the mouth is formed by the atrophy of the oral epithelium over the summit of the erupting tooth. The junction of the mouth epithelium with the epithelial attachment is called the gingival margin. In such an erupting tooth, the epithelial att,achment ideally extends to the cementoenamel junction.
gingival on the
Z?.-Photomicrogrsph papila in which left. The connective
of a ground epithelium (E’p) tissue (CT) shows
mouth of patient and pyorrhea-gross
section of two molar teeth with is attached to the enamel (E) signs of inflammation. D, Dentine.
of age. of oral
She has hygiene.
approximal the tooth
The space formed between the gingival margin and the surface of the enamel or enamel cuticle is called the gingival crevice. The old conception based on clinical evidence was that on eruption the gingira is attached only at the
Yig. 29.-Photomicrograph of a longitudinal decalcified section This patient’s lamer fi\~~ teeth WFW cxcisad in toto by Fig. 28. Masses of tartar are seen adhering to the enxmel :rnd cw~entum. gingival
Fig. 30.-Photomicrograph of the interdental papila in Fig. 29. Masses of tartar (T) Cover the enamel (E) The epithelium of the gingiva (BP) is disintegrating CC). gingival crevice is to be noted at GG.
situated between and extend onto and the abnormal
tW0 iIlCiSOrs the Cementum depth of ‘the
cementoenamel junction, overlapping the enamel as free gingiva and forming a deep gingival crevice. According to newer histologic evidence of an epithelia] attachment to the tooth, the gingival crevice is zero depth in ideal cases (Fig. 26). Fig. 26 is a decalcified section of a molar and the enamel is lost during histologic preparation. But the enamel is retained in a ground section and the epithelial attachment can be demonstrated as in Fig. 27. Such a section is prepared by investing the soft tissues with a rosin to render them hard enough for grinding. Gottlieb and Orban showed that there is a shift in the position of the epithelial attachment on the tooth during its eruption toward occlusal contact, active eruption. Opposing teeth do not remain fixed in position after reaching occlusal contact but continue in eruption to compensate for attrition, in ideal cases. As the bottom of the gingival crevice shifts apically, with peeling off and migration of epithelial attachment, we see stages of passive tooth eruption. The shift of the epithelial attachment must be considered pathologic when it progresses too rapidly and independently of the active eruption of the tooth. As long as the cervical cementum is in functional connection with the bone through periodontal fibers the epithelial attachment does not extend over this area. It is only when the attachment of the periodont,al membrane is severed that the epithelial attachment can shift from the cementoenamel junction onto the surface of the root. It is difficult to determine in many cases whether cessation of cementum and fiber formation is caused by advancing age, lack of vitamins, or by toxic action of bacteria in the gingival crevice. But once a deep gingival crevice or ‘ ‘ pocket ’ ’ has developed, unhygienic conditions are established, inviting tartar deposition in the crevice and inflammation of the gingiva. This eventually leads to destruction of the supporting tissues of the tooth. Fig. 28 is a photograph of the mouth of a patient 29 years old with pyorrhea and extensive dental caries. This is a picture of gross neglect of oral hygiene. Fig. 29 is a low-power photomicrograph showing the five lower anterior teeth in longitudinal section. Masses of tartar are attached to the enamel cuticle and the eementum. There are deep gingival crevices (pockets) due to the irritation of the tartar. Fig. 30 is a photomicrograph at higher magnification, showing the inflamed approximal gingiva between two incisors. Large masses of tartar (T) are noted ; disintegration of the epithelium (EP) is evident and both gingival crevices have been abnormally deepened (GC) and lined with small remnants of tartar. But there is still an epithelial attachment just below the pocket (GC) with a horny (secondary) cuticle on the cementum. There is inflammation in the connective tissue as indicated by the round cell infiltration. Then follows resorption of bone and periodontal fibers and eventual loosening of the tooth.