Hertwig's Epithelial Sheath Research Articles

Taurodontism.

Taurodontism can be defined as a change in tooth shape caused by the failure of Hertwig's epithelial sheath diaphragm to invaginate at the proper horizontal level. An enlarged pulp chamber, apical displacement of the pulpal floor, and no constriction at the level of the cemento-enamel junction are the characteristic features. Although permanent molar teeth are most commonly affected, this change can also be seen in both the permanent and deciduous dentition, unilaterally or bilaterally, and in any combination of teeth or quadrants. Whilst it appears most frequently as an isolated anomaly, its association with several syndromes and abnormalities has also been reported. Despite the clinical challenges, taurodontism has received little attention from clinicians. Due to the prevalence of taurodontism in modern dentitions and the critical need for its true diagnosis and management, this review addresses the etiology, clinical and radiographic features of taurodontism, its association with various syndromes and anomalies, as well as important considerations in various areas of expertise dental treatments of such teeth.

Non Syndromic Generalised Taurodontism in Primary and Permanent teeth -A Case Report

Abstract Taurodontism is a rare dental anomaly defined as a change in shape of the tooth caused by the failure of Hertwig's epithelial sheath to invaginate at the proper horizontal level. The characteristic features of this anomaly are an enlarged pulp chamber with apical displacement of the pulpal floor, and no constriction at the level of the Cemento-enamel junction. They occur most frequently as an isolated anomaly or it may be associated with certain syndromes. Permanent dentition is more commonly affected than deciduous dentition, seen unilaterally or bilaterally, and in any combination of teeth or quadrants. This case report describes a case of generalised taurodontism involving all the deciduous molars and permanent molars in a 7 years old male patient. How to cite this article: Bharath .KP, Priya, Poornima .P, Neena I.E. Non Syndromic Generalised Taurodontism in Primary and Permanent teeth -A Case Report. CODS J Dent 2015;7: 90-92

Immunolocalization of TAK1, TAB1, and p38 in the developing rat molar

In tooth development, transforming growth factor beta (TGF-β) and bone morphogenetic protein (BMP) are involved in cell differentiation and matrix protein production. TGF-β and BMP have two signaling pathways: the Smad pathway and the non-Smad pathway. However, only a few studies have focused on the non-Smad pathway in tooth development. TGF-β-activated kinase 1 (TAK1) is activated by TGF-β or BMP and binds to TAK1-binding protein (TAB1), activating p38 or c-Jun N-terminal kinase (JNK), forming the non-Smad signaling pathway. In this study, we examined the distribution of these kinases, TGF-β receptor 1 (TGF-β-R1), BMP receptor-1B (BMPR-1B) and Smad4 in cells of the rat molar germ histochemically, in order to investigate the signaling pathway in each type of cell. Immunostaining for TGF-β-R1, BMPR-1B, Smad4, TAK1, TAB1 and phosphorylated-p38 (p-p38) showed similar reactions. In the cervical loop, reactions were clearer than in other enamel epithelium. In the inner enamel epithelium, signal increased with differentiation into ameloblasts, became strongest in the secretory stage, and decreased rapidly in the maturation stage. Signal also increased upon differentiation from preodontoblasts to odontoblasts. In Hertwig's epithelial sheath, with the exception of BMPR-1B, reactions were stronger in the later stage, showing more enamel protein secretion than in the early stage. However, no clear reaction corresponding to phosphorylated-JNK was observed in any type of cell. These results suggest that TGF-β or BMP is involved in the induction of differentiation of inner enamel epithelium cells into ameloblasts, and preodontoblast differentiation into odontoblasts, the regulation of cervical loop cell proliferation, the elongation or regulation of the epithelial sheath, and the secretion of enamel protein and dentin matrix protein through the non-Smad signaling pathway via TAK1, TAB1 and p38 as well as Smad signaling pathways in the rat molar germ.

Immunohistochemical Examination for the Distribution of Podoplanin-Expressing Cells in Developing Mouse Molar Tooth Germs

We recently reported the expression of podoplanin in the apical bud of adult mouse incisal tooth. This study was aimed to investigate the distribution of podoplanin-expressing cells in mouse tooth germs at several developing stages. At the bud stage podoplanin was expressed in oral mucous epithelia and in a tooth bud. At the cap stage podoplanin was expressed on inner and outer enamel epithelia but not in mesenchymal cells expressing the neural crest stem cell marker nestin. At the early bell stage nestin and podoplanin were expressed in cervical loop and odontoblasts. At the root formation stage both nestin and podoplanin were weakly expressed in odontoblasts generating radicular dentin. Podoplanin expression was also found in the Hertwig epithelial sheath. These results suggest that epithelial cells of developing tooth germ acquire the ability to express nestin, and that tooth germ epithelial cells maintain the ability to express podoplanin in oral mucous epithelia. The expression of podoplanin in odontoblasts was induced as tooth germ development advanced, but was suppressed with the completion of the primary dentin, suggesting that podoplanin may be involved in the cell growth of odontoblasts. Nestin may function as an intermediate filament that binds podoplanin in odontoblasts.

“Taurodontism” an endodontic challenge: a case report

Taurodontism can be defined as a change in tooth shape caused by the failure of the Hertwig's epithelial sheath diaphragm to invaginate at the proper horizontal level. An enlarged pulp chamber, apical displacement of the pulpal floor, and no constriction at the level of the cementoenamel junction are the characteristic features. Permanent molars are most commonly affected. Endodontic treatment of a taurodont tooth is challenging and requires special handling because of the proximity and apical displacement of the roots. Here, we report a case in which endodontic treatment of the maxillary right first molar with taurodontism was performed. In this case, the maxillary right second molar and maxillary left first and second molars were also taurodont teeth.

A new approach to root formation.

In endodontics, treatment of an open apex tooth with necrotic pulp is a problem. It seems that with promotion of remnants of Hertwig's epithelial sheath or rest of malassez accompany with a good irrigation of root canal we can expect root formation. (Iranian Endodontic Journal 2008;3:42-43).

Taurodontism: a review of the condition and endodontic treatment challenges

Taurodontism can be defined as a change in tooth shape caused by the failure of Hertwig's epithelial sheath diaphragm to invaginate at the proper horizontal level. An enlarged pulp chamber, apical displacement of the pulpal floor, and no constriction at the level of the cementoenamel junction are the characteristic features. Although permanent molar teeth are most commonly affected, this change can also be seen in both the permanent and deciduous dentition, unilaterally or bilaterally, and in any combination of teeth or quadrants. Whilst it appears most frequently as an isolated anomaly, its association with several syndromes and abnormalities has also been reported. The literature on taurodontism in the context of endodontics up to March 2007 was reviewed using PubMed, MEDLINE and Cumulative Index to Nursing & Allied Health Literature. Despite the clinical challenges in endodontic therapy, taurodontism has received little attention from clinicians. In performing root canal treatment on such teeth, one should appreciate the complexity of the root canal system, canal obliteration and configuration, and the potential for additional root canal systems. Careful exploration of the grooves between all orifices particularly with magnification, use of ultrasonic irrigation; and a modified filling technique are of particular use.

Evagination and Invagination of the Oral Epithelium during Tooth Development in Alligator Mississippiensis

The early development of alligator odontogenesis shows embryological 'the Biogenetic law'. The swelling of the oral epithelium formed in the medial nasal process. It contained many cell deaths without teeth development at (Ferguson) stage16. This stage compared to the horny teeth of Lamprey. At stages 17-19, the dental lamina developed from the oral epithelium and the enamel organ-like structure raised from it. The teeth developed just under the organ is composed of only collagen, so called dentine teeth. This stage compared to the fish tooth with the absence of enamel organ. After stage 27, the enamel organ completely developed and formed the tooth crown with the enamel and dentine. The completed teeth had long roots about 2 to 3 times longer than the crown height. Hertwig's epithelial sheath grew opposite to the enamel organ. All these observations suggested that the crown forming enamel organ and Hertwig's epithelial sheath developed in the symmetry through evagination (swelling) and invagination of the oral epithelium, respectively, based on the concept of the body plan.

Bacteriologic study of the contents of radicular cysts

Radicular cysts are, most often, oral tissue cysts. The basis for their development are the remains of Hertwig's epithelial sheath, which is stimulated to proliferate by infectious agents, mostly periapical granuloma or pulp necrosis. The cyst contents can be liquid, either clear or milky-white, or thick and yellowish-caseous pus. Bacteriological and histopathological researches have shown that development of radicular cysts from periapical lesions is a consequence of penetration of normal bacterial flora of the oral cavity. The study has bacteriologically examined contents of 50 radicular cysts. They were operated between 1993 and 1995 at the Department of Oral Surgery of the Outpatient Health Care Centre Banja Luka. Pathological substrate of the cyst was punctured by a sterile needle and bacteriologically examined at the Microbiological Laboratory of the Clinical Centre in Banja Luka. Radicular cysts contents most often consisted of: alpha-hemolytic streptococcus, Streptococcus pneumoniae, Staphyilococcus epidermidis, Streptococcus group B and alpha-hemolytic Streptococcus. It is concluded that normal oral cavity flora is present in the cysts contents, the infection penetrated through the root canal to periapex, where, due to the course of time, radicular cyst develops from periapical granuloma.

Pathological evaluation of the effects of intentional disocclusion and overloading occlusion in odontogenesis disorders in N-methylnitrosourea-treated hamsters.

This study compares the effects of disocclusion and overloading occlusion on dental lesions. Ten-day-old Syrian hamsters were divided into 4 groups: group I, untreated animals; group II, animals whose hemilateral incisors were disoccluded; group III, N-methylnitrosourea (MNU)-treated animals; and group IV, MNU-treated animals whose hemilateral incisors were disoccluded. The ipsilateral maxillary and mandibular incisors were repetitively cut with diamond discs. The hamster is easier to anesthetize. Animals received a 0.2% solution of MNU (10 mg/kg body weight) intragastrically twice a week for 16 wk. All the cut mandibular incisors and the MNU-treated uncut mandibular incisors showed lack of iron deposition on the enamel surface. The eruption rate was significantly higher in the cut disoccluded incisors of groups II and IV (p < 0.05) and significantly lower in the uncut overloaded incisors of groups II and IV (p < 0.05). In the cut mandibular incisors of group IV, the degree of the disturbance of odontogenesis and the atypical proliferation of odontogenic epithelium were more prominent (p < 0.02), and the dental lesions occurred earlier. Histologically, the disturbed Hertwig's epithelial sheath and the Hertwig's epithelial sheath-like transformed U-shaped part and enamel organ seemed to lead to disturbances of amelogenesis and detinogenesis as well as to atypical proliferation of odontogenic epithelium nests. Thus, this method of disocclusion of the incisors of rodents may represent a useful model for the investigation of the effects of various agents on tooth formation over a short experimental period.

A confocal laser scanning microscopic study of the immunofluorescent localization of fibronectin in the odontoblast layer of human teeth

The distribution of fibronectin in dental pulp was studied in developing and developed human teeth by indirect immunofluorescence using a confocal laser scanning microscope. In the apical region of developing teeth, intense fluorescence was found along the basement membrane facing the mesenchyme of Hertwig's epithelial sheath and first-formed (mantle) predentine. With further elongation of odontoblasts, fibronectin was observed between the cells, appearing as corkscrew fibres passing from the pulp into predentine parallel to the long axis of the odontoblasts. In the coronal region of developing and developed teeth a similar distribution of fibronectin was observed in the odontoblast layer. At the border zone between odontoblasts and predentine the reaction was intense, but was weak in the predentine itself. In the calcified dentinal matrix it had disappeared completely, except for the area along the dentinal tubules. The results demonstrate that fibronectin is present in the odontoblast layer during all stages of dentinogenesis. Fibronectin-positive fibrous structures between odontoblasts probably correspond to von Korff fibres, and are closely related to odontoblast differentiation and dentinogenesis.

歯根未完成永久歯の移動に伴う歯根の発育, 根尖閉鎖に関する光顕的ならびに電顕的研究

Immature permanent teeth of young adult dogs were displaced by placing force of 50g and 200g on them and the effect of displacement on root growth and apical closure was investigated light and electron microscopically. The results were as follows : 1. Change in the 50g group Although deformation of Hertwig's epithelial sheath was seen on the compression area light microscopically in short term cases, the root apex was closed by connecting cellular cementum and growth of dental root by formation of root dentine was observed in the end. Apposition of cementum however was decreased compared with the control group. Scanning electron microscopy showed large and small resorptive lacunae of the alveolar bone and apposition of new bone. Transmission electron microscopy showed marked destruction of cells one to two weeks after displacement. The tension area did not show deformation of Hertwig's epithelial sheath but showed root growth and apical closure light microscopically. Scanning electron microscopy did not show resorption of the dental root or of the alveolar bone. Transmission electron microscopy showed cementblasts, osteoblasts and fibroblasts of high cell activity. After maturation, however, cells of low activity were found. 2. Change in 200g group On the compression area, deformation of Hertwig's epithelial sheath was seen light microscopically and root growth and apical closure were delayed as compared with the 50g group. Scanning electron microscopy showed restoration of the resorption findings of the dental root and alveolar bone. Transmission electron microscopy showed marked appearances of osteoclasts, marked organelle, and active ruffled border. On the tension area, the light microscopic findings were about the same as those of the 50g group and deformation of Hertwig's epithelial sheath was not seen and apposition of new trabeculae in the alveolar bone in particular was marked. Scanning electron microscopy did not show resorption of dental root or of the alveolar bone. Transmission electron microscopy showed rough surfaced endoplasmic reticulum, cementblasts with developed Goldi apparatus, osteoblasts, and fibroblasts. Characteristic suggestive of high cell activity was seen. After maturation of dental root, organelle decreased. On the basis of the foregoing findings, although displacement of immature permanent teeth by continuous force of 50 to 200g caused deformation of Hertwig's epithelial sheath and poor formation of dentine and cementum in the apical region to some extent, it was clear that periodontal membrane fibers were functionally arranged between the teeth and the surrounding alveolar bone.

Morphological and immunocytochemical characterization of cultured rat incisor cervical epithelial cells

Epithelial cells from the cervical loop of the rat incisor were isolated by co-culture of apical expiants with growth-arrested 3T3 fibroblasts. The epithelial phenotype of the expanding outgrowths was confirmed 10 days after the seeding of the explants by phase-contrast microscopy and immunocytochemical identification of cytokeratins. After 3 weeks in culture, the epithelial cells covered the entire surface of the coverslips and were then passaged. Subcultures gave rise to a confluent sheet within 10–12 days. Light and electron microscopy showed that confluent cervical epithelial cells generally reconstituted a bi-layered structure similar to Hertwig's epithelial sheath. Epithelial cells from the rat palate, cultured and subcultured according to the same procedure, organized themselves in 5–6 cell layers, the upper cells having generally a squamous morphology. Synthesis of extracellular matrix molecules by rat incisor cervical epithelial cells was studied with specific antibodies. These cells failed to produce type I collagen, but synthesized all the major basement membrane components (type IV collagen, laminin, heparan sulphate proteoglycan and fibronectin). These observations suggest that the culture conditions allowed the reconstitution of a typical Hertwig's epithelial sheath by rat incisor cervical epithelial cells.

Early mouse molar root development: Cellular changes and distribution of fibronectin, laminin and type-IV collagen

We analysed epithelial-mesenchymal interactions that occur during the early stages of the formation of mouse molar roots using light and electron microscopy. Morphological changes observed in the cells of Hertwig's epithelial sheath, the pulp and the follicular mesenchyme are described. The Hertwig's epithelial cells lose their cuboidal form and become flattened, apparently intermixing with the cells of the follicular mesenchyme. At the light- and electron-microscope levels, immunoperoxidase techniques were used to localize fibronectin, laminin and type-IV collagen. These appear to be closely associated with cell differentiation and matrix deposition in developing tooth roots. In addition, at the ultrastructural level, intracellular immunoreactivity was detected. The rough endoplasmic reticulum and nuclear envelope of some cells of the periodontal ligament facing the acellular cementum exhibited specific reactivity with laminin and type-IV collagen. Moreover, these periodontal ligament cells express keratin, but not vimentin, filaments. Our results demonstrate that Hertwig's epithelial cells maintain their capacity to synthesize laminin and type-IV collagen, as well as to express keratin filaments, despite basement membrane fragmentation and the disorganization of Hertwig's epithelial sheath. Thus, some Hertwig's epithelial cells remain in the periodontal ligament intermixed with follicular mesenchyme cells.

Apexogenesis に伴う歯髄内神経線維の消長に関する実験的研究

Pulpotomy with calcium hydroxide was performed on teeth with incomplete root of young dogs, and growth of root and apical closure thereafter, or changes in pulpal nerve fibers at what is called Apexogenesis were investigated neurohistologically. The results were as follows : 1. On 7th day postoperatively, regenerated nerve fibers extended to the matrix formed directly under the boundary zone of the plane of pulp cross section. On 14th day, degenerated nerve fibers remaining in the living pulp were nearly nil. This stage corresponded to non-apposition stage of cellular cementum of root apex in normal growth of root, and medium and small nerve bundles, single nerve fibers, and minute nerve fibers with Schwann cells were seen ascending straight mainly in the region between the tips of Hertwig's epithelial sheath in the pulp of the root. 2. On 35th day, the dentin barrier was formed about evenly. Regenerated nerve fibers and their branches extended toward the periphery, branched out further on reaching the periphery, and formed relatively sparse nerve plexuses. This stage corresponded to apposition stage of cellular cementum of root apex in normal growth of root, and was characterized by degenerative findings of tortuous courses and moniliform in the nerve fibers penetrating between cementoblasts and in the nerve fibers penetrating through gaps where future side branches would be. 3. On 95th day, formation of dentin barrier and root dentin was stabilized. At the same time, nerve plexuses at the periphery of dentin barrier were dense and nerve endings were abundant. Many individual nerve fibers had about the same width as the contrast. This stage corresponded to terminal stage of root formation, and was characterized by arrangement into bundles of nerve fibers penetrating into the pulp, penetration together with blood vessels of mainly large, medium, and small nerve bundles through apical branches, and by absence of minute nerve fibers with Schwann's cells, and reduction in single nerve fibers.

歯根未完成歯の根尖閉鎖に関する実験的研究

Immature permanent teeth, numbering 72 in 34 young mongrel dogs, were given root canal filling with Calvital, a calcium hydroxide, after pulp extirpation. Radiological and histopathological investigations were then made of apexification and the healing process. The results were as follows : 1. In radiological investigations, the apex showed continuity of radiopacity on the 35th day after treatment and much denser radiopacity after the 62nd day in the control groups. In the experimental groups, appearances of radiopacity were observed from the 62nd day and these were recognized more clearly on the 180th day. 2. In histopathological findings between the 1st and the 3rd day after treatment, Hertwig's epithelial sheaths and the external layer of the periodontal membrane were necrotic. Calcific clods appeared in the necrotic layers and in the adjacent healthy tissue and showed a positive von Kossa reaction. 3. Between the 7th and the 14th day, part of the necrotic layers dissolved and was resorpted and giant cells appeared. The calcific clods gradually decreased in number and in size. 4. On the 21st day, apical closure by cellular cementum, though irregular because of many and thick accessary roots, was observed in the control groups. In the experimental groups, many giant cells appeared around the necrotic tissue, but the periodontal membrane showed a trend toward recovery. Many young fibers and capillary vessels were observed. 5. On the 35th day, the calcific clods seen to the 21st day disappeared. In the external layer of the periodontal membrane under the residual medication, thick bundles of collagen fibers were seen running vertically toward the root canal. This finding was recognized as the commencement of calcification. Arrangement of the periodontal membrane was irregular in comparison with that in the control groups. 6. On the 62nd day, apexification by cementum-like tissue began and nearly complete apexification was observed in part. Around the hard tissue formed, cementoblast-like cells were seen and collagen fibers penetrating from the periodontal membrane were observed between these cells. 7. On the 180th day, the cementum-like tissues formed became thicker and accessary-root-like tissues were observed in these tissues. However, no complete apexification was observed. 8. No dentin formation was observed in any of the experimental groups, nor was growth of root observed. 9. Hertwig's epithelial sheath became necrotic on the 3rd day and disappeared thereafter. Its relation with apexification and growth of root was not recognized.

幼若犬永久歯歯胚および周囲組織に対する Betatron 電子線限局照射の影響に関する実験的研究

As a means of radiation treatment in the field of oral surgery, high energy electronic therapy employing Betatron and Linac has recently been taken. In the present study, the changes given to the permanent tooth germ and its surrounding tissues of the young dog when Betatron electronic rays was radiated partially and limitedly were investigated roentogenologically and histopathologically. Fifty-eight young dog of about three months old were used as a experimental animals. A dose of 4 MeV, Betatron electronic rays, 600 rad and 1200 rad were applied to the 3rd primary molar and the 4 th permanent premolar germ, respectively in the right mandible. The results obtained were as follows : 1. In the irradiated groups of 600 and 1200 rad, the disturbance of enamel formation were observed on and after the 21st day after the irradiation. 2. Since the 21st day after the irradiation, pulp cells around the cervical end of dentin formation appeared to arrange radially and meet at right angle with collagenous fibers being developed within immature pulp tissue. 3. In the 1200 rad does group, a destruction of the Hertwig's epithelial sheath was observed on the 14st day after the irradiation and on the 30th day, the sheath disappearred. About that time, osteoblast-like cells different from odontoblast occurred in the situation, and an osteodentin began to from already. 4. In the 600 rad dose group, although troubles in dentin and pulp tissues were little observed, enamel hypoplasia was noticed only. These observation might suggested that the radiosensitivity of the ameloblasts derived from the ectoderm was higher than the other. 5. In the 1200 rad dose group, enamel hypoplasia, osteodentin formation, reticular atrophy of pulp and shortening of tooth root were found. These findings was not so vary from the results of predecessors conducted over 1000 R experiments. 6. Both in the 600 and 1200 rad dose group, the growth of the permanent tooth showed remarkably delay and severe crowding within the mandible was observed. 7. In the primary tooth, slight radio-active troubles, such as pulp congestion and the decrease in the number of pulp cells were observed. 8. In the 600 rad dose group, the mandible was not affected at all, but in the 1200 rad one their showed congestion of bone marrow and fibrous degeneration of bone tissue and so on.

乳歯および永久歯歯胚を含む下顎骨有形片の即時再移植に関する実験的研究

As regards to the study on the transplantation of the developing teeth was made and reported by Apfel (1950), Langdon and Young (1953), Fong (1953), Hirakawa (1955), Yamaguchi (1964) and others. On the other hand, a new concept of immediate reconstruction of the mandibule following the surgical resection of the mandibular block was discovered by Marino, Turco and Craviotto (1949). Since exploitation of this article, the supplementary study was made and described by Stuteville (1950), Huebsch and Wickstrom (1951), Nagai (1952), Hirakawa (1952) and others. However, the detailed report as regards to the healing process and fate of the tooth germs that was involved in the replanted mandibular block was not yet published. At this time, the author proved to determine and recognize the attitude of tooth germs, the changes of deciduous teeth and the healing process of mandibular bone block that was immediately replanted mandibular bone after resection of complete solution of continuity in infant dogs. The study was made by patho-histological and roentogenological courses. The results on this article are summarized as follows. 1. Untill about 10 postoperative days, I observed the blood clots filling the space between the replanted mandibular bone block and the recipient bone edges. Reactive cells infiltration were appeared in the dental papilla, the dental sac, the deciduous tooth pulp and the periodontal membrane, added to this findings, I recognized disordered arrangement of the odontoblasts and the inner-outer enamel cells. 2. From 10 to 14 days after the operation, the blood clots showed the tendency of organization. Following this, there was a decrease of the inflammatory reaction in the dental papilla, the pulp and the periodontal membrane. 3. About 21 days after the operation, they showed remarkable regeneration and growing development of Hertwig's epithelial sheath. Inner-outer enamel cells was shown arrangement alike the normal structures. Also, both replanted bone and recipient bone was shown complete union with the formation of new bone. Still more, the replanted mandibular bone block was reconstructed both functionary and morphologically day and days. 4. Thereafter, the tooth germs was growed into the developing teeth accompanied with growth of infant dogs. At the period of 120 days after the operation, the germs were developed into the tooth with a nearly normal structure histologically and roentogenologically and showed the tendency of eruption itself. The findings and facts described above, I believe the new knowledge in this field. The results of this article were beliebed to give the important suggestions on the treatment of jaw bone injury of the children and infants.

Hertwig's sheath in the rat incisor.

A study was undertaken to reveal the histomorphologic nature of Hertwig's epithelial root sheath in the mandibular incisor of the young Sprague‐Dawley rat. Hertwig's epithelial sheath was found to be composed of three distinct cell layers: (1) inner dental epithelium, (2) stellate reticulum, and (3) outer dental epithelium. The inner dental epithelium and outer dental epithelium were simple cuboidal to low columnar cell layers whereas the stellate reticulum was a stratified squamous layer. As one proceeded incisally from the basal loop, the three cell layer diminished to two with the disappearance of the stellate reticulum. Further forward, the two cell layer became a single layer with the absence of the outer dental epithelium. The remaining inner dental epithelium eventually became discontinuous and was replaced by connective tissue elements. The inner dental epithelium and outer dental epithelium of the intact sheath were separated from the adjacent connective tissue by a distinct non‐metachromatic, PAS‐alcian blue positive basement membrane. The membrane next to the inner dental epithelium was followed incisally onto the external surface of the dentin. Epithelial rests were not identifiable in the periodontal ligament. In other words, an island of epithelial cells surrounded by a distinct basement membrane was not observed.

AN ELECTRON MICROSCOPIC STUDY OF THE CEMENTUM, SHARPEY'S FIBERS AND PERIODONTAL LIGAMENT IN THE RAT INCISOR.

AbstractThe rat incisor is a continuously erupting, so‐called “rootless,” tooth. It differs from human or similar “rooted” teeth in that its generative cells persist at the base of the tooth. These cells are derived from the invagination of oral epithelium which forms the dental lamina and the developing dental organ. This epithelium becomes a vase‐shaped structure surrounding a condensation of connective tissue elements called the dental papilla. The cells on the labial aspect of the epithelial dental organ specialize to form enamel. The cells on the lateral and lingual aspects remain undifferentiated and are called Hertwig's epithelial sheath. The odontoblasts are derived from the dental papilla. The hard tissues of the tooth form continuously at the base, and the tooth erupts into the oral cavity where the incisal edge is worn by abrasive action during gnawing (v. Brunn, 1887; Addison and Appleton, '15; Schour and Massler, '42).Hertwig's sheath extends from the base of the tooth to a short distance beyond the point where dentin formation commences. At this level the epithelial cells separate and the sheath disintegrates. As these cells separate, cementoblasts and collagen fibers appear adjacent to the newly forming dentinal surface. Apatite crystals are deposited in the matrix of collagen fibers and a thin layer of cementum results (Selvig, '63). Since Hertwig's sheath is persistent at the base of the tooth, this initial phase of cementogenesis is continuous.In rooted teeth, on the other hand, initial cementogenesis is completed when the root becomes fully formed, and Hertwig's sheath has been expended. There may be subsequent progressing depositions of cementum to form one or several strata. This initial stratum is called “primary cementum” and the successive layers are called “secondary cementum.” Primary cementum contains fine fibrils arranged radially (fiber cementum). Secondary cementum generally contains cells (osteocementum) but it may be acellular. Regardless of category, cementum contains matrix (intrinsic) fibers, and also, in most cases, Sharpey's (extrinsic) fibers (Lehner and Plenk, '36; Egli, '46; Held, '51; Schmid, '51; Sicher, '62).During the course of eruption of the rat incisor, the periodontal fibers which are attached to the cementum move with the tooth until they somehow undergo dissolution prior to the entry of the tooth into the oral cavity. A similar picture is obtained during the more limited period of eruption of rooted teeth. The mechanisms of formation, reorganization and dissolution of the periodontal fibers are all matters of biologic importance. Moreover, investigations of these phenomena may have some bearing upon understanding those pathologic processes which involve the detachment of the gingiva from teeth and the exfoliation of teeth from their sockets.