Tooth development 2

Tooth development

1. Tooth development II Dr. Samah khaled

2. 5. Late bell stage: 1. Dental lamina: The breakpoint that makes enamel organ transfers from early to late bell is Deposition of the first layer of dentine Mesenchyme of dental sac invades the lateral dental lamina leading to its break down. Remnants of the dental lamina may persist in the gingival and the jaw and they are called epithelial rests of Serres ( Serres ' pearls) . These remnants may form: Eruption cyst: small cyst over the developing tooth that may delay eruption. May give rise to odontoma . May be activated to form supernumerary teeth.

3. ( Serres ' pearls)

4. Eruption cyst Odontoma Supernumerary tooth

5. 2. Enamel organ: It increases in size. Its cervical portion gives rise to epithelial root sheath of Hertwig (HERS).

6. A. Outer dental epithelium: N utrition to enamel organ enter via 2 routs: Dental papilla Dental sac through O.E.E When the first layer of dentin is laid down the nutrition of the dental organ via the dental papilla will stop. O.E.E become flattened. They become low cuboidal with high nuclear/cytoplasmic ratio (little cytoplasm). Folding of the smooth surface of outer enamel epithelium to increase its surface area. At the region of these folds the dental sac sends many capillary loops, to provide a rich nutritional supply. O.E.E develop microvilli, cytoplasmic, vesicles and increased number of mitochondria at the end facing the capillary loops for active transport of materials .

7. B. Inner dental epithelium: Under the influence of the first formed dentin layer the inner enamel epithelium will be stimulated to be differentiated into tall columnar cells that will produce enamel matrix: called ameloblasts . Ameloblasts 4 – 5 microns in diameter 40 microns in length in cross section they are hexagonal. They are attached by junctional complexes laterally and by desmosomes to the stratum intermedium . The boundary between inner dental epithelium and odontoblasts outlines the future ADJ.

8. Induction Inner dental epithelium Proximal end Distal end Odontoblasts 8 Cell free zone Cell rich zone Centriole Nucleus Golgi apparatus Mitochondria

9. Reciprocal Induction Odontoblast Dentin matrix Ameloblast Enamel matrix Dentin

10. STRATUM INTER . STELLATE RETICULUM INNER DENT. EPITH. PREDENTIN ODONTOBLASTS LATE BELL STAGE ( Appostion stage)

11. Stellate reticulum: The space needed for the developing enamel will be gained from: The shrinkage of the stellate reticulum by the loss of the intercellular fluid. Becomes hardly distinguished from the stratum intermedium . The shrinkage begins at the height of the cusp or the incisal edge and progresses cervically . Stratum intermedium : These layers show strong reaction for alkaline phosphatase enzyme , which is needed for the mineralization of the enamel. Also, the presence of well developed cytoplasmic organelles, acid mucopolysacharides and glycogen deposits indicate a high degree of metabolic activity.

12. 1. The single rooted tooth: When the enamel and dentine formation have reached the future amelocemental junction. The cervical loop (the inner & outer enamel epithelium begin to grow deeper into the surrounding ectomesenchyme of the dental sac. It elongates and moves away from the newly completed crown area to enclose more of the dental papilla tissue forming Hertwig's epithelial root sheath (HERS).

13. At the first the epithelial root sheath bends at the future cemento enamel junction into horizontal plane known as epithelial diaphragm. This diaphragm produces narrowing of the wide cervical opening of the tooth germ. The plane of the diaphragm remains relatively fixed during the root development and growth. The proliferation of the cells of the epithelial diaphragm is accompanied by proliferation of the cells of connective tissue of the pulp, which occurs in the area adjacent to the diaphragm.

14. The epithelial sheath of Hertwig proliferates coronally to the diaphragm in a vertical direction. The cells of the inner enamel epithelium forming the sheath of Hertwig remain short and induce the undifferentiated mesenchymal cells of the dental papilla to differentiate into odontoblasts . So the function of the root sheath is to mold the shape of the root and initiate dentine formation. After dentine deposition the connective tissue of the dental sac proliferates and invades the Hertwig epithelial root sheath dividing it into a network of epithelial strands. These strands are moved away from the surface of dentine so that the connective tissue cells of dental sac in contact with the outer surface of dentine are differentiated into cementoblasts which deposit cementum on the dentine surface.

15. The rapid epithelial sheath proliferation, dentine formation and epithelial sheath destruction explain the fact that the HERS cannot be seen as a continuous layer in the developing root. The wide apical foreman is reduced first to the width of the diaphragmatic opening and later is further narrowed by apposition of dentine and cementum at the apex of the root.

16. The epithelial strands may undergo degeneration; remnants may persist in the periodontal ligament in the form of network or isolated islands known as epithelial rests of Malassez . Epithelial rests of Malassez are source of epithelial lining of dental cyst that develops in reaction to inflammation of periodontal ligament.

17. 2. The multirooted teeth: The deciduous and permanent molars and some premolars have more than one root. Their roots are formed like the single rooted tooth till it reaches the level of bi-or trifurcation where the epithelial diaphragm proliferates horizontally producing tongue like extensions (2 tongue extensions in case of 2 rooted tooth and 3 in the three rooted tooth). At the region of future bifurcation of the roots the free ends of these horizontal epithelial extensions grow towards each other and fuse dividing the wide opening into 2 or 3 partitions. The odontoblasts differentiate along the diaphragm, and the pulpal surface of the extended epithelial bridges form dentin, and on the periphery of each opening, root development follows in the same way as described for single rooted teeth.

18. Clinical considerations Tooth formation is dependent on both oral epithelial and adjacent mesenchymal cells for development. Factors such as: X-rays. Nutritional deficiencies. Drugs. Change the ability of these cells to function, thus affecting tooth development.

19. 1. Enamel pearls If the cells of the epithelial root sheath of Hertwig remain adherent to the dentin surface, they may differentiate into ameloblasts and produce enamel , called enamel pearls. They appear as small, spherical enamel projections especially at the cemento -enamel junction (CEJ) or in the furcation area in molars. They usually cause periodontal problems. Enamel pearl in deciduous teeth may cause delayed exfoliation of primary teeth because of slower process of enamel resorption. This may lead to deviation of erupting permanent molars.

20. 2. Bare dentin If the epithelium root sheath of Hertwig is delayed in its separation from the dentin, a zone of the root is devoid of cementum. In about 10% of teeth, the cemento -enamel junction consists only of a layer of dentin without enamel and cementum. Dentin is sensitive when exposed to oral environment patient feel pain with different foods and drinks. Enamel Cementum Dentin

21. 3. Intermediate cementum: If the continuity of the Hertwig's root sheath is broken after odontoblastic differentiation and before dentine formation , intermediate cementum is developed. It occurs at apical 2/3 of premolars and molars roots and rare in incisors and deciduous teeth.

22. 4. Accessory root canal If the epithelial root sheath of Hertwig is broken before odontoblastic differentiation and dentin formation a defect in the dentinal wall of the pulp chamber or root canal will result. Disturbance in the fusion of the tongue like extension of the diaphragm causes defects in the pulpal floor. Large blood vessel may disturb the course of the root sheath lead to accessory root canal. These defects account for the development of accessory root canals opening on the periodontal surface of the root.

23. The stages of tooth development

24. 1. Initiation The dental lamina and tooth buds represent that part of the oral epithelium that has powers for tooth formation. Histophysiological stages of tooth development Absence of either single tooth or multiple teeth (partial anodontia ) most frequently, upper 2, upper and lower 8,due to lack of initiation Complete lack of teeth ( anodontia ) as in Ectodermal Dysplasia syndrome. Abnormal initiation may result in development of single or multiple supernumerary teeth.

25. 2. Proliferation Proliferative activity is found in dental lamina, bud, cap, early bell stage and late bell stage where the dental matrices are not yet deposited. The unequal growth by mitotic division causes regular changes in size and proportions of the growing tooth germ. Any disturbance in this stage causes dental problems ranging from; absence of teeth, disturbed tooth formation, or giving supernumerary teeth

26. 3. Histodifferentiation . This phase reaches its highest development in the bell stage just before the beginning of matrix formation. So the differentiation of ameloblasts and stratum intermedium is essential for enamel formation , also the differentiation of odontoblasts is important for the producing of dentine . In case of vitamin A deficiency, ameloblasts fail to differentiate properly so their organizing influence is disturbed and atypical dentin called osteodentin is formed.

27. 4 . Morphodifferentiation The morphologic pattern and relative size of the future tooth is started at the early bell stage when the inner enamel epithelium arranges them on the basement membrane. At late bell stage the morphodifferentiation is established by outlining the future amelodentinal junction and amelocemental junction. The morphology of the tooth is performed at this stage by differential growth of the formative cells just prior to matrix deposition.

28. In Morphodifferentiation stage, endocrine disturbances affect size or form of teeth without affecting ameloblasts and odontoblasts functions resulted in underdeveloped or extra structures in the tooth. Also delayed eruption in hypopituitarism and hypothyroidism causes small clinical crown mistaken as small anatomical crown. Twining Extra roots or cusps Peg or mal formed tooth

29. 5. Apposition The apposition is the deposition of the matrix of the hard structures. It is confined to the late bell stage. The matrix is deposited by the cells along the site outlined the formative cells at the end of morphodifferentiation . The dentine and enamel matrices are deposited in a rhythmic manner. Enamel hypocacification Enamel hypoplasia

30. Function of the enamel organ Outer enamel epithelium: It limits the boundary of the enamel organ. Active transport of materials specially, after hard dental tissue formation as it becomes folded to facilitate the passage of nutrient material to the enamel organ. It picks up the calcium salts from the dental sac to either the stellate reticulum or stratum intermedium . It forms with the inner dental epithelium the epithelial root sheath of Hertwig which is responsible for root formation .

31. Stellate reticulum: Acts as a buffer against physical forces that may distort the configuration of the developing amelodentinal junction giving rise to gross morphologic changes, so maintains tooth shape It seems to permit only a limited flow of nutritional elements from the outlying blood vessels to the formative cells, so it acts as a store house for the nutritive materials. It keeps room for the developing enamel and supports its production as they shrink and lose their intercellular fluid to bring the ameloblasts close to the blood vessels situated outside the outer enamel epithelium.

32. Stratum intermedium : The function of this layer is not understood. It is believed to control fluid diffusion into and out of the ameloblasts i.e. transfer Ca from blood vessels to ameloblasts . It provides the enamel organ with proteins (alkaline phosphatase, Ca -Mg ATPase) needed for mineralization. Those cells are histologically and histochemically distinct from the cells with inner enamel epithelium, but both layers considered as a single functional unit responsible for supporting the production and mineralization of enamel.

33. Inner enamel epithelium: Morphodifferentiation function as it determines the form and size of the crown and root portions of the tooth Organizing function as it exerts an organizing influence induction on the undifferentiated cells of the dental papilla to differentiate into odontoblasts . Formative functions as it differentiated into ameloblasts (by reciprocal induction) that lays down enamel matrix and helps in its mineralization. This layer is arranged in a pattern to determine the future morphology of the amelodentinal junction (A.D.J) and the crown. It forms with the outer dental epithelium the epithelial root sheath of Hertwig which is responsible for root formation. Nutritive function as it shares in the transport of the nutritive materials from the dental papilla to the enamel organ before dental hard tissue formation.

34. Protective function as after the full enamel thickness is deposited it secretes an organic layer called primary enamel cuticle. A protective covering to enamel of unerupted tooth against resorption and preventing precipitation of cementum. Protective function also as it forms with the other layers of the dental organ the reduced enamel epithelium which protects the enamel surface until the tooth erupts. Contribute to the formation of the dentogingival junction of erupted teeth.

35. Function of dental papilla and dental sac The dental papilla gives rise to: Dentine Dental pulp The dental sac gives rise to: Cementum Periodontal ligament Alveolar bone proper

36. Function and fate of dental lamina: The maxillary and mandibular dental laminae give rise to a total 52 tooth buds or tooth germ , 20 for the primary teeth and 32 for the permanent teeth in 3 phases. Initiation of the entire deciduous dentition that occurs during the 2nd month (I.U.L), at first the enamel organ of the deciduous teeth maintains a broad connection to the dental lamina in the cap stage. in the early bell stage it begins to breakdown by mesenchymal invasion, which first penetrates its central portion and divides it into the dental lamina proper and lateral dental lamina. At the late bell stage complete disintegration of the dental lamina occurs.

37. The dental lamina proper proliferates at its deeper margin and gives rise to successional lamina which forms the primordium of the permanent successors lingual to the enamel organs of the deciduous teeth, and occurs about 5(M.I.U.L) for the permanent central incisors and 10 months of age for the second premolars. The permanent molars which have no deciduous predecessors arise directly from the distal extension of the dental lamina distal to the primary second molars. The time of initiation of the first permanent molar is about 4(M.I.U.L), for the second molar it is one year and the third molar four years. It is thus evident that the activity of the dental lamina extends over a period of about five years and disintegrates completely or remains as epithelial rests of Serres .

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