Development of teeth


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Development of teeth

  1. 1. DEVELOPMENT OF TEETH Development and growth of the crown initially and the not later results in the crown occupying an intraoral position and root occupying an intraosseous position in which it is anchored in a bony socket. Six stages are explained for the tooth development and growth process namely: 1. Lamina or initiation stage – in which germinal tissue is formed (dental). 2. Bud stage – Here the analge responsible for enamel development forms. 3. Cap stage. 4. Bell stage. 5. Apposition and calcification stage. 6. Tooth eruption. 1
  2. 2. Further it is to be noted that the dental tissues not only vary in size and shape but also in their development pattern as well as period of growth and development. The development of tooth is a study of prime importance since many of clinical conditions are associated with disturbances in developmental process. The study of development of teeth involves many complex biological processes. This includes epithelial mesenchymal interactions, differentiation, morphogenesis, fibrillogenesis and mineralization. It is at around 6.5 weeks of gestation or when embryo is 13-14 mm in length stomatodeum i.e. primitive oral cavity when is examined under light electron microscopy shows a primitive 4 to 5 layered epithelial cells covering a band of connective tissue. This connective tissue based on its site of origin from neural crest is termed as ectomesenchyme. The ectomesenchyme here consists of a few spindle shaped cells separated by a gelatinous ground substance. 2
  3. 3. Primary Epithelial Band (Nevy 1970, Ruch J.V. 1984) It is around the 6.5 weeks of embryonic life, there appears a continuous band of thickened epithelium around the mouth in place of future upper and lower jaws. It occurs by fusion of separate plates of thickened epithelium. These epithelial thickenings are roughly hose shoe shaped and correspond in position to the future dental of upper and lower jaws (Nevy et al 1970). This resulting thickening of epithelium is not mainly because of increased proliferative activity of the cells but rather is because of change in the orientation of cleavage plane of dividing cells. These bands of epithelium (upper and lower) are termed as primary epithelial bands. Hence formed primary and band very quickly given rise to two subdivisions, the vestibular lamina and dental lamina. The division of band in to these two 3
  4. 4. layers is so fast that majority of investigators consider them (laminae) as separate entities. Vestibular Lamina (Nevy et al 1970) If coronal section of a developing embryo at 6 weeks is examined at head region, no sulcus or vestibule are seen clinically. It is the vestibular lamina that proliferates within the mesenchymal tissues and forms a vestibule or depression between developing jaws and cheeks. The formation of vestibule occurs by enlargement and degeneration of cells of vestibular lamina. Dental Lamina It’s the first stage of tooth development increased mitotic activity in a specific portion of stomatodeal ectoderm of both upper and lower arches produces prominent thicknenings that dips into underlying ectomesenchyme. The epithelial thickening seen now produces two horse shoe shaped bands defining the prospective upper and lower dental arches in the 4
  5. 5. stomatodeum. This germinal band of ectodermal epithelium circumscribing the future maxillary and mandibular arches is termed as Dental Lamina. At certain points along dental lamina each representing the location of one of the 10 mandibular and 10 maxillary deciduous teeth the ectodermal cells that grow into underlying mesenchyme. Each of these little down growth represents the beginning of enamel organ of the tooth bud of a deciduous tooth. It is found that the mitotic index as well as epithelial replication rate is slower when compared to that in underlying ectomesenchyme. Thus the down growth of epithelium is also a result of a growth of ectomesenchyme (Tencate 1970). Once the cells from tooth bud proliferating, they assume different types of -----. Now the bud resemble cap shape. The ectomesenchymal cells in the cap proliferative more rapidly 5
  6. 6. and appear more dense than surrounding structures. This represents beginning of formation of dental papilla. Now surrounding dental papilla and enamel organ a third layer of cells develop. This third layer is called as dental sac. This consists of ectomesenchymal cells and fibers that surrounds the dental papilla and enamel organ. The function of individual structures will be discussed later. Successional Laminae The portion of dental lamina adjacent to developing tooth anlage retains its connection with the lingual aspect of tooth primordium via Lateral Lamina. The free terminal end of dental lamina begins to proliferate at the end of forth month IU. This newly established growth center is known as successional lamina. It provides anlage for permanent teeth replacing deciduous teeth. Parent Dental Laminae 6
  7. 7. In the 8th week of tooth development tooth anlagen for 10 primary teeth is produced by the dental lamina. This lamina also provides germ for the permanent molars which does not have any predecessors. Because of this the dental lamina providing for the formation of I, II and III molars is called as Parent Lamina or Lamina for Permanent Molars. The mechanism involved simply is one of the continued distal growth i.e. the dental lamina after having established the growth centers for 10 primary molars on either jaws keep growing on distal aspect. The growth of this keeps in pace with growth of the dental arches. The buds of first permanent molars appear at 4 month of time. That of II and III molars appear after birth (9 month and 1 year post natally respectively). Thus activity of dental lamina begins at 6.5 weeks IV and lasts for about 4-5 years postnatally. Stage II or Bud stage: 7
  8. 8. Within days after formation of dental lamina knoblike structures start appearing on the vestibular aspect of dental lamina in ten specific approximating areas in maxillary and mandibular arches. These are the primordial for the deciduous teeth and are variously referred to as tooth buds, tooth primordial or dental anlagen. The first primordial to form are those of incisors of mandibular arch (at the end of seventh week ambryo 17mm in length). Then the maxillary incisor buds start appearing. At the 8th week when embryo is about 25mm in length, the buds for all primary teeth are formed. Since the main function of certain epithelial cells of tooth bud is to form enamel those cells constitute enamel organ which is critical to normal tooth development. In the bud stage, the enamel organ consists of a peripherally arranged low columnar cells and centrally located 8
  9. 9. polygonal cells. Now many of the cells of tooth bud and the surrounding ectomesenchyme undergo mitosis. As a result of this mitosis and the migration certain cells from neural crest into the area of ectomesenchyme, cells surrounding the tooth bud condense. The area of ectomesenchymal condensation immediately adjacent to enamel organ is called Dental Papilla. Now the ectomesenchymal condensation that surrounds both dental papilla and enamel organ is known as Dental Sac. Dental Papilla and sac form (pulp and dentin) and (cementum and periodontium) respectively. At this stage the cells of dental papilla start invaginating in the center of bud and next stage i.e. cap stage begins. 9
  10. 10. Bud Stage Stage II or Cap Stage The cells of this dental primordium starts proliferating and growing. As these proliferation continues the intercellular spaces are enlarged particularly those of the core. This proliferation is not uniform in all the areas of tooth bud. Instead an unequal growth is seen in the bud. Like more amount of growth is seen in the inferior border of bud. This leads to an inward growth of mesenchyme below the bud with expanding growth of mesenchyme more and more amounts of mesenchyme is accumulation in the enlarging concavity and the epithelial bud is rapidly transformed in to a cap or cup shaped structure. The connective tissue bordering the lining epithelium of the cap is dental papilla. Continued cell movements or rearrangements effected by growth forces result in a change in the shape of the cap. Now the cells in the tooth forming structure are no more of uniform size or 10
  11. 11. shape. The cells lining the concavity of the cap become low columnar and is called inner enamel epithelium. The cells lining the convea portion of the cap are cuboidal in shape and are called as outer enamel epithelium. The polygonal cells that are present between inner and outer enamel epithelium begin to separate as more amount of fluid starts collecting between them and form a cellular network called as stellate reticulum. This cells in stellate reticulum assume a branched reticular pattern. The spaces between these reticular forms are filled with a mucoid fluid rich in albumin, which gives the stellate reticulum a cushion like consistency that may support and protect, delicate enamel forming cells. Enamel knot and Cord The cells in the center of enamel organ are densely packed and form enamel knot. The knot projects in part towards dental papilla, so that the center of epithelial invagination shows a knoblike structure that is bordered by the labial and lingual enamel 11
  12. 12. grooves. At the same time in the increasingly high enamel organ a vertical extension of the enamel organ occurs called as enamel cord. Both these structures are temporary that disappear before enamel formation begins. The functions of enamel knot and used may be to act as reservoirs of dividing cells for growing enamel organ. Dental Papilla The changes in the dental papilla occurs concomitantly with the development of epithelial enamel organ. Although epithelization exerts a deminating influence over the adjacent connective tissue condensation of connective tissue is not a passive crowding by proliferating epithelium. D.P. shows active budding of capillaries and mitotic figures and its peripheral cells adjacent to inner enamel organ proliferative and differentiate into odontoblasts. 12
  13. 13. Dental Sac Concomitant with the development of enamel organ and dental papilla there is a marginal condensation in the ectomesenchyme surrounding the enamel organ and D.P. gradually a more fibrous and dense layer develops which terms in to dental sac (D.S.). The cells of D.P., D.S., and E.O. are formative structures for entire tooth and supporting structures. Bell stage or IV stage As the invagination of epithelium deepens and margins continue to grow, four different types of epithelial cells can be distinguished and it assumes a bell shape. The fourth layer comes from differentiation of stellate reticulum into another layer of cells called stratum intermedium. Hence the four layers of the tooth bud include: 1. Outer enamel epithelium. 13
  14. 14. 2. Stratum intermedium. 3. Stellate reticulum and 4. Inner enamel epithelium. Inner Enamel Epithelium The inner enamel epithelium consists of a single layer of epithelium that differentiate into tall columnar cells prior to amelogenesis. These full columnar cells are called as ameloblasts. These cells are 4-5µm in diameter and 40µm in length. These elongated cells are attached to one another by junctional complexes laterally and by means of desmosomes with cells of stellate reticulum. The cells of inner enamel epithelium has a organizing influence over cells of (etcomesenchyme of D.P.) which gets converted into odontoblasts. Stratum Intermedium 14
  15. 15. A few layers of squamous cells form stratum intermedium between inner enamel epithelium and stellate reticulum. These cells are closely attached by desmosomes and gap junctions. They possess a high degree of metabolic activity which is indicated by increased cytoplasmic organelles acid mucopolysaccharides and glycogen deposits. This layer is essential for enamel formation and it is absent in the part which takes part in root formation. Stellate Reticulum The stellate reticulum expands further mainly by an P.D. accumulation of intracellular fluid. These cells are star shaped with pointed ends in contact with one another. This layer collapses prior to amelogenesis thereby reducing the distance between nutrient capillaries outside the outer enamel expansion and ameloblasts. At this stage these cells are very difficult in distinguishing from those of it intermedium. The change begins at the height of cusp tips or at 15
  16. 16. incisal edges, and proceeds cervically as the formation of enamel progresses. 16
  17. 17. Outer Enamel Epithelium The cells of outer enamel epithelium flatten to a low cuboidal form. At the end of bell stage preparatory to and during amelogenesis, the O.E. epithelization which was smooth becomes folded into papillae which extend into stratum intermedium. In these papillary extensions blood capillaries carrying rich nutrients from dental sac are found. Thus nutrition is provided to avascular enamel organ by this modification for its intense metabolic activity. Dental lamina In all these areas except in the areas of permanent molars, the dental lamina proliferates at its deep end contributing for permanent successors. Dental papilla At this stage, before the enamel organ begins to produce enamel, the peripheral cells of mesenchyme proliferate and under the organizing influence of ameloblasts convert into odontoblasts. These odontoblasts are cuboidal in form in the initial stage. Later 17
  18. 18. they assume a columnar form and acquire specific potential to produce dentin. The basement membrane that separates enamel organ from dental papilla just prior to dentin formation is called as membrane preformation. Dental Sac Before formation of dental tissues begins the dental sac shows circular arrangement of its fibers and resembles a capsular structure with the development of root, the fibers get embedded into cementum to alveolar bone and converted into periodontal ligament. The transformation of cap stage to bell stage occurs when the embryo is about 100-160mm in length (Provenza 1986). Advanced Bell Stage: During advanced bell stage, the boundary between inner enamel epithelization and odontoblasts outlines the future dentino- 18
  19. 19. enamel junction. In addition, the cervical portion of enamel organ gives rise to epithelial sheath of Heartwig. Hertwig’s epithelial root sheath and root formation: The development of root formation begins after the enamel and dentin formation has reached the future cemento-enamel junction. The enamel organ plays an important role in root development by forming Hertwig’s epithelial root sheath which molds the shape of the roots and initiates radicular dentin formation. The Hertwigs root sheath includes only outer and inner enamel epithelium and thus does not involve of intermedium and st. reticulum. The cells of inner enamel epithelium remain short and normally do not produce enamel. These cells will induce radicular cells to get converted into odontoblasts. These odontoblasts shorts laying down dentin. When the first layer if dentin has been laid down, the epithelium and Hertwigs root sheath looses its continuity and its close relation to 19
  20. 20. the root surface. Its epithelial remnants may persist on the surface of root as group or clusters of cells or tubules which are found in periodontal ligament as cell rests of malasses. There is a pronounced difference in the development of Hertwig’s epithelial root sheath in teeth with single root and in teeth with two or more roots. Prior to the beginning of root formation the sheath bonds into a horizontal plane forming epithelial diaphragm. This horizontal bonding causes narrowing of wide cervical opening. The plane of diaphragm remains relatively fixed during growth and development of root. The proliferation of epithelial diaphragm is accompanied by proliferation of cells of connective tissue of the pulp which occurs in the area adjacent to pulp diaphragm. The free end of diaphragm does not itself grow into the connective tissue but the epithelium present coronally to diaphragm proliferates. Now the differentiation of odontoblasts and formation 20
  21. 21. of dentin follows lengthening of root sheath. At the same time, the connective tissue from dental sac proliferates and divides the root sheath into epithelial strands. These by the D.S. connective tissue comes in contact with underlying dentin. These cells from D.S. once they come in contact with dentin differentiates into cementoblasts and deposit cementum on the dentin. In the last stage of epithelial development proliferation of epithelium lags behind that of pulpal connective tissue. The apical foramen is at first established to the width of Hertwigs epithelial root sheath. Later the dentin deposition at a rapid rate causes narrowing of apical foramen to its normal size. Differential growth of epithelial diaphragm in multirooted teeth causes division of root trunk into two or three roots. During the general growth of enamel organ expansion of cervical opening occurs in such a way that small tongue like extensions develop from horizontal diaphragm. Two such extension occurs in lower molars and 3 extensions from upper molars. Before the division of root trunk occurs the free ends of these extensions grow towards 21
  22. 22. each other and fuse. The cervical opening of these teeth is hence divided into two or 3 openings. On the pulpal surface the dividing epithelium starts dentin formation and on the periphery of each opening root development follows as in case of single rooted teeth. Histopathology and Clinical Considerations: A number of growth processes participate in the progressive development of teeth. Except for initiation which is a momentary process each of these processes overlap each other and many are continuous throughout the various morphologic stages of odontogenesis. These processes include: 1. Initiation. 2. Proliferation. 3. Histodifferentiation. 4. Morpho differentiation. 5. Apposition. Initiation: Dental laminae and associated tooth buds represent those parts of oral epithelium that have the potential for tooth formation. 22
  23. 23. Specific cells in horse shoe shaped lamina have the potential to form enamel organ of certain teeth by responding to certain factors that initiate or induce tooth development. Different teeth are initiated at definite times. Initiation or induction requires ectomesenchymal epithelial interaction. The mechanism of such as interaction is not clearly understood. However it has been demonstrated that dental papilla mesenchyme can induce or instruct tooth epithelium and even non tooth epithelium to form enamel. The lack of initiation results in loss of a single tooth or multiple teeth (Anodontia). On the other hand abnormal initiation may lead to development of single or multiple supernumerary teeth. Proliferation In this stage enhanced proliferative activity from initiation stage results successively in bud, cup and bell stages. The proliferative growth causes changes in proportions and size of growing enamel organ (tooth germ). 23
  24. 24. Any disturbance during these stages may result in defective dental problems ranging from absence of tooth to disturbed tooth formation. Histo-differentiation This succeds the proliferative stage. The cells of tooth germ in proliferative stage undergo definitive functional and morphologic changes and acquire their functional assignment. They differentiate and give up their capacity to multiply as they assume new functions. This law governs all differentiating cells. This stage is seen at its peak in bell stage when cells of developing enamel organ differentiate into cells of enamel and dentin formation. In the process of histodifferentiation under the organizing influence of inner enamel epithelisation cells the cells of adjacent connective tissue differentiate to form odontoblasts. These odontoblasts lay down dentin. This process of dentin formation causes differentiation of cells of I and E into ameloblasts. These 24
  25. 25. ameloblasts now lay down a thin layer of enamel adjacent to dentin formed. Thus the formation of dentin proceeds and essential to enamel formation. In vitamin A deficiency the ameloblasts fail to differentiate properly. Consequently their organizing influence on adjacent mesenchymal tissue is disturbed . this may result in formation of a typical dentin known as osteodentin. Morpho differentiation: The morphologic pattern or basic form and size of the tooth is established by morphodifferentiation i.e. by differential growth. It is therefore proliferation is very important stage in tooth development. The dentino-enamel junction and dentino-enamel are different and characteristics of particular teeth and act as blue print pattern. In conformity with those pattern ameloblasts, odontoblasts and cementoblasts deposit enamel, dentin and cementum 25
  26. 26. respectively and thus give the completed tooth a particular shape and size. Any disturbances during the process of morphodifferentiation may result in size and shape disturbances. Example: - Supernumerary cusps or roots. - Twinning of teeth. - Loss of cusps. - Disturbed shape – peg shaped incisors, Hutchinsons incisors. Apposition: Apposition is the process of deposition of matrix of hard dental structures. Appositional growth of enamel and dentin is a layer like deposition an extracellular matrix which later gets calcified. This type of growth is additive. The appositional growth is characterized by regular and rhythmic deposition of extracellular matrix which is of itself incapable of further growth. This process of apposition occurs in periods of alternating rest and activity. 26
  27. 27. Disturbances during the process of apposition and calcification may result in enamel hypoplasia, enamel hypocalcification etc. These result in formation of teeth. 27