DEVELOPMENT OF TOOTH Student: LE THI KIM TUYEN CONTENTS 1. Introduction 2. Dental lamina 3.Vestibular lamina 4. Tooth development 5. Devolopmental stages - Bud stage - Cap stage - Bell stage - Advanced bell stage 6. Hertwig’s epithelial root sheath and root formation 7. Conclusion INTRODUCTION • Tooth formation occur 6th week of intrauterine life with the formation of primary epithelial band. At about 7th week the primary epithelial band divides into a lingual process called dental lamina & a buccal process called vestibular lamina. All deciduous teeth arises from dental lamina, later the permanent successors arise from its lingual extention & permanent molars from its distal extension. • The primitive oral cavity, or stomodeum, is lined by stratified squamous epithelium called the oral ectoderm. • The oral ectoderm contacts the endoderm of the foregut to form the buccopharyngeal membrane • Membrane ruptures at about 27th day of gestation and the primitive oral cavity establishes a connection with the foregut • Most of the connective tissue cells underlying the oral ectoderm are of neural crest or ectomesenchyme in origin • These cells instruct the overlying ectoderm to start the tooth development, which begins in he anterior portion of the future maxilla & mandible and process posteriorly. DENTAL LAMINA • 2-3 week after the rupture of buccopharyngeal membrane, certain areas of basal cells of oral ectoderm proliferate rapidly, leading to the formation of primary epithelial band. • The band invades the underlying ectomesenchyme along each of the horse-shoe shapes future dental arches • At about 7th week the primary epithelial band divides into an inner (lingual) process called Dental Lamina & an outer (buccal) process called Vestibular Lamina • The dental lamina serves as the primordium for the ectodermal portion of deciduous teeth • Later during the development of jaws, permanent molars arise directly from the distal extention of the dental lamina. • The successors of the deciduous teeth develop from a lingual extension of the free end of the dental lamina opposite to the enamel organ of each deciduous tooth • The lingual extension of the dental lamina is named the successional lamina and develops from the fifth month in utero (permanent central incisor) to the tenth month of age (second premolar). Oral epithelium Enamel organ A Initiation (Bud stage) B C D Proliferation Morphodifferentiation Apposition (Cap stage) Histodifferentiation (Bell stage) Growth Calcification E (Before emergence) F (After emergence) Eruption G H Attrition FATE OF DENTAL LAMINA • It is evident that the total activity of the dental lamina extends over a period of at least 5 years. • As the teeth continue to develop, they lose their connection with the dental lamina • They later break up by mesenchymal invasion, which is at first incomplete and does not perforate the total thickness of the lamina. • Fragmentation of the dental lamina progresses toward the developing enamel organ • Any particular portion of the dental lamina function for a much briefer period since only a relatively short time elapses after initiation of tooth development before he dental lamina begins to degenerateactive • However the dental lamina may still be active in the third molar region after it has disappear elsewhere, except for occasional epithelial remnant. VESTIBULAR LAMINA • Labial and buccal to the dental lamina in each dental arch, another epithelial thickening develops independently • It is the vestibular lamina, also termed the lip furrow band • Subsequently hollows and forms the oral vestibule between the alveolar portion of the jaws and the lips and cheeks TOOTH DEVELOPMENT • At certain points along the dental lamina, each representing the location of one of the 10 mandibular and 10 maxillary deciduous teeth, the ectodermal cells multiply still more rapidly and form little knobs that grow into the underlying mesenchyme • Each of these little down growths from the dental lamina represents the beginning of the enamel organ of the tooth bud of a deciduous tooth. • First to appear are those of the anterior mandibular region. • As the cell proliferation occurs each enamel organ takes a shape that resembles a cap DENTAL PAPILLA On the inside of the cap, the ectomesenchymal cells increase in number. The tissue appears more dense than the surrounding mesenchyme and represents the beginning of the dental papilla. DENTAL SAC / DENTAL FOLLICE • Surrounding the combined enamel organ and dental papilla, the third part of the tooth bud forms. It is the dental sac or dental follicle, and it consists of ectomesenchymal cells and fibers that surround the dental papilla and the enamel organ • Thus the tooth germ consists of the ectodermal component-the enamel organ and the ectomesenchymal components-the dental papilla and the dental follicle. • During and after these developments the shape of the enamel organ continues to change • The depression occupied by the dental papilla deepens until the enamel organ assumes a shape resembling a bell • The dental lamina becomes longer and thinner and finally breaks up and the tooth bud loses its connection with the epithelium of the primitive oral cavity. DEVELOPMENT STAGES Morphologial stages 1. 2. 3. 4. 5. 6. Dental lamina Bud stage Cap stage Early bell stage Advanced bell stage Formation of enamel and dentin matrix Physiologial procecesses Initiation Proliferation Histodiffereniation Morphodifferentiation Apposition BUD STAGAE / PROLIFERATION • This is the initial stage of tooth formation where enamel organ resembles a small bud • During the bud stage, the enamel organ conisist of peripherally located low columnar cells & centrally located polygonal cells • The surrounding mesenchymal cells proliferate, which resulfs in their condensation in two areas • The area of condensation immediately below the enamel organ is the dental papilla • The ectomesenchymal condensation that surround the tooth bud & the dental papilla is the tooth sac • The dental papilla as well as the dental sac are not well defined during the bud stage, they become more denfined during the subsequent cap & bell stages • The cells of the dental papilla form the dentin and pulp while the dental sac forms cementum & periodontal ligament. CAP STAGE Outer and inner enamel epithelium - The peripheral cells of the cap stage are cuboidal, cover the convexity of the “cap,” and are called the outer enamel (dental) epithelium. - The cells in the concavity of the “cap” become tall, columnar cells and represent the inner enamel (dental) epithelium - The outer enamel epithelium is separated from the dental sac, and the inner enamel epithelium from the dental papilla, by a delicate basement membrane. Stellate reticulum - Polygonal cells located in the center of the epithelial enamel organ, between the outer and inner enamel epithelia, begin to separate due to water being drawn into the enamel organ from the surrounding dental papilla - As a result the polygonal cells become star shaped but maintain con- tact with each other by their cytoplasmic process - As these star- shaped cells form a cellular network, they are called the stellate reticulum. - The cells in the center of the enamel organ are densely packed and form the enamel knot - This knot projects in part toward the underlying dental papilla - At the same time a vertical extension of the enamel knot, called the enamel cord occurs - The function of the enamel knot and cord may act as a reservoir of dividing cells for the growing enamel organ. BELL STAGE • As the invagination of the epithelium deepens and its margins continue to grow, the enamel organ assumes a bell shape. • In the bell stage crown shape is determined. It was thought that the shape of the crown is due to the pressure exerted by the growing dental papilla cells on the inner enamel epithelium. • The inner enamel epithelial cells which lie in the future cusp tip or incisor region stop driving earlier and begin to differentiate first. • The pressure exerted by the continuous cell division on these differentiating cells from other areas of the enamel organ cause these cells to be pushed out into the enamel organ in the form of a cusp tip. The cells in another future cusp area begin to differentiate, and by a similar process results in a cusp tip form. Inner enamel epithelium • The inner enamel epithelium consists of a single layer of cells that differentiate prior to amelogenesis into tall columnar cells called ameloblasts • These elongated cells are attached to one another by junctional complexes laterally and to cells in the stratum intermedium by desmosomes. • The cells of the inner enamel epithelium exert an organizing influence on the underlying mesenchymal cells in the dental papilla, which later differentiate into odontoblasts. Stratum intermedium • A few layers of squamous cells form the stratum intermedium, between the inner enamel epithelium and the stellate reticulum • These cells are closely attached by desmosomes and gap junctions. • This layer seems to be essential to enamel formation Stellate reticulum • The stellate reticulum expands further, mainly by an increase in the amount of intercellular fluid. • The cells are star shaped, with long processes that anastomose with those of adjacent cells • As the enamel formation starts, the stellate reticulum collapses to a narrow zone there by reducing the distance between the outer & inner enamel epithelium. Outer enamel epithelium • The cells of the outer enamel epithelium flatten to a low cuboidal form. • The outer enamel epithelium is thrown into folds which are rich capilla network, this provides a source of nutrition for the enamel organ • Before the inner enamel epithelium begins to produce enamel. Peripheral cells of the dental papilla differentiate into odontoblasts • These cuboidal cells later assumes a columnar form & produce dentin. Dental lamina • The dental lamina is seen to extend lingually and is termed successional dental lamina as it gives rise to enamel organs of permanent successors of deciduous teeth • The enamel organs of deciduous teeth in the bell stage show successional lamina and their permanent successor teeth in the bud stage Dental papilla • The dental papilla is enclosed in the invaginated portion of the enamel organ. • The basement membrane that separates the enamel organ and the dental papilla just prior to dentin formation is called the membrana preformativa. Dental sac • Before formation of dental tissues begins, the dental sac shows a circular arrangement of its fibers and resembles a capsular structure. With the development of the root, the fibers of the dental sac differentiate into the periodontal fibers that become embedded in the developing cementum and alveolar bone. Advanced bell stage - This stage is characterized by the commencement of mineralization and root formation. During the advanced bell stage, the boundary between inner enamel epithelium and odontoblasts outlines the future dentinoenamel junction . The formation of dentin occurs first as a layer along the future dentinoenamel junction in the region of future cusps and proceeds pulpally and apically. HERTWIG’S EPITHELIAL ROOT SHEATH AND ROOT INFOMATION 6 • The development of the roots begins after enamel and dentin formation has reached the future cementoenamel junction. • Hertwig’s root sheath consists of the outer and inner enamel epithelia only, and there- fore it does not include the stratum intermedium and stellate reticulum. • As the first layer of dentin has been laid down, the epithelial root sheath loses its structural continuity and its close relation to the surface of the root • Its remnants persist as an epithelial network of strands or clumps near the external surface of the root. • These epithelial remnants are found in the periodontal ligament of erupted teeth and are called rests of Malassez • Prior to the beginning of root formation, the root sheath forms the epithelial diaphragm • The outer and inner enamel epithelia bend at the future cementoenamel junction into a horizontal plane, narrowing the wide cervical opening of the tooth germ • The proliferation of the cells of the epithelial diaphragm is accompanied by proliferation of the cells of the connective tissue of the pulp, which occurs in the area adjacent to the diaphragm • The free end of the diaphragm does not grow into the connective tissue, but the epithelium proliferates coronal to the epithelial diaphragm • The connective tissue of the dental sac surrounding the root sheath proliferates and invades the continuous double epithelial layer dividing it into a network of epithelial strands • The rapid sequence of proliferation and destruction of Hertwig’s root sheath explains the fact that it cannot be seen as a continuous layer on the surface of the developing root • The wide apical foramen is reduced first to the width of the diaphragmatic opening itself and later is further nar- rowed by apposition of dentin and cementum to the apex of the root Epithelial rests • Differential growth of the epithelial diaphragm in multirooted teeth causes the division of the root trunk into two or three roots • During the general growth of the enamel organ the expansion of its cervical opening occurs in such a way that long tongue like extensions of the horizontal diaphragm develop • Before division of the root trunk occurs, the free ends of these horizontal epithelial flaps grow toward each other and fuse • The single cervical opening of the coronal enamel organ is then divided into two or three openings On the pulpal surface of the dividing epithelial bridges, dentin formation starts On the periph- ery of each opening, root development follows in the same way as described for single-rooted teeth CONCLUSION Since development of tooth forms the base of dentistry, a thorough understanding and a sound knowledge is required by a dentist regarding the development stages of tooth & the anomalies related to it, so as to identify & treat them in a proper fashion. 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