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Lec.( 1 ) Oral histology dr. lubna al khafaji Development of the Tooth and Its Supporting Tissues This discusses the histologic aspect of tooth development and the coming together of the different tissues that form the tooth and its surrounding tissues. What are the signals mediating the initial steps in tooth development? A signaling molecule originating from the oral epithelium of the first branchial arch results in the expression of both of (transcription factors) Lhx-6 and Lhx-7 genes in the neural crest– derived ectomesenchyme of the oral portion of the first branchial arch as early as day 9 of gestation , The earliest histologic indication of tooth development is at day 11 of gestation, which is marked by a thickening of the epithelium where tooth formation will occur on the oral surface of the first branchial arch After about 37 days of development, a continuous band of thickened epithelium forms around the mouth in the presumptive upper and lower jaws. These bands are roughly horseshoe-shaped and correspond in position to the future dental arches of the upper and lower jaw. Each band of epithelium, called the primary epithelial band, quickly gives rise to two subdivisions which in grow into the underlying mesenchyme colonized by neural crest cells. These are: 1- The dental lamina, which forms first, on the anterior aspect of the dental lamina, continued and localized proliferative activity leads to the formation of a series of epithelial outgrowths into the mesenchyme at sites corresponding to the positions of the future 1 deciduous teeth. Ectomesenchymal cells accumulate around these outgrowths. From this point, tooth development proceeds in three stages: the bud, cap, and bell. 2- The vestibular lamina, which forms shortly afterward and is positioned just in front of the dental lamina (Labial and buccal) . The vestibule forms as a result of the proliferation of the vestibular lamina into the ectomesenchyme soon after formation of the dental lamina. The cells of the vestibular lamina rapidly enlarge and then degenerate to form hollows or cleft and forms the oral vestibule between the alveolar portion of the jaws (tooth- bearing area) and the lips and cheeks. also termed the lip furrow band. Fate of dental lamina It is evident that the total activity of the dental lamina extends over a period of at least 5 years. Any particular portion of the dental lamina functions for a much briefer period since only a relatively short time elapses after initiation of tooth development before the dental lamina begins to degenerate at that particular location. However, the dental lamina may still be active in the third molar region after it has disappeared elsewhere, except for occasional epithelial remnants. 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. Remnants of the dental lamina persist as epithelial pearls or islands within the jaw as well as in the gingiva. These are referred to as cell rest of Serres. 2 Histophysiology of tooth developmet 1- Initiation: The dental laminae and associated tooth buds represent those parts of the oral epithelium that have the potential for tooth formation. Different teeth are initiated at definite times. Initiation induction requires ectomesenchymal–epithelial interaction. 2- Proliferation: Enhanced proliferative activity ensues at the points of initiation and results successively in the bud, cap, and bell stages of the odontogenic organ. Proliferative growth causes regular changes in the size and proportions of the growing tooth germ. 3- Histodifferentiation: The formative cells of the tooth germs developing during the proliferative stage undergo definite morphologic as well as functional changes and acquire their functional assignment (the appositional growth potential). 4- Morphodifferentiation: The morphologic pattern, or basic form and relative size of the future tooth, is established by morphodifferentiation, that is, by differential growth. Morphodifferentiation therefore is impossible without proliferation. The advanced bell stage marks not only active histodifferentiation but also an important stage of morphodifferentiation in the crown, outlining the future dentinoenamel junction. 5- Apposition: is the deposition of the matrix of the hard dental structures. 3 Developmental stages of tooth are: 1- BUD STAGE The epithelium of the dental laminae is separated from the underlying ectomesenchyme by a basement membrane The bud stage is represented by the first epithelial incursion into the ectomesenchyme of the jaw The epithelial cells show little if any change in shape or function. The supporting ectomesenchymal cells are packed closely beneath and around the epithelial bud As the epithelial bud continues to proliferate into the ectomesenchyme, cellular density increases immediately adjacent to the epithelial outgrowth . This process is classically referred to as a condensation of the ectomesenchyme. 2- CAP STAGE As the tooth bud grows larger, it drags along with it part of the dental lamina; so from that point on, the developing tooth is tethered to the dental lamina by an extension called the lateral lamina The epithelial outgrowth, which superficially resembles a cap sitting on a ball of condensed ectomesenchyme, is referred to widely as the dental organ but actually should be called the enamel organ (epithelial part of the tooth germ) , because it eventually will form the enamel of the tooth. The enamel niche is an apparent structure in histologic sections, created because the dental lamina is a sheet rather than a single strand and often contains a concavity filled with connective tissue. A section through this arrangement creates the impression that the tooth germ has a double attachment to the oral epithelium by two separate strands . The ball of condensed ectomesenchymal cells, called the dental papilla, will form the dentin and pulp. 4 The condensed ectomesenchyme limiting the dental papilla and encapsulating the enamel organ—the dental follicle or sac—gives rise to the supporting tissues of the tooth. Because the enamel organ sits over the dental papilla like a cap, this stage of tooth development is known as the cap stage. The enamel organ, dental papilla, and dental follicle together constitute the dental organ or tooth germ. Important developmental changes begin late in the cap stage and continue during the transition of the tooth germ from cap to bell. Through these changes, termed histodifferentiation: a mass of similar epithelial cells transforms itself into morphologically and functionally distinct components. Outer enamel epithelium: consists of a single layer of cuboidal cells, separated from the surrounding connective tissue of the dental sac by a delicate basement membrane. Prior to the formation of hard structures, this regular arrangement is maintained only in the cervical parts of the enamel organ. At the highest convexity of the organ become irregular in shape and cannot be distinguished easily from the outer portion of the stellate reticulum. The capillaries in the connective tissue surrounding the epithelial enamel organ proliferate and protrude toward it. Immediately before enamel formation commences, capillaries may even indent the stellate reticulum. This increased vascularity ensures a rich metabolism when a plentiful supply of substances from the bloodstream to the inner enamel epithelium is required . During enamel formation, cells of the outer enamel epithelium develop villi and cytoplasmic vesicles and large numbers of mitochondria, all indicating cell specialization for the active transport of materials. The capillaries in contact with the outer enamel epithelium show areas with very thin walls, a structural modification also commonly found in areas of active transport The cells in the center of the enamel organ synthesize and secrete glycosaminoglycans into the extracellular compartment between the epithelial cells. Glycosaminoglycans are hydrophilic and so pull water into the enamel organ. The increasing amount of fluid increases the volume of the extracellular compartment of the enamel organ, and the central cells are forced apart. Because they retain connections with each other through their desmosomal contacts, they become star-shaped .The center of the enamel organ thus is termed the stellate reticulum. 5 Enamel knot Enamel knots are clusters of nondividing epithelial cells visible in sections of molar cap stage tooth germs. Enamel cord: In some planes of section, one can see cells extending from the enamel knot across the stellate reticulum to the outer enamel epithelium .This structure is referred to as the enamel cord; while it could be: 1- part of the enamel knot organizational center, 2-It could also be related anatomically to the site where the lateral lamina attaches to the enamel organ cap. This gives the stellate reticulum a cushion like consistency and acts as a shock absorber that may support and protect the delicate enamel-forming cells. There are temporary structures (transitory structures) that disappear before enamel formation begins: 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, so that the center of the epithelial invagination shows a slightly knob like enlargement that is bordered by the labial and lingual enamel grooves . At the same time a vertical extension of the enamel knot, called the enamel cord occurs. When the enamel cord extends to meet the outer enamel epithelium it is termed as enamel septum, for it would divide the stellate reticulum into two parts. The outer enamel epithelium at the point of meeting shows a small depression and this is termed enamel navel as it resembles the umbilicus. The function of the enamel knot and cord: 1- May act as a reservoir of dividing cells for the growing enamel organ. 2- Recent studies have shown that enamel knot acts as a signaling center as many important growth factors are expressed by the cells of the enamel knot and thus they play an important part in determining the shape of the tooth.

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