Lec. 3 Dr. Ali H. Murad Development of the tooth

Dental development is initiated when 1st arch epithelium is combined with the neural crest cells. However, after 12 days of development, 1st arch epithelium loses this odontogenic potential, which then assumed by the ectomesenchyme & elicits tooth formation. After 37 days of development, a continuous band of thickened epithelium form around the mouth in the presumptive upper & lower jaws. Each band of epithelium, called the primary epithelial band, very quickly gives rise to two subdivisions, the vestibular lamina & the dental lamina.

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Within the dental lamina, continued & localized proliferative activity leads to the formation of a series of epithelial ingrowths into the ectomesenchyme at sites corresponding to the positions of the future deciduous teeth. From this ingrowth, tooth development proceeds in 3 stages: bud, cap, & bell. These terms are described for the morphology of the developing tooth germ.

1-Bud stage Is represent the 1st epithelial incursion in to the ectomesenchyme of the jaw.Is a round, localized growth of epithelial cells surrounded by proliferating mesenchymal cells. The epithelial show little change in shape or function.

2-Cap stage (proliferation stage) The epithelial ingrowth gradually enlarged, it gains a concave surface resembles a cap sitting on a ball of condensed ectomesenchyme. The epithelial cells now termed enamel organ & remain attached to the dental lamina. The ball of condensed mesenchymal cells called dental papilla, forms the dentin & pulp. The condensed ectomesenchyme limiting the dental papilla & encapsulating the enamel organ called, dental follicle, which give the supporting tissues of the tooh. Enamel organ, dental papilla, & dental follicle together constitute the tooth germ

3-Bell stage (histo & morphodifferentiation stage) Continued growth of the tooth germ leads to the next stage of tooth development

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the bell stage. The bell stage so called because the enamel organ resembles a bell. Important developmental changes continue, these changes, termed histodifferentiation & morphdifferentiation, in which the epithelial cells transform it into morphologically & functionally distinct component. The cells of enamel organ differentiated to: 1-outer enamel epithelium, which cover the enamel organ 2-inner enamel epithelium, which become the ameloblasts that form the enamel of the tooth crown 3-stellate reticulum, which are star shaped with processes attached to each other. 4-stratum intermedium, these lie adjacent to the inner enamel epithelium. They assist ameloblasts in the enamel formation.

Two other important events take place during the bell stage: 1- the dental lamina joining the tooth germ to the oral epith. Breaks up into discrete islands of epithelial cells, thus separating the developing tooth from the oral epithelium. 2- the inner enamel epithelium fold, making it possible to recognize the shape of the future crown pattern of the tooth.

Development of the dental papilla Densely packed cells characterized the dental papilla. This is obvious even in the early bud stage, during which cells proliferate around the enlarging tooth buds at the leading edge of the dental lamina. This cells density is maintained as the enamel organ grows. Cells of the dental papilla are fibroblast (appear to be in a delicate reticulum). Blood vessels appear early in the dental papilla, along with nerve fibers associated with these vessels.

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Crown formation The next step in the development of the tooth, late in the bell stage, is formation of the two principal hard tissues of the tooth; the dentin & the enamel. The small columnar cells of the inner enamel epith. elongate, becoming tall & columnar with their nuclei aligned adjacent to the stratum intermedium & away from the dental papilla. As these morphological changes occur in the cells of the inner enamel epith., changes also occur within the adjacent dental papilla. The undifferentiated cells in the dental papilla increase rapidly in size & ultimately differentiate in to odontoblast (dentin forming cells).

The odontoblasts, as they differentiate, begin to elaborate the organic matrix of dentin, collagen & ground substance. As the organic matrix is deposited, the odontoblasts move toward the center of the dental papilla, leaving behind a cytoplasmic extension around which dentin is formed. After the 1st dentin has been formed, & only then, the cells of the inner enamel epith. differentiate further and termed ameloblast, producing an organic matrix against the newly formed dentin. Almost immediately, this organic matrix is partially mineralized & becomes the enamel of the crown. The enamel forming cells (ameloblasts) move away from the dentin, leaving behind an even-thickness of enamel. 4

It has been stated that odontoblasts differentiate under an organizing influence stemming from the cells of the inner enamel epith., on the other hand, the enamel formation can’t begin until some dentin has formed (reciprocal induction).

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