Information on the mechanism that governs tooth shape, size, and location in the developing jaw is sparse. One hypothesis holds that exogenous signals from the ectomesenchyme and epithelium of the embryonic jaw induce a specific pattern of cellular development that leads to tooth formation. A second hypothesis is that a specific tooth type develops from select clones of cells. The objective of this review is to consider these ideas in light of recent studies on genes that regulate embryonic patterning. Special attention is directed at the notion that there is restricted expression of specialized genes. These genes are expressed in overlapping domains along the jaw axis. Genes expressed within a region provide a unique homeobox code that serves to specify a tooth type and ultimately determines tooth form and position in the developing jaw. Expression of this genetic 'bar code' is carefully controlled by the dental epithelium, neural crest-derived cells, and growth factors. The latter agents also probably serve as epithelial signals that activate events that lead to odontogenic differentiation and morphogenesis.
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