Mammalian teeth develop from two types of cells: stomodeal ectoderm, which forms ameloblasts, and cranial neural-crest-derived (ecto) mesenchyme cells, which form odontoblasts and cementoblasts. These two cell types, juxtaposed in the developing oral cavity, interact to control the entire process of tooth initiation, morphogenesis, and cytodifferentiation. Cell-cell signaling pathways and their target nuclear factors have been identified as key mediators of the progressively complex exchange of information between ectoderm and ectomesenchyme. The constantly changing direction of the reciprocal signaling and cell responses between ectoderm and ectomesenchyme enables cells to monitor their relative spatial positions and differentiated states continuously. The least understood of the early processes in tooth development are morphogenesis and patterning. From a seemingly uniform layer of oral ectoderm and underlying mass of ectomesenchyme, different types (shapes) of teeth develop in different positions. Tooth type is determined very early in development, before the overt onset of morphogenesis. Thus, the early ectoderm-ectomesenchyme cell interactions must in some way either create or respond to positional differences in the jaw primordia.
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