Figure 3. Tooth formation. (A) Tooth with enamel. Through the reciprocal interactions between dental epithelial cells and the underlying ectomesenchymal cells, odontoblasts (Od) develop from the ectomesenchymal cells. As odontoblasts retreat centripetally (shaded arrow), these cells deposit the ECM proteins for dentin between the odontoblast layer and the dental epithelial cell layer. Subsequently, ameloblasts (Am), now fully differentiated from the dental epithelial cells, lay down the ECM proteins for enamel on the mineralizing dentin surface. Enamel thus grows centrifugally away from the pulp (closed arrow). As enamel grows to its final thickness, enamel structural proteins are degraded and resorbed by ameloblasts, and further deposition of calcium and phosphates facilitates enamel maturation. (B) Tooth with enameloid. The histogenesis of teleost teeth is considerably different from that of tetrapod teeth covered with enamel. Initially, the ECM proteins for enameloid, not dentin, are secreted by both the inner dental epithelial (IDE) cells and odontoblasts and deposited below the basal lamina abutting the IDE cells (closed arrow). While the activity of the IDE cells diminishes, odontoblasts continue to retreat centripetally (shaded arrow) and deposit ECM proteins for dentin. Major ECM proteins secreted by ameloblasts, odontoblasts, and the IDE cells are shown at the bottom. Note that the deposition of AMTN and ODAM is known only at the enamel maturation stage.