Tooth morphogenesis and the differentiation of ameloblasts.

All vertebrate organs are formed from several cell types, and it is currently believed that interactions between the different components constitute the most important mechanism in the regulation of organ morphogenesis. In developing teeth morphogenetic interactions occur between the epithelium covering the facial processes and the underlying neural crest-derived mesenchyme. Morphogenesis is accompanied by differentiation of the various dental cell types, including the ameloblasts. Although ameloblasts differentiate terminally and start the deposition of enamel matrix only after the completion of crown morphogenesis, there is increasing evidence suggesting that the segregation of the ameloblast cell lineage may start much earlier. For example, the down-regulation of the North receptor, which in some other developmental system is associated with cell fate determination, is already seen in the dental epithelium prior to the bud stage. It is not known to what extent the differentiation of ameloblasts depends on tooth morphogenesis, and whether the same mesenchymal signals regulate morphogenesis and cell differentiation. There is evidence that growth factors act as morphogenetic signals. Bone morphogenetic proteins and fibroblast growth factors appear to regulate the initiation of tooth development, as well as the morphogenesis of the crown shape. However, the molecular nature of the signals regulating the advancing specialization of the cells in the ameloblast cell lineage remains unknown.