Molecular genetics of tooth morphogenesis and patterning: The right shape in the right place

Development of the mammalian tooth has for many years served as a useful mo del system for the study of cell-cell interactions in organogenesis. Early development of teeth (tooth buds) shows many morphological and molecular si milarities with early development of other organs such as the lung, hair, k idney, etc. There has been much progress toward understanding epithelial/me senchymal cell signaling in tooth germ formation. Advances in understanding the formation of different shares of teeth (morphogenesis) at their correc t positions in the jaws (patterning) has, until recently, been less forthco ming. We review here the latest ideas on the control of odontogenic pattern ing and morphogenesis. The stages of early tooth development are well-defin ed histologically and have been described in numerous textbooks. The progre ssion from localized thickenings of oral epithelium to bud, cap, and bell s tages provides an adequate description of the gross morphological changes s een in the epithelial cells of early developing tooth germs. Less obvious a re the concomitant changes taking place in the dental (ecto)mesenchymal cel ls which originate from the cranial neural crest and which condense around the tooth bud epithelium. However, it is very clear that these mesenchymal cells are equal partners with epithelium during the early stages of tooth g erm formation and undergo complex changes which, although not obvious histo logically, are revealed with molecular (gene) probes. Genes identified as b eing important for the early communication between the epithelial and ectom esenchymal cells mainly comprise those which code for proteins which act as secreted signals between the cells (ligands) and those that code for nucle ar proteins that act to control gene expression in response to the signals. Little is presently known about the changes in structural proteins such as cell adhesion molecules which are involved in mediating the physical inter actions between cells and generating the morphological changes.