Correlation between apoptosis distribution and BMP-2 and BMP-4 expression in vestigial tooth primordia in mice.

The eutherian dental formula consists of three incisors, one canine, four premolars and three molars in each dental quadrant. Muroid evolution led to a reduction in the number of teeth, with one incisor separated from three molars by a long diastema. However, seven vestigial tooth primordia (D1-5, R1-2) have been detected in the embryonic diastemal area of the mouse maxilla between embryonic days (ED) 12.5 and 13.5. Computer-aided 3D reconstructions were used to analyse the temporo-spatial pattern of apoptosis during regression in the two largest and most distal vestiges (R1, R2). These structures have been widely considered as the primordium of the first upper molar and, accordingly, related molecular data have been interpreted exclusively in terms of progressive molar development. The spatial distribution of epithelial apoptosis, which affected the R1 and R2 rudiments in two consecutive waves on ED 12.5 and 13.5, respectively, was compared with our earlier data on expression of genes encoding bone morphogenetic proteins (BMP-2 and BMP-4). Similar temporo-spatial patterns of apoptosis and expression of BMP, specifically confined to the epithelium of the rudimentary tooth primordia, strongly support involvement of BMPs in the regulation of epithelial apoptosis during odontogenesis.