THE LIFE-HISTORY OF AN EMBRYONIC SIGNALING CENTER - BMP-4 INDUCES P21AND IS ASSOCIATED WITH APOPTOSIS IN THE MOUSE TOOTH ENAMEL KNOT

The enamel knot, a transient epithelial structure, appears at the onse t of mammalian tooth shape development, Until now the morphological, c ellular and molecular events leading to the formation and disappearanc e of the enamel knot have not been described, Here we report that the cessation of cell proliferation in the enamel knot in mouse molar teet h is linked with the expression of the cyclin-dependent kinase inhibit or p21, We show that p21 expression is induced by bone morphogenetic p rotein 4 (BMP-4) in isolated dental epithelia, As Bmp-4 is expressed o nly in the underlying dental mesenchyme at the onset of the enamel kno t formation, these results support the role of the cyclin-dependent ki nase inhibitors as inducible cell differentiation factors in epithelia l-mesenchymal interactions, Furthermore, we show that the expression o f p21 in the enamel knot is followed by Bmp-l expression, and subseque ntly by apoptosis of the differentiated enamel knot cells. Three-dimen sional reconstructions of serial sections after in situ hybridization and Tunel-staining indicated an exact codistribution of Bmp-4 transcri pts and apoptotic cells. Apoptosis was stimulated by BMP-4 in isolated dental epithelia, but only in one third of the explants, We conclude that Bmp-1 may be involved both in the induction of the epithelial ena mel knot, as a mesenchymal inducer of epithelial cyclin-dependent kina se inhibitors, and later in the termination of the enamel knot signali ng functions by participating in the regulation of programmed cell dea th, These results show that the life history of the enamel knot is int imately linked to the initiation of tooth shape development and suppor t the role of the enamel knot as an embryonic signaling center.