Conservation of early odontogenic signaling pathways in Aves

Teeth have been missing from birds (Aves) for at least 60 million years. Ho wever, in the chick oral cavity a rudiment forms that resembles the lamina stage of the mammalian molar tooth germ. We have addressed the molecular ba sis for this secondary loss of tooth formation in Aves by analyzing in chic k embryos the status of molecular pathways known to regulate mouse tooth de velopment. Similar to the mouse dental lamina, expression of Fgf8, Pitx2, B arx1, and Pax9 defines a potential chick odontogenic region. However, the e xpression of three molecules involved in tooth initiation, Bmp4, Msx1, and Msx2, are absent from the presumptive chick dental lamina. In chick mandibl es, exogenous bone morphogenetic protein (BMP) induces Msx expression and t ogether with fibroblast growth factor promotes the development of Sonic hed gehog expressing epithelial structures. Distinct epithelial appendages also were induced when chick mandibular epithelium was recombined with a tissue source of BMPs and fibroblast growth factors, chick skin mesenchyme. These results show that, although latent, the early signaling pathways involved in odontogenesis remain inducible in Aves and suggest that loss of odontoge nic Bmp4 expression may be responsible for the early arrest of tooth develo pment in living birds.