MITSIADIS, TA; FONTAINE-PERUS, J; CHERAUD, Y; KINGS COLLEGE LONDON, SE1 9RT LONDON, UK; CNRS U6018, NANTES UNIVERSITY, 44322 NANTES, FRANCE ; CNRS U6018, NANTES UNIVERSITY, 44322 NANTES, FRANCE: RECOVERY OF TEETH IN BIRDS

Recent advances in molecular and developmental genetics have provided tools for understanding evolutionary changes in the nature of the epithelial-mesenchymal interactions regulating the patterned outgrowth of the tooth primordia. Tissue recombination experiments in mice have identified the oral epithelium as providing the instructive information for the initiation of tooth development. Teeth were lost in birds 70 to 80 million years ago. Despite their disappearance, a number of gene products and the requisite tissue interactions needed for tooth formation are found in the avian oral region. Current thinking holds that it is the avian cranial neural crest-derived mesenchyme that has lost odontogenic capacity, whereas the oral epithelium retains the signaling properties required to induce odontogenesis. In order to investigate the odontogenic capacity of ectomesenchyme, we have used neural tube transplantations from mice to chick embryos to replace the chick neural crest cell populations with mouse neural crest cells. The mouse/chick chimeras obtained show evidence of tooth formation. The broader implications of these data are that craniofacial morphogenesis is initially directed by species-specific mesenchymal signals interplaying with common epithelial signals.