Chen 9603081 Tooth development provides an excellent model for studying epithelial-mesenchymal tissue interactions leading to organogenesis during vertebrate development. While teeth have been missing from Aves for nearly 100 million years, in the avian oral cavity there does exist a residual structure resembling the tooth rudiment at the lamina stage in reptiles and mammals. This arrest of tooth development in Aves is believed to be due to the failed interactions between the presumptive dental epithelium and mesenchyme. A similar phenotype is observed in the mouse Msxl and Msx2 double mutants in which tooth development is also arrested at the lamina stage. To date there is no information available on the arrest of tooth development and its underlying molecular mechanisms in Aves. This proposal is aimed at defining the failure of epithelial-mesenchymal interactions and to determine the molecular mechanisms underlying the arrest of avian tooth development, with a goal of generating an avian tooth bud in vitro and in viva. In situ hybridization, in vitro tissue recombination, in vitro organ culture will be employed to examine the expression of important genes, to localize the odontogenic defect to the tissue compartment in the chick presumptive tooth germ, and to verify the conservation of signaling pathways in the dental mesenchyme between mouse and chick. A possible conservation of signaling pathways in the formation of tooth and limb will be also verified by replacing chick limb AER with mouse dental epithelium and vice versa. Chick CAM culture and mouse intraocular graft techniques will be used to enhance chick tooth germ development. Effort will also be made to generate a chick tooth germ by restoring gene expression in the chick presumptive tooth germ in vitro and in viva with retroviral infection. The long-term goal of this study is to substantiate our understanding of the molecular basis of epithelial-mesenchymal interactions during organogenesis.
