Although failure of placental function is one of the most common causes of embryonic death during gestation in humans and other vertebrates, little is known about how development of these extraembryonic tissues is regulated. The ability to induce mutations in the mouse provides an opportunity for studying the complex interactions during placental development on a defined genetic background. The experiments outlined in this proposal focus on a series of radiation-induced deletions which identify through complementation and embryological studies a 20-kb region of mouse chromosome 7(MMU7) containing a gene(s) needed for normal development of the extraembryonic ectoderm, the progenitor to the embryonic portion of the placenta. Embryos homozygous for deletions removing this region, known as the extraembryonic development (exed) region, display a small, pyknotic extraembryonic ectoderm at embryonic day (E) 7.5. By contrast, the embryonic ectoderm appears normal in size and morphology at this time, but development and growth does not progress and the embryos are resorbed by E8.0. The specific aims presented in this proposal will concentrate on molecular approaches for identifying and evaluating the functional significance of candidate genes as well as a continued analysis of the biology of the mutation. Three primary questions will be addressed. The first focuses on whether exed affects extraembryonic ectoderm development only or whether there is also a direct effect on development of the embryonic ectoderm. This question will be addressed using tetraploid-diploid aggregations to produce chimeric embryos with wild-type extraembryonic ectoderm and a mutant embryo. The second question focuses on the significance of the 20-kb exed critical region which is defined by an area of non-overlap between two larger deletions. It is not known whether removal of the 20-kb critical region is both necessary and sufficient to cause the exed phenotype, or whether other genes outside the region are also required to be removed to produce the phenotype. To clarify this issue, the 20- kb critical region will be deleted from a wild-type MMU7 by gene targeting strategies and embryos homozygous for this small deletion will be examined for the exed phenotype. Finally, the search will begin for the exed gene by analysis of the 20-kb critical region of MMU7.