The normal proliferation and differentiation of trophoblast precursors is essential for reproduction in placental mammals. The trophoblast is a unique cell type which both protects the fetus from immunological attack by the mother and provides nutrition to the entire conceptus through the placenta. Thus, an understanding of trophoblast origin and function will facilitate attempts to control fetal wastage and intrauterine growth retardation in humans. The objective of this research is to determine the mechanisms of origin and differentiation of the trophoblast precursors, the trophectoderm cells in the blastocyst and their diploid descendants in subsequent embryonic stages. We address two aspects of trophectoderm development that can be approached using the embryos of laboratory mice as models for peri-implantation mammalian development. The first objective is to determine the developmental stages at which progenitors of the trophectoderm lineage become distinct from those of the fetal cell populations, and the sequence of events by which the separate tissues of the trophectoderm lineage are established. This will be accomplished by a cell lineage analysis using microinjection of intracellular tracers, infection with retroviruses encoding a histochemically-detectable product, and labeling the entire trophectoderm layer with a cell surface marker. This analysis is the first use of mammalian retroviruses to study lineage during the peri-implantation period, using an indicator gene under the regulation of an internal promotor. The second objective is to examine the relationship between commitment and specific gene expression in trophoblast development. This will be accomplished by determining the precise time, location and functional role of trophectoderm-specific cytokeratin synthesis in early blastocysts using microinjected polyclonal and monoclonal antibodies to cytokeratins. Also, recombinant DNAs containing a cytokeratin promoter and a histochemically-detectable indicator gene will be microinjected to determine the earliest time and location of trophectoderm-specific gene expression. These studies will be the first analysis of cytokeratin function in an intact, fluid-transporting epithelium. Also, the DNA studies will be the first use of the transgenic mouse technique to analyze tissue-specific gene regulation in preimplantation embryos. These molecular events will be correlated with the cellular events of allocation and commitment during blastocyst growth. The results should provide a detailed understanding of the origin of trophoblast in mouse embryos that is applicable to placental mammals in general.
