The purpose of this study is to analyze presomitic embryogenesis with special emphasis on cellular and extracellular factors that determine the transition of pluripotent and developmentally uncommitted cells of the blastocyst to irreversibly committed cells, in the early somitic embryo. Due to the inaccessibility of the early postimplantation mammalian embryos developing in utero, culture in vitro under defined conditions will be used throughout the entire study. Targeted biochemical and microsurgical interventions will be performed on embryos in order to elucidate the role of various determinants of normal embryogenesis. We shall analyze the morphogenetic role of selected nuclear, cytoplasmic, cell surface and extracellular components present in the preimplantation embryo or developing during early postimplantational embryogenesis. The ultimate goal will be to gain insight to the interplay of these determinants and how they lead to an orderly initiation of organogenesis. The results of these studies will provide a basis for the better understanding of normal embryonic development and why embryos in the early presomitic embryogenesis are most susceptible to teratogenesis.