Selective activation of developmentally regulated genes heralds the transition from totipotent embryonic cells to differentiated cell types. Differentiation of embryos and embryonal carcinoma cells (ECC) in vitro gives rise to entodermal cells which synthesize a basement membrane glycoprotein, laminin. Basement membrane has been implicated in morphogenetic events which regulate cellular aggregation and organization of the embryo. We propose to characterize the structure of laminin and produce antibodies specific for laminin. Isolated laminin and specific antisera will be used to analyze the role of laminin in cell attachment and aggregation, search for laminin receptors on ECC and entodermal cells and probe the time of appearance and role of laminin during normal embryogenesis. We intend to clone the laminin gene(s) using recombinant DNA technology. Isolated genes will be used to study different levels of gene control (DNA rearrangement, RNA processing, changes in mRNA synthesis) during the transition from totipotent to differentiated entodermal cells.