Cytoskeletal proteins play a major role in regulating growth and development by influencing the shape, orientation, polarization, locomotion and division of cells. The aim of this study is to elucidate the role of the plasma-membrane associated filaments during development of the early mouse embryo. The organization, composition and function of the filamentous network will be examined during fertilization, compaction and implantation using a combination of ultrastructural techniques. Subplasmalemmal filaments will be visualized in "membrane lawns" that are prepared by attaching eggs and embryos to cover slips and shearing off the upper portion of the cells to leave the adherent membrane with its associated filaments. Replicas of the lawns, as well as conventional thin sections, will be examined by TEM. Immuno- TEM techniques will be used to identify myosin, tubulin and spectrin; actin filaments will be identified by labelling with heavy meromyosin. Correlative studies with the cytoskeletal inhibitors nocodazole and cytochalasin D are designed to elucidate the function of the filaments. Plasma membrane-associated filaments will be studied in: 1) The egg with the aim of elucidating the mechanism whereby chromosomes influence the accumulation of actin filaments, promote microtubule growth, and function in the formation of the polar body; 2) Compacting blastomeres to determine the role of the filaments in surface polarization and blastomere spreading; 3) Blastocysts to define the role of the filaments in adhesion and spreading of the trophoblast.