This research program is directed at an understanding of cell-cell and cell-substratum interactions during animal morphogenesis. An analysis of morphogenetic mechanisms is central to our understanding of embryonic development. The proposed research examines the morphogenesis of the primary mesenchyme cells (PMCs), a population of migratory cells in the sea urchin embryo. These cells undergo a predictable sequence of movements that can be directly observed in vivo and are amenable to biochemical and immunological analysis. The specific goals of the project are threefold: 1)To examine the mechanisms that underlie the directionality of PMC migration, by a combination of approaches including isolation of embryonic extracellular matrices, micromanipulation of PMCs, cell ablation by UV microsurgery, and electron microscopy, 2)To analyze the molecular interactions that take place between the PMCs and their substratum, the embryonic basal lamina. We will generate polyclonal and monoclonal antibodies against the basal lamina and use these reagents to analyze the spatial distribution of antigenic determinants during the period of PMC migration. We will also use these reagents to search for substrate molecules that have a functional role in PMC migration, by microinjecting the antibodies into living embryos and by testing their effects on PMCs migrating in vitro, 3) To examine cell-cell interactions during PMC morphogenesis. We will use biochemical, immunological, and microsurgical methods to elucidate the mechanism by which the PMCs prevent secondary mesenchyme cells (SMCs) from expressing a skeletogenic phenotype. We will focus on the timing of events during this interaction and the nature of the signal transmitted by the PMCs. In addition, we will attempt to develop and in vitro system to study the PMC-SMC interaction.