During development multiple signaling pathways and morphogenetic movements are active to pattern the embryo. This process consists of the progressive specialization of cells to give rise to the diversity of cell types fond in the adult organism. The first step in this process is the establishment of the three embryonic germ layers, endoderm, mesoderm and ectoderm during gastrulation. The overall goal of this research is to understand how cells become progressively committed to a specific germ layer. This knowledge will contribute to the fundamental understanding of how cell fates are established and how it integrates with patterning of the embryo. This proposal examines the signaling and response mechanism of cells during dorsal mesoderm induction which leads to the formation of somites. The first specific aim will examine the activity of the dorsalizing signal which is responsible for instructing ventral-lateral mesoderm to differentiate as somitic tissue. Somite development is a lengthy process that begins at gastrulation and continues into neuralation as the posterior axis develops. Heterochronic grafts will be used to determine the length to which this signal is active during development. This will reveal whether this signal is active throughout the progressive formation of somites or whether this signal is active only at the onset of somite formation to initiate the process. To understand how cells are instructed to form somites, the second specific aim examines the cell behaviors of grafted cells and neighboring cells by time-lapse video micrography. The third specific aim examines the competence of cells to respond to this dorsalizing signal. Preliminary results suggest that cells from the neural ectoderm remain competent to form mesodermal cell types as late as the end of gastrulation. To further examine this observation, we will perform heterochronic grafts of cells from the anterior and posterior neural ectoderm, epidermis and ventral-lateral mesoderm. We will examine how changes in cell competence relate to changes in gene expression and how together this leads to patterning of the vertebrate embryo. The last aim is to provide undergraduate and graduate students from under-represented groups an opportunity to acquire essential skills that will prepare them for future careers in biomedical research.