The goal of this study is to identify the mechanisms regulating early endodermal cell migration and understand how cell movements contribute to the overall formation of the gastrointestinal tract. Although much progress has been made in identifying the transcription factors that specify endodermal cells, much less is known about the cell biological processes that occur subsequently, including the onset of cell motility. Shortly after specification, endodermal cells internalize to form the inner layer of the body during a process known as gastrulation. Using the zebrafish embryo as a model system, I will examine two aspects of endodermal cell migration - random directionality and contact-dependent repulsion. For these studies, I will use high-resolution fluorescence imaging of live embryos and a novel transgenic line that I developed that fluorescently labels the actin cytoskeleton. Preliminary evidence suggests that Nodal, a signaling protein already known to induce endodermal fate, can promote endodermal migration in a directionally random manner. I will characterize the effects of Nodal signaling on the actin cytoskeleton within endodermal cells, and I will identify the actin regulatory proteins that function to promote Nodal-induced motility. In the second specific aim, I will examine the mechanisms regulating contact-dependent repulsion. This is a process whereby migrating endodermal cells are repelled away from each other after making brief contact. Using both candidate and microarray- based approaches, I will attempt to identify the cell surface receptors that initiate contact repulsion as well as the cytoplasmic signaling proteins that act downstream of these receptors. Together, directionally random motility and contact repulsion appear to drive the dispersal of endodermal cells across the surface of the early embryo, after which these cells coalesce into a single gut tube. To understand the significance of this initial dispersal, I will use gain and loss of function techniques to interfere with normal endodermal migratory behavior and assess the effects on subsequent development of the gastrointestinal tract. In the course of this study, I will gain further expertise in vertebrate gut development, whole-embryo time-lapse imaging, and quantitative methods for examining in vivo cell migration. My primary mentor, Dr. Didier Stainier, and my co-mentor, Dr. Orion Weiner, will provide additional training in these areas to help me achieve my research goals. This study will shed light on a poorly appreciated aspect of gastrointestinal development and will form the basis for a future R-level application as I continue to explore the relationship between cell migration and organ development.