This work represents a continuation of our program using zebrafish genetics and embryology to understand the earliest embryonic steps in fashioning the cardiovascular system. We focus here in particular upon the molecular pathways for assembly of the first artery and vein in the embryo. This process, termed vasculogenesis, is not well understood. We use as linchpins for this work two mutations we discovered in a zebrafish genetic screen. gridlock is a mutation affecting the aorta. We positionally cloned the gridlock gene. It is expressed in the aorta but not the vein, and is a novel member of the HES family of transcription factors. The second mutation, cloche , is a mutation that prevents formation of both blood and vessels, appearing to act as an essential element in generation of the elusive hemangioblast, a cell proposed to give rise both to the earliest blood and vessels in the embryo. Specific Aim 1: We will use laser-activated dye uncaging to define the origin and migratory track of angioblasts giving rise to the first artery and vein, both in wild-type and mutants defective in vasculogenesis. Specific Aim 2: In other systems, HES genes are known to function in driving cell fate decisions, and are the downstream effectors of the cell surface receptor, Notch. We propose that gri acts in a pathway which drives angioblasts to become pre-arterial or pre-venous even prior to generation of functioning vessels, and to do so as part of a pathway involving the cell surface Notch receptor. We will test this hypothesis by examination of the cellular and molecular effects of increasing or reducing grl and molecular components of the Notch cascade. Specific Aim 3: We will complete the positional cloning of cloche, and begin to examine its mechanism of action. This work defines pathways critical to normal vessel formation. These genes will be of importance as candidate genes for congenital vascular disorders such as aortic coarctation. It will also be of importance to examine these developmental pathways in the context of adult vessel injury, repair, and collateralization.