The cardiovascular system is the first functional organ of the vertebrate embryo. It forms by two distint but related processes, vasculogenesis and angiogenesis (Risau and Flamme, 1995). During vasculogenesis, endothelial progenitor cells (angioblasts) differentiate in the mesodermal compartments of embryonic and extra-embryonic tissues to form a primary capillary plexus. Subsequently, the primitive vasculature is refined by sprouting and non-sprouting angiogenesis, and undergoes extensive remodeling into small and large vessels (Risau, 1997). The functional maturation of blood vessels is accompanied by the recruitment of perivascular cells, such as pericytes and smooth cells. The embryonic vascular system develops in anticipation of the demands of the growing embryo for oxygen and nutrients. In contrast, angiogenesis in the adult organism occurs in response to the metabolical requirements of tissues, and is triggered by hypoxia. Deregulated angiogenesis plays an important role in the pathogenesis of a variety of diseases, including retinopathy and solid tumor growth (Folkman, 1995).