Vascular Development

This chapter focuses on the cellular and molecular mechanisms that regulate embryonic vascular development. Cellular and molecular characterization of embryonic endothelial cells has significantly improved the understanding of blood vessel development and behavior. Together, classical observations and more recent molecular and genetic approaches have helped to elucidate many aspects of vascular specification, differentiation, and remodeling. The cardiovascular system is the first functional organ formed during embryogenesis, arising long before other organs and tissues. Abnormalities in its assembly or function almost invariably lead to embryonic lethality. The reason that cardiovascular function is critical to the survival of higher organisms lies in the fact that every cell must receive nutrition and eliminate wastes via blood vessels. Studies have demonstrated that the cells in complex tissues are generally located within about 100-200 μm of an endothelial-lined blood vessel, which is the diffusion limit for oxygen, otherwise they perish from starvation and/or asphyxiation. During formation, the general pattern of the embryonic vascular system is strikingly stereotyped within a species, and highly conserved between different vertebrate species. The defining cell type of the vascular system is the endothelial cell, which forms the seamless lining of the entire circulatory system, including the heart and all arteries and veins, large and small. Given the architectural conservation of the vasculature between species, it is likely that endothelial cells arise and assemble following genetically hard-wired molecular cues that are conserved across vertebrates.