Development of the embryonic vascular system.

In this article, we survey the mechanisms involved in early embryonic angiogenesis. The first embryonic blood vessels are formed exclusively by endothelial cells. Therefore, the emergence and behavior of this cell type is the center of this article. We discuss both intra- and extraembryonic angiogenesis and the various modes of capillary formation. The high plasticity and migratory potential of endothelial cells and their precursors, the angioblists, are outlined. The promoting and inhibiting influences of the extracellular matrix on the behavior of angioblasts are a matter of concern, as is also the question of embryonic angiogenic factors. Cell-cell interactions that may lead to organ-specific differentiation of endothelial cells are mainly discussed in the context of blood-brain barrier formation and development of fenestrated capillaries. The last section deals with the development of the vascular wall.