Embryonic Development of Blood Vessels

Abstract

The first step in the formation of blood vessels is the emergence of their inner lining cells, the endothelial cells (EC). These cells then associate into tubular structures and other cells types, notably smooth muscle cells or pericytes, organize themselves around them to make the vessel wall. Structural specializations such as tight junctions in vessels of the blood-brain barrier or regionalized. adhesion molecules are acquired by differentiating EC. The development of new blood vessels is an uncommon physiological event in adults. It occurs during development of the corpus luteum or the placenta, during wound healing and regeneration or during tumorigenesis. The latter phenomenon has been elected by many groups for detailed morphological and biochemical analysis. Various in vivo and in vitro models have been devised to study this process. It could be shown that endothelial cells do not arise de novo in the adult but derive from already established blood vessels. The basement membrane surrounding the capillaries breaks down; endothelial cells undergo mitogenesis and migrate into the tumor under the effect of chemoattractive and mitogenic factors (review in Folkman, 1985). This process has been designated as angiogenesis and several growth factors have been identified with angiogenic properties. The quiescent state normal in most physiological situations is maintained through an interplay between endothelial cells and pericytes (Sato et al., 1990).