Signaling vascular morphogenesis and maintenance.

Blood vessels are constructed by two processes: vasculogenesis, whereby a primitive vascular network is established during embryogenesis from multipotential mesenchymal progenitors, and angiogenesis, in which preexisting vessels (both in embryo and adult) send out capillary sprouts to produce new vessels (1-3). Endothelial cells are centrally involved in each process: They migrate and proliferate and then assemble into tubes with tight cell-cell connections to contain the blood. Peri-endothelial support cells are recruited to encase the endothelial tubes, providing maintenance and modulatory functions to the vessels; such cells include pericytes for small capillaries, smooth muscle cells for larger vessels, and myocardial cells in the heart.