The multifactorial regulation of vessel assembly. The upper panel summarizes observations obtained using tissue culture models and knockout mice. Formation of a primary capillary network is initiated by VEGF-induced proliferation and migration of endothelial cells (ECs). Endothelial secretion of PDGF-B attracts mesenchymal cells, which contact the EC. Contact between the ECs and mesenchymal cells activates TGF-β1, which suppresses endothelial proliferation and migration, induces mural cell differentiation, and is associated with vessel maturation. The lower panel illustrates findings in experimental models in which levels of angiopoietins were manipulated by exogenous administration. Addition of Ang1 results in stabilization of vessels in the absence of pericytes, whereas addition of Ang2 induces vessel regression in the absence of VEGF (–) and sprouting in the presence of VEGF (+).