Vascular development: cellular and molecular regulation.

The vascular system forms through a combination of vasculogenesis and angiogenesis. In vasculogenesis, vessels form de novo via the assembly of endothelial precursors called angioblasts, whereas in angiogenesis new vessels arise by migration and proliferation of endothelial cells from preexisting vessels. Although the two processes are distinct in some respects, recent evidence suggests that they share a number of regulatory mechanisms. The identification of a number of defined growth factors, observations of genetically manipulated mice, and the recognition of the importance of cell-cell interactions have greatly expanded our understanding of the regulation of vascularization. The paracrine actions of a variety of polypeptide growth factors, including platelet-derived growth factor, vascular endothelial growth factor, transforming growth factor-beta, and the angiopoietins, appear to be orchestrated in a complex sequence of steps that lead to the development of the adult vascular system. Thus, communication between the forming vasculature and the tissue parenchyma, as well as interactions among cells of the vascular wall, all appear to influence vascular development and growth.