The role of pericytes in angiogenesis

Pericytes are branched cells embedded within the basement membrane of capillaries and post-capillary venules. They provide an incomplete investment to endothelial cells, thus reinforcing vascular structure and regulating microvascular blood flow. Pericytes exert an important role on endothelial cell proliferation, migration and stabilization. Endothelial cells, in turn, stimulate expansion and activation of the pericyte precursor cell population. The balance between the number of endothelial cells and pericytes is highly controlled by a series of signaling pathway mechanisms operating in an autocrine and/or paracrine manner. In this review, we will first examine the molecular aspects of the pericyte activating factors secreted by endothelial cells, such as platelet derived growth factor B (PDGF-B), vascular endothelial growth factor (VEGF), transforming growth factor beta (TGF-beta) and angiopoietins (Angs), as well as signaling pathways involving Notch and ephrins. We will then consider the complex and multivarious contribution of pericytes to the different aspects of angiogenesis with particular emphasis on the potential role of these cells as targets in tumor therapy.