Circulating Endothelial Progenitor Cells and Adult Vasculogenesis

Postnatal neovascularization (NV) has previously been considered synonymous with proliferation and migration of pre-existing, fully differentiated endothelial cells resident within parent vessels. The finding that circulating endothelial progenitor cells (EPCs) may home to sites of NV and differentiate in situ is consistent with vasculogenesis, a critical paradigm for the establishment of vascular networks in the embryo. While the percent contributions of angiogenesis and vasculogenesis to postnatal NV remain to be clarified, our observations in the eye, together with recent reports from other investigators in other tissues and pathologies, support that growth and development of new blood vessels in the adult are not restricted to angiogenesis but encompass both embryonic and adult mechanisms. As a corollary, augmented or retarded NV, whether endogenous or iatrogenic, likely includes enhancement or impairment of vasculogenesis. In this chapter, the molecular and cellular factors that play a role in EPC involvement in NV are discussed.