Vascular progenitor cells: Origin and mechanisms of mobilization, differentiation, integration, and vasculogenesis

The recent discovery of progenitor cells in peripheral blood that can differentiate into endothelial or vascular smooth muscle cells has led to the re-evaluation of many traditionally held beliefs about vascular biology. Most notably, concepts of vascular regeneration and repair, previously considered limited to the proliferation of existing differentiated cells within vascular tissue, have been expanded to include the potential for postnatal vasculogenesis. These cells have since been identified in the bone marrow, heart, skeletal muscle, and other peripheral tissues, including the vasculature itself. The significance of these cells lies not only in developing our understanding of normal vascular biology, but also in the insights they may provide into vascular diseases such as atherosclerosis. In addition, a potential role in therapeutics has already been explored in early clinical trials in humans. The mechanisms underlying the mobilization, target tissue integration, differentiation, and the observed therapeutic benefits of these cells are now being elucidated. It is these mechanisms, and the current understanding of the lineage of these cells, that constitutes the focus of this review.