The discovery of endothelial progenitor cells. An historical review.

Although the earliest sites of hematopoietic cell and endothelial cell differentiation in the yolk sac blood islands were identified about 100 years ago, cells with hemangioblast properties have not yet been identified in vivo. Endothelial cells differentiate from angioblasts in the embryo and from endothelial progenitor cells, mesoangioblasts and multipotent adult progenitor cells in the adult bone marrow. Endothelial progenitor cells (EPC) were initially described by Asahara et al. [Asahara T, Murohara T, Sullivan A, et al. Isolation of putative progenitor endothelial cells for angiogenesis. Science 1997;275:964-7.], and the past few years have seen a rapid expansion of our knowledge of EPC biology. Prior to the discovery of this cell type, new vessel formation was believed to occur to proliferation of existing endothelial cells. These findings have overturned the previous dogma that vasculogenesis can only occur during embryogenesis. Questions persist regarding their functional characteristics, as well as the precise panel of cell surface markers that define this cell population.