Putative endothelial progenitor cells were initially discovered in 1997 by Asahara and colleagues. The hypothesis driving their research was that a common precursor may give rise to the hematopoietic cells and angioblasts that condense to form blood islands during the initial phase of vascular development, and that it may be possible to identify an adult progenitor cell using the shared markers between the hematopoietic cells and angioblasts, namely CD34 and Flk-1 (or VEGFR2). CD34+ blood cells were magnetically sorted to a purity of 15.7%, and VEGFR2 cells were sorted to a purity of 20.0%. When CD34+ cells were plated on fibronectin, they adhered and proliferated for 4 weeks, whereas the CD34- cells did not. Plating the positive and negative populations together increased cell proliferation, cluster formation and also formation of tube-like structures. Further, these plated CD34+ cells took up acetylated-LDL and expressed higher levels of endothelial cell markers compared to freshly sorted CD34+ cells. Most importantly, the plated CD34+ cells were found co-localized with CD31+ cells at sites of new blood vessel growth in a rodent model of hindlimb ischemia. Together, these results demonstrated an endothelial phenotype, behavior, and apparent contribution to new blood vessel growth by the isolated CD34+ cells. Further, this was the first evidence of the existence of a circulating endothelial progenitor which could possibly participate in the growth of new vasculature. These findings motivated many researchers to equivalently search for and prove the existence of an endothelial progenitor cell.
In the following years, many papers were published about these putative endothelial progenitor cells, but as findings varied widely, it became clear that different cell types were all being referred to as ‘endothelial progenitors.’ There is still no single, agreed upon definition of an endothelial progenitor by surface markers or gene ex
CD34+ cells and Subsets
Since the initially discovered EPCs were defined based on ex
Circulating Angiogenic Cells (CACs)
Rather than using surface molecule ex
Outgrowth Endothelial Cells (OECs)
The last population of ‘endothelial progenitors’ that has been widely investigated does contain cells that exhibit true progenitor behavior, and are called Outgrowth Endothelial Cells (OECs), or sometimes Endothelial Colony Forming Cells. These cells were named because of their late-outgrowth behavior, with colonies appearing 10-14 days after isolation from cord blood or bone marrow, as opposed to the early outgrowth by CACs. In contrast to the low or absent proliferative potential of the CAC population, OECs exhibited a high-proliferative potential and formed colonies in single cell re-plating assays, as would be expected of a true progenitor. These cells were shown to originate in the bone marrow, but not from a CD133+ or CD45+ population, nor did they express CD14. OECs demonstrated close similarities to endothelial cells in their surface marker ex