Endothelial progenitor cells are a controversial and hypothetical population of rare cells believed to circulate in the blood with the ability to differentiate into endothelial cells, the cells that make up the lining of blood vessels. The process by which blood vessels are born de novo from endothelial progenitor cells is known as vasculogenesis. Most of vasculogenesis occurs in utero during embryologic development. Endothelial progenitor cells, if existing in adults, were therefore first believed to be angioblasts, which are the stem cells that form blood vessels during embryogenesis. Endothelial progenitor cells are hypothesized to participate in pathologic angiogenesis such as that found in retinopathy and tumor growth. While embryonic angioblasts have been known to exist for many years, adult endothelial progenitor cells were first believed to be characterized in the 1990s after Asahara and colleagues published that a purified population of CD34-expressing cells isolated from the blood of adult mice could purportedly differentiate into endothelial cells in vitro.[1] Recent discoveries in both mice[2] and humans[3] have led scientists and researchers to question the existence of endothelial progenitor cells. It is therefore now believed that the cells Asahara and colleagues have studied are hematopoietic cells that never become endothelial cells.[4] These controversial findings have led to a significant amount of confusion and conflict in this field of biology. It is well known that various cytokines, growth factors, and hormones cause hematopoietic cells, and by association endothelial progenitor cells, to be mobilized into the peripheral circulation, ultimately homing to regions of angiogenesis.[5]作者: 待鹤 时间: 2010-1-23 16:02