Human Vascular Progenitor Cells

During the process of human embryonic development, blood islands develop alongside the endoderm, which segregate into individual hemangioblasts that are surrounded by flattened endothelial precursor cells. The hemangioblasts mature into the first blood cells, while the endothelial precursors develop into blood vessel endothelium. New vascular formation, termed vasculogenesis, takes place within the embryo. The role of blood cell production is taken over by a series of embryonic organs, such as the liver, spleen, thymus, and bone marrow. The bipotential hemangioblast produces the primitive erythroid and endothelial progenitor cells, and the hemogenic endothelium gives rise to hematopoietic stem cells and endothelial progenitors. This chapter discusses the ability of these progenitors of hematopoiesis (blood cell repopulation), vasculogenesis or angiogenesis. The role of human embryonic stem cells as a source for vascular progenitors is also discussed. Human embryoid bodies (hEBs) are formed by the spontaneous differentiation of hESCs, and comprise multilineage tissues from endodermal, ectodermal, and mesodermal origin. Several experimental procedures have been developed to explore the endothelial potential of hESCs. Undifferentiated human embryonic stem cells (hESCs) form teratomas once injected into severe combined immunodeficient (SCID) mice. During teratoma formation from hESCs, two parallel vascular processes occur: angiogenesis of host vasculature into the forming human teratoma; and vasculogenesis of spontaneously differentiating hESCs. In further studies of human vasculogenesis, various vascular markers should be examined and evaluated in the course of spontaneous differentiation.