Segregation of the embryonic vascular and hemopoietic systems

The origin of endothelial cells and their subsequent assembly into the prim ary vascular system have been mostly analyzed in the avian embryo. Followin g the discovery of specific growth factors and their cognate receptors, the molecular mechanisms underlying these processes have been unraveled in bot h birds and mammals. In particular, experimental studies of the angiogenic vascular endothelial growth factor (VEGF) and its receptors, carried out in both vertebrate classes, have provided significant insight into the develo pmental biology of endothelial cells. The VEGF receptor VEGFR2 is the earli est marker known to be expressed by endothelial precursor cells of avian an d mouse embryos. Based on the localization of VEGFR2(+) cells in the avian embryo and on clonal culture experiments, two types of endothelial precurso r cells can be distinguished from gastrulation stages onward: posterior mes odermal VEGFR2+ hemangioblasts, which have the capacity to differentiate in to endothelial and hemopoietic cells, and anterior VEGFR2+ angioblasts, whi ch can only give rise to endothelial cells.