Endothelial cell precursors in the avian embryo.

Whereas the origin and migration of endothelial cells (ECs) have been studi ed primarily in the avian embryo, the molecular mechanisms governing these events in birds and mammals were unraveled following the identification of specific growth factors and of their receptors. In particular, analytic and experimental studies of vascular endothelial growth factor (VEGF) and its receptors have provided significant insights into the developmental biology of the vascular system. VEGFR2 is the earliest marker expressed by EC prec ursors in chickens and mice. Based on the localization of VEGFR2-positive c ells in the avian embryo and on findings from clonal culture experiments, t wo types of EC precursors can be differentiated as early as the gastrulatio n stage, namely posterior mesoderm hemangioblasts capable of differentiatin g into both ECs and hematopoietic cells and anterior angioblasts capable of yielding only ECs.