VASCULOGENESIS

Induction by fibroblast growth factors of mesoderm during gastrulation leads to blood-forming tissue, including angioblasts and hemopoietic cells, that together constitute the blood islands of the yolk sac. The differentiation of angioblasts from mesoderm and the formation of pri mitive blood vessels from angioblasts at or near the site of their ori gin are the two distinct steps during the onset of vascularization tha t are defined as vasculogenesis. Vascular endothelial growth factor an d its high-affinity receptor tyrosine kinase flk-1 represent a paracri ne signaling system crucial for the differentiation of endothelial cel ls and the development of the vascular system. Specific cell adhesion molecules such as VE-cadherin and PECAM-1 (CD-31), and transcription f actors such as ets-1, as well as mechanical forces and vascular regres sion and remodeling are involved in the subsequent events of endotheli al cell differentiation, apoptosis, and angiogenesis.