The formation of new vessels during vasculogenesis and angiogenesis. Vasculogenesis, the de novo organization of ECs into vessels in the absence of preexisting vascular structures, takes place during embryogenesis in the blood islands of the yolk sac (pictured) and in the embryo through expression of growth factors, in particular fibroblast growth factor (FGF) and vascular endothelial growth factor (VEGF). The tyrosine receptor kinases, VEGFR-1 (flk-1) and VEGFR-2 (flt-1), are expressed on mesenchymal cells and newly formed ECs, respectively, and are essential for the generation and proliferation of new ECs and the formation of tubal EC structures. Angiogenesis, the continued expansion of the vascular tree, is mediated through the expression of additional tyrosine kinase receptors, tie-2 (tek), which binds to Ang1 and Ang2 (angiopoietins), resulting in the maintenance of mature vessels, the development of new vessels, and the regression of formed vessels in processes dependent on a combination of factors, most notably the presence or absence of growth factors.