Blood Vessels formation at the cellular level.
Hemangioblasts are the first vascular progenitor cells to appear in the posterior primitive streak. These progenitors become restricted to haematopoietic or angiogenic fate after migrating into extra-embryonic sites, namely, the yolk sac and allantois, and into intraembryonic sites in the mesoderm.
In the extra-embyonic sites, the cells give rise to either hematopoietic cells by aggregating to form blood islands that then fuse to generate a primary capillary plexus. The primary capillary plexus undergoes remodeling along with intra-embryonic vessels to form a mature circulatory system.
Intra-embryonic angioblasts, represent endothelial predecessors, and migrate as well and aggregate to form the dorsal aorta and cardinal vein.
Mural cells (pericytes and smooth-muscle cells) proliferate and differentiate in response to TGF-β signaling and are recruited to vessels by PDGF secreted by endothelial cells.
Vasculogenesis is the first stage of vascular development, in which a network of blood vessels is created. Angiogenesis comprises the second stage and involves remodeling of the primary vascular plexus into a network of large and small vessels arteries and veins.