By the third week of gestation the vascular system begins to develop with the formation of blood islands. The embryo's oxygen needs can no longer be met by diffusion alone.

Blood vessel formation begins in the outer wall of the yolk sac. The yok sac endoderm causes the induction of numerous blood islands in the splanchnic mesoderm. The blood islands contain and are founded by stem cells called hemangioblasts.

Hemangioblasts are bipotential stem cells and can either give rise to angioblast (endothelial cells) or hematopoietic (blood-forming) cells. However once a hemangioblasts commits to one of these lineages, it loses the capacity to form other cell types. (1)

Later hemangioblasts will migrate and hematopoiesis shifts to its adult location inside the embryo.

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Blood Islands on the area vasculosa of 33 hour chick embryo.

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Formation of vascular network in 56 hour chick embryo.

FORMATION OF CAPILLARIES, ARTERIES, AND VEINS

The formation and fusion of these hemangioblastic blood islands to form capillary networks is known as vasculogenesis.

New arteries and veins are formed through elongation and branching of existing vessels in a process called angiogenesis.

Angioblasts assemble into simple tubes during vasculogenesis. These tubes are elongated during angiogenesis.