What are the mechanisms of blood vessel formation?

New blood vessels are formed by two main processes: angiogenesis and vasculogenesis. Vasculogenesis is the in situ differentiation and growth of blood vessels from mesoderm derived hemangioblasts, leading to the formation of the circulatory system. The circulatory system, or vascular network, is formed through differentiation and the migratory precursors of endothelial cells and smooth muscle cells. Angiogenesis is the process in which new blood vessels are formed from pre-existing blood vessels, permitting the delivery of nutrients and oxygen to the body’s tissues. In vasculogenesis, new blood cells are derived from hemangioblasts, which differentiate into angioblast cells. Angioblasts are cells that are not fully matured into endothelial cells. In contrast, during angiogenesis new capillaries are formed from pre-existing blood vessels, which are lined with endothelial cells. These endothelial cells multiply in number to permit the new capillaries to form. 

A third mechanism for blood vessel formation is arteriogenesis. The term was established to differentiate the formation of collateral arteries from angiogenesis, which primarily occurs in the ischemic flow-dependent tissue. Artergionies is the process of the remodeling of pre-existing anastomoses to mature arteries. During development, it is joined by the migration of smooth muscle cells and pericytes from the epicardium. These vessels can grow significantly, enough to take over the role of a large artery when blocked.