Angiogenesis

Angiogenesis is the formation of new capillaries that sprout from existing blood vessels. It differs from vasculogenesis, the assembly of the first vessels from precursor cells in the embryo. When tissue runs short of oxygen, its cells release signals, chiefly vascular endothelial growth factor (VEGF). The endothelial cells lining nearby vessels then digest the surrounding matrix, migrate and divide to form a sprout, which hollows into a tube, joins other vessels and is stabilized by supporting cells called pericytes.

In healthy adults most vessels are quiet, held in check by a balance of stimulating and inhibiting signals. Angiogenesis switches on where needed: in wound healing, in the regrowing uterine lining each menstrual cycle, in the placenta and in muscle with endurance training. Too little contributes to poor healing and ischemic disease, while abnormal vessel growth drives wet age-related macular degeneration and diabetic eye disease.

Tumors depend on it. A cluster of cancer cells cannot grow beyond a few millimeters without its own blood supply, so tumors tip the balance by releasing pro-angiogenic factors, a change known as the angiogenic switch. The new, leaky vessels feed the tumor and give its cells routes to spread. Angiogenesis inhibitors, such as antibodies against VEGF and drugs that block its receptors, are used to treat several cancers and eye diseases.