The walls of blood vessels are formed by vascular endothelial cells. These cells rarely divide, doing so only about once every 3 years on average. However, when the situation requires it, angiogenesis can stimulate them to divide.

Angiogenesis is regulated by both activator and inhibitor molecules. Normally, the inhibitors predominate, blocking growth. Should a need for new blood vessels arise, angiogenesis activators increase in number and inhibitors decrease. This prompts the growth and division of vascular endothelial cells and, ultimately, the formation of new blood vessels.

Before the 1960s, cancer researchers believed that the blood supply reached tumors simply because pre-existing blood vessels dilated. But later experiments showed that angiogenesis--the growth of the new blood vessels--is necessary for cancerous tumors to keep growing and spreading.

Source: National Cancer Institute www.cancer.gov