Capillaries--the tiny vessels that form the interface between the arterial and venous circulations--extend into all the tissues of the body, replenishing nutrients and carrying off waste products. Under most conditions, capillaries do not increase in size or number because the endothelial cells that line these vessels do not divide. But in certain circumstances, such as during menstruation or in wound repair, they do. This proliferation of endothelial cells, causing the formation of new capillaries, is called angiogenesis or neovascularization. It is typically tightly controlled and short-lived, usually turning off one to three weeks after its function has been accomplished.
Judah Folkman first hypothesized in the 1960s that angiogenesis is also integral to the complex biology that enables and encourages the growth of tumors and other forms of cancer. Folkman has spent the last four decades validating this hypothesis, beginning with a publication in the New England Journal of Medicine in 1971. In this paper, he proposed the revolutionary concept that tumors are unable to grow beyond a certain size unless they have a dedicated blood supply, and that successful tumors secrete an unknown substance (which he then called tumor angiogenesis factor, or TAF) that encourages new blood vessel growth. The process of angiogenesis, Folkman argued, helps transform a tumor from a small cluster of mutated cells to a large, malignant growth. In the years since, Folkman and scores of other researchers have demonstrated that not just one, but many such factors exist, and that they stimulate neovascularization.