The Role of Vascular Endothelial Growth Factor in Tumor Angiogenesis

Angiogenesis, the sprouting of capillaries from pre-existing vessels, is of fundamental importance during embryonic vascular development and during certain physiological processes in the adult organism, for example menstruation, pregnancy, or wound healing. Moreover, angiogenesis plays a major role in the pathogenesis of several diseases, such as proliferative retinopathy, psoriasis, or solid tumor growth (Folkman, 1995). The growth of solid tumors is largely dependent on the supply of oxygen and nutrients from the blood stream. Small tumors of less than 1–2 mm in diameter are not vascularized, and they can be nourished by simple diffusion. Such avascular tumors do not induce neovascularization, and they rarely metastasize. However, further tumor growth is dependent on a vascular network that is able to fulfill the demands of the growing tumor for oxygen and nutrients. Vascularized tumors induce host vessels to extend vascular sprouts. They have the potential to expand their cell population, and they may eventually metastasize (for review, see Folkman, 1995).