Tumors cells need a rich blood supply in order to grow and metastasize. Angiogenesis (Angio-blood, genesis-creation) is the process by which new blood vessels, called capillaries are formed. Capillaries are lined with endothelial cells.

Normal angiogenesis occurs under very tight physiological regulation when stimulators and inhibitors work in balance with each other. Normally the proliferation rate of endothelial cells is very low, with turnover times for these cells exceeding 1000 days.

Exceptions occur in which the angiogenesis process takes approximately 5-8 days. This is seen in wound healing, embryonic development, the monthly growth of the uterine lining in menstruation women, and also in cancer.

The following link, courtesy of University of Brescia, Italy and Associazione Italiana per la Ricerca sul Cancro, provides a brief introduction to the angiognesis process. Angiogenesis: A Brief Introduction.