Angiogenesis: The process of developing new blood vessels. Angiogenesis is important in the normal development of the embryo and fetus. It also appears important to tumor formation. Certain proteins, including angiostatin and endostatin, secreted by tumors work (at least in mice) by interfering with the blood supply tumors need. Angiostatin is a piece of a larger and very common protein, plasminogen, that the body uses in blood clotting. Endostatin is a piece of a different protein, collagen 18, that is in all blood vessels.