Eventually, strings of new endothelial cells organize into hollow tubes, creating new networks of blood vessels that make tissue growth and repair possible. Most of the time endothelial cells lie dormant. But when needed, short bursts of blood vessel growth occur in localized parts of tissues. New capillary growth is tightly controlled by a finely tuned balance between factors that activate endothelial cell growth and those that inhibit it. At a critical point in the growth of a tumor, the tumor sends out signals to the nearby endothelial cells to activate new blood vessel growth.
Angiogenesis is also related to metastasis.
It is generally true that tumors with higher densities of blood vessels are more likely to metastasize and are correlated with poorer clinical outcomes. Also, the shedding of cells from the primary tumor begins only after the tumor has a full network of blood vessels. Strategies Of the anti-angiogenesis drugs now in clinical trials, some were designed to target specific molecules involved in new blood vessel formation. Several differences between standard chemotherapy and anti-angiogenesis therapy result from the fact that angiogenesis inhibitors target dividing endothelial cells rather than tumor cells. Also, since anti-angiogenic drugs may not necessarily kill tumors, but rather hold them in check indefinitely, the endpoint of early clinical trials may be different than for standard therapies.