All cells need a constant source of oxygen and nutrients such as glucose (sugar). Our cells get their nutrients delivered to them via the blood. Nutrients and oxygen are pumped through the body via the circulatory system. Once in the tissues, the nutrients cross the blood vessel walls and enter the spaces around the cells. Cells need nutrients constantly, and the process by which nutrients float over to cells takes time. In order to make sure that all cells get enough nutrients, our tissues are full of many small blood vessels (capillaries) that can deliver food to within a very short distance of any cell.
Even though cancer cells are abnormal, they still require oxygen and nutrients. The development of blood vessels is an essential step in the growth of a tumor. Without vessels tumors cannot grow to be larger than a small fraction of an inch.(1)(2)(3)(4)(5) When the area around the cells in a tumor starts to get too far from a blood vessel, the oxygen and nutrient levels start to go down. A decrease in oxygen is also called hypoxia. Hypoxia triggers changes in the behavior of the tumor cells.
The tumor cells produce (or cause nearby cells to produce) growth factors that stimulate the formation of blood vessels. Tumors that do not produce (or cause other cells to produce) angiogenesis factors can not grow.(2) One of the most well-studied angiogenesis factors is called vascular endothelial derived growth factor (VEGF). VEGF or other angiogenesis factors produced by tumor cells or nearby cells can cause the development of blood vessels that feed the growing tumor. Because VEGF is a normal signal for the cells forming the blood vessels, they are really just doing their job. The tumor 'tricks' the body into creating new blood vessels. The blood vessels created in this way are not exactly the same as normal blood vessels. Frequently they are less organized and leakier than normal vessels.
Abnormal angiogenesis is not limited to cancer. Other diseases, including macular degeneration, a progressive eye disease, are linked to abnormal development of blood vessels.(4) The process of angiogenesis in cancer is shown in the animation below.