Things have slowed down a bit around here, so it’s time to get back to the Hallmarks of Cancer. Hallmarks 1-4 have been discussed and can be found here, here, here and here, which now brings us to Hallmark #5, which is called “inducing angiogenesis.” 
Every human cell needs oxygen to live. Oxygen is brought into the body through breathing atmospheric air. Oxygen rich blood is transported away from the heart to tissues via big arteries that taper down into smaller vessels called arterioles in the tissue. The oxygen diffuses out of the arteriole and into the tissue. This diffusion of oxygen is only good for spaces of a maximum of 1 mm which is quite tiny as you can see in this picture. 1 mm is the smallest gem on the left. Thus, this means that each tissue in the human body has an intricate network of vessels, both big and small that efficiently deliver oxygen to an area. The only time a normal human body induces angiogenesis is during wound healing and menstruation. This, like all other processes we have discussed, is tightly regulated, and even then angiogenesis is only switched on transiently.
Cancer has found ways to turn angiogenesis on all of the time. Cancer has to induce angiogenesis because of the oxygen diffusion issue. A clump of cancer cells that exceeds 1 mm will not get enough oxygen through diffusion alone. The cancer is not normal tissue and it is not orderly, thus it won’t have blood vessels efficiently delivering oxygen to it, so it must induce the body to make new blood vessels to get the oxygen it needs. Just like there are growth factors for cells, there are growth factors that regulate angiogenesis. When a tumor exceeds 1 mm, it becomes oxygen starved (hypoxic) and starts sending out pro-angiogenesis signals to have nearby cells start making new blood vessels. New blood vessels are formed, but they are disorganized, messy, convoluted and leaky. This is why tumors are “bloody”. This permanent state of angiogenesis in tumors is a perversion of an orderly physiologic process and it sustains the ever enlarging tumor.
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