Tumor Angiogenesis

A tumor consists of a rapidly dividing and growing population of cells that have lost their ability to divide in a controlled fash-ion. Once a tumor lesion exceeds a few millimeters in diam-eter, hypoxia and nutrient deprivation trigger an angiogenic switch that allows the tumor to receive oxygen and nutrients supply from the bloodstream to sustain its growth.1 To achieve this, tumor cells exploit their microenvironment by releas-ing cytokines and growth factors to activate normal, quies-cent cells around them and initiate a cascade of events. These events include the formation of new blood vessels, a process called tumor angiogenesis, as well as a recruitment of new tumor-associated cells such as macrophages and fibroblasts.2–4 Both tumor cells and tumor-associated cells contribute to the vascularization by activating and recruiting endothelial cells within blood vessels that are close to the tumor microenvi