Abstract
Blood vessel formation in tumor is defined as tumor angiogenesis. So far, the most known its mechanism is sprouting, which means formation of blood vessels from existing ones, as a result of the proliferation and migration of endothelial cells. The main mitogenic factor of these cells is vascular endothelial growth factor VEGF, acting by VEGFR-2 receptors. Recent studies have provided knowledge about the ability of tumors to form vessel-like structures. The phenomenon was called vascular mimicry. Tumor cells show a high plasticity and they can undergo differentiation to the ones with phenotype similar to endothelial cells. Each of the known tumor angiogenesis mechanisms is a result of many different factors and cell cooperation in tumor microenvironment. Tumor ability to the heterogeneous vascularization forces developing of complex, anti-angiogenic therapy directed to different molecular and cellular targets. Therapies, used so far, often lead to drug-induced hypoxia, which increases tumor cell aggressiveness and metastasis.