Model schematic illustrating the local and systemic contributions to angiogenesis. The human body is modeled as a series of organ compartments connected in an anatomic manner. The local contribution consists of the proliferation and migration of ECs (orange cells) in response to the local imbalance between angiogenesis stimulators and inhibitors. The angiogenesis stimulators produced by the tumor initiate the systemic pathway as well, which involves the mobilization of EPCs from the bone marrow. EPCs (blue cells) can localize in tumor vessels and contribute to angiogenesis.