Model of vascularization as observed in the human MDA MB231 breast cancer cell tumor grown as a xenograph in a female nude mouse. The well vascularized tumor capsule overlies the tumor cortex. The cortex extends between 100 and 150 μm beneath the capsule. The HIF-α positive region is located in the subcortical region, at distances greater than 150 μm of the capsule. More endothelial pseudopods are located in this subcortical region than are located in the cortical region of the tumor. Even further away from the vascularized capsule is the region of tumor necrosis which contains no blood vessels or endothelial cell pseudopods. This model is consistent with the conclusion that the hypoxic regions of the tumor produce HIF leading to production of vascular endothelial growth factors and sprouting of endothelial cell pseudopods.