Regression of immature blood vessels in a xenografted glioma tumor. (a) A tumor grown under conditions of constitutive high-VEGF expression showing a mixture of α-SMA–positive and α-SMA–negative blood vessels. The α-SMA–negative vessel (arrow) is shown at a higher magnification in the inset (counterstained with H&E) to highlight the integrity of its endothelium. (b and c) A tumor 72 h after VEGF withdrawal. Both covered (right arrows) and uncovered (left arrows) vessels are still observed. However, the uncovered vessel shows clear evidence of disintegration (better seen in c depicting the same vessel in a serial section counterstained with H&E). (d) A tumor 5 days after VEGF withdrawal. Note that most surviving vessels are α-SMA–positive. (e) VMIs were determined, as described in Methods, in high-power fields of sections obtained either before or 5 days after VEGF withdrawal (scoring 270 or 87 vessels, respectively). Calculated VMIs were 0.30 (SEM = 0.04) and 0.94 (SEM = 0.05), respectively. H&E, hematoxylin and eosin; SMA, smooth muscle actin; VEGF, vascular endothelial growth factor; VMI, vessel maturation indices.