Tumor vascularity is reduced in the absence of MMP9 in the C3(1)-Tag model. (A) The CD31+ endothelial cell area was significantly decreased in C3(1)-Tag;Mmp9−/− tumors (P = .01, t test), while (B) the relative vascular coverage by αSMA-positive pericytes was not altered, as determined by double immunofluorescent staining using antibodies against CD31 and αSMA. For each genotype, eight tumors were stained and quantified. (C) Areas of necrosis were observed in Masson’s trichrome–stained tumor sections from C3-Tag;Mmp9+/+ and C3-Tag;Mmp9−/− mice. Representative examples of tumor sections are shown, with total tumor area (red line) and total necrotic area (black line and marked by black arrows) indicated. Scale bar, 2 mm. (D) A non-significant trend for increased necrosis area was observed in C3(1)-Tag;Mmp9−/− (n = 12) compared to C3(1)-Tag;Mmp9+/+ (n = 15) mice by quantification of necrotic areas (P = .1, t test).