Tumor cell–free Matrigel plug assay confirmed the antiangiogenic effect observed in Wnt1-GL261 tumors. (A) Immunohistochemistry staining for CD31/PECAM-1 (red) and hematoxylin counterstaining (blue) of Matrigel plug sections, supplemented with diluent (control), 400 µg/ml human Wnt1, or 200 µg/ml mouse Dkk1 recombinant proteins. Bar, 200 µm. (B) Quantification of CD31⁺ area in percentage in Matrigel plugs (n = 6 plugs/group, 8 representative pictures/plug; **, P < 0.001). Error bars indicate SEM.