VEGF enhances angiogenesis during bone formation elicited by MDSCs expressing BMP4. (a and b) CD31 immune staining shows more active capillaries (arrows) branching from the surrounding muscle and growing toward the scaffolds (S) in the BMP4+VEGF group than in the BMP4 group at 4 days PI. (c and d) An extensive network of capillaries (arrows) formed inside the scaffolds of the BMP4+VEGF group, while only a modest level of capillaries formed in the scaffolds of the BMP4 group, at 10 days. (e) Histomorphometry demonstrates that the relative capillary density was significantly higher in the BMP4+VEGF group than in the BMP4 group (*P < 0.01) at 10 days. Magnification: a and b, ×400; c and d, ×100.