VEGF enhances endochondral bone formation elicited by MDSCs expressing BMP4. (a) Radiograph shows augmented bone formation in the BMP4+VEGF (B4+Ve) site compared with the BMP4 (B4) site 4 weeks PI. (b) Quantitative analysis shows that the BMP4+VEGF group produced significantly more bone than the BMP4 group (*P < 0.01, n = 4). (c–f) Alcian Blue staining shows enhanced cartilage formation (purple) at 7 days and accelerated cartilage resorption at 10 days in the BMP4+VEGF group compared with the BMP4 group. (g–j) von Kossa staining shows more extensive cartilage mineralization (black) at 10 days and increased mineralized bone formation (black) at 14 days in the BMP4+VEGF group compared with the BMP4 group. Magnification: c, d, and g–j, ×100; e and f, ×200.