(A) SO staining of WT and Mmp13
−/− fracture callus at 7, 10, 14, and 21 days post-fracture shows that cartilage persists in the Mmp13
−/− callus from 14 through 21 days post-fracture. Scale bar = 1 mm (B) Histomorphometric measurements of total callus volume (TV), total cartilage volume (CV) and total cartilage volume as a proportion of total callus volume (CV/TV) in WT and Mmp13
−/− mice at day 7 (WT n = 6, Mmp13
−/−
n = 6), 10 (WT n = 8, Mmp13
−/−
n = 6), 14 (WT n = 8, Mmp13
−/−
n = 6), 21 (WT n = 6, Mmp13
−/−
n = 6) and 28 (WT n = 6, Mmp13
−/−
n = 6). There is a statistically significant increase in total cartilage volume in Mmp13
−/− calluses compared with WT at day 7 (**p<0.01), 14 (*p<0.05) and 21 (*p<0.05). There is a statistically significant increase in total cartilage volume as a proportion of total callus volume in Mmp13
−/− calluses compared with WT at day 7 (**p<0.01), 14 (*p<0.05) and 21 (*p<0.05). Wilcoxon test, bars represent means ± S.D. At 21 days post-fracture, all Mmp13−/− calluses contained cartilage as compared to 1/3 of WT. At 28 days post-fracture, only Mmp13−/− calluses (1/3) still contained cartilage.