Chondrogenic EGFR deficiency accelerates PTOA progression in adult mice. (A) WT and Egfr iCKOAcanER mice received tamoxifen injections right before 6 N loading at 2 months of age. Their knee joints were harvested 2 weeks and 1 month later. H, harvest; T, tamoxifen injection. (B) Immunostaining of p-EGFR in uninjured articular cartilage from WT and Egfr iCKOAcanER mice at 2 weeks after loading. Scale bars, 50 μm. (C) Safranin-O/Fast green-stained sections of WT and Egfr iCKOAcanER cartilage at indicated time points. Scale bars, 100 μm. (D) Cartilage injury length was quantified (n = 6/group). (E) Mankin score of WT and Egfr iCKOAcanER mouse joints at 1 month after loading was quantified (n = 6/group). (F) H&E-stained sections of WT and Egfr iCKOAcanER synovium at indicated time points. Scale bars, 50 μm. (G) Synovitis score was quantified (n = 6/group). (H) Representative safranin-O/Fast green-stained sections of WT and Egfr iCKOAcanER joints at 1 month after loading. Arrows point to heterotopic ossification area in synovium. Scale bars, 100 μm. F, femur; M, meniscus; S, synovium; T, tibia. The comparisons were conducted by two-way ANOVA with Tukey–Kramer multiple comparison test.