MMP-1, MMP-3, and MMP-13 expression. (A-D) MMP-1; (E-H) MMP-3; (I-L) MMP-13 (A,E,I) ACL from young normal knee; (B,F,J) ACL from aging knee; (C,G,K) fibroblast-like cell aggregates in the degenerated ACL; (D,H,L) chondrocyte-like cell aggregates in the degenerated ACL. (Original magnification ×40). The graph (M) represents the percentage (mean ± SEM) of MMP-1-, MMP-3-, and MMP-13-positive cells in each group. **P < 0.01. (N) Western blotting for MMP-1, MMP-3, and MMP-13 (n = 3 for each category).