α-SMA and STRO-1 expression. (A-D) α-SMA; (E-H) STRO-1. (A,E) ACL from young normal knee; (B,F) ACL from aging knee; (C,G) fibroblast-like cell aggregates in the degenerated ACL; (D,H) chondrocyte-like cell aggregates in the degenerated ACL. Black arrows, positive cells; white arrows, negative cells. (Original magnification ×40). (I) The graph represents the percentage (mean ± SEM) of α-SMA- and STRO-1-positive cells. **P < 0.01.