Collagen types I, II, and III. (A-D) Collagen type I, (E-H) collagen type II, (I-L) collagen type III (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. Most collagen bundles were type I collagen positive (A-D), however, staining intensity of the ECM around chondrocyte-like cell aggregates was lower (black arrows). Type II collagen-positive area is observed around chondrocyte-like cell aggregates (white arrows). In the normal ACL, type III collagen is located within the loose connective tissue (black arrowheads) that divides the collagen fibrils of the ligament into small bundles but not dense collagenous tissues. In the degenerated ACL, type III collagen (white arrowheads). (Original magnification ×40).