Representative histopathology of ankle joints from wild-type mice and mice deficient for MMP-13. (A), (B) Day 0 (before serum transfer): notice the even and clear joint space (js) and smooth articular cartilage (ac). (C), (D) Day 8 (post serum transfer): (C) wild-type joints display the fibrovascular synovial and periarticular proliferation (fp), erosion of ac, intra-articular exudates and osteolysis in severely affected joints (arrows), whereas (D) joints of mice deficient for matrix metalloproteinase-13 (MMP-13) are nearly free of inflammation and the synovium retains its relatively acellular composition (arrows). Images were taken at 40× and 100× magnification. KO, knockout.