Photomicrographs showing the histopathologic and immunohistochemical features of arthritis induced by peptidoglycan. (a) Normal histopathologic appearance of a mouse knee joint following injection with phosphate-buffered saline. (b) Histopathology of an arthritic knee joint of a mouse 3 days after intra-articular inoculation with peptidoglycan, showing infiltration of inflammatory cells in synovial tissue and granulomas within the synovium. (c) Immunohistochemistry of an arthritic knee joint, showing synovial expansion of Mac-1-expressing cells three days after intra-articular inoculation with peptidoglycan. JC, joint cavity; C, cartilage; ST, synovial tissue.