Synovium of untreated Tg197 mice at age 8 weeks. (a) Proliferation of the synovial tissue (syn) in the articular cavity (ac) showing macrophage-like type A synoviocytes (sa) with thin cytoplasmic protrusions (arrow) invading the articular surface (as), and closely packed secretory type B synoviocytes (sb) seen in the superficial layer of pannus. (b) The synovial membrane was lined by closely packed, elongated (arrow) or rounded synoviocytes with infiltrating cells (arrowhead) present under the synovium. (c) Degenerating synoviocyte with disintegrated nuclei (arrow) and vacuolated (v) cytoplasm along with disrupted collagen fibres (arrowhead) randomly seen in the synovium. (d) Adherent-type junction (arrow) sealing two synoviocytes with fibrin (f) between them. (e) Synoviocytes appeared flattened, and partially degranulated mast cells (arrow) are seen under the synovium. N = 4 joints; mean percentage of degenerating synoviocytes = 80% of total cells. Bars = 5 μm. c, chondrocytes; N, nucleus.