Prg4 mutants display excessive chondrogenesis in the glenoid fossa and articular disc and increased Has2 expression. Serial parasagittal sections from 3-month-old control (A-E, K) and Prg4–/– (F-J) TMJs were processed for in situ hybridization with isotope-labeled riboprobes for Col II (A, B, F, G), Hyaluronic acid synthase 2 (Has2) (C-E, H-J), and Prg4 (K). Note the increased chondrogenesis in the Prg4–/–- glenoid fossa (F) and thickened disc (G) and increased Has2 expression in the glenoid fossa (H), disk (J), synovial membrane (J), and mandibular condyle (I). (L) Summary of abnormalities identified in TMJ lacking Prg4. Scale bars: 180 µm in A for A-J; 350 µm in K for K. gf, glenoid fossa; di, articular disc; co, mandibular condyle; sm, synovial membrane; sf, superficial layer.