Figure 1.
Morphological changes of the TMJ resembling osteoarthritis-like characteristics in Lubricin null (Prg4−/−) mice.
Top Panels: TMJ diagram indicating the different components of the TMJ. C: condyle, f: fossa, d: disc, uc: upper joint cavity, lc: lower joint cavity. Highlighted in blue the synovial membrane (sm). On the right, lubricin in situ hybridization shows localization of the Prg4 gene primarily in the synovial membrane and upper joint cavity in mice at postnatal day 1 (10× magnification). (A–F) Representative coronal sections of the TMJ of lubricin control (Prg4+/−) (A–C) and null (Prg4−/−) (D–F) mice at 2-, 6- and 9-months of age, stained with H&E. Prg4−/− articular surfaces are irregular (arrowheads), there is an increased number of chondrocytes (dark purple dots) in the condyle (c), increased chondrocyte clusters (red circles in E), decreased number of flat cells in the most superficial layer (arrows) of the condyle, disc (d) and fossa (f), and deposit of cartilage and cellular detritus over the different structures of the joint (brackets). Thickness and cellularity in the disc progressively increases in the Prg4−/− mice compared to Prg4+/− control mice. Pictures are taken with 20× magnification and insets are 40X.
Figure 2.
Synovial hypertrophy in Prg4−/− mice TMJs.
TMJ sections stained for H&E at 2- and 9-months old mice in control (Prg4+/−) and lubricin null mice (Prg4−/−). Synovial membranes in the upper joint cavity are indicated by arrows. Control Prg4+/− mice exhibit a thin synovial lining (A, C, E, and G) compared to the hypertrophied synovium (red bar) observed in Prg4−/− mice (B, D, F, and H) and characteristic of synovitis. An increase in the severity of synovitis is observed over time, as shown in 2-month-old Prg4−/− (B, F) versus 9-month-old Prg4−/− (F, H) mice. Villous digitations (arrowhead), cartilage debris surrounded by synovial membrane (open arrowhead) and detritus rich zones (*) are observed in 9-month-old Prg4−/− mice. These inflammatory changes were seen primarily in the upper joint cavity, located between the disc (d) and fossa (f). (A–D: 10X, E–H: 20X).
Figure 3.
Increased TMJ Osteoarthritis-like characteristics in Prg4−/− mice.
(A–D) Decreased extracellular matrix components in Prg4−/− mice. Paraffin embedded, formalin fixed TMJ sections at 6-months, histologically stained for Safranin O (SO) (A, B) and immunostained for aggrecan neopeptide (C, D) in Prg4+/− and Prg4−/− mice. A black dotted line separates the region of columnar chondrocytes in the condyle (below) from the proliferating chondrocytes (above). Note the abundant SO positive staining (red dots) in the proliferating zone in control Prg4+/- mice and the few positive SO staining in Prg4−/− null mice. As expected, the opposite is observed for aggrecan neopeptide immuno localization, which shows abundant aggregan neopeptide in proliferating chondrocytes, indicating that in Prg4−/− mice aggrecan degradation is increased. Insets in C and D correspond to a magnification of the field indicated in the red dotted line and highlight the extracellular localization of aggrecan neopeptide staining (arrows). (E–J) TRAP staining in 2-, 6- and 9- month TMJ sections. Black arrowheads show TRAP+ multinucleated cells (MNC) in resorptive areas of the condyles stained in red (magnified panel on right). (K) Quantitation of TRAP+ MNC at 2-, 6-, and 9- month-old mice showed an increase in osteoclastogenesis of 38.65%, 41.2%, and 45.8% respectively in Prg4−/− mice, compared to age-matched control mice. Student t-test: *, p<0.05; **, p<0.01. Dotted lines demarcate the surfaces of the condyle and fossa. c: condyle, d: disc, f: fossa.
Figure 4.
Osteoarthritis-like characteristics in TMJ and knee joints in Prg4−/− mice.
(A–D) Representative coronal sections of the TMJ and knee joints of lubricin null (Prg4−/−) mice at 2- and 9- months of age, stained with H&E. At 2 months, the articular surface of the TMJ condyle displays very few superficial flat chondrocytes, whereas, several superficial zone chondrocytes can still be observed at the cartilage surface in the knee joint. (A and B, arrowheads). In both joints, evidence of lightly stained protein deposition across the entire joint surfaces can be observed (Fig. 4 brackets). At 9 months of age, the cartilage surfaces of both joints display disease progression, chondrocytes are absent from the articular surfaces of both the knee and the TMJ, and the protein layer deposited on all surfaces is enlarged and disrupted (Figs. 4 C and D). In addition, in the TMJ there is a large infiltration of synoviocytes deposited on the surface of the condyle (Fig. 4C *). c: condyle, d: disc, fe: femur, t: tibia.