The role of the synovium in repairing cartilage defects

The purpose of this study was to examine the role of the synovium in the transitional zone between the articular cartilage and the synovial membrane in cartilage repair and the relationship between the origin of the repaired cartilage and the grafted synovium. We used 8-week-old Sprague Dawley (SD) rats and green fluorescent protein (GFP) transgenic rats. In study 1, a full-thickness cartilage defect was created at the medial condyle of the femur, and the synovium 5 × 5 mm extending up to the cartilage defect was resected in the left knee (cartilage defect without synovium group) but not resected in the right knee (cartilage defect with intact synovium group). In study 2, after the creation of a full-thickness cartilage defect and resection of the synovium, the synovium of the GFP rats was transplanted into the unilateral knee (cartilage defect with transpl.synovium group). At 2, 4, 6, and 8 weeks after surgery, we evaluated the repaired tissue in cartilage defects histologically and immunohistochemically, and the expression of aggrecan and type II collagen in the repaired tissue was also investigated using reverse transcriptase-polymerase chain reactions (RT-PCR). At 6 and 8 weeks after surgery, the defect was filled with cartilage-like tissue in cartilage defect with intact synovium group and cartilage defect with transpl.synovium group, but not in cartilage defect without synovium group. GFP positive cells were observed in the repaired tissue and the expression of aggrecan and type II collagen was found in cartilage defect with transpl.synovium group. We concluded that the synovium in the transitional zone between the articular cartilage and the grafted synovial membrane invades the cartilage defects where the cells could be detected as GFP-positive cells. Those cells may take part in the repair and may induce chondrogenesis.