Osteophytes represent areas of new cartilage and bone formation in human and experimentally induced osteoarthritis (OA). The present study addressed the production of nitric oxide (NO), vascular endothelial growth factor (VEGF) and the occurrence of apoptosis during osteophyte formation.
Osteophytes in the knee joint of rabbits that developed OA-like lesions following anterior cruciate ligament transection (ACLT) were analysed by histology and immunohistochemistry for NO production, and the presence of VEGF. TUNEL was used to detect DNA fragmentation.
At the joint margins in the interface between cortical bone marrow and periosteal lining growth plate-like formations were detectable as early as 4 weeks after ACLT. By 12 weeks after ACLT osteophytes were visible in 100% of femoral condyles and tibial plateaus. Discrete areas with proliferating chondrocytes, hypertrophic chondrocytes, calcified matrix and vascular invasion were observed. VEGF immunoreactivity was most prominent in hypertrophic chondrocytes 9 weeks after ACLT. Nitrotyrosine immunoreactivity was detected in endothelial cells and in some hypertrophic chondrocytes in the calcified zone 4 weeks after ACLT. After 8 and 12 weeks, positive cells were detected in the hypertrophic and calcified zone. TUNEL-positive cells were seen in blood vessels, and among hypertrophic chondrocytes adjacent to the blood vessels 4 weeks after ACLT. The proliferative zone, pre-hypertrophic zone and hypertrophic zone showed only a few TUNEL positive cells. In contrast, 8 weeks and 12 weeks after ACLT, most hypertrophic chondrocytes, but few proliferative chondrocytes showed DNA fragmentation.
Hypertrophic chondrocytes in osteophytes express VEGF and this can promote vascular invasion of cartilage. The presence of TUNEL-positive cells shows a similar distribution as nitrotyrosine immunoreactivity during all phases of osteophyte development, suggesting that NO production and chondrocyte death are related events in osteophyte formation.