Antigen-induced arthritis in the rabbit: ultrastructural changes at the chondrosynovial junction

Structural changes at the chondrosynovial junction of the lateral border of the lateral femoral condyle have been studied by electron microscopy in rabbits with antigen-induced arthritis of 6 h-27 days duration. Rapid changes in the collagen fibrils of the extracellular matrix in the synovial lining and articular cartilage were noted. Collagen fibrils with unusually large diameters were observed. Overgrowth of cartilage by inflamed synovium was seen within 3-6 days of induction of arthritis and by day 12 the interface between these two tissues was largely indistinguishable. The synovial pannus at this time was fibrotic and infiltrated with plasma cells, lymphocytes and macrophages. Few polymorphonuclear leucocytes were found in the developing pannus. Macrophages were found with extended processes which enveloped neighbouring cells. Some blood vessels had thickened endothelial cells though lymphocytes were not observed in their vicinity. This study reveals the rapidity with which synovial pannus can develop and suggests that there are a number of mechanisms operating to cause cartilage breakdown in antigen-induced arthritis.