An electron microscopic study of the synovial-bone junction in rheumatoid arthritis.

The cellular characteristics of the rheumatoid synovial-bone junction (SBJ) were examined in the electron microscope. Large numbers of mononuclear inflammatory cells along the bone suggested the advance of the rheumatoid inflammatory lesion into bone tissue. The SBJ contained macrophage-rich and osteoclast-rich areas. The presence of the macrophage as a predominant cell type and the marked collagenolysis of the eroded bone matrix suggested that collagenase released by macrophages was responsible for a major portion of the bone erosion. The observation of osteoclasts with ruffled borders, containing free bone crystals in close contact with the bone, indicated that these osteoclasts were stimulated by the rheumatoid synovium. It seems likely that articular bone resorption depends upon the local host response to the macrophage and osteoclast in the rheumatoid joint.