Mast cells in early stages of antigen-induced arthritis in rat knee joints

The occurrence of mast cells has been investigated in inflamed and control knee joints of rats suffering from antigen-induced arthritis, an animal model of rheumatoid arthritis in man. Rats were immunized with methylated bovine serum albumin (mBSA) followed by an intra-articular injection of mBSA (arthritis, right joints) or saline (control, left joints). Rats developed severe acute synovitis associated with cartilage erosion in the arthritic joints, whereas control joints did not show any noticeable changes. Mast cells were counted in synovial and adjacent tissues in cryostat sections of whole knee joints stained with Toluidine Blue O. The area of the synovium of each knee joint was determined using survey photomicrographs and a morphometer. Both total numbers of mast cells and frequency of mast cells in inflamed synovia were decreased after 1 day after induction of arthritis. The frequency of mast cells remained decreased up to 14 days after induction of arthritis. Morphological indications for degranulation of mast cells were never found in inflamed joints. It is concluded, therefore, that mast cells do not play a significant role in the inflammatory process during the early phase of arthritis.