Studies of polyarthritis caused by mycoplasma arthritidis in rats. III. Histopathological findings.

Mycoplasma arthritidis induced polyarthritis in the rat is characterized histologically by four distinct phases. Firstly, two days after infection, early lesions such as vascular changes and alterations in the lining cell layer of synovium are observed. This is followed four days after inoculation by an acute exudative phase with the deposit of fibrin and infiltration by polymorphonuclear granulocytes together with first signs of joint destruction by pannus-like granulation tissue. From about the third week onwards, lymphocytes and plasma cells predominate in the subsynovium, indicating subacute inflammation. In the fourth phase which lasts until the end of the one year observation period, chronic arthritis with destruction of cartilage and bone, ankylosis, and chronic inflammation, sometimes with acute recurrences, were observed in about 25% of the joints. Both the marked tendency to pannus formation and the longevity of the arthritis, make this an interesting, experimental model of chronic polyarthritis, particularly as it has been induced by a peptidoglycan-free microorganism.