Abstract
143 Sprague Dawley rats, ranging from 19-63 weeks of age, were examined pathologically for the occurrence of spontaneous joint lesions. Macroscopically, a white round plaque was observed in the caudocentral portion of the medial femoral condyle or humeral head and sometimes it had a partially detached margin. Microscopic examination revealed a local thickening of the articular cartilage protruding into the subchondral bone, which suggested cessation of endochondral ossification and retention of cartilage. As the lesion advanced there was fissure formation in the thickened cartilage and proliferation of fibrous tissue beneath the fissure. Formation of a cartilage flap and extensive fibrosis with large cysts were also observed in the most advanced cases. Total incidence of the lesion in the distal femur was 22·2% in male and 2·8% in female rats. The change detected in the rats was identical with pathologic descriptions of osteochondrosis in domestic animals.