Early ultrastructural changes of articular cartilage and synovial membrane in experimental vitamin A-induced osteoarthritis.

Objective. To evaluate the sequential ultrastructural changes of the articular cartilage and synovial membrane in the earliest phases of the vitamin A model of osteoarthritis (OA) in the rabbit. Methods. The superficial layer of the weight bearing zone of the articular cartilage and the synovial membrane from femorotibial joints of 12 osteoarthritic rabbits were evaluated 3, 6, and 9 days after the triggering intraarticular injection of 100,000 IU of retinol palmitate. Four uninjected rabbits were used as controls. Results. At 3 days, ultrastructural changes of chondrocytes could be seen (hypertrophic cells with increased lipid droplets, chondrocytes rich in microfilaments and glycogen, and some degenerating cells) with no evident lesions of the matrix. The synovium was similar to that of the control rabbits. At 6 days, chondrocyte changes seemed almost identical to those of 3 days, while the synovial membrane appeared markedly involved, substituted by a single layer of A-type cells lying on fibrous subsynovial tissue in which lymphocytes, mast cells, and blood vessels could be seen. Conspicuous alterations and necrobiosis of the cartilaginous cells characterized later stages (9 days). The intercellular matrix was mainly made up of amorphous material and bundles of collagen fibers. The synovial membrane was transformed into a thick fibrous tissue partially covered with scattered cells no longer distinguishable as A or B type. Conclusion. Our data suggest that in the vitamin A model of OA the initial metabolic changes of the chondrocytes have a pivotal role in determining the relentless cascade of events leading to the full expression of this disease. Moreover, although several studies have been carried out on the early changes in experimental OA, no complete morphological evaluations on the developmental aspect of this model have been available.