An ultrastructural and morphometric study of articular cartilage in the knees of exercised aged rats

Regular physical activity has been promoted as an effective method to prevent functional decline in elders, but there is concern that an increase in physical activity may lead to the development of osteoarthritis in weight-bearing joints. An optimal level of synovial joint use will enhance and maintain articular cartilage structure, composition, and functions, but excessive loading may damage normal cartilage.; Articular cartilage which covers the articular ends of bones provides a shock absorber and load-bearing surface in a movable joint. The tissue is composed of cells, chondrocytes, which synthesize a matrix consisting of collagen and proteoglycans. Weight-bearing properties of the cartilage (including resistance to compression and distribution of load) depend mainly upon its proteoglycans. Joint-loading appears to have a significant effect on proteoglycan synthesis.; The effects of moderate aerobic exercise on chondrocyte ultrastructure were studied in the knee joint articular cartilage of young and aged rats (F1 Hybrids). Synthetic activity of the chondrocytes was quantified by measuring the amount of rough endoplasmic reticulum (RER) from electron micrographs using a stereologic point-counting method.; The superficial zone of articular cartilage in the young exercised rat (which is the surface layer) exhibited significant increases in volume of cells per tissue volume unit, surface area of RER per unit volume of chondrocyte cytoplasm and cartilage tissue, and volume density of RER. In the intermediate and deep zones of articular cartilage in both young and aged exercised rats, no significant differences in RER were detected. No signs of degeneration were observed in the exercised cartilage. However, in the cartilage of aged rats (sedentary and exercised), typical changes such as decreases in cartilage thickness and the sizes and amounts of proteoglycan matrix granules were observed.; The increase in RER activity in the superficial zone of young rats clearly shows that area of articular cartilage is very active in terms of responding to loads and synthesizing proteoglycans. Since no qualitative changes were seen at either the light or electron microscopy levels indicating any osteoarthritic conditions in both young and exercised rats, these observations suggest that moderate aerobic exercise does not cause degenerative joint disease.