Horizontally oriented clusters of multiple chondrons in the superficial zone of ankle, but not knee articular cartilage

Osteoarthritis is a progressive disease that is initiated at the surface of articular cartilage and proceeds to destroy the entire depth of the cartilage. The prevalence of osteoarthritis varies in different joints; e.g., the ankle joint has a very low prevalence of the disease compared to the knee joint. To better understand any inherent differences between the articular cartilage of the ankle and that of the knee that would account for the difference in occurrence of osteoarthritis, studies were undertaken to examine differences between the superficial zones in these two joint cartilages obtained from human donors. Chondrocytes in the superficial zones of the normal ankle (talocrural) and the normal knee (tibiofemoral) joints were identified with a monoclonal antibody specific for the superficial zone protein (SZP). When the chondrocytes from both joints were compared in serial horizontal sections, the chondrocytes in the superficial zone of the knee cartilage were seen either as isolated single cells or as doublets. However, the chondrocytes within the superficial zone of normal ankle cartilage were arranged in planar clusters containing multiple chondrons composed of 2-13 cells. There were no detectable differences in the chondrocyte clusters in the superficial zone of the ankle with respect to age, gender, or site on the cartilage surface. Adjacent to a lesion in an ankle joint with degenerative changes, the clusters were larger, containing up to 22 chondrocytes. This is the first report documenting the presence of multiple chondrons in the superficial zone of normal human adult articular cartilage.