We studied the results of immunostaining for S-100 protein, alpha-smooth muscle actin, muscle specific actin and desmin in articular cartilage specimens obtained during an arthroscopy from eight patients with different degrees of osteoarthritis of the knee joint. In all cases, most of the cartilage cells were strongly positive for S-100 protein. Actin positive chondrocytes were present in four samples showing repair cartilage changes with occurrence of fibrocartilage tissue. Moreover, in one case, we observed typical desmin-positive chondrocytes in the layer of cartilage filling the defect of the articular cartilage surface. The expression of desmin can be regarded as a reaction to trauma or the indication of an inherent abnormality. The chondrocytes probably switched on smooth muscle features during the healing process, because desmin is to a great extent a typical muscle cell marker. This fact could probably support our previous supposition that cartilage cells expressing muscle markers could be designated as myochondroblasts and myochondrocytes analogously to the terminology of myofibroblasts. It is possible that during the healing of the cartilage defects, such a transformation of the immunophenotype of the cartilage cells is quite frequent, but it could also be only transient nature only

        Key words: articular cartilage – osteochondritis – actin-positive chondrocytes – desmin positive chondrocytes – myochondrocytes