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Various osteogenic tumours and bone producing tumour-like lesions of bone were examined for S-100 protein using the immunostaining methods. The positive reaction for S-100 protein of some bone cells was detected not only in osteoblastic osteosarcomas and osteoblastomas, but also in osteoplastic reactive lesions, fibrous dysplasia, Paget's disease and in the tissue of the bony callus. The positive reaction for osteocalcin in these cells showed, that they may be of osteoblastic and osteocytic lineage. The S-100 protein positive bone cells have to be differentiated from chondrocytes persisting in new-formed bone trabecullae. On the one hand S-100 protein positive osteocytes and osteoblasts may represent transient form of osteocytes histogenetically related to chondrocytes, but we were not able to prove such suggestion. Therefore, S-100 protein positivity can be caused by polyclonal character of the used antibody, that is a mixture of three antibodies, S-100 A1, A6 and anti B2. The specificity of these three components differs depending on the histogenesis of cells and their functional status. Such explanation is supported by the results of our study, because we also observed intense positivity of bone cells in the reaction with monoclonal antibody against S-100A6. In contrast, no staining of bone cells was detected with monoclonal antibodies against S-100 A1 and B2.

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.