The expression of alpha smooth muscle actin, muscle specific actin, desmin, h-caldesmon, and calponin was studied immunohistochemically in the following soft tissue and bone tumours and tumour-like lesions: muscle fibromatosis, inflammatory pseudotumours, chondroblastoma, enchondroma, chondrosarcoma, fibrous dysplasia, ossifying myositis, osteoblastoma, convential osteosarcoma, leiomyoma and leiomyosarcoma. Tumours and tumour-like lesions with myofibroblastic cells, osteoblasts and chondroblasts frequently exhibited intensive immunoreactivity for the muscle markers, and therefore, some of them may occasionally be confused with leiomyoma and leiomyosarcoma. Calponin does not help to differentiate various mesenchymal tumours expressing muscle markers, because it also stains intensively myofibroblasts, osteoblasts and chondroblasts. We confirmed that h-caldesmon was expressed intensely in leiomyomas and leiomyosarcomas, and never in the other tumours examined, with the exception of three chondroblastomas. The results have shown that h-caldesmon is a rather specific and sensitive marker for smooth muscle tumours, but it can also stain some actin positive myochondroblasts. It is possible that the positivity of h-caldesmon in some chondroblastomas is due to their complete myogenic transdifferentiation, and so we use the term myochondroblasts and myochondrocytes for designation of such S-100 protein, actin, and h-caldesmon positive cells.

        Key words: actin – calponin – h-caldesmon – mesenchymal tumours – bone – soft tissue – myochondroblasts