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The myofibroblast: a study of normal, reactive and neoplastic tissues, with an emphasis on ultrastructure. part 2 - tumours and tumour-like lesions.This paper describes the ultrastructure of the commoner myofibroblastic tumours and tumour-like lesions. The objective is to complement mainstream pathology texts, which have concentrated on the clinical and light microscopy features of these lesions and which have arguably but understandably somewhat neglected electron microscopy as an ancillary diagnostic tool and a technique for investigating tumour cell biology. Ultrastructural features are described of nodular fasciitis, the myofibromatoses (including Dupuytren's disease), inflammatory myofibroblastic tumour, post-operative spindle cell nodule, fibroma of tendon sheath, fibrous pseudotumour, benign fibrous histiocytoma, atypical fibroxanthoma, dermatofibrosarcoma protuberans, myofibrosarcoma (myofibroblastic sarcoma), malignant fibrous histiocytoma (pleomorphic myofibrosarcoma), epithelioid sarcoma and spindle-cell carcinoma. Fibrosarcoma and leiomyosarcoma are illustrated for comparison. The fibronexus is emphasised as an important marker for the most confident diagnosis of myofibrosarcoma. Some pathologists accept a light microscope definition, which includes alpha-smooth-muscle actin positivity, h-caldesmon negativity and, in some cases, desmin positivity. Caution in the interpretation of desmin staining in a possible myofibroblastic lesion is urged, since, in combination with an ultrastructurally identified lamina, it more probably suggests true smooth-muscle differentiation. Myofibroblastoma and angiomyofibroblastoma are examples of tumours argued on the basis of ultrastructural findings (sometimes in combination with desmin staining) to be primitively differentiated smooth-muscle cell rather than myofibroblastic proliferations.
