Expression of muscle-specific actins and myosin in light microscopically undifferentiated small and dark cell malignancies of soft tissues.

Eighteen light microscopically undifferentiated small and dark cell malignancies, previously studied ultrastructurally and immunohistochemically in terms of desmin, vimentin and myoglobin expression, were analyzed using mono- and polyclonal antibodies to muscle-specific isoforms of myosin and actin. For comparison, 10 characteristic rhabdomyosarcomas, 5 alveolar and 5 embryonal, were included in the study. A polyclonal antibody to skeletal myosin produced an indistinct staining in all the alveolar and embryonal rhabdomyosarcomas. The staining was most prominent in well-differentiated rhabdomyoblastic tumor cells. In the analysis of the small and dark cell malignancies, this antibody produced a weak and indistinct positivity which can not be interpreted with certainty as an expression of muscle-specific properties. An antibody directed at the alpha and gamma isoforms of actin, which are present in smooth and striated muscle, produced a distinct positive staining in all the alveolar and embryonal rhabdomyosarcomas and in 8/18 small and dark cell malignancies, 7 of which were also shown to express desmin. An antibody directed at the alfa-smooth muscle isoform of actin did not produce any positive staining in any of the tumors. The present study indicates that both muscle-specific actin and desmin can be expressed in tumors lacking ultrastructural evidence of a rhabdomyoblastic differentiation and that the combined use of monoclonal antibodies to desmin and muscle-specific actin is of value when it comes to recognizing rhabdomyosarcomas within the group of undifferentiated small and dark cell malignancies of soft tissue tumors.