Immunophenotype and ultrastructure of alveolar soft part sarcoma

Nine cases of alveolar soft part sarcoma were studied in terms of their immunoreactivity towards intermediate filaments and muscle markers, and their ultrastructure with regard to muscle features and the cytoplasmic crystals characteristic of this tumour. Inconsistent expression of alpha-smooth muscle actin, vimentin and desmin were found. No unambiguous muscle cell characteristics were identified by electron microscopy, although multiple subplasmalemmal densities with overlying lamina were emphasised as a typical but not specific muscle feature. Classical alveolar soft part sarcoma crystals at the ultrastructural level were absent throughout, but pseudo-crystals with a pre-crystalline finely filamentous content were numerous in one case. The results were integrated into a comprehensive comparison with the literature. It is clear that immunohistochemical evidence of striated muscle differentiation is detectable in some but not all cases of ASPS. The detection of Myo D1 protein in the small numbers of ASPSs so far studied for this gene product raises the possibility that in the majority of ASPSs this is the only evidence of early striated muscle differentiation and that only in the minority is it able to induce desmin production. Study of a substantial series for Myo D1 is required to explore this possibility.