Electron microscopic evidence suggesting the myogenous derivation of the so-called alveolar soft part sarcoma

Electron microscopic study has revealed distinctly different granules in cells comprising alveolar soft part sarcoma, carotid body tumor (nonchromaffin paraganglioma), malignant melanoma, and granular cell myoblastoma which allow for their differential diagnosis. This, as well as other pertinent information, militate against views that implicate a histogenetic relationship between alveolar soft part sarcoma and anatomical analogues of these alveolated neoplasms or those of neutral origin. On the other hand, the cytoplasmic crystalloids encountered in alveolar soft part sarcoma exhibit a striking morphological similarity to the rods observed in cells of benign rhabdomyoma and nemaline myopathy as well as Z-band material and tropomyosin B. Differences noted between these structures, as well as rods observed in examples of alveolar or pleomorphic rhabdomyosarcoma, do not appear sufficiently significant to compromise the conclusion that the alveolar soft part sarcoma most likely represents a unique type of rhabdomyosarcoma.