The ultrastructure of poorly differentiated rhabdomyosarcomas: a case report and literature review.

The ultrastructure of a case of rhabdomyosarcoma is reported, and the world literature on the ultrastructure of rhabdomyosarcomas is reviewed. The reported case was first observed as an undifferentiated malignancy originating in the maxillary antrum of a 46-year-old white woman and later showed differentiated cells in metastatic sites. A specific feature of the sarcomere within a representative malignant cell is necessary and sufficient for diagnosis of rhabdomyosarcoma by ultrastructure. Examples of specific features include thick and thin filaments in a hexagonal array, an unambiguous Z line, or A bands with H and M bands. The presence of thick and thin filaments without additional features of the sarcomere indicate a myogenous origin, either leiomyosarcoma or rhabdomyosarcoma. Other ultrastructural features observed in rhabdomyosarcomas, for which the diagnosis is confirmed by the presence of diagnostic ultrastructural features or by the presence of cross-striations observed by light microscopy, include large irregularly shaped nuclei with projections and invaginations, free ribosomes closely associated with filaments, rough endoplasmic reticulum that is often dilated, glycogen, and external lamina, prominent nucleoli, and pleomorphic mitochondria. When this spectrum of features is present, the possibility of rhabdomyosarcoma cannot be excluded, and a diligent search for the diagnostic components of the sarcomere should be made. Small intercellular junctions, pinocytotic vesicles, and paranuclear cilia occurred in the poorly differentiated metastasis of this case; the presence of these nonspecific features permits neither diagnosis nor exclusion of rhabdomyosarcoma.