Ultrastructural characterization of poorly differentiated rhabdomyosarcomas

Twenty-five rhabdomyosarcomas, histopathologically and immunocytochemically classified as embryonal (12 cases), alveolar (11 cases), and pleomorphic (two cases), were studied by routine and immunogold electron microscopy, to identify, in poorly differentiated cells, reproducible markers useful for diagnostic purposes. Rhabdomyoblasts with rudimentary sarcomeres were observed in 12 cases (48%). Large cells (named oval cells) with an oval shape and an eccentric nucleus, like rhabdomyoblasts but lacking organized myofilaments, were observed in 20 cases (80%). By immunogold electron microscopy, performed on four of 25 cases, both oval cells and rhabdomyoblasts showed abundant intermediate filaments positive for desmin. Oval cells may be used as markers of poorly differentiated rhabdomyosarcomas.