Rhabdomyosarcoma in children: ultrastructural study of 31 cases.

Thirty-one cases of rhabdomyosarcoma in children were studied by electron microscopy. All of the tumors examined fit into a continuum of progressive cellular differentiation mimicking that of normal myogenesis, with increasingly differentiated examples being decreasingly common. We conclude that all childhood forms of rhabdomyosarcoma are essentially embryonal tumors, with many not being sufficiently differentiated at the cellular level to allow positive ultrastructural diagnosis.