Abstract
Six cases of orbital rhabdomyosarcoma were studied with the electron microscope. Tumor cells (rhabdomyoblasts) could be classified into three fine-structural types: (1) Myofibrillar cells had large amounts of cytoplasm containing characteristic well-differentiated banded myofibrils. Each myofibril consisted of thick and thin myofilaments grouped in parallel array. (2) Myofilamentous cells had moderate amounts of cytoplasm containing thick and thin myofilaments in poorly ordered array with indistinct banding and little tendency toward myofibril formation. (3) Nonspecific cells usually had small amounts of cytoplasm with a few widely scattered, randomly oriented thin filaments only. There was no tendency in this cell type to form filament aggregates. It is suggested that electron microscopy may contribute to the specific diagnosis of rhabdomyosarcoma by detecting actomyosin filaments which could not be detected with light microscopy. If possible, the histology of an orbital biopsy should be supplemented by a fine-structure study when rhabdomyosarcoma is being considered in the differential diagnosis.