Embryonal rhabdomyosarcoma of the prostate. An ultrastructural study

An ultrastructural study of an embryonal rhabdomyosarcoma arising from the prostate of a 9‐year‐old boy is reported. Cross striations in neoplastic cells were difficult to find under the light microscope. The electron microscope, however, revealed large cells with poorly formed but easily recognizable myofibrils. There were numerous small cells devoid of myofilaments in the vicinity as well. Their cytoplasmic contents were of two types: one type had abundant free ribosomes while the other had rough endoplasmic reticulum and Golgi apparatus. Irrespective of type, the small cells were associated either with the large cells containing myofilaments or with collagen fibrils. From their disposition and ultrastructural characteristics, it was difficult to determine which type of the small cells represented the precursor of the large cells containing myofibrils and which belonged to the stromal component of the neoplasm.