Embryonal Rhabdomyosarcoma : Microscopic
Poorly-differentiated embryonal rhabdomyosarcomas (RMS) resemble developing muscle at 5 to 8 weeks' gestation. They are composed of small, round or spindle-shaped cells with hyperchromatic nuclei and scant cytoplasm as seen in this image. Mitotic activity is brisk. Rhabdomyoblasts with abundant eosinophilic cytoplasm are either absent or present in isolated pockets within the tumor. Glycogen is universally present in all types of rhabdomyosarcomas. Multinucleated giant cells are rare in embryonal RMS, unlike alveolar RMS where they are frequent. Rhabdomyosarcomas of genitourinary tract and retroperitoneum sometimes show foci of cartilaginous or osseous differentiation.