The ultrastructure of alveolar and embryonal rhabdomyosarcoma. A correlative light and electron microscopic study of 17 cases.

An ultrastructural study is presented of 8 alveolar rhabdomyosarcomas and 8 embryonal rhabdomyosarcomas, and one mesenchymoma with predominantly rhabdomyoblastic differentiation. Thin and thick myofilaments and Z-lines or Z-like densities were found in all. Intermediate filaments were found in 4 alveolar and 4 embryonal rhabdomyosarcomas. A correlation was noted between the light- and electron microscopic appearance of the tumours regarding the degree of differentiation. A wide differentiation spectrum of the tumour cells was observed which could be correlated partly with normal fetal myogenesis. Ultrastructural differences were demonstrated in the alveolar and embryonal types. Electron microscopy is considered to play an important role in the characterization and diagnosis of rhabdomyosarcoma and is a valuable tool in the differential diagnosis.