Rhabdomyoma: ultrastructural features and distribution of desmin, muscle type of intermediate filament protein.

A case of adult type rhabdomyoma in the neck region of a 77-year-old man is presented. In electron microscopy, some tumour cells showed irregularly orientated sarcomere formation with distinct Z-bands. All cells contained rod-like inclusions with attached thin cytoplasmic filaments. When stained with antibodies against desmin many tumour cells showed a diffuse peroxidase reaction, but the tumour also contained cells with cross striations. After phosphotungstic acid hematoxylin staining, dense cytoplasmic inclusions were seen; these structures showed strong affinity to antibodies against desmin. Our results apparently confirm the suggestion that the rod-like inclusions in rhabdomyoma cells are modified Z-bands. The desmin staining patterns in the present tumour may represent different stages in disorderly proliferation and degeneration of rhabdomyoma cells.