Rhabdomyoma of the soft palate. Fine structural details of a highly differentiated muscle tumor.

An adult rhabdomyoma was light and electron microscopically studied. The lesional cells presented well-known structural details, such as abundance of mitochondria and glycogen, myofilaments and myofibrils, hypertrophied Z-bands and masses of Z-band material. Triads were randomly scattered in the cytoplasm and also related to the individual sarcomeres. In sarcomeres the triads were regularly placed near to the A-I-junctions. This peculiarity of mammalian skeletal muscle fibers may yield a criterion to distinguish between cardiac and extracardiac rhabdomyomas. Circumscribed surface areas of the tumor cells were provided with elaborate infoldings of plasma membrane and basal lamina. These areas were interpreted as imitating myotendinous junctions. Satellite cells were regularly found.