Rhabdomyoma of the heart: ultrastructural study of three cases.

The electron microscopic features of three primary cardiac rhabdomyomas were studied for the first time. Distinct striated muscle cells arranged in a disorderly fashion are described. The cases demonstrated a spectrum of possible myobibril differentiation with one case having only Z-bands and myosin filaments present while the other two had well formed thick and thin filaments with all of the bands present including Z, I, A, H, and M. Glycogen bodies, numerous mitochondria with tubular cristae, lipid deposits, and peculiar striated structures resembling zebra bodies were also seen. Desmonsomal attachments suggestive of Purkinje-type fibers were seen in all of the cases and were the only types seen in cases 2 and 3. In addition, intercalated discs characteristic of myocardial fibers were noted in case 1. This finding still leaves the exact histogenesis of the lesion uncertain although origin from two types of fibers is possible.