Ultrastructure of rhabdomyoma

Ultrastructural studies of rhabdomyoma, a rare benign human tumor, confirm its origin from skeletal muscle and differentiate it from other tumors, such as granular cell myoblastoma with which it may be confused by light microscopy. Evidence is presented that characteristic “crystals” observed by light microscopy in rhabdomyoma are in fact modified and hypertrophied Z-bands similar to the rods of nemaline myopathy. The structural characteristics of myofibrils, satellite cells, T-system, paracrystalline inclusions, glycogen, and mitochondria are described in rhabdomyoma. Many parallels were found between the fine structure of the tumor and that of normal and pathological muscle, especially denervation atrophy of muscle with which a special relationship is suggested.