[Ultrastructural organization of cell characteristics relevant to diagnosis in rhabdomyosarcomas]

Rhabdomyosarcomas may be easily diagnosed by light microscopy when tumor cells exhibit cross striations typical of skeletal muscle cells. But in many cases the histological diagnosis is difficult because the tumors are predominantly composed of undifferentiated elements and only single cells can be seen which show an eosinophilic cytoplasm suggesting different steps of a possible rhabdomyoblastic differentiation. Using the electron microscope seven embryonal rhabdomyosarcomas, one alveolar rhabdomyosarcoma including one lymph node metastasis and one pleomorphic rhabdomyosarcoma were analysed in order to study the submicroscopical organization of diagnostic cellular features, resp. the distribution and arrangement of cytoplasmic filaments. The following lines of cellular differentiation could be distinguished: 1. Development of differentiated myoblasts and satellite cells: -- primitive, undifferentiated tumor cells: small round cells with scanty cytoplasm containing few thin filaments (4 to 6 nm) and intermediate-type filaments (10 nm), -- round or slightly spindle-shaped myoblast-like tumor cells: cells with moderate cytoplasm exhibiting irregularly arranged thin and intermediate-type filaments and only some thick filaments (15 nm), -- myotube-like cells: long extended cells (strap shaped cells) revealing thin and thick filaments, Z-line material and different stages of myofibrillar organization, -- well-differentiated myoblasts: long extended cells showing typical cross-striations which correspond to well-formed sarcomeres with I-bands (Z-lines with extending thin filaments) and A-bands, which are subdivided into H- and M-bands, -- satellite cells of typical ultrastructure associated with differentiated myoblasts by a common basement membrane present in one case. 2. Development of aberrant myoblasts and giant cells: -- round myoblasts: cells with increased cytoplasm containing thin and thick filaments, primitive Z-lines, which were not organized into sarcomeres as well as unaligned sarcomeres, -- large round myoblasts and giant cells: cells with abundant cytoplasm containing irregularly distributed thin and thick filaments, primitive Z-lines and haphazardly arranged sarcomeres, which did not appear as cross-striations by light microscopy. The ultrastructure of tumor cells is discussed with regard to the degree of differentiation and their light microscopic appearance. The scale of cellular features in rhabdomyosarcomas could be also correlated with normal fetal myogenesis. Furthermore this confirms the important role of electron microscopy in the differential diagnosis of other small, primitive and dark-cell tumors.