[Ultrastructure of the cellular elements of mouse rhabdomyosarcoma]

The ultrastructure of different types of cell elements of rhabdomyosarcoma clones induced by 20-methylcholanthrene in mice CC57W is described. Along with general signs of a decreased differentiation typical for blastic cells of different histogenesis (rough endoplasmic reticulum, an abundance of free ribosomes, etc) some types of cell elements of myoblasts are characterized by specific features such as the character of anastomoses, fibrillary structures, etc. Morover, as a pathognomonic sign for tumorous myoblasts at the ultrastructural level could be only the presence of myofibrils with periodic aggragations like Z-bands in their cytoplasm. Since within one clone population both cells with and without myofibrils were encountered, the absence of the latter should not be considered to be a criterion for denying the myogenic nature of such cell elements.