Fine structure of dividing cells and of nondividing, differentiating cells of nickel sulfide-induced rhabdomyosarcomas.

Cells of nickel sulfide-induced rhabdomyosarcomas were studied with the electron microscope. Cells in the proliferative compartment were mononucleate and exhibited neither myofilaments nor other features of normal muscle cell differentiation. Cells in the nonproliferative compartment had myofilaments and differentiated similarly to the normal fetal and early postnatal muscle cells. The tumor cells did not differentiate beyond the initial stage of formation of triads and of the smooth endoplasmic reticulum. Transitional forms between undifferentiated and differentiated tumor cells were observed. Only a fraction of the progeny of the dividing cells seemed to differentiate; the remaining fraction did not and remained in the proliferative cell compartment as stem cells. The ratio between these cells and the descendants of the dividing cells that, by differentiation, became part of the nonproliferative cell compartment was a factor in the growth of the rhabdomyosarcomas. The pattern of fetal differentiation of the rhabdomyosarcomas was analogous to that of diethylnitrosamine-induced hepatomas studied previously in this laboratory.