Osteosarcoma cells in tissue culture. III. Actin filament distribution

Microfilaments, especially actin, have been demonstrated in a variety of noncontractile cells. In earlier studies, the quantity as well as the distribution of these microfilaments has been used to differentiate normal murine fibroblasts from virally transformed cells. This study examines these differences in cells from spontaneously occurring, human osteosarcomas and normal human fibroblasts by transmission election microscopy and heavy meromyosin (HMM) decoration. Both the tumor cells and the normal fibroblasts were found to have subcortical bands of 7-nm microfilaments that labelled with HMM and were 150-300 nm in thickness. There was a central reticular pattern of microfilaments predominantly composed of 7-nm filaments in the fibroblasts but with a larger proportion of 10-nm filaments in the osteosarcoma cells. Arrowhead formation was present after mild Triton X-100 extraction and HMM decoration on only the 7-nm microfilaments, in both types of cells. Differences in the quantity of 10-nm filaments between cells in culture from spontaneously occurring human sarcomas and normal fibroblasts may account for differential surface responses, e.g., contact inhibition and saturation density. Unlike some evidence from viral transformation models, the data from this study do not support the hypothesis that tumorgenicity is linked mechanistically to decreases in polymerized cellular actin, at least not in cells from spontaneously occurring human sarcomas. The cellular behavior of human sarcomas, both in vitro and in vivo, may be a manifestation of differences both in structural proteins and cell surface proteins.