Two distinct types of SV40-transformed human amnion cells.

Human epithelial amnion cells infected in primary culture with simian virus 40 transform to give rise to two morphologically different cells (T and R). R cell foci appear later, after infection, and the proportion of R to T cell foci increases when older amnion cultures are in fected. T cells may be easily subcultured by routine trypsinization methods. The more slowly dividing R cells are very resistant to treatment with enzymes and chelating agents, and require special subcultivation procedures. Cultures of both transformed cell types produce SV40; R cell cultures show higher virus titers and contain more virus-producing cells. Chromosome numbers of R cells are near tetraploid in the early stages of cultivation. However, T cells remain in the diploid range for longer periods. R cells do not enter a typical "crisis" stage.