Reversion of the transformed phenotype to the parental phenotype by subcultivation of X‐ray‐transformed C3H/10T1/2 cells at low cell density

Transformed cells induced by X‐irradiation of a stable contact‐inhibited line of C3H mouse embryo cells (10T½) display an altered morphology (fibroblastic), a decreased sensitivity to contact inhibition, a high cell saturation density, a decreased sensitivity to interferon and a capacity to form colonies in agar and malignant tumors in mice. Under certain experimental conditions, serial passage of transformed cells (from cloned lines) at low cell density (i.e., 400 cells per 25 cm 2 flask) was accompanied by a reversion from the transformed phenotype to a phenotype characteristic of the parental untransformed 10T½ cells: epithelioid morphology, low cell saturation density, increased sensitivity to interferon and failure to form colonies in agarose or tumors in mice. The revertent phenotype was stable as long as these cells were passed at low cell density. Passage at high cell density, however, resulted in a back reversion to the transformed phenotype. When transformed cells were seeded at a much lower cell density, so that only a few colonies appeared per Petri dish, reversion to the parental phenotype occurred at the first passage and appeared stable. Back reversion was no longer observed even when these cells were passed at high cell density. The experimental results suggested that reversion of the transformed to the parental phenotype was not due to a selection of a pre‐existing population of cells with the parental phenotype but rather to a change in the phenotype of the entire cell population.