Cellular mechanisms of transformation of BALB/3T3 A31-1-1 cells [corrected] by gamma-irradiation.

In order to clarify the mechanism of transformation, the effect of cell generation number on the time of appearance of transformed cells was examined in gamma-ray-irradiated BALB/3T3 A31-1-1 cells. The results showed that an increased number of cell generations (produced by seeding of cells at lower cell densities as well as under dispersed conditions) was important for increasing the rate of transformation. By a procedure in which irradiated cells in dishes were dispersed by trypsinization and recultured in the original dishes after cultivation for a certain number of days, we expected to establish the so-called "expression period." The rationale was that the number of transformed foci would increase after the appearance of a transformed cell due to its expansion. The expression period was shown to be 1.5 weeks on average. The time of appearance of transformed cells, however, varied from as early as about 1 week to 2.5 weeks. When cells were seeded at higher or lower cell densities and examined by the technique of dispersion and reculture, the expression periods were almost the same. Thus, it is probable that transformed cells can appear before or after the confluent state is reached.