Transformation of BALB/c-3T3 cells: I. Investigation of experimental parameters that influence detection of spontaneous transformation.

The frequency of spontaneous morphological transformation is an important variable in measuring chemical-induced transformation in BALB/c-3T3 clone A-31-1-13 cell cultures. Data from 110 experiments, which included benzo[a]pyrene control groups and other chemical treatment groups, were analyzed for factors that influenced spontaneous transformation. Spontaneous transformants demonstrated a continuum of morphological variants (type I, II, and III foci) that fit a normal distribution if converted to log10. The magnitude of transformation depended on the ampule of cryopreserved cells and the serum lot. Although the average frequency was approximately 0.71 x 10(-6) (type III foci/cell that survived and proliferated to confluence), the absolute number of foci/vessel increased in proportion to the surface area of the culture vessel. Thus, the frequency of spontaneous transformation was directly related to the cumulative number of mitoses that occurred in forming the contact-inhibited monolayer. These data are consistent with a hypothesis that spontaneous transformation in BALB/c-3T3 cells is a mutational event or some other single-step phenomenon.