Frequent appearance of radiation-induced transformation in junctional areas of colonies.

Morphological observations on induction of in vitro transformation were made in X-irradiated mouse embryonic 10T-1/2 cell line. Morphology of the cells growing in interior of the developing colony was almost uniform, whereas giant, morphologically altered, and transformed cells appeared in the contact area between two and three neighboring colonies. All of 10 transformed foci appeared in the contrast area between 2 and 3 neighboring colonies. This suggests that the contact of colonies was an important process for the expression of radiation-induced transformation.