Differences between "spontaneous" and induced sister-chromatid exchanges with fixation time and their chromosome localization.

Mean "spontaneous" SCE values have been investigated in synchronized CHO and S3/4 fibroblasts and in phytostimulated human peripheral lymphocytes. Significant increases were found at progressive fixation times following BrdU introduction in all cultures. Similar increases were observed with fixation times in SCEs induced by pulse treatment with methyl methane sulphonate (MMS) at various stages in the S period in CHO cells. In the fibroblast cultures these increases ranged from 20 to 80% with an 8-h interval between fixations. The percentage increase depended on the real proliferation occurring during this fixation interval. In the lymphocyte cultures, fixation intervals raning from 22 to 54 h yielded increases of 40 to 80%. It is proposed that when SCE yields are compared in the SCE test for potentially damaging agents, the stages in cell proliferation expressed as frequencies of first, second, and third mitoses after BrdU incorporation should be taken into account. The distribution of "spontaneous" and MMS-induced SCE's have been examined in marker chromosomes of the CHO cells. Although "spontaneous" SCE's are nonrandomly distributed, they do not appear to show a high incidence over any specific type of banded region. MMS-induced SCEs, on the other hand, are preferentially located over those chromosome sites in replication at the time of pulse treatment.