Cell sensitivity to transplacental mutagenesis by N-ethyl-N-nitrosourea isgreatest during early gestation in the Syrian hamster

The extremely high rate of cell division that occurs during early embryogen esis is hypothesized to predispose to high rates of mutation after chemical exposure, We tested this supposition experimentally. To probe the variatio n in susceptibility to mutation induction as a function of gestation stage, somatic cells of the developing Syrian hamster were isolated after transpl acental treatment with N-ethyl-N-nitrosourea (ENU), Mutants were quantified using either 6-thioguanine (6-TG) or diphtheria toxin (DT) as selective ag ents. Several different approaches were used. In one, three litters were ex posed on each gestation day and fetuses were removed on day 13. Maximum fet al sensitivity to ENU's genotoxic action was noted when treatment was at da ys 8 and 9, fewer mutants being obtained with earlier and later exposures. To compensate for the low numbers of target cells early in gestation, this experiment was repeated using larger numbers of litters exposed at the earl ier time points, and the highest mutation frequency was now found to occur after treatment on gestation days 6 and 7, In the second approach, mutation s were quantified in cells harvested 24 h after transplacental ENU exposure . Hen again, embryos exposed at earlier times of gestation were more suscep tible than those treated at later periods. Based on the total cell numbers in embryos and fetuses at each gestation day, we conclude that mutation fre quency is maximal on day 6, corresponding to the primitive streak stage wit h extremely high rates of cell division. (C) 1999 Elsevier Science B.V. All rights reserved.