. Analysis of genomic instability in the embryos All gastrulas from a number of litters were pooled for cytogenetic analysis. In the CF1 strain, a total of 330 gastrulas from 47 females were used for these analyses. As shown in Table 8, about 13% of the gastrula cells from control CF1 mice contained at least one chromosome anomaly. The great majority of anomalies consisted in chromatid gaps and, to a lower extent, chromatid fragments (Fig. 2e). There were no chromosome-type aberrations, except for one Robertsonian fusion. No increase of chromosome
damage was found in the first generation gastrulas of CF1 females that had been X-irradiated with 0.2 or 0.4 Gy at the embryonic 1- cell stage. Although some chromosome fragments and gaps were observed, the numbers of chromatid-type aberrations again largely exceeded those of the chromosome-type aberrations (Table 8). In the 0.4 Gy group, one Robertsonian fusion was observed, like in the control group. Interestingly, apparent joining of two or four chromosomes at or proximal to their centromeres was observed in one metaphase of the 0.2 Gy group (Fig. 2c) and an even more particular association between four chromosomes was found in one metaphase of the 0.4 Gy group (Fig. 2d). The cytogenetic approach used in this study did not allow to further characterize these anomalies that were classified as “chromosome fusions”, together with the Robertsonian fusions. The frequencies of abnormal cells and the total numbers of chromatid and chromosome aberrations per 100 cells were remarkably similar between control gastrulas and gastrulas from females X-irradiated with either dose.