Changeable aspects of radiation-induced chromosome aberrations during early development of the zygote

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To understand changeable aspects of radiation-induced chromosome aberrations (CAs) during the embryonic development, mouse pie-implantation and post-implantation embryos derived from gamma-irradiated (2,4 Gy) spermatozoa were studied cytogenetically and embryologically.1.Ahighly successful method for chmmosome preparation of mouse 1-cell to 4-cell embryos was established.2.A total number of 1,170 zygotes were karyotyped in irradiated and control groups. The zygotes with structural chromosome aberrations (SCAs) showed a clear dose-dependent increase, reaching 36% in the 4 Gy group. No dose-dependent increase of aneuploidy was found. The most predominant type of CA was chromosome breaks/fragments (60%) which was followed by dicentric chromosomes (25%).3.A total number of 396 2-cell embryos were karyotyped. The incidence of embryos with SCAs was nearly the same as that in 1-cell embryos, but about 90% of them converted into mosaic-type SCA. The incidence of SCA-derived aneuploidy was also significantly high in the irradiated groups.4.A total number of 537 4-cell embryos were karyotyped. A great majority of the embryos with SCA were of mosaic-type. The incidence of aneuploidy became decreased, suggesting that they converted again into mosaic type.5.About 50 to 60% of embryos with SCA reached to blastocyst stage, showing no developmental arrest or delay.6.Chromosome analysis was carried out with 2,240 somatic cells obtained from 85 live embryos (10〜11-day old). None of embryos was found to be chromosomally abnormal. The incidence of post-implantation death was significantly high in the irradiated groups, almost all the dead embryos were being in the state of degenerated conceptus. These suggest that selection of the embryos with CA mainly takes place immediately after implantation.7.No increase of embryonic death or developmental anomalies was observed in 16〜17-day-old fetuses (survey of 52 pregnant females), supporting the above-mentioned suggestion.