Paternally inherited effects of gamma radiation on mouse preimplantation development detected by the chimera assay.

It has previously been shown that type B spermatogonia in male mice treated with 0.05 Gy of X rays undergo an alteration expressed by progeny embryos as a cellular proliferation disadvantage in a chimera assay. We wished to obtain information on the assay's detection limit to ionizing radiation and on the radiosensitive target in male germ cells. Male mice were briefly irradiated with 137Cs gamma rays at nominal absorbed doses of 0.0, 0.0015, 0.005, 0.010, or 0.05 Gy and then mated for the next 8 weeks to untreated females. Four-cell embryos from treated males (experimental embryos) were paired with FITC-labeled embryos from untreated males (control embryos) to form aggregation chimeras. The chimeras were cultured for 30-40 h and examined under phase-contrast and UV illumination for the number of unlabeled cells (from the experimental embryo) and total chimera cell number, which were then expressed as "proliferation ratios" (No. unlabeled cells/total chimera cell No.). Significant decreases in proliferation ratios were observed at postirradiation weeks 4, 6, and 7 for the 0.01-Gy dose group and at weeks 5-6 for the 0.05-Gy dose group. In addition, significantly lower ratios were observed with early and mid four-cell embryos, but not with late four-cell embryos. These results suggest that mouse male germ cells express a radiosensitive target(s) whose detection limit by the assay lies at an absorbed dose between 0.005 and 0.010 Gy for brief gamma irradiation and whose effect on embryonic cell proliferation might decay by the second cleavage.