Radiosensitive target in the early mouse embryo exposed to very low doses of ionizing radiation.

We exposed mouse preimplantation embryos in vitro to either tritiated water (HTO) or tritiated thymidine (TdR) to determine whether the radiosensitive target was nuclear or extranuclear for embryonic cell proliferation disadvantage in the mouse embryo chimera assay. 8-cell embryos were incubated in either HTO or TdR for 2 h and paired with non-irradiated control embryos to form chimeras. Chimeras were cultured for an average of 20.2 h to allow for 2-3 cell cycles and then partially dissociated to obtain the number of progeny cells contributed by the two partner embryos for each chimera. These values were expressed as a "proliferation ratio" (number of cells from the irradiated embryo: total number of cells in the chimera). A ratio significantly less than 0.50 indicates that the experimental embryo expressed an embryonic cell proliferation disadvantage, which is the endpoint of this assay. The activity concentrations of HTO and TdR were adjusted so that both would deliver comparable mean absorbed nuclear doses during the combined initial 2-h irradiation incubation and subsequent 20.2 h chimera incubation periods. Although nuclear doses were comparable under these conditions, the extranuclear dose delivered by the uniformly distributed HTO was about 100 times greater than the extranuclear dose delivered by TdR for each given nuclear dose. Consequently, obtaining mean TdR proliferation ratios < or = mean HTO proliferation ratios would be evidence for a nuclear target while obtaining mean HTO proliferation ratios < mean TdR proliferation ratios would be evidence for an extranuclear target. TdR consistently produced lower mean proliferation ratios over a range of doses from 0.14 Gy to 0.43 Gy. Therefore, we conclude that the radiosensitive target for this endpoint is nuclear.