Preimplantation growth delay and micronucleus formation after in vivo exposure of mouse zygotes to fast neutrons.

Mouse zygotes were irradiated with fast neutrons (0.06 to 1.00 Gy) 1 h after conception and examined at various intervals (24 to 100 h after conception) for embryonic development and micronucleus formation. The frequency of micronuclei per cell increased linearly with dose in 2-cell embryos observed at 24 h after conception and in 4-cell and 8-cell embryos at 48 h after conception. Compared with X rays, the relative biological effectiveness of neutrons for the induction of micronuclei per embryo was 2.5 at 24 h after conception and 3.5 at 48 h after conception. Neutron-induced micronucleus formation was accompanied by morphological growth delay and a significant decrease in the number of cells in the embryos. An inverse relationship was found between the number of cells in embryos and the number of micronuclei when observed at 48 h after conception following irradiation with 0.12 to 1.00 Gy and at 78 h after conception following exposure to 0.50 Gy. The effect of neutron irradiation on embryonic development was likely to be mediated by cell death, as suggested by a significantly increased dead cell index in blastocysts following irradiation of zygotes.