This study analyzes the radiosensitivity of preimplantation mouse embryos in vitro by culturing them to early postimplantation stages after irradiation. Embryos were removed from the oviducts at the 2-cell stage and cultured to the 4-cell, 8-cell, morula, or blastocyst stage for irradiation. After further culture, embryos were scored for blastocyst formation, hatching, trophoblast outgrowth, and inner cell mass (ICM) growth. Probit analysis of dose response for blastulation indicated that radiosensitivity increased between the 2-cell and 4-cell stages and decreased at the 8-cell stage. Irradiation of morulae did not prevent blastulation but impaired hatching (${\rm ED}_{50}=339\pm 39\ \text{rads}$), trophoblast outgrowth (${\rm ED}_{50}=269\pm 21\ \text{rads}$), and particularly ICM growth (${\rm ED}_{50}=109\pm 17\ \text{rads}$). After irradiation at the blastocyst stage, embryos were much more capable of hatching and trophoblast outgrowth (${\rm ED}_{50}>1600\ \text{rads}$) than when irradiated at other stages but were still incapable of ICM growth (${\rm ED}_{50}=180\ \text{rads}$). Developmental failure of embryos irradiated at the 2-cell, 4-cell, and 8-cell stages was a combination of preimplantation and postimplantation deaths, whereas failure of embryos irradiated at the morula and blastocyst stages occurred predominantly postimplantation. The different sensitivities of trophoblast and ICM are consistent with the unique growth patterns of these two tissues.

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