Influence of some methodological factors on the radiosensitivity of the mouse zygote

The experiments reported here were undertaken to investigate the influence of some methodological factors on the radiosensitivity of the mouse zygote. The following factors were studied: 1) the use of natural or hormone-stimulated ovulation; 2) the procedure followed for fertilization:mating overnight, or only during a short period in the morning after all oocytes have been ovulated, in vitro fertilization; 3) the type of irradiation, i.e., in vivo or in vitro irradiation. The radiosensitivity of the zygotes was estimated under the different experimental conditions by measuring the ability of the irradiated embryos to cleave and to develop further to the blastocyst stage. Our results suggest that the protocols used for mating and fertilization probably have a greater influence on embryonic survival following irradiation than the use of gonadotropins to stimulate ovulation. The highest degree of synchrony in the development of the embryos is achieved by restricting mating to a short period or by using in vitro fertilization. The very low LD50s obtained under such synchronous conditions confirm the high radiosensitivity of the mouse zygote at the early pronuclear stage. Comparison between the effects of in vivo and in vitro irradiation does not indicate a greater radiosensitivity of the embryo irradiated in vitro in comparison to the embryo irradiated in vivo.