DNA Synthesis in Rat Blastocysts X-Irradiated during Delayed Implantation

Sprague-Dawley rats were exposed to 300 or 500 R acute whole-body x-irradiation on day 5 of castration-induced delayed pregnancy, and blastocysts were examined at various times thereafter. Both doses of radiation produced a temporary inhibition of mitosis in the embryonic cells. Although mitosis resumed approximately 12 hr after exposure, irradiated embryos failed to grow at a rate comparable to that of sham-irradiated controls. Control embryos attained a maximum size of approximately 85 cells before entering the mitotic dormancy characteristic of delayed implantation in the castrate progesterone-treated rat. Maximum sizes of 71 and 64 cells were attained by embryos exposed to 300 R and 500 R, respectively. Autoradiographic evaluation of [3H] thymidine incorporation revealed a significantly greater percentage of labeled cells in irradiated embryos compared to controls at 2.5 and 5 hr after exposure to either dose. A maximum of 62% of the cells from control embryos incorporated [3H] thymidine during a 1 hr in vitro incubation, whereas 74% of the cells of embryos examined 2.5 hr after 300 R, and 85% of the cells of embryos examined 5 hr after 500 R were labeled. The percentage of labeled cells in irradiated embryos did not return to control levels until 12 hr after exposure, and then tended to fluctuate around control values from 12 to 101 hr after exposure. The data are consistent with the hypothesis that the 12-hr mitotic inhibition observed in the cells of irradiated embryos may be due in part to a prolongation of the time spent in S phase of the cell cycle.