Zygotes from superovulated hybrid C57BL/10J × SJL/J F1 mice were flushed from the oviducts 24 hr after the HCG injection. Pronuclear stage embryos were selected, cultured, and observed for a total of six days in microdrops of a chemically defined medium at 37°C in an atmosphere of 90% nitrogen, 5% oxygen, 5% carbon dioxide. At 28 hr after the HCG injection, the late pronuclear stage embryos were irradiated at exposures ranging from 50-600 R using a 280 kVp Picker X-ray machine (80 R/min, 0.5 mm Cu + Al parabolic filters). As determined by logistic analysis of the survival data, the ED50 values were 757 ± 69 R for development to the two-cell stage and 126 ± 27 R for development to the expanded blastocyst stage. An analysis of the times of the first two cleavages following irradiation indicated a dose-dependent cleavage delay of 0.85 min/R in the second cleavage following late pronuclear irradiation from exposures up to 600 R. These experiments illustrate that immediate arrest of cleavage, delay in the subsequent cleavages, and failure to undergo initial differentiation all result from irradiation of the pronuclear zygote. Use of the in vitro culture system allows a precise quantification of the degree of radiation-induced cleavage delay.

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