Cell Cycle-Dependent Radiosensitivity in Two-Cell Mouse Embryos in Culture

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Abstract
The radiosensitivity in embryo systems varies depending on factors such as genetic background, O2 environment, developmental stage and age of the embryo in cell cycle. The involvement of cell cycle age in radiosensitivity of 2-cell mouse embryos was investigated. The doses needed for 50% killing of blastocyst formation in vitro (LD50) of X-rays for the 2-cell mouse embryos in culture were measured during their cell cycle. The cell cycle in the 2-cell embryos was quite peculiar; the cell cycle time of 18 h was divided into a long DNA post synthesis phase (G2) plus mitosis (M) of 14 h and a short DNA synthesis phase (S) of 4 h. Apparently the LD50 varies roughly from 100 to 600 rad within the cell cycle. A major factor in determining the sensitivity to ionizing radiation of 2-cell mouse embryos in vitro and perhaps in vivo is their position in the cell division cycle at the time of irradiation.