Changes in radiosensitivity of the in vitro fertilized mouse ova durin zygotic stage from fertilization to first cleavage.

The radiosensitivity of mouse zygotes fertilized in vitro (BC3F1 ovum x ICR sperm) to X-rays has been measured as a function of time from fertilization to first cleavage. The dose of X-rays (LD50) required to prevent development of 50% of the zygotes to the blastocyst stage in vitro varied markedly depending on the time of irradiation from about 40 to 400 R. Sensitivity is relatively low shortly after sperm entry and becomes extremely high (LD50, 40 R) at the stage just before pronuclear formation (4 to 6 hr after insemination). In the later pronuclear stage (12 hr of fertilization) the sensitivity becomes low (LD50, about 400 R), and it increases again just before first cleavage.