The radio-sensitivity of oocytes at different stages of maturation

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Abstract
The viability of the zygote after fertile mating has been used as an index to assess the radio-sensitivity of oocytes in Graafian follicles at different times before expected ovulation, and at known stages of the maturation process. Radio-sensitivity increases as the interval between irradiation and expected ovulation decreases; the increase is progressive as the oocyte passes from the late dictyate (diplotene) phase through the successive stages of maturation, at least up to congression of chromosomes at first metaphase. It is estimated that radio-sensitivity increases by about ten times between the earliest and the latest developmental stages studied, on the day of pro-oestrus. Oocytes in "medium-sized" follicles (irradiated 5 to 35 days before fertilization) are highly radio-resistant. The estimated LD50 for oocytes at this stage is about three times higher than that for pre-ovulatory oocytes some 14 to 15 h before expected ovulation. The total difference between the radio-sensitivity of oocytes in "medium-sized" or growing follicles, and that of oocytes at first metaphase, is of the order of x 30. X-irradiation is followed by superovulation and a decrease in the mean size of the corpora lutea of pregnancy. In general, the number of corpora lutea is inversely correlated with their mean size. The degree of superovulation is partially dependent on (a) the dose of X-rays and (b) the developmental stage of the oocytes at the time of irradiation. The radio-sensitivity of the fertilized ovum, 8 to 12 h after ovulation, is lower than that of pre-ovulatory oocytes which have reached or passed first metaphase. Subsequently, the sensitivity of the zygote decreases with advancing age.