Symposium on Atomic Energy in Animal Science: Radiation Effects on Gonadal Development in Farm Animals

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Abstract
In the prenatal mammal, prior to the sexual differentiation of the gonad, sublethal doses of radiation cause no marked effect on the reproductive capacity of viable offspring. Approximately 4 days after sexual differentiation (day 14 of gestation), 150 r of gamma rays will sterilize the prenatal male rat. The gonocyte of the rat retains this high degree of radiosensitivity until approximately the third day of postnatal life at which time many of the primitive germ cells complete their morphogenesis and become spermatogonia. Amounts of radiation necessary to sterilize the male from this point on are super-lethal. Maximum male germ-cell sensitivity in swine and cattle extends from approximately days 50 and 80 prenatally, respectively, to days 70 to 80 postnatally in each species. Bull and boar differ from the rat in that sublethal doses of radiation only diminish their sperm-producing capacities and do not cause sterility. Germ-cell radiosensitivity in the female varies with the stage of the meiotic prophase. On day 15 of gestation, 100 r of X-radiation administered to the oogonia of the rat decreased germ-cell numbers to about 15% of control values. The same dose administered on the 17th day of gestation, when many of the germ cells are in leptotene of the meiotic prophase, reduced the number of germ cells to 50% of control. Following irradiation at day 19, with a preponderance of oocytes in zygotene, 90% of the oocytes survived and on the day of birth, with the majority of the oocytes in pachytene, a 60% oocyte survival rate was noted. On the fifth day after birth the oocytes enter their stage of rest or dictyate and 100 r will again almost effect their complete destruction. From the 5-17th day after birth 150 r of gamma rays will sterilize the female rat, but with the advent of marked follicle growth 300 to 400 r is required to affect lifetime reproductive performance materially. The wide species differences in oocyte sensitivity are probably related to the stage in the meiotic prophase at which the oocytes of the various species "rest". The morphology of the resting bovine oocyte is typical of pachytene and the morphology of the resting porcine oocyte is typical of diplotene. Both pachytene and diplotene oocytes are comparatively refractive to radiation, thus probably accounting for the radioresistance seen in the oocytes of these species. Oocytes in dictyate are first seen in growing and vesicular follicles of porcine and bovine ovaries, respectively.