Fertility of female mice irradiated with fission neutrons.

Abstract

The mouse ovary is an exceptionally radiosensitive organ. Several studies have shown that single whole-body exposures of 75 to 100 rads of x-radiation cause complete sterility in the adult female mouse (1, 2). This effect appears to be permanent. In the male, on the other hand, subjected to doses several times higher, sterility may be only temporary (3). The immature oocyte is particularly sensitive to low doses of radiation; the mature oocyte appears to be more radioresistant (4). Thus, after low exposures, several litters may be obtained before permanent sterility occurs. It is therefore imperative, in fertility studies, to measure total reproductive performance. During the past decade, a group at Argonne National Laboratory has been investigating the biological effects of external irradiation on different organisms (5). The work of the Neutron Toxicity Program has been concerned largely with the effects of fission neutrons and Co60 gamma rays on mammals (6). The present experiment was undertaken to determine the effects of single brief doses of fission neutrons on the reproductive capacity of the female mouse. Methods and Materials Female mice, CF. No. 1 strain, five to six weeks of age, were obtained from Carworth Farms and isolated in our laboratory until two to three months old. The animals were irradiated with doses of fission neutrons, from 20 to 150 rads,2 in single brief whole-body exposures. All were exposed in the gamma-neutron radiation chamber (7) at Argonne's heavy-water research reactor, CP-5. The intensity of the neutron beam was approximately 5 rads per minute, so that the exposures took thirty minutes or less. Within one week after irradiation, the exposed female mice were mated with normal, fertile, unirradiated males. Four females were caged continuously with a single male. When a mouse became pregnant, she was isolated and allowed to bear and wean her young. Such females were then returned to males for a second breeding test. Six such tests were successively completed within thirteen months of the original neutron exposures. The young produced from these matings were retained for a study of the effects of neutron irradiation of young adult female mice on the length of life of their progeny. Results The results of the first six tests are listed in Table I in terms of the number of pregnancies, the percentage of parous females, and the average litter size. Even after 150 rads, the highest neutron dose utilized, some mice produced viable offspring, at least in the first breeding test after irradiation. However, exposure to a dose of 75 rads or more produced complete sterility after one or two matings. A single irradiation of 50 rads caused a significant and progressive decrease in the percentage of pregnancies and in the average size of litters. The mice exposed to 20 rads of fission neutrons did not differ significantly from the unirradiated controls during the first three mating periods.