CAN X-RAYS BE USED TO INCREASE LITTER SIZE IN THE MOUSE?

¹ Department of Health, Education, and Welfare, Public Health Service, Food and Drug Administration, Bureau of Radiological Health, Rockville, Maryland 20852

1. The average litter size for normal CF₁ mice is close to 10.5, varying in these experiments from 9.75 to 11.73. Separate groups of controls were provided for each of the three major experiments.

2. Resorptions were found even among the control groups, with an average of 1.5%. Other anomalies were not encountered, even in offspring from irradiated ova. Resorptions following X-radiation of ovarian ova never exceeded 7%, which is still within the normal range.

3. Depending upon the experiments, sexually mature females were either taken at random with regard to their estrous cycle, or vaginal smears were made and the females categorized to select the particular phase of estrous cycle of concern.

4. Among litters from matings immediately after (whole-body) X-radiation from 10 to 200r, the variations in resorptions over exposure levels was minimal, while variation in average litter size was within the range of normal experience for these mice.

5. Even among mice mated one week after exposure the differences in litter sizes were small and the statistical significance tenuous.

6. It was found that even a maximum exposure of 200r altered neither the resorption rate or production of anomalies, nor induced any significant increase in litter size.

7. When the various phases of the estrous cycle were segregated for irradiation to 100r, it was found that those in estrus showed somewhat smaller mean litter size than those which were X-irradiated during diestrus or metestrus. However, when compared with the control group, which probably included all phases of the estrous cycle, there was no statistically significant change.

8. On the basis of this study of some 8000 offspring, there is no evidence of significant change in ova production following whole-body X-radiation from 10r to 200r of mature CF₁ female mice, and certainly no evidence of superovulation.