Effects of continuous gamma radiation on the life span of mice.

Virgin female mice, CF#1 strain, were continuously irradiated with 60Co gamma rays at a dose-rate of 6 R/day from conception to natural death. A marked life shortening was found for the irradiated group. The mean life span of irradiated group was 384•}25 days (the mean accu mulated dose being 2307.7 R), while that of the non-irradiated control group was 642•}12 days. To demonstrate the somatic effects of radiation on mice, a series of experi ments were carried out with mice which were bred in an environment of con tinuous irradiation at a low dose-rate throughout the entire duration of their lives, namely from conception until full growth or until natural death 1-3>. The present paper reports the effects of continuous irradiation on the life span of mice. The animals used were CF#1 virgin female mice. In the irradiated group, female mice were continuously exposed through the entire duration of their lives including intra-uterine life. For this purpose, the female parents with vaginal plugs after mating were transferred into the gamma room, and allowed to gestate and given birth under the continuous irradiation of gamma-rays at a low dose-rate. After birth, the female progenies were kept in the gamma room until their natural deaths. The radiation source used was 1.7 Ci of 60Co. Each day the source was removed during 2 hours for the maintenance of the mice, and then replaced for continuous irradiation during 22 hours at a dose-rate of 4.54x 10-3R/min, i, e., a daily dose of 6 R/22 hr.-day. In this manner, they were exposed to a total dose of from 174 to 4122 R for a period of 9 to 667 days old including the intra-uterine period, respectively, by which time all had died naturally. The control group was main tained in the same manner as the irradiated group but without exposure. The control and irradiated groups consisted of 45 and 51 mice, respectively, which were randomly selected at birth. Table 1. Survival data for the control and irradiated groups. The results are given in Table 1. The cumulative percent survival is plotted as a function of the number of the days after birth in Fig. 1. The first death among the control group occurred at 458 days old, and the last mouse died at 803 days old. The rest was distributed between 458 and 803 days old (Table 1). Juvenil death (preweaning death) is usually found in the control population as previously reported'), but this was not observed in the present series. In the irradiated series, four out of the fifty-one mice (7.8%) were dead between 9 to 11 days old, and the majority of the other deaths occurred in the period from 134 to 667 days old (Table 1). The mean life span for the control was 642±12 days, which for the irradiated group it was 384±25 days (mean accumulated dose 2307.7 R) including the juvenile death. From these results, the mean life span for the latter was markedly shortened (about 258 days) as compared with that of the control, and the difference was statistically significant (t=8.65, P<0.001). In the course of the experiments, it was found that mammary and ovarian