1136 RADIATION BIOLOGY 



decide which of these two tissues is the more radiosensitive unless it 

 is stipulated whether one is concerned with acute or chronic injury. 

 Another type of difference is shown after irradiation of ovary and testis. 

 The ovary may be permanently sterilized for the production of ova and 

 thus indirectly of ovarian hormones, while in the testis spermatogenesis 

 can return after complete but temporary sterilization, and the production 

 of androgen is at no time diminished. In fact, in the testis, spermato- 

 cytogenesis is relatively radiosensitive and spermiogenesis is radio- 

 resistant, but the organ as a whole is potentially radioresistant for both 

 external and internal secretion. A third kind of reaction is that of the 

 lens with its lack of acute damage after moderate irradiation, but which, 

 after a long latent period, may show irreversible cataract formation from 

 such a dose. Thus we must distinguish between acute and chronic 

 radiation effects, and it must be determined in each case whether there 

 is a potential for recovery or not. 



In the gonads there is clear-cut evidence of the cumulative effect of 

 repeated small doses of irradiation. A similar but less intense effect has 

 been demonstrated in other organs, as the damage which results from 

 localized radioactive isotopes and as the damage in spleen, thymus, and 

 bone marrow after long-continued (eight to sixteen months) daily doses 

 of 4.4 or 8.8 r per day. Variations on these dose-time relations, as well 

 as the length of recovery periods between exposures, have not been 

 thoroughly studied. In the crypt epithelium of the mouse, repeated 

 daily exposure to 60 or 80 r per day of total-body X rays leads to a definite 

 but small degree of acquired radioresistance (M. A. Bloom, 1950). 



There is considerable evidence in favor of an indirect effect of irradia- 

 tion. The symptoms of "radiation sickness" substantiate strongly the 

 existence of such an effect. From morphological data it would seem 

 that the best evidence rests on the following observations: (1) Radiation 

 elicits a greater response in a particular organ when the same amount is 

 applied to the whole body than when it is applied locally; and (2) local 

 irradiation sometimes produces destructive changes in distant tissues. 

 These conclusions are based on innumerable investigations carried out to 

 study other aspects of radiation, and also on specific studies (Halber- 

 staedter and Ickowicz, 1947; Jolles, 1950; Ssipowsky, 1934-35; Van 

 Dyke and Huff, 1949). For example, in the thymus, pyknosis of the 

 cells is more intense after total-body irradiation than after irradiation of 

 either the superior or inferior part of the body (in each instance, 2000 r of 

 X rays) (Halberstaedter and Ickowicz, 1947). Atrophy of the lead- 

 protected thymus and spleen of rats takes place when other regions of 

 the body are irradiated with several thousand roentgens of X rays 

 (Leblond and Segal, 1942). However, when rats have been adrenal- 

 ectomized several days before irradiation, this indirect effect on thymus 

 and spleen does not occur. These findings have been corroborated 



