1132 RADIATION BIOLOGY 



the adrenal cortex. The latent period preceding degenerative changes in 

 both may also be significant. In these experiments the glomerular zone, 

 which is probably the main source of regeneration of the cortex, is not 

 damaged, the changes occurring mainly in the zona fasciculata and zona 

 reticularis. However, some investigators report no changes in the 

 adrenals of rabbits and guinea pigs (Strauss, 1921). In general the 

 medulla seems more resistant than the cortex. 



No clear-cut changes have been observed after injection of radioactive 

 isotopes, although some of these, such as plutonium, radium, and 

 Zr 93 -Cb 93 , lodge in the organ. After insertion of radium needles, how- 

 ever, necrosis does occur. 



Hypophysis. Most of the effects of irradiation in this organ seem to 

 center in the anterior lobe, as degenerative changes in the other portions 

 of the gland have been described only after very heavy irradiation. It is 

 claimed that 3000 to 10,000 r at 180 kv produces a loss of body weight 

 (Fehr, 1936). 



The reported hypophyseal changes are very indefinite and are usually 

 described as swelling and increase or decrease of acidophils (Fehr, 1936; 

 Podljaschuk, 1928). Young rats exposed to 2000 to 2500 r have been 

 said to show a reduction of 50 to 75 per cent in these cells and numerous 

 pyknotic nuclei in the chromophils (Lawrence, Nelson, and Wilson, 1937). 

 However, other reports hold that there are no changes in the cells of the 

 hypophysis (Fehr, 1936; Lacassagne and Gricouroff, 1941). Irradiation 

 of the hypophysis of young animals seems to depress the growth and 

 development of the animals. 



Larger doses administered over the whole head do not change the 

 histological structure of the anterior hypophysis but are said to produce 

 an increase in its content of thyrotropic and gonadotropic hormones 

 (Grumbrecht et al., 1938). 



Thyroid. It is almost universally agreed that the thyroid is moderately 

 radioresistant. Radium implants caused hemorrhage and necrosis of 

 the thyroid (Bower and Clark, 1923). Repeated X irradiation produced 

 no changes with doses lower than those leading to lesions of the skin 

 (Walters, Anson, and Ivy, 1931). Some diminution of hyperplasia was 

 noted in opossums after irradiation (Walters, Anson, and Ivy, 1932). 



However, very large amounts of I 131 are markedly effective in cancer of 

 the thyroid and also in removing symptoms in some cases of hyper- 

 thyroidism. 



Parathyroid. Little is known of the effects of radiation on the 

 parathyroid. 



DISCUSSION 



It is of considerable theoretical and practical importance to evaluate 

 the relative radiosensitivities of the different kinds of cells and organs in 



