PHYSICAL AND BIOLOGICAL FACTORS 929 



tissues known to differ in sensitivity. It has been suggested that radio- 

 sensitivity may be related to the ratio of the nuclear and cytoplasmic 

 nucleic acids, since the highest ratio is found in lymphoid tissue and the 

 lowest in resistant cells (Brues and Rietz, 1951). Proliferating tissues 

 probably show, in general, an increase in DNA per cell (Price and Laird, 

 1950). It is well known, however, that ribonucleic acid (RNA) also 

 tends to be increased in proliferating cells. 



That certain types of recovery may be faster in cells which are pre- 

 sumed to have a more rapid turnover is indicated by the finding that 

 intestinal epithelium and lymphoid tissue exhibit less interference of 

 mitotic activity after sublethal X irradiation than the less sensitive skin 

 and adrenal tissue (Knowlton and Hempelmann, 1949). On the other 

 hand, although thyroid administration increases the mitotic index of 

 mouse epidermis, and presumably its rate of metabolism, it does not 

 affect the response to irradiation, similar changes being observed in the 

 epidermis of control and thyroid-fed irradiated mice (W. W. Smith, 1951). 



In general, the hematopoietic and germinal tissues are the most sensi- 

 tive to radiation. These are followed by the intestinal epithelium, skin, 

 and connective tissue. Bone and glands are relatively radioresistant 

 while muscle and nerve are the least sensitive (Warren, 1942; Henshaw 

 and Snider, 1946; Warren and Bowers, 1950). There seems to be no 

 relation between the susceptibility of different tissues and their basal 

 oxygen consumption. Brain and kidney have higher rates of respiration 

 than spleen; yet the former are relatively radioresistant, while the latter 

 is radiosensitive. Although it has been shown that polyploidy protects 

 certain simple organisms against radiation damage (Latarjet and 

 Ephrussi, 1949; Clark and Kelly, 1950), there does not appear to be any 

 reason to think that this is an important factor in radiosensitivity of the 

 several animal tissues. Polyploid cells occur in the liver, but they are 

 the exception rather than the rule and probably do not account for the 

 apparent resistance of this organ to radiation. 



Although radiosensitivity appears to be related to the life span, growth 

 rate, and differentiation of tissue, this is only part of the story. For 

 example, with the onset of mitosis germinating wheat seedlings become 

 more resistant to the growth-retarding effect of X radiation (Henshaw 

 and Francis, 1935), and in the eggs of Drosophila sensitivity does not 

 exactly parallel the rate of division (Packard, 1930). Furthermore, 

 radiosensitivity is not correlated with the number of premitotic or 

 mitotic nuclei in the roots of Vicia faba (Mottram, 1935b). The most 

 sensitive cells in the testis are the spermatogonia rather than the sperma- 

 tocytes, which manifest greater mitotic activity (Bloom, 1947). The 

 rapidly growing squamous cell epithelioma is fairly resistant to radiation, 

 while the more slowly growing basal-cell tumor is sensitive (Packard, 

 1930). There are other exceptions; the regenerating liver does not show 



