404 INFLUENCE OF EXPOSURE FACTORS 



division, at which time injured cells appeared. With neutron irradiation, 

 degenerating cells appeared in large numbers even before definite re- 

 covery of the mitotic process had taken place. This generalized necrotic 

 effect was even more evident with alpha-particle radiation. 



The other line of evidence comes from the experiments using protrac- 

 tion and fractionation, where it appeared that organisms recovered from 

 x-ray effects much more rapidly than they did from those of neutrons. If 

 cells not active in mitosis are more selectively resistant to x-radiation, it 

 is understandable that such cells will then permit rapid replacement of 

 injured and destroyed cells. It appears paradoxical at first that the two 

 systems in the mouse that are most sensitive to chronic exposures of 

 neutrons represent, on one hand, rapidly growing tissue such as the 

 gonad, and, on the other hand, slowly growing tissue, the lens. Also, 

 the ovary is deep in the body and the lens is superficial in location. 

 Again, the bone marrow, actively proliferative, does not appear to be as 

 relatively sensitive to neutrons as is the testis or ovary. The changes in 

 the skin are not different at first, but later (63, 64) the injury, especially 

 in deeper portions, is more pronounced and irreversible after the neutron 

 exposure. 



The situation may not be quite as confused as it seems. It may be 

 that all tissues are more sensitive to neutrons, but effects of small daily 

 exposures are more evident in the lens because this organ does not have 

 the ability to discharge dead cells from its system (as does the skin, for 

 example) and the accumulated effect becomes visible. Although the 

 testis and ovary are able to discharge dead cells, they are unable to re- 

 place them from other parts of the body. The lack of special sensitivity 

 to neutrons of hematopoietic and bone tissue may be more apparent 

 than real. The regenerative stimuli for blood cells are probably so regu- 

 lated by body requirements that the population following small daily 

 exposures may not reflect the complete damage to this system. The 

 more complete story might come from a study of depletion time, and in 

 fact it might be just some factor of aplasia that brings on death more 

 early in the neutron-treated animals. Neither the pronounced neutron 

 effect on the lens nor that on the gonads is particularly hazardous to 

 the life of the organism. However, they may reflect the degree of damage 

 produced, but not apparent, in other organs of the body. Organisms 

 surviving neutron exposures may be, therefore, more susceptible to 

 trauma and infection than one would expect from exposures to x-rays 

 tRat had a similar early effect on chances for survival. If the lethal 

 action of radiation on mammalian cells is like that on certain plant cells, 

 then an explanation of the differential recovery rates (that is, of photon 

 irradiations over those of neutrons and alpha particles) may be at hand. 



