Some Notes on Aging 345 



In radiation effect, radiation exposure is related directly to enhancement 

 of degenerative change, thus simulating the effect of aging. The similarity 

 may be due in part to the random destruction of cells and partly to the alteration 

 of function of cells. Within certain cells such as the marrow and the lymphatic 

 tissue, or in embryologic development, radiation over a wide range of dose 

 and for several species of mammals destroys approximately three cells out 

 of every 1000 cells per roentgen of whole-body exposure (7c). At less than lethal 

 exposures, this random destruction of cells proportionally to radiation exposure 

 does not have a lasting effect upon the blood-forming tissues, since these cells 

 rapidly regenerate. However, the average lethal dose of whole-body radiation 

 exposure is estimated to involve a 50 per cent reduction in these cells. Some- 

 what the same changes occur in other body cells, the degree being dependent 

 upon radiation sensitivity. (Some cells are known to be much more resistant 

 to radiation than blood-forming cells.) The effects of radiation in diminishing 

 the numbers of cells also seem to be about the same upon mammalian germinal 

 cells as on blood-forming tissues; in both tissues, approximately two to three 

 cells are affected per 1000 cells per roentgen. Thus it appears that each roentgen 

 of exposure to tissues like the blood-forming system, the gonads, and the develop- 

 ing embryo may have about equal probabiUty of either kilhng the cell directly or 

 altering its chromosomal structure if it survives. Such changes may be sus- 

 pected as having a role in inducing age change in the somatic tissues. 



Leukemia induction by radiation, as evidenced from the analysis of Court- 

 Brown and Doll and others (7a,b,c,d), is increased proportionally to radiation 

 exposure. These changes are such that approximately 50 r of whole-body 

 exposure produces a frequency of leukemia equal to its natural incidence and the 

 effect is proportional to dose over a wide range. This is in remarkable agreement 

 with genetic change in mammals ; here, too, an exposure of 50 r produces approxi- 

 mately the same number of mutations as occur naturally in one generation. 



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