CHROMOSOME ABERRATIONS IN LIVER CELLS 255 



plotted as a function of age. Although the curve represents only 2 points, 

 previous work substantiates the tendency of aberrations induced by X-rays 

 to decrease slowly with time. On the same graph is plotted the data of Fig. 1 

 in order that a comparison can be made between the effects of chronic and 

 acute irradiation. At 7-5 r/day it takes 31 days to accumulate 234 r; if 

 chromosome aberrations form and are retained for small doses as they are for 

 large doses (follow a single hit curve), the chronically irradiated mice should 

 have accumulated 58% aberrations at 31 days. In other words, under this 

 assumption the chronically irradiated animals should have followed a curve 

 shown by the dotted line in Fig. 4. Since the experimental curve is very far 

 from the same slope as this dotted curve, it follows that the assumption was 

 not correct. It then seems justified to conclude that the chromosome aberra- 

 tions foUow a multihit curve as a fimction of dose. In other words, chromo- 

 some Jiealing must take place. 



DISCUSSION 



The fact that the aberrations do not decrease and even increase for a time 

 following neutron irradiation is consistent with the fact that there is little or 

 no recovery from neutron irradiation as far as life-shortening is concerned 

 (Curtis, 1961). The chromosomal injury was very severe and most cells scored 

 as abnormal had multiple bridges and fragments. If a large fraction of these 

 cells were so badly injured that they could not be forced into cell division for 

 some time, then the initial scorings would show an abnormally low aberration 

 frequency because of the abnormally large percentage of normal cells 

 imdergoing mitosis. In time, the injured cells apparently recover the power to 

 undergo mitosis and thus the percentage abnormality increases. 



The finduig that chromosomal damage by X- or y-rays is a multihit 

 phenomenon, implying a chromosomal recovery mechanism, is consistent with 

 several other well-known radiobiological phenomena. First, it has been shown 

 (Steffenson and Arnason, 1954) that for X-irradiation of Tradescantia and 

 other plant material, there is a similar effect. Secondly, it has been shown 

 (Russell and Russell, 1959) there is an effect of dose-rate in mutation induc- 

 tion by X-rays in mice. Thirdly, for X- or y-rays very small doses are pro- 

 portionately only about one-quarter as effective in decreasing the life- 

 expectancy in mice as are large doses (Curtis, 1961). The present data do not 

 allow an accurate estimate of the proportionately lower effectiveness of small 

 doses, but it must be close to a factor of 4. 



These facts support the tentative hypothesis that the acceleration of 

 ageing by radiation is due entirely to the induction of mutations in somatic 

 cells. This follows from the fact that both with X- and y-rays and with 

 neutrons, chromosome aberrations correspond to the degree of life-shortening 



