198 Alma Howard 



Effect of irradiation on DNA metabolism in some 

 mammalian tissues 



An examination of the literature shows that very wide 

 differences exist between tissues in their response to irradia- 

 tion with regard to its effect on DNA metabolism. This is 

 especially clearly shown in the work of Kelly and co-workers 

 (1955), who measured the incorporation of ^^P into DNA of 

 mouse small intestine, spleen, liver, bone marrow ("carcass"), 

 and two transplanted tumours after X-irradiation at four 

 dose-levels. Informative time curves were obtained by 

 sacrificing animals from 2 hours to 5 days after irradiation, 

 the isotope being injected at fixed short times before sacrifice. 

 The responses of the tissues were so different, both in dose and 

 time-response, that no general statement can be made about 

 them other than that a depression was always seen, and that it 

 was apparent at the earliest times after irradiation that were 

 studied. In the small intestine, for example, the maximal 

 effect of 800 r was seen at 3 hours, with full recovery at about 

 1 day and nearly three times the normal rate of incorporation 

 at 2 days. Bone marrow incorporation, on the other hand, 

 showed a maximal effect only at about 3 days after 800 r, and 

 was still very low at 5 days. The different response of the two 

 tumours, a mammary carcinoma and a lymphosarcoma, was 

 especially striking. They had approximately the same growth- 

 rate, mitotic index, and short-term incorporation of ^^P before 

 irradiation. After 800 r the mammary carcinoma showed a 

 reduction in incorporation to about one half, but no change in 

 weight or histological appearance beyond a decrease in mitotic 

 index. The lymphosarcoma showed reduction in incorpora-. 

 tion to 4 per cent of normal at one day, with later apparent 

 recovery; there was marked involution, and a large amount 

 of cell death. It is very clear that there had been important 

 changes in the cell population, and the authors point out 

 that most of the effects they observed in this and other tissues 

 could be explained by such changes. The reduction in 

 amount of various phosphorus compounds, including DNA, 

 in rabbit bone marrow after gamma-ray doses in the mean 



