90 Discussion 



in Dr. Van Bekkum's presentation: have we in the lymphocytes a 

 different system as far as sensitivity towards radiation is concerned ? 



Louiit: In our laboratory we have seen, in other tissues besides lymph 

 tissues, the break-up of cells before mitotic division, with a dose of about 

 150 r. I do not think it is confined to this particular system. 



Mitchell: This acute cytolysis is seen in some of the cells of the highly 

 radiosensitive and radio-curable basal-cell carcinoma of the skin ; even 

 after 50 r within about 45 minutes areas of liquefaction have been seen. 



Holmes: It may be relevant to Dr. Roller's question to describe an 

 experiment we carried out for Dr. Trowell. He asked us to estimate the 

 DNA synthesis in lymph glands where he was quite sure there was 

 no mitosis. We found more uptake of ^^p into the DNA fraction than 

 one would expect in a resting tissue. It seemed just possible that even 

 if no division was occurring the DNA was undergoing more change than 

 it does in other tissues. 



Bracket: Dr. Van Bekkum, is there any contamination of nuclear 

 material in your mitochondria, for instance is there any DNA? Could 

 it be that, when there is this destruction of nuclei, material originating 

 from degenerating nuclei might mix with your mitochondrial fraction? 

 According to Allfrey and Mirsky (1955, Nature, Lond., 176, 1042), 

 isolated thymus nuclei might be the site of some sort of oxidative 

 phosphorylation. It may be specially important, if that is true, to 

 know whether you have any nuclear contamination or not. 



Van Bekkum: I cannot believe that it is possible, as Mirsky claims, 

 to completely separate the nuclei from the cytoplasm in a tissue such 

 as the thymus. As far as I can remember, Allfrey and Mirsky did not 

 demonstrate phosphorylation in isolated liver cell nuclei, which are 

 much easier to obtain, and in my opinion that is what should be done. 



Bracket: I quite agree with you that that should also be done. 



Van Bekkum: I am not convinced that the experiments of Allfrey 

 and Mirsky have proved beyond doubt that nuclei or nuclear fragments 

 are capable of oxidative phosphorylation. I am sure that even our 

 control preparations do contain a certain amount of nuclear fragments. 

 I have not been able to avoid that by varying the methods of homo- 

 genization and differential centrifugation. Therefore we have spent a 

 considerable amount of time on the in vivo studies with radioactive 

 phosphate and we have interpreted the results as supporting our 

 in vitro findings. 



Howard: Coming back to the correlation of the histological effects in 

 the spleen, it is clear that lymphocytes can be killed by very small doses 

 of radiation. The dead cells take a little time to appear in the tissue, 

 but I do not think we have to assume that there is no change in the cell 

 population until the dead cells appear. There are changes taking place 

 as soon as there is a mitotic arrest. You only have to suppose that the 

 function of the mitochondria has something to do with the cell cycle, 

 or perhaps even that cytochrome c is bound up with the cell cycle in 

 some way, and you will expect that an hour or two hours after irradia- 

 tion, when there has been a blocking of cells entering mitosis, this might 

 be reflected in the activity of the mitochondria. So I think these histo- 



