General Discussion 301 



irradiated with 100 r the oxidative and phosphorylating power of its 

 mitochondria gets markedly reduced and that 50 r suffice to obtain 

 similar effects in the thymus. But if we expose rats to much larger 

 doses the mitochondria extracted from their liver go on to oxidize 

 and phosphorylate at a normal rate. Now you can of course say 

 that the composition of the liver differs from that of the spleen and 

 thymus; that the liver contains constituents having a powerful 

 protecting effect which prevent radiation energy reaching the 

 sensitive spots. You may also say that the enzymatic pattern 

 differs in the liver from that of the spleen or thymus, recalling the 

 considerations put forward by Prof. Krebs and that conditions for 

 enzyme inactivation are more favourable in the last-mentioned 

 organs. But the above-mentioned difference is not only shown when 

 comparing liver with spleen or thymus but also other moderately 

 sensitive organs with very sensitive ones. Thus we have to consider 

 the possibility that inactivation of enzymes is preceded by cell 

 lesion. As to the latter, a conspicuous parallelism is shown between 

 radiation sensitivity and rate of DNA formation in the organ con- 

 sidered. You can easily find exceptions to this regularity. Lym- 

 phocytes in which no DNA formation takes place are an example, 

 some plant seeds, some plants. In view of the very great number of 

 parameters involved it is very difficult to find any regularity which is 

 valid without exception. It has been known for many years that 

 cells exposed to irradiation often die when trying to divide. Li 

 emphasized this point in his classical book. Our knowledge of this 

 type of cell death was enlarged by investigations reported by Forss- 

 berg in his address. He has shown that by irradiating mice with 

 ascites tumour the formation of DNA in the tumour cells is blocked, 

 at the same time synthesis of several metabolites goes on. Such a 

 selective influence is bound to have serious consequences. DNA 

 formation being obstructed, the cell cannot divide and thus it has to 

 accommodate all additionally formed metabolites in the mother cell 

 which correspondingly swells. At a later stage, when the power of the 

 cell to synthesize DNA recovers, the cell can divide. But division of 

 such an abnormal cell is often fatal. 



When considering the above-mentioned exceptions we must take 

 into account the fact that exposed cells may die unconnected with 

 division processes. Even unexposed erythrocytes die and their 

 death may be accelerated by interference with oxygen supply, 

 production of haemolysing substances and other agencies. It was 

 shown by Howard and Pelc that irradiation may interfere with a 

 very late phase of the mitotic cycle in which the full DNA complement 

 of the cell is already reached, and Dr. Howard told us that she con- 

 siders this type of interference to be the primary one, a view against 



