82 D. W. VAN Bekkum 



These consisted of pyknosis, fragmentation and vacuolization 

 of the nuclei of a small number of thymocytes. The percentage 

 of degenerated nuclei was 3-7 and 8-9 in two representative 

 sections against 1-0 and 1-7 in sections of control glands. 

 The number of mitotic figures was decreased by about 50 per 

 cent in the irradiated group. This amount of nuclear degenera- 

 tion seems rather small to explain the observed changes of the 

 mitochondrial functions. It was concluded that the oxidative 

 phosphorylation in these tissues represents a cytoplasmic 

 function the radiosensitivity of which is comparable to that 

 of the nuclei. 



The interval between irradiation and the appearance 

 of the effect 



Because of the possible relation between nuclear metabolism 

 and oxidative phosphorylation it seemed of interest to investi- 

 gate whether the disturbance of the latter becomes discern- 

 ible prior to or after the beginning of nuclear degeneration. 

 This was studied on spleen tissue after a total body dose 

 of 700 r (van Bekkum and Vos, 1955). Measurements were 

 made at 15 minutes, 30 minutes, 1, 2 and 4 hours after 

 irradiation. The earliest significant depression of phosphate 

 uptake was found at 2 hours (Fig. 1), when oxygen consump- 

 tion was only slightly depressed. The histological findings 

 may be summarized as follows: from 15 minutes up to 2 hours 

 after irradiation the mitotic frequency was greatly diminished, 

 at 4 hours some reappearance of mitotic figures was noted. 

 Signs of nuclear degeneration were absent or dubious at 15 

 and 30 minutes after irradiation. After 1 hour early stages of 

 nuclear degeneration became clearly discernible in a few 

 cells. At 2 hours nuclear degeneration was present in 10-20 

 per cent of the lymphocytes and at 4 hours the majority of 

 the cells showed pyknosis or nuclear fragmentation. Nuclear 

 damage could thus clearly be observed before a significant 

 decrease of phosphorylation was demonstrable. 



This, however, cannot be taken as proof that nuclear degen- 

 eration precedes the mitochondrial lesion. Because individual 



