Radiation and Regenerating Rat Liver 227 



Mrs. Kelly. Small doses of irradiation, in our case 450 r, do 

 not have any obvious immediate inhibitory action on DNA 

 formation if given during synthesis, although they delay 

 mitosis and cause chromosome breaks. 



As it had been shown by the work of Pelc and Howard 

 (1953) and of Kelly and co-workers (1955) that small doses 

 given before the beginning of synthesis have a marked delay- 

 ing action on DNA formation, we carried out irradiations with 

 450 r at 12 hours after hepatectomy. It can be seen from 

 Fig. 1 that the increased rate of synthesis normally begins by 

 15 hours. The irradiation delayed the increased DNA forma- 

 tion and the onset of mitosis for about 10 hours; at the time 

 when the control liver showed a very high rate of synthesis 

 (at about 24 hours after hepatectomy) the difference between 

 the control and the irradiated tissue was very great. We 

 found that we could demonstrate the effect of 450 r of X-ray 

 irradiation at any stage of liver regeneration if we waited 

 9-12 hours for the difference between the control and irradi- 

 ated liver to become obvious. This gave direct confirmation of 

 the work carried out by Pelc and Howard on the bean root 

 with radioautographic techniques. 



Recovery (or partial recovery), as measured by DNA 

 synthesis and mitosis, comes at about 36 hours after hepa- 

 tectomy. It is obviously difficult to say whether the cells con- 

 cerned in synthesis and division are the same cells which were 

 forced to delay their activity, or whether another group of 

 cells has taken their place. It has been supposed that cells not 

 yet preparing for DNA synthesis might be unaffected by the 

 irradiation and that these undamaged cells could enter into 

 DNA synthesis and mitosis in their turn, causing the apparent 

 recovery. We have been able to show that this is not the case. 

 Irradiation with 450 r before hepatectomy shows the same 

 inhibition and recovery at the same time (Fig. 1) as are shown 

 by irradiation 12 hours after, so that it is not possible to 

 picture an early interphase stage which is unaffected by the 

 irradiation. In three cases irradiation was carried out 24 

 hours before hepatectomy; here there is some recovery in the 



