538 



L. G. LAJTHA 



G 2 



Mitotic 

 / DNA \ndzx 



/ Specific 



Activity 



10 20 30 



Hours after partial hepatecto/ny 



Fig. 2. Scheme of events in regenerating liver. 



and Kelly. 51 The essence of the findings is that relatively small doses of 

 radiation (150-450 r.) given within the first 10-12 hours after partial hepa- 

 tectomy (i.e. before the onset of increased DNA synthesis) delay the onset 

 of DNA synthesis, while once DNA synthesis has begun, much larger doses 

 are required (1000-2000 r.) to produce any measurable depression. The de- 

 gree of depression in the middle of synthesis following 2200 r. is of the order 

 of 50 %. 48 A certain amount of disagreement between different laboratories 

 (as to the degree and optimum time for depression) may be caused by the 

 difference of the time scale of events after hepatectomy in different strains 

 of animals, and also in differences in technique, i.e. surgical partial hepa- 

 tectomy or carbon tetrachloride poisoning, used to initiate liver regenera- 

 tion. 



Work on regenerating liver was to some extent initiated by the pioneer 

 observation of Howard and Pelc that in bean root cells small doses of radia- 

 tion may delay the onset of DNA synthesis in individual cells, while not 

 affecting synthesis in cells in which it has already begun. 6 This observation, 

 which has a great significance in understanding radiation effects on DNA 

 synthesis, will be discussed at length later in this chapter. 



4. Effects of Irradiation in Vitro 



It has been held for a long time, that radiosensitivity of tissues, in terms 

 of morphological damage or cell death, is less pronounced under in vitro 

 than under in vivo conditions. Several factors have been held responsible 

 for this observation, prominently among them is the lower oxygen tension 

 in vitro than in vivo. This, in most conditions, is undoubtedly true, and the 



51 L. S. Kelly, J. D. Hirsch, G. Beach, and W. Palmer, Cancer Research 17, 117 (1957). 



52 C. D. Jardetsky, C. P. Barnum, and H. Vermund, J. Biol. Chem. 222, 421 (1956). 

 63 R. E. Beltz, J. V. Laicker, and R. V. Potter, Cancer Research 17, 688 (1957). 



