THE RESPONSE OF TISSUES TO CONTINUOUS IRRADIATION 217 



that the decrease in generation time is the result of the more slowly dividing 

 cells speeding up, or of latent cell population called into division, and not of 

 any change in minimum generation time. 



We have not found any evidence of increased radio-resistance in the blood- 

 forming tissues after exposure at 50 rads/day. The effect of a single dose of 

 acute radiation appears, if anything, to be slightly more severe than in 

 normal animals. 



DISCUSSION 



It is not yet possible to come to any j&rm conclusion about the reason for 

 the different response of gut and bone-marrow to continuous irradiation. 

 Possible reasons are: 



(i) a different radiosensitivity of the precursor cells with respect to 

 continuous irradiation, 



(ii) a difference in the homeostatic forces which influence speed and 

 efficiency of regeneration following the radiation injury, 



or (iii) a difference in the significance of general tissue damage produced by 

 radiation. 



With regard to (i), the generation time could be a very important factor. 

 With a shorter generation time less radiation dose wiU be accumulated between 

 divisions, which would lead to a decrease in radiosensitivity if there were a 

 substantial "wiping out" of damage at division. To determine whether this is 

 the case requires much more data than we have at present on the response of 

 cell popidations with different mean generation times. The extent of the 

 accumulation of chromosomal or other damage in the stem-cell compartment 

 of tissues is obviously one of the major factors that has to be taken into 

 account, and more work on tissues of low proliferation rate (such as the liver 

 prior to partial hepatectomy) is required. 



Generation time, in stem-cell and in multiplicative compartments, will 

 also be one of the factors influencing (ii), the speed of regeneration, but there 

 will be many other factors as well, such as the nature and efficiency of feed- 

 back mechanisms. The greater tolerance of the red cell system to continuous 

 irradiation, as compared with the white cell system, could be the result of a 

 more rapid and efficient homeostatic mechanism. For an assessment of the 

 importance of this factor in comparing the responses of gut and blood- 

 forming tissues much more information is required with regard to both 

 systems. However, it could be that the rapid recovery observed in the gut 

 after injury results from a homeostatic mechanism not based on feed-back, 

 but on a normally very high proliferation rate together with a short cell 

 precursor line. In the blood-forming tissues, feed-back mechanisms and a 



