214 



L. F. LAMERTON 



intestine of the rat. This suggestion receives some support from the observa- 

 tion that the number of cell fragments in the cvypts decreases with time under 

 continuous irradiation. If there is, in fact, an increase in radio-resistance in 

 this tissue under contmuous irradiation, the mechanism is unknown. An 

 increase in polyploidy has been looked for, but not fomid (Kember et al, 1962). 



20 

 2 12 



'1^ 



Mitotic count 





Fig. 1. Response of small intestine of young rat (6 to 8 weeks old) to a continuous dose- 

 rate of 415 rads per day from a ^^^Cs source. Data obtained from squash preparations of crypts. 



It is not known how long the gut wiU maintain function at a dose-rate of 

 415 rads/day, but in studies after 100 days irradiation at 50 rads/day, a nor- 

 mal mitotic index has been found with evidence of only a small reduction in 

 crypt population. The generation time of the cells under these conditions is at 

 present being measured. 



RESPONSE OF THE BLOOD-FORMING TISSUES 



The blood-forming tissues behave very differently from the gut under 

 continuous irradiation. The dose-rate at which they maintain fimction is 

 much lower than for the gut, and there is also some evidence that they 

 respond by a substantial decrease in mean generation time of the cells. The 

 dose-rates which can be tolerated for considerable periods of time appear to 

 differ for the different types of blood-forming tissues (Lamerton et al., 1960); 

 erythropoietic tissue being the most resistant type. At 415 and 176 rads/day 

 the count of white cells and platelets in the blood falls rapidly without any 

 evidence of stabilization, and studies with ^^Fe indicate that the erythropoietic 

 activity also collaj)ses rapidly. At 84 rads/day there is an initial fall in white 

 count and platelets followed by a transient recovery preceding the final fall 

 to 50 or 60 days when the animal dies. However, even terminally, when white 

 cell count and platelet count are very low, ^^Fe studies show a net erythro- 

 poietic activity little different from normal, though there may be some 

 difference in the relative activities of spleen and bone-marrow. This point wiU. 

 have to be investigated. 



