22 



H. MAISIN 



irradiated with 500 r died of medullary syndrome. It was also interesting 

 because different authors had bound radioresistance with higher mitotic 

 activity not only in radiosensitive organs but even more in radioresistant ones 

 (Knowlton and Hempelmann, 1949). As bone-marrow is highly radiosensitive 

 it was of the utmost interest to evaluate its reaction to low doses particularly 

 in our rats because such doses do not involve the appearance of radioresis- 

 tance. We thought it best to estimate the marrow activity by using three 

 tests: the variation of the total number of marrow cells; the incorporation of 

 ^^Fe in the red cells (Maisin, 1959) and the enumeration of reticulocytes in the 

 peripheral blood. 



Number of marroiv cells 



The results obtained in the rats pre-irradiated with 1 r and 0-1 r and in the 

 control non-irradiated rats after different periods of time and their statistical 

 significance (evaluated by t-test) are presented in Tables III and IV. 



Table III. Total number of nucleated bone-marrow cells per mg marrow 



Time after 



irradiation 



(days) 



Ir 



Controls 



Significance 

 (t-test) 



Table IV. Total number of nucleated bone-marroiv cells per mg marrow 



Time after 



irradiation 



(days) 



0-1 r 



Controls 



Significance 

 (t-test) 



After 1 r, the total number of cells per mg of marrow is statistically 

 increased at the 28th, 56tli and 98th days; after 14 days, there is no difference. 

 The difference in number of cells in favour of the rats irradiated with 1 r is 

 also interesting by the 84th day and slightly significant by the 42nd day. 



