RADIOACTIVE TRACERS 



869 



corpuscles of the human marrow are more radiosensitive than the late 

 forms, those cells which survive radiation for 16 to 24 hours show no 

 impairment in their ability to take up ^^Fe in vitro. Nor did irradiation 

 of rabbits with 100 to 1000 r affect the uptake of ^sFe by the cells. They 

 state, in accordance with the results discussed above: "It appears that 

 the extreme radiosensitivity (of haemoglobin synthesis) shown in vivo 



1000000 



iDOOOO - 



10 000 



1000 



100 



7 9 II 13 15 



Doys after irradiation 



jFig. 12. Erythrocyte counts in the lymph of rats exposed to 



650—750 r of X-rays. 



is more likely to be due to impairment of the rate of erythrocyte form- 

 ation (mitosis) than to the impairment of haemoglobin synthesis." 



In addition to interfering with the formation of red corpuscles and 

 shortening the life-time of circulating erythrocytes, irradiation may 

 make the capillary wall permeable to erythrocytes. These leak out 

 into the interspaces (Wish, Furth, Sheppard and Storey, 1952), 

 and into the lymph flow (Ross, Furth and Bigelow, 1952) as can be 

 seen in Fig. 12, and soon get haemolysed. Thus the candle is burned at 

 both ends. No new red corpuscles are formed which should replace those 

 which terminated their life-cycle and even some of those circulating 

 can leak out. The untightness of the capillary wall is presumably due to 

 thrombopenia, as a minimum platelet level seems to be necessary to 

 keep the capillaries in a physiological state. Radiation thrombopenia is 

 due to the same causes as the haemopoietic arrest and as far as inter- 

 ference with mitotic processes is responsible for the latter, this inter- 

 ference is responsible for thrombopenia as well. 



Exposure of dogs to 500 r was found to lead within three days to a 

 mean decrease of the red corpuscle volume by 12 per cent. Erythropoietic 



