RADIOACTIVE TRACERS 



867 



As long as incomplete erythropoietic cells are present the laying down 

 of haemoglobin into these is going on which follows from i he fact that 

 it takes one to two days beforp the full effect of the haemopoietic arrest 

 gets visible by a depressed ^**Fe content of the circulating haemoglobin. 



Exposure of the mouse to 400 r leads after 30 minutes to a depression 

 of the mitotic figure of the erythrocytic series of the bone marrow to one 

 fourth of its normal value (Knowlton and Widner, 1950). 



Cell destruction 



In tadpoles exposed to 500 r haematopoietic cell destruction was found 

 to be directly correlated with the diminished rate of cell division during 

 the same time (Schjeide and Allen, 1951). When, however, massive 



Pc change in"Fe spec, acr, 

 due to exposure to 500 r 



Fig. 10. Effect of X radiation on incorporation of ^^Fe. 



doses of 10,000 to 20,000 r were applied, the rate of haematopoietic cell 

 destruction surpassed that of cell division, indicating cell destruction 

 independent of mitotic processes. Lasnitzki (1943), exposing tissue 

 cultures of avian fibroblasts to 1000 r of X-rays of low ionization density, 

 could not observe degenerated cells 30 minutes after exposure; but when 

 she exposed the tissue cultures to 1000 rep of |S-rays, having a high ion- 

 ization density, she could observe 3 per cent degenerated cells immediately. 

 These degenerated cells were due to an immediate effect both on mitotic 

 and resting cells. 



Cell death may be entirely disconnected from mitotic processes and 

 also from interference with the DNA molecule. This is shown among 



