1068 



RADIATION BIOLOGY 



tissue occurs even though cysteine or estrogens have been given prophy- 

 lactically. The decreased effect observed in the peripheral blood is due 

 to a more rapid regeneration of the hematopoietic tissue from the more 

 radioresistant precursors such as reticular cells, whereas the effect 

 observed when phenylhydrazine is given prophylactically is that hemato- 

 poietic tissue, including reticular cells, blast forms, and perhaps even more 

 mature forms, survives and is immediately capable of accelerated produc- 

 tion. Regeneration of granulocytic tissue is more rapid than regenera- 

 tion of lymphatic tissue when cysteine has been given prophylactically if 

 the peripheral blood can be used as a measure of the production of 

 granulocytes and lymphocytes. On the other hand, phenylhydrazine, 

 although producing a panhyperplasia of the bone marrow, produces a 

 more extreme erythroblastic hyperplasia which persists even after irradia- 

 tion. Erythropoiesis is the major initial regenerative activity of the 

 bone marrow under these conditions. 



"THERAPEUTIC" MEASURES 



Death of animals exposed to single total-body dosages of ionizing 

 radiations in the lethal range is assumed to be due to the destruction of 

 certain tissues in the body and failure of functional reconstitution of one 



Table 16-5. Survival of Mice Exposed to Various Dosages of X Radiation 



WITH AND WITHOUT SHIELDING OF THE SURGICALLY EXTERIORIZED SPLEEN 



a Technique differed in that the artery at the distal tip of the spleen was not severed, 

 and thus no infarct occurred in the spleen. 



or more of the tissues (e.g., the hematopoietic system) that individually 

 or collectively are vital to the survival of the animals. Although many 

 reasons for the importance of the blood-forming tissue to survival can be 

 enumerated, it cannot be stated with certainty that death or survival of 

 the irradiated animal invariably depends on the direct or indirect results 



