930 RADIATION BIOLOGY 



evidence of increased sensitivity (Braes and Rietz, 1951), and erythro- 

 blast vulnerability to radiation injury is not enhanced by an increase in 

 mitotic activity; in fact, the hyperplastic erythroid tissue shows less 

 injury than the normal (Jacobson et at., 1948). 



There are equally convincing arguments in regard to primitiveness and 

 radiosensitivity. Susceptibility of the developing ovum of the rabbit 

 does not depend exclusively on its differentiation (Bloom, 1947). More- 

 over, the primitive reticular cells are exceedingly resistant although the 

 blast cells, which can develop from reticular cells, are quite sensitive 

 (Bloom, 1947; Tullis, 1949). Finally, the highly differentiated nerve cell 

 is radioresistant while the polymorphonuclear leukocyte, though well 

 differentiated, is fairly sensitive (Warren, 1942). It would appear that 

 many factors act to influence responsiveness of tissue under various con- 

 ditions of growth and differentiation. 



SPECIES SENSITIVITY 



The lethal dose of ionizing radiation for the whole animal varies not 

 only among the different species but also among animals of the same 

 species. The dosage of total-body X radiation required to kill 50 per 

 cent of adult warm-blooded mammals within 30 days ranges from 200 to 

 800 r (Dowdy, 1949). In Table 14-1 are presented the acute lethal 



Table 14-1. Comparison of the 30-day LD 6 o for a Total-body Dosage of Hard 



X Rays" 

 Animal LD 50 , r 



Guinea pig 200-400 



Swine 275 



Dog 325 



Goat 350 



Monkey 500 



Mouse 400-600 



Rat 600-700 



Hamster 700 



Rabbit 800 



a These dosages are approximate and may vary from time to time in the same or 

 different laboratories (refer to text). 



dosages for a number of laboratory animals. The absolute roentgen 

 values will differ somewhat depending upon the strain, conditions of 

 exposure, and maintenance, but the order of sensitivity remains the 

 same. The LD 50 for man is questionable; best estimates place it between 

 the LD 50 for the goat and mouse. Of interest is the radioresistance of the 

 bat, a hibernating mammal, whose life span under laboratory conditions 

 is shortened only after dosages of about 15,000 r (D. E. Smith et at., 

 1951). On the other hand, the LD 50 for the goldfish is only 850 r (Prosser 

 et al., 1947a). 



