492 PAUL S. HENSHAW 



0.0002 r 

 Fig. 2. Levels of exposure for consideration. 



to be of the same type as those resulting from the larger doses, but less 

 precipitous. 



Doses of 2,000 r applied to the whole body of mammals leads to death 

 in 2 to 3 weeks, but it is more due to damage in intestinal epithelium and 

 hemopoietic organs than in the nervous system. Doses of 2,000 r are espe- 

 cially interesting, since they are below the level required to cause the so- 

 called nervous system death, yet are sufficient to cause death in days or 

 weeks. The key to the situation appears to lie in the fact that this level of 

 dosage causes extensive damage, and in some cases complete destruction, 

 of the growing or progenitive tissues. 



This is illustrated most diagrammatically in the skin (Fig. 3). Skin epi- 

 thelium has at its base a germinal layer (stem cells), which is the source 

 of cells giving rise to functional squama, the dry protective outer layer. 

 Cells originating in the germinal layer mature or differentiate as they move 

 outward to form the squama. True to a general finding in radiobiology, the 

 more primitive proliferating cells of the germinal layer are the most sensitive 

 to radiation. When cells of this layer are damaged, the renewal or replace- 

 ment process is impaired. When all are destroyed, stopping replacement 

 altogether, skin breakdown with development of an open lesion is inevitable. 

 This arises as surface cells wear away and disappear. 



The germinal layer of the testis involves similar renewal and maturation 

 processes, the renewal elements being the most sensitive to radiation. The 

 same situation exists with respect to intestinal epithelium, hemopoietic tis- 

 sues, and the lens of the eye (Henshaw, 1958), each tending to show failure 

 of some kind (sterility, anemia, poor assimilation, cataracts) following 2,000 

 r of exposure. 



When doses are somewhat less than 2,000 r or when only parts of the 

 body are exposed, death may not occur, and often there is recovery in the 

 form of regrowth of depleted progenitive elements, the regrowth stemming 



