1056 RADIATION BIOLOGY 



recovery processes continued for a period of several weeks. In a few 

 animals the increase in leukocytes approached normal, but on an average 

 it amounted to approximately one-half normal. Beginning at approxi- 

 mately the twelfth week and continuing to the twenty-fourth week, one 

 animal after another developed a recurrent pancytopenia and died. 

 There was a persistent increase in the percentage of circulating reticulo- 

 cytes, and the color index also increased. Hemorrhage appeared to play 

 no important role in the terminal picture since gross or microscopic evi- 

 dence of hemorrhage was minimal or absent. 



Group c. The third pattern, c, was one of complete recovery. The 

 initial postirradiation hematologic findings were identical with those of 

 the other two patterns. Recovery from the anemia began approximately 

 six weeks after the termination of the exposure. A rapid increase of all 

 cellular elements during the subsequent six weeks was followed by a 

 gradual approach to normal. This was achieved most rapidly by the 

 heterophils and was delayed longest in the platelets. However, full 

 recovery took place over a period of many months. It is of interest to 

 note that, although the increased red-cell diameter (macrocytosis) per- 

 sisted for about sixty weeks, reticulocytes returned to normal after 

 approximately ten weeks. 



The histologic examination of animals that died in the three groups 

 may be summarized as follows: The bone marrow of the animals of 

 group a showed a slight-to-considerable decrease in cellularity with an 

 approximately normal proportion of the different cellular components. 

 In the animals that died in this group and in those of group b that died 

 earliest, the degree of cellularity of the bone marrow was normal or nearly 

 normal or even hyperplastic, but the proportion of the various cellular 

 components was not normal. In the animals of group c the bone marrow 

 was comparable to that of nonirradiated controls. Hemorrhage played 

 no significant role in the development of the terminal anemia in these 

 three groups of animals since gross or microscopic evidence of hemorrhage 

 was minimal or absent. 



HEMATOLOGIC EFFECTS OF EXPOSURE 

 TO INTERNALLY ORIGINATING RADIATION (RADIOELEMENTS) 



So far as the peripheral blood is concerned, the sensitivity and rapidity 

 of the effect on blood-forming tissue of the radioelements and subsequent 

 reflection in the hematologic picture depend on the dosage and the extent 

 of localization within hematopoietic tissue. With all the radioisotopes 

 studied by Jacobson, Marks, and Lorenz (1949), whether they localized 

 in bone or more generally throughout the body, a reduction in the 

 lymphocyte values of the peripheral blood was found to be the most 

 sensitive indicator of acute or subacute effect. This was also true for 



