1106 RADIATION BIOLOGY 



severe depletion of the organ in 4 days. In contrast to large single doses 

 of external irradiation in which there is marked regeneration, there is none 

 in those areas of the thymus in the range of long-continued bombardment 

 from isotopes in nearby tissues. 



SPLEEN 



The spleen is made up of lymphatic tissue which is typical in its 

 white pulp and atypical in its red pulp. The white pulp ensheaths the 

 arteries; the red pulp surrounds the radicles of the veins and consists 

 mainly of reticular cells and fibers and a few lymphocytes ; it is permeated 

 by circulating blood. Since lymphopoiesis occurs in the while pulp and, 

 in some mammals, ectopic myelopoiesis in the red pulp, the effects of 

 ionizing radiations on these functions may be compared here within a 

 single organ. The white pulp, essentially dense lymphatic tissue with 

 nodules, reacts in a fashion similar to that of lymph nodes. 



After a single exposure of rabbits to 800 r of total-body X irradiation, 

 the spleen is rapidly reduced in size during the first day, and for a week or 

 longer it remains small and dense, with relatively small and inconspicuous 

 areas of white pulp, lacking nodules or germinal centers (Murray, 1948a). 

 Neither the germinal centers nor the white pulp as a whole recovers 

 appreciably until about two weeks after treatment. Mitosis stops in a 

 few minutes, and within the next few hours most of the lymphocytes 

 throughout the white pulp die and the germinal centers, normally rich in 

 medium-sized lymphocytes, are obliterated as such, leaving dense masses 

 of reticular cells. In the red pulp, lymphocytes suffer the same change 

 as in the white pulp, and the relatively few erythroblasts and myelocytes 

 present show damage at early intervals and are absent at 5 and 9 days. 

 The period from 3 to 17 hours is one of rapid phagocytosis by reticular 

 cells of the debris from this widespread degeneration in both white and 

 red pulp and from invading heterophil leukocytes. With the removal of 

 the dead cells, the spleen, particularly the white pulp, shrinks, leaving 

 the reticular cells prominent. A phase of relative inactivity ensues (1 to 

 9 days) with further condensation of the reticular cells, the few abortive 

 attempts at regeneration being by transformation of reticular cells. 

 Finally (10 days to four weeks) a phase of reconstitution takes place 

 through mitotic activity of the lymphocytes. The regeneration of 

 erythroblasts and myelocytes and new formation of megakaryocytes 

 occur after the resumption of lymphopoiesis. 



On the other hand, in mice and rats receiving 350 and 600 r (total- 

 body), respectively, erythropoiesis is renewed much earlier. It expands 

 from a few foci in the red pulp to such an extent that it sometimes 

 encroaches upon the white pulp and apparently interferes with regenera- 

 tion there. Megakaryocytes, which are numerous in the normal spleen 



