HISTOLOGICAL CHANGES AFTER IRRADIATION 1101 



gradient of injury occurs with X rays and, to a less extent, with slow 

 neutrons, but not with fast neutrons. Secondary rays, emitted by the 

 bone spicules of the metaphysis after X irradiation and to a less extent 

 after slow-neutron bombardment, may perhaps account for these differ- 

 ences. 



Both depletion and regeneration take place more slowly in response to 

 the radioactive isotopes than to any of the external ionizing radiations. 

 Slow degeneration explains the absence of cellular debris, and delayed 

 regeneration is undoubtedly the effect of the continued bombardment by 

 long-lived isotopes. 



In addition to the degenerative changes characteristic of bone marrow 

 subjected to external irradiation, an atypical fibrous bone develops from 

 spindle cells in the marrow cavity after administration of many of the 

 radioactive isotopes. This will be described in the section on bone. 



Study of aspirated vertebral marrow from atomic bomb casualties at 

 Hiroshima disclosed the disappearance, within the first week, of myeloid 

 tissue and the presence of atypical cells, many of them resembling plasma 

 cells. These findings are confirmed and extended by histological sections 

 of the long bones and sternum obtained at autopsy, in which multiplica- 

 tion of reticular cells is seen at 12 days (Liebow et al, 1949). The failure 

 to find in the Hiroshima material the early degeneration of erythroblasts, 

 previously described on the basis of closely spaced specimens from 

 laboratory animals (M. A. Bloom and W. Bloom, 1947), probably rests 

 on the scanty material obtained during the first weeks after the explosion 

 of the bomb. In the authors' opinion, the residual foci of erythroblasts 

 described in the Hiroshima material may well be foci of regenerating 

 erythroblasts. 



It may not be amiss to point out here that a variety of toxins can 

 duplicate in detail the bone marrow changes which follow irradiation. 



LYMPHATIC ORGANS 



Lymphatic tissue is composed of a framework of reticular cells and 

 fibers with vast numbers of large, medium-sized, and small lymphocytes 

 filling the meshes. The relative proportions of lymphocytes to stroma 

 determine whether the lymphatic tissue in a given location is loose, dense, 

 or nodular (Maximow and Bloom, 1948). 



Lymph nodes and the accumulations of lymphatic tissue in other 

 organs (tonsils, solitary nodules and Peyer's patches of the intestinal 

 tract, and appendix) are exceedingly susceptible to ionizing radiations, 

 and, as in the other hematopoietic organs, the degree of injury is propor- 

 tional to the dose, within certain ranges. Damage is evidenced by 

 massive destruction of lymphocytes of all sizes with consequent striking 

 changes in the lymphatic nodules and reduction in size of the organ. 



