HISTOLOGICAL CHANGES AFTER IRRADIATION 1099 



BONE MARROW 



The framework of active mammalian marrow (myeloid tissue) con- 

 sists of blood sinusoids lined with phagocytic littoral cells and a loose 

 stroma of reticular fibers with embedded fat cells and primitive and 

 phagocytic reticular cells. In the meshes of this framework are closely 

 packed groups of free cells: hemocytoblasts (free stem cells), erythro- 

 blasts (precursors of red blood corpuscles), myelocytes (precursors of 

 granular leukocytes), and megakaryocytes. In the normal adult hemo- 

 cytoblasts are rare. 



Ionizing radiations cause destruction of the free hematopoietic cells, 

 and their replacement by dilated sinuses and a gelatinous or fatty type of 

 marrow (M. A. Bloom, 1948; Dunlap, 1948; Heineke, 1903; Lacassagne 

 and Gricouroff, 1941; Martland, 1931; Pappenheim and Plesch, 1912; 

 Tullis, 1949). The bone marrow is much more susceptible to acute 

 radiation damage than many of the other organs, and the changes pro- 

 duced are easily detectable at the LD 50 30-day level and, in some species, 

 even well below this (M. A. Bloom, 1948). Thus, after 800 r total-body 

 X irradiation in the rabbit, initial damage is revealed by a cessation of 

 mitosis half an hour after treatment and a progressive degeneration of 

 hematopoietic cells, already striking at 3 hours. Some cells die very 

 soon, as evidenced by pyknosis and fragmentation of their nuclei, and 

 are then phagocytosed by macrophages; others assume abnormal forms 

 with swollen or distorted, often multilobed, nuclei, and, after an interval, 

 these too may degenerate. The first of the blood-forming cells to be 

 destroyed are the erythroblasts and hemocytoblasts, next the myelocytes, 

 and finally the megakaryocytes. In each series the younger cells are 

 more sensitive than the older forms of the same cell type. Thus, in the 

 red-cell-forming series the basophil erythroblasts are more sensitive than 

 the polychromatophil, and these in turn than the orthochromatic ones. 

 There is also some erythrophagocytosis during this period. By 4 or 5 

 days the marrow is almost completely devoid of immature red and white 

 blood corpuscles. At no stage is there any visible damage to reticular 

 cells. 



The acute destructive effects on the myelocytes are quickly reflected 

 in the drop in granular leukocytes in the circulating blood. However, 

 the acute destruction of erythroblasts is not reflected in a similar rapid 

 drop in the erythrocytes of the blood, since the latter are much longer 

 lived. The effects of irradiation on the peripheral blood cells and blood 



cells and a gelatinous intercellular substance, in addition to an occasional macrophage 

 (as in the center of the field) and some collapsed vessels. (C) After 10 days, the mar- 

 row shows a focus of regenerating hematopoiesis. Note the exceedingly large, young 

 cells with darkly stained cytoplasm (basophilia) and the great prominence of the 

 nucleoli. These cells are somewhat larger than the normal. 1030 X. 



