962 RADIATION BIOLOGY 



of various radioelements may be attributed to their anatomical distribu- 

 tion and the specific ionization and energy of the radiating particles. 



The histological picture is characterized by disappearance of mitotic 

 figures, cell degeneration, and death, leading, a few days later, to aplasia 

 of the marrow and lymphoid tissue (Bloom and Jacobson, 1948). Edema 

 and hemorrhage with subsequent fatty infiltration of the hypoplastic 

 marrow cavity may occur several days after irradiation (Lutwak-Mann 

 and Gunz, 1949). The early chemical changes in the blood-forming 

 tissues are obscure. A transitory increase in oxygen consumption, 

 carbon dioxide production, and hemin synthesis has been seen in marrow 

 homogenates prepared from rabbits immediately after 800 r of X irradia- 

 tion (Altman et al., 1951). Synthesis of RNA and DNA by rabbit bone 

 marrow is inhibited, however, while that of protein is unaffected, during 

 the five hours following similar irradiation (Abrams, 1951). An early 

 increase in respiration has not been observed in irradiated rat thymus 

 (Barron, 1946). Respiration and anaerobic glycolysis of marrow and 

 thymus decline below the preirradiation level during the first few days 

 after exposure; recovery is apparent around the tenth day (Lutwak-Mann 

 and Gunz, 1949; Barron, 1946). Nucleic acid phosphorus in the marrow 

 reaches a minimum value between the second and fourth days and is still 

 below normal at two weeks (Lutwak-Mann and Gunz, 1949). It is of 

 interest that fatty acid synthesis is increased soon after X irradiation; 

 this may account, in part, for the deposition of fat in the hypoplastic 

 marrow cavity (Altman et al., 1951). 



Greatest sensitivity in the marrow is shown by the erythrocytic pre- 

 cursors (M. A. Bloom, 1948; Rosenthal, Pickering, and Goldschmidt, 

 1950; Denstad, 1941). Precursors of granulocytes and platelets are 

 somewhat less sensitive, and the primitive reticular cells are quite 

 resistant. Erythropoietic sensitivity is reflected physiologically by an 

 early decrease in the uptake of iron by bone marrow. Twenty-four 

 hours after exposure to 250 r, iron uptake in the rat may be decreased by a 

 factor of 10 (Hennessy and Huff, 1950). In contrast to these findings, 

 recovery from a standard anemia produced by phlebotomy immediately 

 prior to irradiation with 200 r is only slightly less rapid in irradiated than 

 in control cats (Valentine and Pearce, 1951). On the basis of functional 

 impairment, it also appears that erythropoietic tissue is less sensitive to 

 radiation injury than myelopoietic, but this may be a consequence of the 

 preferential influence of phlebotomy on the relative recovery rates of the 

 two tissues. Since cells of the reticulum appear to be the common 

 progenitor, recovery may depend upon the factors involved in their transi- 

 tion to the various cell types. Formation of new cells from the reticular 

 cells is said to be inhibited completely during the first week following 

 median lethal irradiation (Rosenthal, Pickering, and Goldschmidt, 1950). 



The blood-forming organs become actively hematopoietic within one to 



