HISTOLOGICAL CHANGES AFTER IRRADIATION 1107 



of these animals, are greatly decreased by 3 days but begin to reappear 

 in mice at 5 days after 350 r and in rats after three weeks following 600 r. 



The sequence and type of effects are similar with lower doses, but fewer 

 cells die, the inactive period is shorter, and regeneration is more rapid. 

 The normal structural pattern shows little disruption below 175 r, and 

 damage to lymphocytes is not seen below 25 r. In different species, 

 damage to the spleen is of the same magnitude for the same size dose, 

 irrespective of differences in the LD 50 for these species (Murray, 1948a). 



Fast and slow neutrons resemble X rays in their qualitative effects on 

 the spleen. Equivalent fractions of the LD 5 o 30-day level produce similar 

 effects. Estimations of their biological effectiveness on the spleen are, 

 roughly, X/fast neutrons = 4; X/slow neutrons = 0.85. 



Daily doses of 80 r of total-body X rays severely deplete the mouse 

 spleen by 20 treatments, but erythropoiesis is definitely elevated after 24 

 treatments. Depletion is less than after 350 r (total-body) given in a 

 single dose. 



Seven hundred roentgens of y rays (from an external source of radium) 

 administered over a three-month period at the rate of 8.8 r per 8-hour day 

 causes less damage to the spleen of guinea pigs than a single total-body 

 dose of 175 r of X rays (Lorenz, Heston, Jacobson, et at., 1953). A similar 

 experiment with mice shows a gradual shrinkage of the organ with pro- 

 gressive depletion of white pulp (Spargo et al., 1951). In the red pulp 

 an increase in erythropoiesis begins at 6 months after 8.8 r per day and 

 becomes progressively greater up to 16 months, with hemocytoblasts 

 predominating at the latter interval. After 4.4 r per day erythropoiesis 

 increases only at 14 and 16 months. In all these chronically irradiated 

 mice there is a progressive increase in mast cells, reaching a maximum in 

 the 8.8-r group of 4.5 times the number in controls. 



Autoradiographs show all isotopes more concentrated in the red pulp 

 than in the white, the deposition often being especially heavy at the 

 transition between the two zones. With Zr 93 -Cb 93 , Y 91 , and radium there 

 is some deposition in the white pulp, mainly around the arterioles. 



When radioactive isotopes lodge, even temporarily, in the spleen, the 

 effects, with one important exception, are similar to those produced by 

 external irradiation and consist in depletion of white pulp and shrinkage 

 of the organ. Indeed, the changes due to administration of appropriate 

 amounts of Na 24 cannot be told from those due to X rays. However, the 

 injection of bone-seeking isotopes such as Y 91 , Sr 89 , Ba 140 -La 140 , plutonium, 

 or radium into small mammals causes an extreme depletion of hemato- 

 poietic cells in the bone marrow, and the red pulp of the spleen develops 

 an accelerated hematopoiesis. Zr 93 -Cb 93 , however, in addition to produc- 

 ing a marked depletion of splenic white pulp, differs from the other bone- 

 seeking (3 emitters in not eliciting a marked compensatory hematopoiesis 

 in red pulp. In fact, erythrophagocytosis is striking in the red pulp 



