NATURE OF SPLEEN-BONE MARROW RADIATION RECOVERY FACTOR 



is effective. Thus, we have found that homogenates of spleens from young 

 G-57 black mice do not protect X-irradiated (640 r) C-57 black mice 

 against death, whereas a single injection of 8mg (wet weight) of homo- 

 logous bone marrow affords protection in 80 per cent of the animals after 

 700 r. One may infer a generalization from such results — namely, that in 

 a given animal species or strain, the homologous post-protective factor is 

 always present in the bone marrow ; and that in some species or strains 

 (for example, LAf^ mice), it is also present in the spleen — presumably associ- 

 ated with primitive haematopoietic function. According to Cronkite 

 et al^^ mouse spleen ' normally shows extensive extramedullary myelopoiesis, 

 resembling myeloid metaplasia in other species'. Likewise, Friedell and 

 Salerno^** conclude from their internal radioisotope studies that 'the 

 spleen is much more important in support of the haematopoietic system in 

 the mouse than in the rat'. Cronkite^^ states that there is no evidence 

 that you can get protective activity with the spleen unless it is a site of 

 myelopoiesis. It seems likely, then, that the spleen is a source of the 

 recovery factor only in so far as it may contain certain bone marrow-like 

 cells (presumably primitive myelopoietic cells). It follows from these 

 considerations that the protective principle of spleen and bone marrow is 

 derived (or is a product of) the same cell type, and is probably identical 

 chemically. The experimental data presented above provide some support 

 for this concept. 



If the radiation recovery factor in bone marrow and that in spleen are 

 functionally identical, and if the marrow provides the largest pool of the 

 factor, then it may be anticipated that the surgical removal of the spleen 

 from a mouse would not necessarily result in any increase in the sensitivity 

 of the animal to whole-body X-irradiation. This would be especially true 

 if a sufficient time interval between splenectomy and radiation exposure 

 occurred — presumably permitting readjustment of bone marrow haemato- 

 poietic function to the absence of spleen. To conclude from the results of 

 splenectomy experiments in mice that the unshielded spleen contains no 

 radiation recovery factor ^^ is unjustified on logical grounds — it appears to 

 the authors — as well as on the basis of the experimental fact that injection 

 of mouse spleen homogenates into otherwise lethally X-irradiated mice 

 affords them protection against death. 



SUMMARY 



Since previous data indicate that the radiation recovery factor in the nuclear 

 fraction of mouse spleen homogenates is inactivated by treatment with 

 DNA-ase and with trypsin, it was pertinent to test the hypothesis that intact 

 viable spleen cells are not susceptible to the action of these enzymes. Fresh 

 spleen cells were obtained from gently minced (with fine scissors) spleen 

 fragments, and incubated with DNA-ase at 25° C for 30 minutes. Under 

 these conditions only 2 per cent of the total cellular DNA was liberated 

 into the supernatant. In contrast, as much as 45 per cent of the total 

 cellular DNA was solubilized by the action of DNA-ase on spleen homo- 

 genates containing the same cell count as the cell suspension. These data 

 support the view that DNA, as present in intact, non-homogenized spleen 

 cells is not available as a substrate for added DNA-ase. 



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