HEMATOLOGIC EFFECTS OF RADIATION 1073 



injury (Jacobson, Simmons, Marks, and Eldredge, 1951; Jacobson, Sim- 

 mons, Marks, Gaston, et al., 1951), but certain tissues and, in particular, 

 the hematopoietic system have a greater potential production per unit 

 volume. 



There are several differences between head or limb shielding and spleen 

 implantation (to be described later) and spleen shielding. According to 

 generally accepted concepts, the reduction in volume dose when the head, 

 limb, or intestine is shielded must be considered as playing a role in the 

 reduction of mortality since these structures represent a fairly large pro- 

 portion of the body weight (15, 7.5, and 12.5 per cent, respectively). On 

 the other hand, the shielded spleen weighs only 0.1 g (0.005 per cent of the 

 body weight) and spleen implants weigh 0.010 g (0.0005 per cent of the 

 body weight) which eliminates from consideration the volume-dose factor 

 of shielded spleen or implanted spleen. The fact that the head, hind 

 limbs, and intestine contain reticuloendothelial tissue and other tissue of 

 mesenchymal origin is probably more important than the volume-dose 

 consideration. 



Postirradiation Spleen Trans-plantation. Transplantation of spleens 

 (total weight, 10 100 mg) from baby or adult mice into the peritoneal 

 cavity of mice within 2 hours after exposure of the recipient adult mice to 

 1025 r of total-body X irradiation significantly increases the survival 

 (ca. 50 per cent) of the irradiated mice and hastens regeneration of 

 hematopoietic tissue (Jacobson, Simmons, Marks, and Eldredge, 1951; 

 Jacobson, Simmons, Marks, Gaston, et at., 1951). Transplantation of 

 spleens into the peritoneal cavity of mice 1 or 2 days after exposure to 

 1025 r of total-body X irradiation likewise enhances survival (ca. 25 per 

 cent) but not as effectively as earlier transplantation. Implantation of 

 muscle into the peritoneal cavity after exposure of mice to 1025 r of total- 

 body X irradiation has no beneficial effect on survival. 



If splenectomy is performed in mice prior to irradiation, followed by 

 implantation of fresh spleens into the peritoneal cavity after irradiation, 

 survival is enhanced, indicating that an intact spleen is not required to 

 make the transplant effective. Surgical removal of the transplanted 

 spleens from the peritoneal cavity of mice 1 and 2 days after the irradia- 

 tion-transplant procedure has invariably been followed by death of the 

 animals. It is assumed that the ineffectiveness of this procedure is due 

 to the fact that the implanted spleens are not vascularized prior to 

 removal and are therefore incapable of elaboration and distribution of 

 the factor. Gross and microscopic observations on mice surviving the 

 irradiation-transplant procedure reveal that the implanted spleen or 

 spleens have vascularized and eventually appear as normal splenic tissue. 

 Revascularization and reconstitution of the implanted spleen are usually 

 well under way by the sixth day after implantation. Transplantation of 

 splenic tissue 2 days after irradiation is admittedly less effective in 



