Inverse Relationship between Splenomegaly and Stem Cell Compartment Size in Mice Treated with Nitrogen Mustard

Following the administration of similar doses of nitrogen mustard (4 mg/kg) to different strains of mice, wide variations in the subsequent degree of splenomegaly were observed, implying strain differences in the role of the spleen in the compensatory erythropoietic response to haematopoietic stress. This investigation was undertaken to determine whether or not these differences were related to the size of the haematopoietic stem cell compartment size in the various strains of mice. Groups of 4 different strains of mice (Swiss Webster, A/J, C57BL/6J and CS1/ASH) were injected i.v. with nitrogen mustard (4 mg/kg body weight) and autopsied at regular intervals up to 20 d post‐injection. At autopsy, the wet weight of the spleen was determined. Subsequently, groups of the same 4 strains of mice were exposed to single doses of wholebody γ‐irradiation in the range of 500–900 rads. 9 d after γ‐irradiation the mice were autopsied, their spleens removed, and the number of endogenous spleen colonies determined. The greatest degree of splenomegaly was observed in the C57bl/6J mice. The Swiss Webster mice showed no splenomegaly during the time period studied. There existed a linear inverse relationship between the maximum degree of splenomegaly observed and the dose of wholebody γ‐irradiation required to completely eliminate endogenous spleen colonies. This data is in accord with the hypothesis that there exists an inverse relationship between the extent of splenomegaly observed following haematopoietic stress and the haematopoietic stem cell compartment size.