Haemopoietic spleen colony formation in the rat: effect of 89Sr-induced bone marrow aplasia.

Regulation of spleen-colony formation, a clonal assay for haemopoietic stem cells, is very different in mice and rats. In the rat, there is an involution of the colony-forming ability of the spleen during infantile development, whereas in mice the ability is maintained throughout life. In our re-evaluation of endogenous spleen colony formation in rats after graded doses of total-body irradiation, colonies ceased to appear after 12 weeks of age. However, histological sections showed that there were tiny colonies growing in the spleen, even at 20 weeks. These microscopical colonies developed into visible colonies when the rats were given treatments that increased the haemopoietic requirement. Thus, the amount of 59FeCl3 incorporated into the spleen increased to compensate for a decrease in uptake by the bone marrow. Rats in which bone-marrow activity was inhibited by 89SrCl2, showed extensive colony formation in the spleen, even after 12 weeks of age, when the endogenous colonies were induced by a sublethal dose of radiation. About twice as much 59Fe acetate activity was incorporated into the spleens of the experimental animals than in those of control rats. These findings imply that spleen-colony formation responds to the haemopoietic requirement of the spleen rather than that of the bone marrow. Furthermore, the requirement of the spleen seems to be much smaller in rats than in mice, because bone-marrow capacity for haemopoiesis is relatively larger in rats.