Studies on the Regulation of Hemopoietic Spleen Colonies

The in vivo intrasplenic cloning of haemopoietic cells was used as an experimental system for the study of some aspects concerned with the regulationof erythropoiesis. Experiments on the effects of polycythemia on the formationof erythroid clones demonstrated that the "feedback" suppression of such cloneswas inhibited if the polycythemic animals were kept under hypoxic conditions. Overloading with oxygen is, thus, an essential factor in the polycythemia-induced suppression of erythroid clones. Polycythemic animals whichwere transferred to hypoxic conditions for only 48 hours before the terminationof the experiment showed the formation of erythroid clones. Analysis of cloneformation following such short exposures to hypoxia suggests that the endogenous erythropoietin which had been functioning in this situation actednot upon the single clone-forming cell, but rather on small cell populationswhich derived from the clone-forming cells in the absence of erythropoietin.There is, therefore, an early, erythropoietin-independent phase of replicationof the clone-forming cells. The application of erythopoietin only during this"early" phase of replication resulted in the formation of erythroid clones, which,following the discontinuation of erythropoietin, disappeared from the spleen.Similar results were obtained when the action of endogenous erythropoietinwas terminated by subjecting animals to polycythemic conditions a few daysafter the injection of bone marrow. It appears that in both these cases thereduction in the number of spleen colonies following discontinuation of erythropoietin activity is due to the total maturation of the erythroid colonies,ending in the evacuation from the spleen of the mature red blood cells. Thedifferentiation of the cloned cells is, thus, causally related to the cessation ofthe replicating effect of erythropoietin.

Submitted on June 22, 1966 Accepted on November 24, 1966