Abstract
The in vivo intrasplenic cloning of haemopoietic cells was used as an experimental system for the study of some aspects concerned with the regulation of erythropoiesis. Experiments on the effects of polycythemia on the formation of erythroid clones demonstrated that the "feedback" suppression of such clones was 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 which were transferred to hypoxic conditions for only 48 hours before the termination of the experiment showed the formation of erythroid clones. Analysis of clone formation following such short exposures to hypoxia suggests that the endogenous erythropoietin which had been functioning in this situation acted not upon the single clone-forming cell, but rather on small cell populations which derived from the clone-forming cells in the absence of erythropoietin. There is, therefore, an early, erythropoietin-independent phase of replication of 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 erythropoietin was terminated by subjecting animals to polycythemic conditions a few days after the injection of bone marrow. It appears that in both these cases the reduction 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. The differentiation of the cloned cells is, thus, causally related to the cessation of the replicating effect of erythropoietin.
