Regulation of Erythropoiesis (XXIV). Studies on the Post-Hypoxic "Rebound" Phase

After exposure to hypoxia there was only a moderate stimulation of erythropoiesis in a strain of mouse which produces minimal amounts of erythropoietin in response to such a stimulus. This was associated with minimal changes in bone marrow CFU’s, but there was a striking decrease in splenic CFU’s. This splenic depopulation seemed related to a specific effect of hypoxia rather than stem cell depletion secondary to erythroid differentiation. On return to ambient pressure there was a prompt onset of splenic red cell aplasia, but only a gradual decrease in marrow erythropoiesis. Despite the prompt splenic red cell aplasia, there was only a slow return to splenic CFU’s to normal levels during the first 4-5 days. Thereafter a slight rebound or overshoot was noted. Although differentiated erythroid elements were still present in the marrow on the fourth post-hypoxic day, a moderate rebound in marrow CFU’s to 150 per cent of control was evident. These findings might best be explained by a migration of stem cells during the post-hypoxic interval. Marrow lymphocytes decreased during hypoxia and rose gradually there-after following, rather than preceeding, the increase in bone marrow CFU’s.