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Effects of human recombinant erythropoietin on differentiation and distribution of erythroid progenitor cells on murine medullary and splenic erythropoiesis during hypoxia and post-hypoxia.

Hemopoietic cells, the extracellular matrix, growth factors and the microenvironment are involved in the regulation of hemopoiesis. Although the regulation of erythropoiesis is well understood at the cellular level in vivo and in vitro, the role of hemopoietic sites of erythroid progenitors production has not been well defined in both steady state conditions and in stress erythropoiesis. In this study we examined the qualitative erythroid differentiation and quantitative changes of the erythroid progenitors in different erythropoietic organs during erythropoiesis of stress in a hypoxia-induced polycythemia and post-hypoxic changes in a mice model. Chronic intermittent exposure to hypobaric hypoxia induced polycythemia in mice and the post-hypoxic period was characterized by total suppression of erythropoiesis. The number and distribution in hemopoietic sites of Immature Erythroid Burst (BFU-EI), Mature Erythroid Burst (BFU-EM) and Erythroid Colony Forming Units (CFU-E) was evaluated in bone marrow and spleen of hypoxic and post-hypoxic mice after removal from the chamber. The number of BFU-EI and CFU-E, was evaluated in both femoral bone marrow and spleen of ex-hypoxic polycythemic mice, at two times intervals after the end of hypoxia. We found that in both bone marrow and spleen, the kinetics of the CFU-E pool was characterized by a sharp fall from above normal to lower than normal levels. BFU-EM increased from normal to higher than normal levels. These results have been correlated with both erythropoietin (EPO) and the erythropoietic activity. The results show that EPO levels largely control both the differentiation and the amplification of the CFU-E pool and they suggest that EPO may acts as a "survival factor" at the CFU-E level and/or increase the flow of cells from BFU-E to CFU-E. After the termination of the period of hypoxia and during post-hypoxia there was a reduction in EPO production which subsequently caused a depletion of the CFU-E population, indicating that the size of the CFU-E pool is EPO-dependent. After the injection of 1U of recombinant human erythropoietin (rHuEPO) the size of that pool was increased and the pool of BFU-EI was decreased. It is noteworthy that our studies show that the spleen functions as a large reservoir of erythroid precursors for hypoxia-induced stress erythropoiesis.