Evidence for proliferation of erythroid progenitor cells in the absence of added erythropoietin.

The cellular mechanisms underlying the requirement of leukocyte conditioned medium for human erythroid colony formation has been studied. Using photographic methods it was found that erythroid progenitor cells were able to form small colonies in the absence of added erythropoietin (epo). The addition of epo at this time leads to further growth and hemoglobinization of cells within the colony. Leukocyte conditioned medium was found to be required for this early proliferative stage of erythroid colony formation.