Abstract
An assay method for erythropoietin-sensitive cells has been developed based on the fact that cells of mouse fetal liver respond to erythropoietin in vitro by increased heme synthesis. To determine whether or not hemopoietic colony-forming stem cells are identical with erythropoietin-sensitive cells, C3H/Bi 13-day fetal liver cells were separated into fractions on the basis of size by unit gravity sedimentation through a 1-2 per cent bovine serum albumin gradient. The cell fractions obtained were assayed for erythropoietin-sensitive cells by the present method and for spleen colony-forming cells by the method of Till and McCulloch. It was found that the modal sedimentation velocity of erythropoietin-sensitive cells was greater than that of the spleen colony-forming cells of mouse fetal liver, showing that these two classes of cells are distinct.
