Erythropoietin is absolutely required for human primitive embryonic erythropoiesis. (A) Methylcellulose colony assay in various growth factor combinations. GPA⁺CD41⁺ cells were sorted at day 12 of hES/OP9 coculture and tested for colony-forming activity in the presence of different combinations of cytokines, as indicated. Cytokines were used at the following concentrations: EPO, 2 U/mL; TPO, 50 ng/mL; SCF, 50 ng/mL; and IL-3, 100 U/mL. Colonies were scored at day 9 of culture. The mean ± SD of 3 experiments is presented. (B) Morphology of Ery-P colonies grown with EPO alone (left) or with EPO + IL-3 (right). Note the difference in colony sizes.