PrlR is as efficient as CHI in supporting EpoR^−/− CFU-e colony formation. (A) Each determination is the mean ± SEM of four independent experiments. Fetal liver cells were cultured in methylcellulose medium containing 500 ng/mL ovine prolactin, 100 ng/mL rat SCF (rSCF), and 10% PDS. Between 3 and 7 EpoR^−/− livers or livers from wild-type littermates were used per experiment. In the presence of Epo, an average of 5,000 CFU-e colonies per wild-type fetal liver, and 12 CFU-e colonies per EpoR^−/− fetal liver were obtained. Wild-type and EpoR^−/− fetal livers contained an average of 2.5 × 10⁵ and 5 × 10⁴ nucleated cells per liver, respectively. (▧), EpoR^−/−; (▪), EpoR^+/− or ^+/+. (B) EpoR^−/− CFU-e colonies supported by PrlR are qualitatively similar to EpoR^−/− CFU-e colonies supported by CHI or wild-type CFU-e colonies differentiating in response to Epo. Scale bar: 100 μm.