Expression of Epo, SCF, and cortisol and their receptors. (A) Primary definitive proerythroblasts (ProEs) express receptors for Epo (EpoR), glucocorticoids (Nr3c1), and SCF (Kit). In contrast, primary primitive ProEs, derived from E9.5 YS, express similar levels of EpoR but nearly undetectable levels of Nr3c1 and Kit (mean ± SEM; N = 3; *P < .001, t test). (B) ESREs expressed high levels of cMyb transcripts compared with fetal liver and bone marrow definitive proerythroblasts. cMyb transcripts were not detected in primitive proerythroblasts. (C) Epo and SCF transcripts are expressed at higher levels in the mid-gestation fetal liver than in the adult bone marrow (BM) or kidney (mean ± SEM; N = 3). (D) Cortisol levels were detected by ELISA in fluid from embryonic and adult mouse tissues (mean ± SEM; N = 3). BD indicates below detection.