During erythropoiesis, erythroid progenitor cells proliferate in response to erythropoietin (Epo) and progressively differentiate, which encompasses an increasing accumulation of iron-bound hemoglobin. The impact of Epo on these dynamic processes remained to be resolved. By combining the development of a time-collapsed super SILAC method with transcriptome analysis we examined Epo-induced alterations in the proteome of erythroid progenitor cells at the colony-forming unit erythroid stage (CFU E). We showed that the timing of a major increase in the transcription factors GATA1 and TAL1 precedes the cessation of cell cycle progression but coincides with a massive upregulation of enzymes of the heme biosynthetic pathway. A failure of this upregulation due to the absence of the Epo receptor resulted in massive iron accumulation in the fetal liver and dilation of the maternal vasculature in the placenta. Thus, Epo-induced dynamic proteome adaptations in CFU E cells not only prevent anemia but also severe iron-intoxication in embryos. Overall design: Heterozygous erythropoietin receptor null (EpoR+/-) BALB/c mice were obtained from Harvey F. Lodish (Whitehead institute for Biomedical Research Cambridge, Massachusetts). To obtain EpoR-/- embryos EpoR+/- mice were breed and the embryos were genotyped by using gene-specific primers (5'-GCACTGAGTGTGTTCTG-3', 5'-GCCTCA CACTCT TCACC-3' and 5'-GCTGCTAAAGCGCATGC-3'). Fetal livers from EpoR-/- BALB/c mice embryos at 13.5 gestational day were dissected from the uteri of the sacrificed mice. Fetal liver cells were re-suspended in 500 μl 0.3% BSA/PBS, passed through 40 μm cell strainers (BD Biosciences) and subsequently treated with 10 ml Red Blood Cell Lysis Buffer (Sigma-Aldrich) to remove erythrocytes. To isolate CFU E cells, a negative selection was performed, in which the fetal liver cells of 40 - 50 livers were incubated for 30 min at 4°C with 10 μl rat antibodies against the following murine surface markers: GR1, CD41, CD11b, CD14, CD45, CD45R/B220, CD4, CD8 (all BD Biosciences), TER-119 (from A. Müller, Würzburg, Germany) and with the rat monoclonal antibody YBM/42 (from S. M. Watt, Oxford, UK). Cells were washed three times in 0.3% BSA/PBS and incubated for 30 min at 4°C with anti-rat antibody-coupled magnetic beads (BD Biosciences). MACS columns (Miltenyi Biotech) were applied for sorting according to the manufacturer’s instructions. Sorted immediately used for microarray analysis.