In vitro differentiation of definitive erythroid cells from the mid-gestation murine fetus. Erythroid lineage differentiation begins with hematopoietic stem cell (HSC) commitment to differentiation (first solid arrow) and ends with enucleation (dashed arrow) of the erythroblast (EB) to form the reticulocyte (Retic). Stages shown are part of a continuum with multiple cell divisions at and between the successive stages. Stages below the dashed line are committed to erythroid differentiation, whereas those above the dashed line have potential to differentiate along nonerythroid lineages. The open reflexive arrows designate stages with long-term or extensive self-renewal capacity. Orange and red designate terminal differentiation stages with progressive hemoglobin accumulation. CMP indicates common myeloid progenitor; MEP, megakaryocytic-erythroid progenitor; BFU-E, burst-forming unit–erythroid; CFU-E, colony-forming unit–erythroid; and PROEB, proerythroblast.