Cell cultures initiated from embryonic tissues are capable of yielding extensively proliferating erythroid cells. (A) Cells were grown in erythroid expansion media. Erythroid cells derived from adult bone marrow proliferated ∼ 10³-fold, whereas those from the 3 independent E9.5 yolk sac cultures each proliferated > 10¹²-fold. (B) Cells were grown as in panel A but were maintained at indicated cell concentrations. Cells exhibit a slowed growth rate and increased death at concentrations of ≥ 4 × 10⁶ cells/mL or higher, but no changes in kinetics were observed at concentrations < 2 × 10⁶cells/mL. (C) Most of the extensively proliferating cells resemble proerythroblasts (ProE) and basophilic erythroblasts (BasoE). The cultures also contain a small number of polychromatophilic (PolyE) and orthochromatic (OrthoE) erythroblasts, as well as reticulocytes (mean ± SEM; N = 16). (D) Ter119 and Kit levels of ex vivo extensively proliferating erythroblasts, adult bone marrow (BM) cells, and E12.5 fetal liver (FL) cells. Most extensively proliferating erythroblasts are Kit^high/Ter119^low. Kit^high/Ter119^low cell populations (red circles), with similar forward scatter (FSC) characteristics are found in the adult marrow and E12.5 fetal liver. One of 3 representative experiments is shown. (E) Extensively proliferating erythroid cells express small amounts of adult (β1 and β2), but no embryonic (ϵy and βH1), β globin transcripts (mean ± SEM; N = 3). In contrast, circulating blood cells from E12.5 of gestation, composed predominantly of primitive erythroblasts, express both embryonic and adult β-globin gene transcripts.