Flow cytometry analysis of fetal liver erythroblasts cultured in vitro. Fetal liver cells were stained with biotinylated anti-TER119 mAb, and TER119^- progenitor-rich cells were purified. Purified TER119^- cells (approximately 70% to 80% R1 and R2 cells; essentially no R3 to R5 cells) were cultured in vitro for 1 day on fibronectin-coated plates in medium containing serum and Epo. Epo was removed from culture at the end of 1 day. The differentiation profiles of cultured cells were examined by flow cytometry and benzidine-Giemsa stain. The arrowhead indicates an extruded nucleus, and the arrow indicates an enucleated reticulocyte. Scale bar: 20 μm.