Terminal differentiation of human erythroid progenitors. Proliferation kinetics (A) and volume decrease (B) during differentiation in the presence or absence of 3% human serum (HS) was monitored daily by an electronic cell counter. (C) Quantitation of hemoglobin accumulation (for photometric assay, see Kowenz et al²⁵  and “Materials and methods”); 500 AU corresponds to 11.0 pg hemoglobin. Error bars in panels A-C represent the SD of mean, n = 5. (D) Cells before (left) and after differentiation for 6 days in the absence (middle panel) or presence of HS (right panel) were cytocentrifuged onto slides and stained with neutral benzidine (to detect hemoglobin; brownish stain) plus histologic dyes (see “Materials and methods”). Image acquisition was performed as described for Figure 1. (E) Kinetics of changes in cell size distribution during differentiation of human erythroblasts without serum (upper panel) and with HS (lower panel). (F) Expression of erythroid cell-surface markers in proliferating and differentiating erythroblasts (left panel, labeled “control”: differentiation without serum; right panel, differentiation in the presence of 3% HS). Proliferating cells or erythroblasts induced to differentiate for 6 days were washed with PBS/1% FCS, stained with fluorescently labeled antibodies against c-Kit (CD117), transferrin receptor (CD71), and glycophorin A (GPA) and subjected to fluorescence-activated cell sorting (FACS) analysis. Orange graphs are FACS profiles of self-renewing erythroblasts; red graphs are cell-surface antigen expression in differentiating cells; light gray histograms indicate background fluorescence.