ESREs are capable of terminal erythroid maturation. (A) Proliferating ESREs transferred from erythroid expansion media to erythroid maturation media mature from proerythroblasts to orthochromatic erythroblasts and enucleated erythrocytes over 3 days. The lower panel is a composite of cells from a single photograph of a cytospin preparation. (B) The kinetics of ESRE maturation. Over the course of 3 days ESREs transition from immature erythroblasts into > 90% enucleated erythrocytes (mean ± SEM; N = 16 independent maturation cultures). (C) Proliferating ESREs have a high FSC and are Kit^highTer119^low cells (upper left). After maturation, the cells decrease in size (FSC; upper right compared with lower right), down-regulate Kit, and up-regulate Ter119 (upper left compared with lower left). One of 3 representative experiments is shown. (D) ESREs grown in StemPro34-based erythroid expansion media, when transferred to erythroid maturation media, yield an 8-fold increase in cell number over the course of 3 days (mean ± SEM; N = 36). ESREs grown in StemSpan-based erythroid expansion media, when transferred to erythroid maturation media, yield a 19-fold increase in cell number over the course of 3 days (mean ± SEM; N = 13).