Characterization of erythroid differentiation. Time course including bright field microscopy images (×40) of Wright-Giemsa–stained cytospins and cell pellets (top) and FACS plots showing cell surface marker expression (bottom) at successive stages during erythroid differentiation. The time points shown correspond to the time points illustrated in Figure 1B. Day-15 cells represent HSPCs; suspension cultures were sampled at days 20, 22, and 25 to obtain progenitors at intermediate stages of erythroid specification; and day-29 cells represent BFU-E cells picked from Methocult cultures after 14 days of erythroid specification. Black arrows indicate examples of proerythroblasts (ProE), basophilic erythroblasts (Baso), polychromatic erythroblasts (Poly), and orthochromatic erythroblasts. FACS plot showing unstained cells is included as a reference below. Uniform gating based on day-29 cells was maintained throughout the figure.