In vitro differentiation of erythroid cells derived from ES cells, fetal liver, and bone marrow. (A) Morphology of ES-EPs before and after 3 days of differentiation, as analyzed on cytospins by hematologic and benzidine staining. Original magnification, × 63. Differentiation kinetics of ES-EPs expanded for 16 days and 94 days, and erythroblasts derived from bone marrow and fetal liver were analyzed by measuring (B) accumulation of hemoglobin, (C) cell size, and (D) cell proliferation.