A model system for in vitro erythropoiesis of normal human CD34⁺ progenitor cells. (A) Highly purified CD34⁺ cells were placed in culture as described in “Materials and methods” and assessed over time for total cell number. (B) Cells stained with Wright-Giemsa showing pronormoblasts (day 7), basophilic normoblasts (day 13), orthochromatic normoblasts, one of which is in the process of enucleating (day 17), and enucleated reticulocytes and orthochromatic normoblasts (day 18). Scale bar is 5 μm. (C) The percentage of enucleated reticulocytes in culture at the indicated times. (D) FACS analysis showing the percentage of cells expressing CD34, CD36, or glycophorin A cell surface markers. Error bars indicate SD.