Outgrowth of erythroid progenitors from umbilical cord blood. Left panels show changes in cell size distribution during proliferation of erythroblast cultures for 45 days in optimized medium (Table 1). Cell volume changes were monitored with an electronic cell analyzer. In the right panels, at the times indicated, cells were cytocentrifuged onto glass slides and stained with cytologic dyes. During the first few days, multiple cell types like leukocytes, macrophages, and only few erythroblasts are present, the proportion of which increases on culture in specific erythroid factors (Epo, Dex, murine SCF, and IGF-1). At day 10, the majority of cells in the rapidly proliferating cultures (mitoses indicated by arrows) show an erythroblast morphology. Cells stained with May-Grünwald and Giemsa were visualized using an Axiovert 10 microscope (Zeiss, Oberkochen, Germany) equipped with a 63 × oil-immersion objective (numerical aperture 44-07-61; Zeiss). At 630 × original magnification, images were captured with a Sony 3CCD color video camera (Sony, Tokyo, Japan) and prepared for publication with IP Lab Spectrum P software v.3.1.1 (Signal Analytics, Vienna, VA).