Sorted R1 to R5 cells that represent erythroblasts at different developmental stages. E14.5 fetal liver cells were double stained for TER119 and CD71 and sorted into R1 to R5 populations as described in Figure 1. The left panels show May-Grunwald Giemsa–stained cytospin preparations of cells sorted from each region of R1 to R5. Representative cells from 2 to 5 fields are shown in each region. These are predominantly primitive progenitor cells (including mature BFU-Es and CFU-Es) in R1, proerythroblasts and early basophilic erythroblasts in R2, early and late basophilic erythroblasts in R3, chromatophilic and orthochromatophilic erythroblasts in R4, and late orthochromatophilic erythroblasts and reticulocytes in R5. The right panels show the cytospin preparations of R1 to R5 cells processed by benzidine-Giemsa stain. Representative cells from several fields are shown in each region. Cells from the R1 and R2 regions are benzidine-negative. R3 is a mixture of benzidine-negative and benzidine-positive cells. Cells sorted from the R4 and R5 regions are all benzidine-positive. Arrows indicate enucleated reticulocytes. Scale bar: 20 μm.