Maturation of primitive erythroblasts is reflected in their expression of Ter119 and CD71. (A) Increase in Ter119 expression on the surface of live GFP⁺ cells from the circulation of E9.5 to E17.5 embryos. These changes are displayed in the color-coded composite density plot on the right. Axes indicate relative logarithmic fluorescence units for Ter119-PE (y-axis) and GFP (x-axis). (B) Density plot of a typical CD71 and Ter119 staining pattern on viable cells dispersed from wild-type whole E14.5 fetal liver. Axes indicate relative logarithmic fluorescence units for Ter119-APC (x-axis) and CD71-PE (y-axis). Regions R1 to R5 are defined by their characteristic CD71 and Ter119 staining pattern of their cells. The cells in each region can be classified by morphology as follows: primitive progenitor cells and proerythroblasts (R1), proerythroblasts and early basophilic erythroblasts (R2), early and late basophilic erythroblasts (R3), chromatophilic and orthochromatophilic erythroblasts (R4), and late orthochromatophilic erythroblasts and reticulocytes (R5).⁵⁰  (C) Density plots of FACS-sorted GFP⁺ cells from E.9.5 and GFP⁻ cells from E9.5, E12.5, and E14.5 embryos. As observed for unfractionated fetal liver cells (B), the circulating cells could be divided into 5 populations (R1 to R5). The greatest numbers of immature cells (R1 and R2) were found at E9.5. By E14.5, significantly larger numbers of mature EryP/GFP⁺ and EryD/GFP⁻ cells (R4 and R5) were seen. Note that, although most EryPs were strongly GFP⁺, there was some heterogeneity in the level of expression of the transgene and this accounts for the appearance of a tail in the plot on the left.