Monitoring enucleation of the primitive erythroid population using the cell-permeable DNA-binding dye DRAQ5. (A) Blood cells were harvested from the circulation of GFP⁺ E13.5 embryos, incubated with DRAQ5, and sorted according to expression of GFP and uptake of DRAQ5. DRAQ5 uptake was a clear indicator of the presence of a nucleus in each cell, as revealed by Giemsa staining (bottom). No fluorescent signal was detected in the GFP⁻ cell populations. (B) DRAQ5 profiles of GFP⁺ cells harvested from E9.5 (when all EryP/GFP⁺ cells were DRAQ5^high) to E18.5 (when almost all EryP/GFP⁺ cells were DRAQ5^neg). (C) Decline in numbers of nucleated EryP (•) versus the increase in numbers of enucleated EryP (○) in the circulation during embryogenesis, expressed as mean ± SEM. The total numbers of EryP remained stable throughout gestation.