Proteomic analyses highlight a significant loss of organelle protein components as primitive BasoEs at E10.5 transition into OrthoEs at E12.5. (A) Erythroid precursors were isolated from the bloodstream of murine embryos at E10.5 and E12.5 and analyzed by LC-MS/MS proteomics. A widespread loss of organelle protein components, especially in mitochondria, ribosomes, and spliceosome and proteasome components, occurs during terminal erythroid maturation (further detailed view of these pathways is provided in supplemental Figures 3-8). (B) Representative imaging flow cytometric images of E10.5 and E12.5 primitive erythroblasts stained with the erythroid marker Ter119, the DNA stain Hoechst 33342, the mitochondrial stain MitoTracker Deep Red (MtDR), and the RNA stain thiazole orange (TO). (C) Histogram plots of cell and nuclear areas (upper panels) and RNA and mitochondrial signal intensity distributions of 13 064 E10.5 and 10 648 E12.5 erythroid precursors. BF, brightfield.