TfR2 is required for erythroid mitochondrial integrity. (A) Summary of imaging flow cytometry studies showing relative MitoTracker Deep Red FM mean fluorescence intensities in lentiviral-transduced K562 cells (n = 4; *P < .05). (B) Transmission electron microscopy of bone marrow from adult wild-type and Tfr2^Y245X mutant mice, depicting erythroid progenitors (top) and granulocytic progenitors (bottom). (C) Ultrastructural quantitation of mean mitochondrial size in erythroid and granulocytic progenitors from wild-type and Tfr2^Y245X bone marrow as in (B) (number of cells analyzed = 15-23 per group; ***P < .001; ns, not significant). (D) Transmission electron microscopy of erythroid progenitors transduced with lentiviral shRNA constructs. (E) Ultrastructural enumeration of electron-dense foci per mitochondrion in transduced erythroid progenitors as in (D) (number of cells analyzed = 10-13 per group; *P < .05). (F) Total heme content in erythroid progenitors transduced with lentiviral shRNA constructs (n = 3 per group; **P < .01). EV, empty vector.