Transmission electron microscopy. (A–D) WT β cells with secretory vesicles containing electron-dense core granules (A), mitochondria with typical cristae (B, C, arrowheads), and endoplasmic reticulum (D, arrowheads). (E–H) B-Cyb5r3 knockout β cells with increased clear electron-dense immature vesicles (E), abnormal mitochondrial cristae (F–H, arrowheads), with occasional proteinaceous matrix precipitates (H, arrow), and swollen cisternae (H, arrowheads). Reference bar in A, B, C, E, F, G = 250 nm; in D and H = 100 nm. (n = 3 mice and 50 cells per group).