Ultrastructural examination of Whirlin knockout central nervous system tissues reveals organelle accumulation in myelinated axons. Electron micrographs of internodal cross sections (A–C) and nodal-paranodal longitudinal (D–F) regions in spinal cords from wild-type (7 week-old, A, D) and Whrn knockout mice (7 week-old, B, E; 3 month-old, C, F). Overall cellular organization is conserved between wild-type and Whrn knockout and sciatic nerve fibers with tightly compacted myelin around each axon. Enrichment of mitochondria (flat arrowheads) and occasional myelin ruffling is observed in Whrn knockout mice compared to wild-type. At higher magnification, the wild-type (G) paranodal loops have characteristic electron-dense septa (concave arrowheads) and parallel arrays of cytoskeletal elements. In contrast, Whrn knockout CNS fibers accumulate organelles (flat arrowheads), particularly mitochondria and transport vesicles, and have paranodal septa (H, I, concave arrowheads) that are less defined. Also, electron micrographs from mice show cerebellar Purkinje myelinated axons (J–L) running through the granular layer. This region reveals axonal swellings filled with densely-packed organelles (flat arrowheads), particularly mitochondria and vesicles, in 7 week-old (K) and 3 month-old (L) Whrn knockout animals compared to 7 week-old wild-type (J). Scale bars: A–F, 2 μm, G–I, 400 nm, J–L: 1 μm.