Electron microscopy observation of cross sections of medaka fish motor nerve fibers. Cross sections of the wild type (A, C) and the pmp22 fish (B, D-N) are shown. The pmp22 fish myelin sheath, D, was thinner than the wild type, C. The attenuated fiber (B; black arrow), enlarged nuclear (B; white arrow, H, I; white arrowhead), large periaxonal space (B, K; white arrowhead), granules (B, K; back arrowhead) and loose basal lamina (B, L; black dashed arrow) were observed in the pmp22 fish PNS. (E) Hypermyelination (black arrowhead) and outfolding (white arrowhead). (F, G) Infolding (black arrowhead). (H) Bare axon (black arrowhead). (I, J) Myelin uncompaction (black arrowhead). (M, N) Myelin debris (black arrowhead) was observed in macrophages. Bars: 2 μm for A and B, 100 nm for C and D, 500 nm for E-N.