Figure 4. Tfam-SCKOs display early preferential loss of small unmyelinated fibers: a–c, Toluidine blue-stained plastic sections of 1-month-old Tfam-SCKO sciatic nerve and littermate ctrls (a) show that at this age there are no differences in the number of myelinated fibers (b) or the extent of myelination (g-ratio, axon area/fiber area) (c). Scale bar, 25 μm. Error bars indicate SEM. n = 4 mice per genotype. d, Electron micrographs of 1-month-old ctrl and Tfam-SCKO sciatic nerves show that, while large myelinated fibers are mostly normal in Tfam-SCKO mice at this age (arrows), the structure of Remak bundles (arrowheads) is largely abnormal. Scale bar, 2 μm. e1–e4, Compared with ctrl littermates (e1), unmyelinated axons in Tfam-SCKO Remak bundles are found touching one another (e2, e4, stars) and interspersed by pathological SC processes known as bands of Bungner (e2, arrowheads). Free-floating excess basement membrane (e2, e3, arrows) enclosing SC cytoplasm (e3, star) with few or no axons left, indicative of extensive degeneration of unmyelinated axons, is also visible. By 2 months of age, most nonmyelinating SCs are associated with unstructured, degenerating Remak bundles, containing abnormal axons (e4, star) and filled with phagocytic vesicles and membranous debris (e4, arrowhead). Scale bar, 500 nm. f, Representative photographs of epidermal fiber innervation (arrowheads) in the footpads of 2-month-old Tfam-SCKO and littermate ctrl mice. Scale bar, 10 μm. g, Quantification of fiber density in the epidermis shows a 30% decrease in Tfam-SCKO by 2 months of age (p = 0.02, two-tailed Student's t test), indicating early degeneration of small unmyelinated C-fibers. Results reported are normalized to per unit area. Error bars indicate SEM. n = 3 mice per genotype at each age.