Figure 2.

myrf^ue70 mutants display CNS-specific hypomyelination at 6 dpf. A, TEM images of the myelinated tracts in the dorsal (top row) and ventral spinal cord (bottom rows). Scale bar: 1 µm. B, Schematic of the transverse section of a 6 dpf larval zebrafish at the level of the urogenital opening. Inset, transverse section of the spinal cord at the same level. Myelinated (green) axons are located in the ventral and dorsal spinal tracts of the spinal cord (CNS) as well as the posterior lateral line (PNS). m, Mauthner axons. C, TEM images of the posterior lateral line at 6 dpf. Scale bar: 1µm. D, The average number of myelinated axons in one hemi-spinal cord is reduced by 66% in mutants (wild types: 35.29 ± 7.83 myelinated axons, mutants: 12.00 ± 4.34 myelinated axons, p ≤ 0.0001, unpaired t test, N = 7 wild types, N = 8 mutants). E, The number of myelinated axons in the PNS is similar between genotypes (wild types: 7.33 ± 1.53 myelinated axons, mutants: 9.00 ±3.83 myelinated axons, p = 0.52, unpaired t test, N = 3 wild types, N = 4 mutants). F, G-ratio of Mauthner axons in wild-type and mutant siblings (wild types: 0.48 ± 0.009, mutants: 0.80 ± 0.08, p = 0.0009, unpaired t test). For D-F, error bars represent mean ± SD. G, g-ratios for myelinated axons for small caliber (area <0.3 µm²) and large caliber (area >0.3 µm²) myelinated axons. The g-ratio of small caliber axons is similar between groups [wild types: 0.57 (0.52–0.62), mutants: 0.59 (0.52–0.70), p = 0.51, Mann–Whitney test, n = 53 myelinated axons in wild types, n = 17 myelinated axons in mutants error bars represent median and IQR]. The g-ratios for large caliber axons are significantly higher in mutants than wild-type siblings (wild types: 0.60 ± 0.08, mutants: 0.71 ±0.08, p ≤ 0.0001, unpaired t test, n = 33 myelinated axons in wild types, n = 19 myelinated axons in mutants error bars represent mean ± SD).