Remyelination is impaired in old zebrafish. (A) Electron microscopic analysis of the lesion area in the optic nerves of old (>15 months) zebrafish show thin myelin at 28 dpl and even 3 mpl. (B) The percentage of myelinated axons in old fish recovers to a similar level as in controls and young fish. (p > 0.05, n = 3 fish, Kruskal-Wallis test, Dunn’s post test). (C) G-ratios in old zebrafish at 28 dpl and 3 mpl are higher than in unlesioned controls, indicating thin newly formed myelin following demyelination, confirmed by (D) quantification of average G-ratios at different time points post-lesion (p < 0.05, n = 3 fish, Kruskal-Wallis test, Dunn’s post test). Data from young animals in B and D are repeated from Figure 3 for comparison. Mean ± SEM. Scale bar: A = 1 μm.