The axon diameter required to initiate myelination is increased in the dnβ1 mice. (a) Representative electron micrographs show a reduction in the frequency of small myelinated axons in optic nerve sections of P17 dnβ1 mutant mice compared with wild-type mice. Bar: 1 µm. (b) Normal myelin morphology and thickness in P17 optic nerve of dnβ1 mice. The linear regression of the g-ratio measurements for each animal (n = 3) is shown. Solid and dashed lines represent wild-type and mutant animals, respectively, from three litters displayed in blue, green, and red. (c) A reduced number of myelinated small-diameter axons is observed in dnβ1 mutants (open figures) compared with wild-type animals (closed figures). This is especially clear in two of the three litters analyzed, identified by different colors as in b. The scatter plots shown display g-ratios of individual myelinated axons as a function of the respective axon size, with the two parts of the plot corresponding to small and large axons shown separately (in left and right panels, respectively) and the former expanded to show the differences more clearly. Closed and open figures represent wild-type and mutant axons, respectively. (d) Representative graph from one litter shows the percentage of myelinated axons with respect to axon diameter at 0.1 µm intervals for wild-type and dnβ1 mice. A polynomial trendline was adjusted to the data. A reduced percentage of myelination across all axon diameter intervals is present in dnβ1 mutant compared with wild-type mice (*, P < 0.05), a phenotype most evident in small axons below 0.6 µm of diameter (**, P < 0.01). (e) Frequency histogram of the diameter of all the myelinated and total (both myelinated and unmyelinated) axons shows a shift of myelinated axons toward larger diameters in mutant animals, whereas the total distribution of axon diameters remains unchanged.