Figure 4.

Ultrastructural analysis of optic nerves from Tag-1^−/− and Tag-1^−/−;plp^Tg(rTag-1) mice. A, Image from the electron microscope of a cross-sectioned optic nerve from a normal mouse. The dashed lines indicate the areas measured for the calculation of the diameter of the axon with and without the myelin sheath and subsequently the g ratio of the fibers. B, G ratio diagram for the Tag-1^−/− (n = 4) and Tag-1^−/−;plp^Tg(rTag-1) (n = 4) mice. There is a small but statistically significant decrease in the g ratio of Tag-1^−/−;plp^Tg(rTag-1) optic nerve fibers compared with the Tag-1^−/− (Student's t test, Tag-1^−/−, p < 0.05, g₀ = 0.735; Tag-1^−/−;plp^Tg(rTag-1), g₀ = 0.701; >100 axons measured for each animal). C, D, Electron micrographs of ultrathin cross sections from Tag-1^−/− (C) and Tag-1^−/−; plp^Tg(rTag-1) (D) optic nerves. Note the slightly hypomyelinated axons and the reduction in small caliber axons in Tag-1^−/− compared with Tag-1^−/−;plp^Tg(rTag-1) nerve. E, Quantification of the percentage of RGC axons with small (<500 nm), medium (500–1500 nm), and large (>1500 nm) caliber in Tag-1^+/+ (n = 3), Tag-1^−/− (n = 4), and Tag-1^−/−;plp^Tg(rTag-1) (n = 3) optic nerves. Bar graphs depict mean ± SEM. The pool of small caliber axons is increased, whereas the percentage of medium caliber axons is decreased in Tag-1^−/−;plp^Tg(rTag-1) mice compared with homozygous mutants. No statistically significant difference is detected between Tag-1^−/−;plp^Tg(rTag-1) and Tag-1^+/+ mice, suggesting the full rescue of the homozygous mutants phenotype (>200 axons measured for each mouse, unpaired Student's t test, ***p < 0.001, **p < 0.005, *p < 0.05).