Figure 3. Oligodendroglial survival, myelin preservation and white matter integrity inCnp1^Cre/+*Cox10^flox/flox mice.

a, b, By Gallyas’ silver impregnation of myelin at 9 months of age, the corpus callosum and other white matter tracts appear normally developed and stable in mutant mice. c–f, Electron microscopy of high-pressure frozen optic nerve shows intact myelination of CNS axons (c, d), and healthy oligodendroglia nuclei (e, f). A, axon; N, nucleus; M, mitochondria. Scale bars, 0.5 μm. g, Serial COX and SDH histochemistry. Left, in the normal appearing white matter of 9-month-old Cnp1^Cre/+*Cox10^flox/flox mutants (corpus callosum), mitochondrial COX activity yields a brown precipitate (axons and astrocytes, for example). By serial COX and SDH histochemistry, only Cox10-mutant COX⁻ cells are visibly stained for SDH (blue precipitate, white arrows). Middle, about half (48.2 ± 6.5%) of the OLIG2-positive oligodendrocyte lineage cells in the mutant corpus callosum (red nuclei in overlay), that is, mostly mature oligodendrocytes that express CNP1^Cre, are lacking COX activity (blue). Right, there are no COX⁻ cells in the corpus callosum of age-matched controls. Scale bar, 20 μm. h, Left, when combining only COX histochemistry with a marker for mature cells (CC1), mutant oligodendrocytes (corpus callosum; 9 months of age) appear white (arrows), intermingled with COX⁺ neighbouring cells and compartments. Middle, mature oligodendrocytes that are CC1⁺ (green) are filled with abnormally large numbers of SDH⁺ mitochondria (red), and appear in orange in the overlay. Note the near absence of SDH signals from neighbouring cells and compartments. Right, in a separate overlay, the SDH⁺ mitochondria (red) are virtually devoid of COX immunostaining (green) in mutant oligodendrocytes. Scale bar, 10 μm.