Background and purpose: It has been reported that multiple sclerosis has four different neuropathological subtypes, and two of them (type III and IV) are characterized by primary oligodendrocyte loss. However, the exact pathomechanism that lead to oligodendrocyte apoptosis in human demyelinating diseases is still elusive. The copper chelator cuprizone induces primary oligodendrocyte apoptosis and consequent demyelination in well defined areas of the mouse brain. Nevertheless, the precise subcellular events that result in oligodendrocyte cell death in the cuprizone model are still unknown. We aimed to study the ultrastructural alterations that might induce oligodendrocyte apoptosis in the cuprizone experimental demyelination model.

Methods: C57BL/6 mice were given cuprizone for two, 21 and 35 days to induce demyelination to investigate early pathological events, and different stages of demyelination. In addition, mice were given cuprizone for 35 days and were allowed to recover for two or 14 days to study early and late remyelination. After the cuprizone treatment, mice were sacrificed and the corpus callosum, the superior cerebellar peduncle, the optic nerve and the sciatic nerve were studied by electron microscopy.

Results: The ultrastructural analysis revealed that cuprizone induced oligodendrocyte apoptosis is accompanied by the formation of giant mitochondria in the affected cells in the corpus callosum and in the superior cerebellar peduncle. Apoptosis of the myelin producing cells was present through the whole cuprizone challenge. Severe demyelination occurred after three weeks of cuprizone administration associated with massive macrophage infiltration and astrocytosis of the demyelinated areas. Axons and neurons remained unaffected.

Conclusion: The formation of giant mitochondria in myelin producing oligodendrocytes is the first pathological sign in the cuprizone experimental demyelination. Mitochondrium pathology in the cuprizone challenge might serve as a useful model to study the pathomechanism of multiple sclerosis subtypes (III and IV) characterized by primary oligodendrocyte degeneration.