Axonal mitochondria across species adjust in diameter depending on thickness of surrounding myelin

In the central nervous system (CNS), axons and its surrounding myelin sheaths, generated by oligodendrocytes, greatly depend on each other, where oligodendrocytes provide axons with both trophic and metabolic support. Across spices, assessment of the axon-myelin ultrastructure is the key-approach to visualize de- and re-myelination of axons. However, this assessment omits to provide information on axonal homeostasis or how axon-myelin influence one another. Since mitochondria may adjust in size thus mirroring the intracellular physiological and metabolic status we applied this to myelinated axons in the CNS. We herein show that a large axonal mitochondria diameter correlates with thinner surrounding myelin sheaths across different CNS tracts and species, including human. We also show that the relation between axonal mitochondria diameter and surrounding myelin thickness is a valuable measurement to verify advanced remyelination in two commonly used experimental demyelinating models, namely the cuprizone and the lysolecithin (LPC) model. Lastly, we show that axonal mitochondria adjust in diameter in response to the thickness of the axonal surrounding myelin whereas the opposite adaption was absent. In summary, the link between axonal mitochondria diameter and surrounding myelin thickness provide insight on the axon-myelin relation both during homeostasis and pathological conditions. This link is also translational applicable and can thus contribute to a better understanding on how to study remyelination using experimental models.