Myelin Pattern Diversity in Grey Matter

Abstract

Myelin, produced by oligodendrocytes, is a highly specialised membrane structure found in the nervous system. It speeds up signal transmission and provides trophic support for axons. Along the axon, myelin forms individual sheaths, or internodes, that are interrupted by gaps known as the nodes of Ranvier. Internodes can vary in thickness and length, but it is generally assumed that the internodes of a given axon are equally thick and long. Furthermore, the thickness and length of internodes is presumed to be dependent of the underlying axon diameter. However, most of this knowledge is derived from 2D electron microscopy and has yet to be confirmed by 3D electron microscopy. Here, I use two publicly available 3D electron microscopy datasets, one from the hippocampus and one from the cortex, to investigate the relationships between myelin thickness, length, and axon diameter and whether neighbouring internodes can have different thickness. In both datasets, some neighbouring internodes had differences in myelin thickness. While previously reported in white matter, I could not detect a dependency of myelin thickness from axon diameter in hippocampus and cortex. Additionally, internode length was neither correlated with myelin thickness nor axon diameter. Altogether these results show that myelin in the CNS can be more versatile than previously thought and widens our understanding of myelin structure and function.