White Matter Structure: A Microscopist’s View

Publisher Summary The white matter of the central nervous system contains axons, their myelin sheaths, and glial cells. Of the glial populations, oligodendrocytes, which produce the myelin sheath, and their progenitors, are the most abundant. The other major glial cell types are astrocytes and microglia. This chapter provides an overview of these cellular elements, including their function, microstructural features, and their relationship to one another and describes some of the changes that occur in response to damage to these white matter elements. The majority of CNS axons are surrounded by a lipid-rich myelin sheath that renders the tracts white. The contribution of myelin to this coloration can be appreciated by comparing a normal brain with one in which myelin is absent or reduced and in which the “white matter” appears semi-translucent. Microscopic analyzes have provided invaluable, high resolution information about white matter structure, including data on cellular identity, cellular and axonal packing densities, cell morphology, ultrastructural details of cellular constituents, and the consequences of pathology. However, newer, in vivo imaging paradigms are beginning to shed light on cellular dynamics too. Microscopic analysis provides a tool for evaluation of localized white matter structure, in health and disease, and when combined with DTI studies, will provide a better understanding of what these non-invasive water diffusion measurements really mean in terms of tissue microstructure.