Content by Dr. Kathleen Rockland
- Brief Background
- Guide to Myelin-stained datasets
- Sample links to other relevant resources
- Sample links to the relevant primary literature
Myelin was discovered in 1854 by Rudolf Virchow. It refers to the lipid wrapping that occurs around many – but not all – axons. Several variants of myelin wrapping have been identified across phylogeny; and myelination seems to have evolved several times independently in vertebrates, annelids, and crustacea (Hartline and Colman, 2007).
Given its importance, there is a considerable literature on all aspects of myelination: the molecular anatomy of the myelin sheath, its biophysical role in conduction of the nerve impulse, changes in development and disease. Recent data indicate that white matter structure is dynamic and that myelin can itself be regulated by impulse activity. Thus, myelin may have a role in cognitive functions (Fields, 2008).
How to visualize myelin
Normal myelin. The simplest way to visualize myelin is what was used by the early anatomists: direct visualization in the unstained brain. Concentrations of myelinated fibers will look white or whitish. Obvious examples are the great fiber bundles (“commissures”), consisting of axons originating from the left or right hemisphere and terminating in the contralateral hemisphere. In brain sections (thin sections, but also thicker sections (>1.0mm) cut by hand), cortical layer 1 will be perceptibly whitish, consistent with the accumulation of axons in this layer that run parallel to the pia surface.
Histological stains specific for myelin were developed at the end of the 19th century, and used effectively as a research tool by several early investigators of brain architecture (e.g., A.W. Campbell, K. Brodmann, O. and C. Vogt, among others). All of the basic myelin stains (for example, Weil, Heidenhahn) will successfully show the large fiber bundles. The demonstration of finer axonal fascicles within the cortical gray matter or within layer 1 can be more demanding and requires optimization of the staining parameters. Classical Golgi stains do not ordinarily stain myelinated fibers.
Current research papers often use immunocytochemistry for myelin basic protein for high-resolution visualization of myelinated fibers. The Gallyas silver stain is also viewed as high-resolution and sensitive, but the procedure has more steps and is harder to carry out. At a global level, diffusion tensor MRI allows visualization of fiber tracts, in vivo or ex vivo, for postmortem brains. This is increasingly used in assays of developmental or pathological changes.
Electron microcopy of appropriately perfused tissues provides direct visualization of the thickness and composition of the individual myelin sheaths.
Degenerating myelin. Pathological lesions, following stroke or other processes resulting in neuronal death, cause grossly visualized demyelination (see, e.g.here). Postmortem specimens, histologically reacted for degenerating myelin, were thus among the earliest ways to analyze brain connectivity.
Connectivity studies prior to the mid-1970’s heavily relied on tracking degenerating myelin (Marchi and Nauta methods) or degenerating terminations (Fink-Heimer method), subsequent to lesioning cell bodies. Lesions were experimentally made by heat or suction in anesthetized animals; but such methods were limited by problems such as how to avoid collateral damage to axons passing through the lesion site, and how to determine the appropriate survival time for visualizing different populations of degenerating processes. In the 1970’s, lesion-degeneration methods were largely superseded by other anterograde methods, first by autoradiography, then in the 1980’s by kidney bean lectin, followed by biotinylated dextran amine in the 1990’s. Both of these latter tracers are still commonly used.