Axon and Myelin Morphology in Animal and Human Spinal Cord

Abstract

Characterizing precisely the microstructure of axons, their density, size and myelination is of interest for the neuroscientific community, for example to help maximize the outcome of studies on white matter (WM) pathologies of the spinal cord (SC). The existence of a comprehensive and structured database of axonal measurements in healthy and disease models could help the validation of results obtained by different researchers. The purpose of this article is to provide such a database of healthy SC WM, to discuss the potential sources of variability and to suggest avenues for robust and accurate quantification of axon morphometry based on novel acquisition and processing techniques. The article is organized in three sections. The first section reviews morphometric results across species according to range of densities and counts of myelinated axons, axon diameter and myelin thickness, and characteristics of unmyelinated axons in different regions. The second section discusses the sources of variability across studies, such as age, sex, spinal pathways, spinal levels, statistical power and terminology in regard to tracts and protocols. The third section presents new techniques and perspectives that could benefit histology studies. For example, coherent anti-stokes Raman spectroscopy (CARS) imaging can provide sub-micrometric resolution without the need for fixation and staining, while slide scanners and stitching algorithms can provide full cross-sectional area of SC. In combination with these acquisition techniques, automatic segmentation algorithms for delineating axons and myelin sheath can help provide large-scale statistics on axon morphometry.

Single‐ and double‐Diffusion encoding MRI for studying ex vivo apparent axon diameter distribution in spinal cord white matter

“…Quantitatively, the averaged ADs extracted in the present study are in the range of 0.5 to 3 μm, depending on the ROI observed, a range that compares well with that obtained in histology . Note that these values are in the lower range of AADs obtained from early and recent diffusion MRI studies of SCs .…”

“…Quantitatively, the averaged ADs extracted in the present study are in the range of 0.5 to 3 μm, depending on the ROI observed, a range that compares well with that obtained in histology. 79 Note that these values are in the lower range of AADs obtained from early and recent diffusion MRI studies of SCs. 12,47,54,64,80 For example, Benjamini and co-workers have found AADs in the range of 5.5-6.5 μm for the ferret SC, 80 while Komlosh et al found, using similar experimental parameters AADs in the range of 3.4-4.5 μm for pig SC.…”

Effects of fixatives on myelin molecular order probed with RP-CARS microscopy

“…On the other hand, the fixation procedure with the combination of paraformaldehyde and glutaraldehyde seems to alter the molecular spatial distribution of the tissue in the opposite direction, since it decreases both the parameters. This small degradation in the two parameters with respect to the unfixed sample is reasonably a consequence of the well-known tissueshrinkage effect induced by the glutaraldehyde [33]. Moreover, the range of spatial lengths that maximizes the differences in the β-value (1 µm -1.5 µm) corresponds to the typical dimension of the spots composing the punctuated patterns observed exclusively in this condition, thus strongly suggesting that they can be linked to the observed decrease of the βvalue.…”

Axons morphometry in the human spinal cord

“…The morphometry of these fibers is remarkably heterogeneous: while white matter can be thought of as a network of identical cables transmitting action potentials, it appears that the size of these fibers can in fact vary from between 0.1 and 10µm 3 . It also appears that different white matter pathways have different microstructural characteristics 4 , but to date this observation has mostly been qualitative, and there are limited data that clearly describe and quantify these differences, and how much these fibers vary across human populations in the brain 5 and spinal cord 6,7 .…”

“…Another major issue is that the methodology of these pioneer studies is questionable : for instance, the staining based on silver impregnation prevents researchers from differentiating axon and glial processes 16 , leading to an overestimation of the axonal density (axons per mm 2 ) by a factor 10 to 20 16 . An extensive review of axon morphometry in the spinal cord can be found in Saliani et al 2017 7 .…”