The Corticospinal Tract in the Human Medulla Oblongata: a High-Resolution Microscopic Analysis
Maria Morozova1,2, Henriette Rusch2, Carsten Jäger1, Alfred Anwander1, Siawoosh Mohammadi3, Markus Morawski2, Stefan Geyer1
1Max Planck Institute of Human Congnitive and Brain Sciences, Leipzig, Germany, 2Paul Flechsig Institute of Brain Research, University of Leipzig, Leipzig, Germany, 3Department of Systems Neurosciences, University Medical Center Hamburg-Eppendorf, Hamburg, Germany
Recent advances in magnetic resonance imaging (MRI)-based biophysical models facilitate the estimation of white matter properties such as apparent g-ratio, axonal diameter, and axonal density in vivo. G-ratio is the ratio between inner and outer axonal diameter and quantifies the relative myelination of a fibre. The main limitation of biophysical models is the simplification of the underlying tissue composition, e.g., axons are modelled as long cylinders with invariable diameters. This assumption can lead to a systematic over- or underestimation of the derived parameters. Another important limitation is the scale gap between the MRI signal of a voxel (~1mm) and the underlying tissue composition (e.g. axon diameters ~1µm). High-resolution microscopy (250 nm) images of 2D histological data provide a better understanding of the microstructural composition of white matter pathways . Human nervous system tracts vary in their microstructural composition (Nieuwenhuys, Voogd & Van Huijzen, 2007). Comprehensive descriptions of individual tracts are necessary to understand their underlying tissue architecture. In this study, we provide a qualitative analysis based on high-resolution light and electron microscopy images of an entire cross section of the human corticospinal tract (CST) at the level of the medulla oblongata.
We obtained a human medulla oblongata sample at autopsy with prior informed consent (female, 89 years, heart failure, 24 hours post-mortem delay) and approved by the responsible authorities. Following standard procedures, the block was immersion-fixed in 3% formalin and 1% glutaraldehyde in phosphate buffered saline at pH 7.4. We dissected the left corticospinal tract from a 500µm vibratome slice of the sample. This section was contrasted in osmium tetroxide, dehydrated in graded acetones, and embedded in Durcupan resin. Semithin (500 nm) and ultrathin (50 nm) sections of the left CST were cut with an ultramicrotome (Reichert Ultracut S, Leica). Sections for light microscopy were stained with toluidine blue, coverslipped, and digitized with an AxioScan Z1 microscope (Zeiss). Sections for electron microscopy were imaged with a Zeiss TEM912 Omega at x3150 magnification.
The size of the cross section of the examined left CST is approximately 10.3 mm² (Fig.1A). However, this value is not corrected for distortions and volume changes due to histological processing. Most fibres descend in a vertical direction, only occasionally obliquely cut fibres can be found (Fig.1B). Due to a relatively short post-mortem delay and glutaraldehyde containing fixative the majority of myelin sheaths appears well preserved. This was confirmed in electron microscopic images (Fig.2). However, some of the myelin sheaths of the large diameter axons show morphological alterations, such as discontinuities, variations in thickness, and vacuolisation. Such properties are characteristic for autolytic processes, an inevitable consequence of post-mortem delay before the onset of fixation. Throughout the entire CST small diameter axons < 4µm are predominant, however, large diameter axons > 8µm are also present in a small number. This qualitative assessment is in line with a previous quantitative electron microscopic analysis of the CST at the level of the medulla oblongata (Graf von Keyserlingk & Schramm, 1984). Our visual analysis revealed no medio-lateral or dorso-ventral gradients in axonal diameter, density, or g-ratio that might reflect the underlying somatotopical arrangement (i.e. descending fibres to the arm running medially and to the leg laterally).
This study is to our knowledge the first high-resolution light microscopic analysis of an entire cross-section of a fibre tract in the human brain. Further quantitative analyses are necessary to verify these qualitative observations and to validate or, if necessary, rectify estimations from MRI-based biophysical models of these axonal properties.
Cortical Cyto- and Myeloarchitecture 2
White Matter Anatomy, Fiber Pathways and Connectivity 1
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Nieuwenhuys, R., Voogd, J., & Van Huijzen, C. (2008). The human central nervous system, 4th ed. Springer, Berlin.