Hemispheric Asymmetry of Myelin Orientation in the Mouse Auditory Cortex

Academic Background

The mammalian brain exhibits inherent asymmetry, which is particularly evident in the motor system and acoustic communication. For example, both mice and humans show hemispheric differences in auditory processing. This asymmetric organization allows the brain to process information more efficiently and provides redundancy in certain scenarios, such as recovery after stroke. Although functional asymmetries have been extensively studied, the underlying microstructural mechanisms remain unclear. This study aims to reveal structural asymmetries in the mouse auditory cortex (AC) by analyzing myelin orientation and to explore how these asymmetries influence functional specialization in auditory processing.

Source of the Paper

This paper was co-authored by Philip Ruthig, Gesine Fiona Müller, Marion Fink, Nico Scherf, Markus Morawski, and Marc Schönwiesner. The authors are affiliated with multiple research institutions, including Leipzig University, the Max Planck Institute for Human Cognitive and Brain Sciences, and the Université de Montréal. The paper was published in 2025 in the European Journal of Neuroscience under the title Hemispheric Asymmetry of Intracortical Myelin Orientation in the Mouse Auditory Cortex.

Research Process

1. Animal Experiments

The study used 11 adult C57BL/6J-Tg(Thy1-GCaMP6f)GP5.11Dkim/J mice (6 females, 5 males). All experiments were conducted in accordance with animal research regulations in Saxony, Germany.

2. Perfusion and Fixation

Mice were anesthetized and perfused intracardially, first with 0.9% saline and then with 4% paraformaldehyde for fixation. Brains were extracted and stored in 4% paraformaldehyde for 9 days for passive fixation.

3. Staining and Clearing

The staining and clearing process followed the iDISCO+ protocol. Steps included dehydration, bleaching, rehydration, blocking, antibody incubation, and secondary antibody staining. Anti-HuC/HuD antibodies (marking neuronal cell bodies) and anti-MBP antibodies (marking myelin) were used. Clearing was performed using dichloromethane and methanol solutions, with samples ultimately immersed in BABB (benzyl alcohol-benzyl benzoate) for transparency.

4. Data Acquisition

Imaging was performed using a LaVision BioTec UltraMicroscope II, with a resolution of 0.54 × 0.54 × 4 μm. The imaging area included the primary auditory cortex, primary visual cortex, and surrounding regions.

5. Data Preprocessing

Raw data were saved as 3D multi-channel TIFF files for subsequent analysis. Preprocessing involved enhancing local contrast using a custom 3D Gabor spherical shell kernel to detect neuronal cell bodies.

6. Neuronal Distribution Analysis

By calculating the local cell density field around each neuron, the research team found that neuronal distribution in the left and right auditory cortices was symmetric, showing no significant hemispheric differences.

7. Quantification of Myelin Orientation

Myelin orientation was quantified using the structure tensor method. Local dominant directions were calculated for each pixel using a sliding window and corrected based on cortical surface curvature. Results showed significant differences in myelin orientation between the left and right auditory cortices, particularly in layer ²⁄₃ (L2/3).

8. Bayesian Modeling

Bayesian methods were used to model myelin orientation, analyzing the effects of hemisphere and cortical layer. Results indicated that the largest differences in myelin orientation were in L2/3, with the right hemisphere in male mice showing a forward tilt, while the opposite pattern was observed in females.

Key Findings

1. Hemispheric Asymmetry in Myelin Orientation: The study found significant differences in myelin orientation between the left and right auditory cortices, particularly in L2/3. The right hemisphere exhibited more intercolumnar connections in L2/3.

2. Sex Differences: Male mice showed a forward tilt in myelin orientation in the right hemisphere, while females exhibited the opposite pattern. This sex difference was particularly pronounced in the right hemisphere.

3. Symmetry in Neuronal Distribution: Despite the asymmetry in myelin orientation, the distribution of neuronal cell bodies was highly symmetric between the left and right auditory cortices.

Conclusion

This study reveals hemispheric asymmetry in myelin orientation in the mouse auditory cortex, with the most significant differences observed in L2/3. Additionally, sex differences in myelin orientation were identified. These findings suggest that while basic developmental structures (such as cortical columns) remain symmetric between hemispheres, more plastic myelinated axons exhibit diverse hemispheric asymmetries. These asymmetries may contribute to functional specialization in auditory processing, such as in vocal communication or the processing of spectral/temporal complexity.

Research Highlights

1. Innovative Experimental Methods: The study combined light-sheet microscopy and iDISCO+ clearing techniques to achieve high-resolution 3D imaging of the mouse brain.
2. Discovery of Sex Differences: The study is the first to reveal the influence of sex on myelin orientation in the auditory cortex, providing a new perspective for future auditory research.
3. Quantification of Hemispheric Asymmetry: Through Bayesian modeling, the research team precisely quantified hemispheric differences in myelin orientation, offering a structural basis for understanding functional specialization in the auditory cortex.

Research Significance

This study not only deepens our understanding of auditory cortex structure but also provides new directions for future research on auditory function. In particular, the discovery of sex differences highlights the need to consider the influence of sex on auditory processing in future studies. Additionally, the high-resolution imaging techniques and quantitative methods used in this research offer valuable references for other neuroscience studies.