The Uncinate Fasciculus (UF) is a white matter association tract responsible for connecting the frontal and temporal lobes of the brain. Its location, general shape, and estimated projections have been visualized and analyzed at low-resolution via diffusion tensor imaging (DTI) in many studies, allowing for a basic understanding of the pathway and its potential functions. Despite this, the microstructural properties of this white matter highway are still relatively unknown as we can estimate characteristics such as myelination and pathway density from low-resolution imaging, but we cannot gain a deeper understanding of the fine characteristics that allow the pathway to function in health and how those features are disrupted in disease. To address this gap, we examined six post-mortem tissue samples, featuring approximately 12,000 axons of the temporal segment of the UF (adjacent to the amygdala and hippocampus) from neurotypical controls. We used high-resolution microscopy to systematically sample and study the microstructure of myelinated axons to characterize the UF for future comparisons to disease states, like Autism Spectrum Disorder (ASD). We provide novel characterization of the microstructure of the UF in both hemispheres, examining density, size, myelination, and trajectories of axons in healthy adult tissue. Our data revealed hemispheric trends of axon characteristics that align with previous imaging studies, including slight left hemispheric asymmetry in regard to density and myelination and a small increase in trajectory variability in the right hemisphere. In addition, like values found previously in the Corpus Callosum, the distribution of thin and medium axons was much higher than thick axons. Our findings provide a more in-depth understanding of the microstructural characteristics of the UF at the level of single axons, providing context for white matter anisotropy and diffusivity estimates from previous DTI studies through values such as myelin thickness, area fraction, and axon trajectory. This structural framework of this pathway can be used to compare with results from in vivo imaging studies, and to provide a basis for comparison with pathological states in the future. / 2026-06-06T00:00:00Z

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