Heterogeneity of White Matter Structure in the Human Brain

Area of Science:

• Neuroscience
• Neuroanatomy
• Histology

Background:

• White matter (WM) comprises nearly half the human brain, containing long-range axons crucial for brain connectivity.
• Current diffusion MRI methods map major WM pathways but cannot resolve individual axon organization (trajectories, density, orientation).

Purpose of the Study:

• To develop and apply a histological and imaging pipeline for characterizing the 3D organization of individual projection axons in post-mortem human white matter.
• To investigate the regional diversity of axonal architecture in the human brain.

Main Methods:

• A specialized histological and imaging pipeline was optimized for post-mortem human white matter.
• The pipeline enabled visualization of 3D trajectories for densely stained large projection axons (diameter > ~1 μm).
• The approach was applied to multiple centimeter-scale WM regions in an adult human brain.

Main Results:

• Striking regional diversity in axonal organization was observed across different white matter regions.
• Distinct architectural motifs were identified, including meshworks, laminar lattices, and tightly packed fiber bundles.
• Examples include loosely packed meshworks in superficial WM, lattice structures near basal ganglia, and dense bundles in the corpus callosum.

Conclusions:

• The observed axonal patterns suggest local adaptations to spatial constraints, axonal density, and connectivity requirements.
• These findings provide new insights into the principles governing brain connectivity and anatomical optimization.
• The study highlights the need for detailed, region-specific atlases of human white matter architecture.