Micron-resolution fiber mapping in histology independent of sample preparation

Mapping the brain's fiber network is crucial for understanding its
function and malfunction, but resolving nerve trajectories over large
fields of view is challenging. Electron microscopy only studies small
brain volumes, diffusion magnetic resonance imaging (dMRI) has limited
spatial resolution, and polarization microscopy provides unidirectional
orientations in birefringence-preserving tissues. Scattered light imaging
(SLI) has previously enabled micron-resolution mapping of
multi-directional fibers in unstained brain cryo-sections. Here, we show
that using a highly sensitive setup, computational SLI (ComSLI) can map
fiber networks in histology independent of sample preparation, also in
fresh-frozen and formalin-fixed paraffin-embedded (FFPE) tissues,
including whole human brain sections. We showcase this method in new and
archived, animal and human brain sections, for different stains and steps
of sample preparation (in paraffin, deparaffinized, stained). Employing
novel analyses, we convert microscopic orientations to
microstructure-informed fiber orientation distributions (μFODs). Adapting
MR tractography tools, we trace axonal trajectories via orientation
distribution functions and microstructure-derived tractograms, revealing
white and gray matter connectivity. These allow us to identify altered
microstructure in multiple sclerosis and leukoencephalopathy, reveal
deficient tracts in hippocampal sclerosis and Alzheimer's disease,
and show key advantages over dMRI, polarization microscopy, and structure
tensor analysis. Finally, we map fibers in non-brain tissues -including
muscle, bone, and blood vessels- unveiling the tissue's function. Our
cost-effective, versatile approach enables micron-resolution studies of
intricate fiber networks across tissues, species, diseases, and sample
preparations, offering new dimensions to neuroscientific and biomedical
research.