Polarized light microscopy of myeloarchitecture and spatial orientation of fiber tracts in human and non-human brains
Using Polarized Light Imaging (PLI), the birefringence of neuronal tissue, particularly of myelinated nerve fibres enables - without any staining procedures - the visualization of single nerve fibres and fibre tracts at an in-plane resolution of 1.3 mm in whole brain sections. The spatial orientation of fibres and fibre tracts can be calculated using Jones Calculus. We have studied human ex vivo brains, and those of vervet monkeys, rats and mice to demonstrate the myeloarchitecture of nerve fibres within the cortical ribbon and the course of fibre tracts in the white matter.
After an introduction to the method, examples of the spatial orientation of fibre tracts in 60-70 mm thick serial sections through human, vervet monkey, rat and mouse brains will be demonstrated. An example of a first 3D-reconstruction of all fibre tracts in a rat brain will also be shown. PLI also allows the visualization of fibre tracts selected by their spatial orientation (“spot light imaging”). Furthermore, the excellent contrast of all fibres within the cortical ribbon provides a new approach to myeloarchitectonic mapping of the cerebral cortex. Finally, PLI may serve as an independent method for validation of data based on diffusion weighted imaging. In conclusion, PLI provides an unprecedented spatial resolution of the anatomical ground truth of fibre tracts and reveals a hitherto unknown complexity of structural connectivity.