Combined cell body and fiber tract imaging of the brain based on polarized light

Introduction:Polarized Light Imaging (PLI) has been developed to analyze the fiber orientation in the human brain [1,2]. This method enables a spatial resolution of 1.6microns, and thus provides unprecedented perspectives to analyze the spatial orientation and course of myelinated fibers and fiber tracts [2]. In order tolink the fiber architecture of the cerebral cortex to the cytoarchitectonic organization, it is necessary to superimpose the PLI-based fiber architecture of acortical region on the cytoarchitecture of the same region. The aim of the present study is to introduce the methodical basis of this approach, and to applyit to the human entorhinal cortex.Methods:PLI provides three basic image modalities in each unstained cryotome section (thickness: 70 microns): retardation, direction and transmittance [1]. Thefiber orientation maps (FOMs) are derived from retardation and direction images based on the birefringent properties of the tissue. The transmittanceimages represent the overall extinction of the tissue, yielding a high contrast between the less transparent cell bodies and the more transparent neuropilin the grey matter. The neuropil of the grey matter appears lighter due to the lower amount of myelinated fibers and higher density of non-myelinatedfibers as compared to the white matter with its densely packed myelin (Fig. 1a). The HSV color space used in the FOMs codes the fiber azimuth by hue,and the fiber elevation by color saturation (Fig. 1b). Hence the value component of the HSV color space can be used to visualize the cell bodies byweighting the value component of the HSV-FOM by the inverted transmittance, i.e. extinction map (Fig. 1c). After the PLI measurements, the unstainedsections were additionally counterstained with cresyl-violet to visualize exclusively the cytoarchitecture (Fig. 1d)