Oliver Krueger

Depiction of nerve fibers by polarized light microscopy

Knowlegde about the course of nerve fibers in the central nervous system (CNS) plays an important role in science and clinical medicine. Their depiction in the human brain is mostly accomplished by diffusion-weighted magnetic resonance imaging. Verification of those individual results, however, requires a comprehensive depiction of the human connectome, that is, the entirety of fibers in the human CNS. A method to contribute to this goal is the ex-vivo delineation of nerve fibers by polarized light microscopy which takes advantage of the birefringent properties of nerve myelin sheaths.

For this technique, tissue is fixated in formaline, cryo-protected in 50% sucrose, and cut into slices of 100 µm thickness. The slices are examined, without the need of staining, in a microscope with a rotatable polarimeter, i.e., the intensity of the transmitted light is recorded at a high resolution under different azimuths. Phase and amplitude of the recorded intensity course allow inference on the in-plane course of nerve fibers as well as their inclination. For illustration, the results can be depicted in color-coded images.

The results obtained by this method allow qualitative depictions and quantitative estimations of the three-dimensional course of nerve fibers within the examined tissue slices. In comparison to diffusion-weighted MRI data they supply additional information concerning the differentiation of seperate portions of complex fiber bundles, the delineation of small branch-offs, and the discrimination of nerve fibers and grey substance tissue. The method is therefore suitable for the examination of circumscribed problems; it furthermore carries a great potential for the development of automated techniques for tracking fibers over longer courses and multiple slices.