Polarization Controlled Coherent Anti-Stokes Raman Scattering Microscopy for Determination of Structural Order of the Myelin Sheath

Coherent Anti-Stokes Raman Scattering (CARS) is a laser-scanning microscopy technique that generates a strong label-free signal in lipid. The myelin sheath surrounding nerves is up to 80% lipid, and therefore is an ideal candidate for CARS imaging. The long lipid chains forming the myelin wraps have a directional preference when the myelin is healthy, with CH2 bonds aligned parallel to the axis of the nerve. The optical polarization dependence of the CARS signal can be used to probe the orientation of CH2 bonds in the sheath and determine their nanoscopic orientation. As myelin becomes unhealthy the organization of the lipids begins to loosen from their native organized packing structure. In the early stages this is not visible by conventional microscopy, however polarization-resolved CARS can measure the increasing disorder in the arrangement of the bonds before obvious morphological changes can occur. The degree of disorder is also measured to provide a metric of myelin disorganization in disease models.