Refractive index tomography of myelinating glial cells

Abstract

Refractive index tomography as an emerging technique enables the 3D morphological investigation of cells with no marker. Here, refractive index tomographic imaging of myelinating glial cells is presented. Myelin as a signal insulation layer around an axon is formed by the wrapping of Schwann cells or oligodendrocytes. Microscopic investigation of myelination traditionally requires fluorescent markers. Glial cells generally wrap the axon for more than ten layers. This multilayer formation has alternating and uniform layers of protein and lipid. Earlier studies on the structure of the myelin sheath have shown that the thickness period is lower than 20nm including the thickness of the extracellular medium after each layer. Direct observation of an individual layer is not possible (using classical microscopy techniques) due to dimensions being very small compared to the wavelength of the illumination light. However, periodic nature of the layers enables the differentiation of a myelinated axon from an unmyelinated one. Rapid change of the integrated refractive index and the Bragg fiber like structure alters the transmission behavior as a function of wavelength and incidence angle. With the 3D sectioning capability of refractive index tomography, these features can be easily identified.