Myeloarchitecture of the murine neocortex visualized by intrinsic third-harmonic generation

Myelination in the brain has been typically associated with the white matter. Recently, however, cortical myelination has gained much interest not only as a possible lesion of multiple sclerosis (MS) but also for its significance in the plasticity of the central nervous system (CNS). It is urgent to understand the functional role of cortical myelination and its involvement in the MS pathogenesis. Here we demonstrate a novel nonlinear optical contrast, namely third-harmonic generation (THG), as a basis of label-free imaging of cortical myelin in living animals. Using the fixed brain slices of fluorescently labeled transgenic mice, we examined the distribution of myelin in the normal brain. We measured the spatial organization of cortical myelin. Finally, the capability of THG microscopy was tested for visualizing cortical myelin in living, unlabeled animals.