Microscopy of myelination

We present a mini review of the microscopical techniques that may be used to investigate the process of myelination in the tissues of the mammalian central nervous system. The biological background to myelination, the process by which neuronal axons are electrically insulated to achieve higher conduction velocities is outlined, including an introduction to neurons and glia, some background discussion of the purpose of myelination in the mammalian nervous system and an outline of our current understanding of the composition and development of the myelin sheath itself. Specific focus will fall on the interaction of oligodendrocytes with the axons of the central nervous system and the unanswered questions surrounding the formation of the myelin sheath by these cells. The main body of this chapter reviews the various microscopical techniques that have been employed to image the myelination process. Examples drawn from the literature and recent results from our lab have been used to illustrate the current state of the art. We consider the use of light microscopy including the advantages of phase contrast imaging, scanning near field optical microscopy (SNOM) and immuno-fluorescencence methods. Results from transmission and scanning electron microscopy studies of myelination are also outlined and discussed together with data from the atomic force microscope (AFM). Finally we turn to recent results involving the application of the environmental scanning electron microscope (ESEM) in correlative fluorescence microscopy studies of oligodendrocyte co-cultures. This chapter concludes with the authors' thoughts on remaining challenges facing instrument developers and users.