Cell polarity in myelinating glia: From membrane flow to diffusion barriers

Myelin-forming glia are highly polarized cells that synthesize as an extension of their plasma membrane, a multilayered myelin membrane sheath, with a unique protein and lipid composition. In most cells polarity is established by the polarized exocytosis of membrane vesicles to the distinct plasma membrane domains. Since myelin is composed of a stack of tightly packed membrane layers that do not leave sufficient space for the vesicular trafficking, we hypothesize that myelin does not use polarized exocytosis as a primary mechanism, but rather depends on lateral transport of membrane components in the plasma membrane. We suggest a model in which vesicle-mediated transport is confined to the cytoplasmic channels, from where transport to the compacted areas occurs by lateral flow of cargo within the plasma membrane. A diffusion barrier that is formed by MBP and the two adjacent cytoplasmic leaflets of the myelin bilayers acts a molecular sieve and regulates the flow of the components. Finally, we highlight potential mechanism that may contribute to the assembly of specific lipids within myelin. This article is part of a Special Issue entitled Lipids and Vesicular Transport.