Fig. 1 |. Myelin components and turnover.

a, Schematic of myelin, a highly organized structure with well-defined compartments. b, Myelin membranes have aunique constitution of lipidsthatare turned over at different rates. c, Myelin proteins localized to compact membranes (right) are turned over more slowly than proteins in non-compact myelin or at the myelin-axon junction (left; e.g. CNP and MAG, respectively). d, Lipids and proteins are added to mature myelin at the inner tongue and paranodal loops from trafficked vesicles and local translation. e, Membranes are removed from mature myelin via myelinoid bodies and concerted autophagy and endocytosis. Microglia and astrocytes, which phagocytose myelin in other contexts, could be involved in turnover of mature myelin. How the residual myelin sheath remains structurally organized and intact after multiple layers of myelin are removed is not fully understood.