INTRODUCTION

Myelin in the peripheral nervous system (PNS) and central nervous system (CNS) is formed by Schwann cells and oligodendrocytes respectively. The myelin formed by the two glial cell types electrically insulates the axons and restricts the generation of action potentials to the nodes of Ranvier, myelin-free regions separating the myelin segments, allowing for rapid propagation of action potentials along the axons. While there are striking overall similarities in the mechanisms of myelination and the structure and molecular composition of PNS and CNS myelin, there are also important differences between the two glial cell types. For instance, Schwann cells elaborate a single myelin segment around a single axon, while oligodendrocytes may myelinate up to 60 different axons (Figure 2.1). Furthermore, Schwann cells – but not oligodendrocytes – are surrounded by a basal lamina that is continuous with the adjacent internode. In addition, nodes of Ranvier in the PNS are covered by Schwann cell processes, while axons at CNS nodes are bare. A characteristic feature of CNS myelin not present in PNS myelin is the so-called radial component, interlamellar claudin-11-positive strands spanning the myelin. There is also a slight difference in the periodicity of mature compact CNS and PNS myelin, and important differences in the molecular composition of CNS and PNS myelin. For instance, proteolipid protein (PLP) is the major myelin protein in the CNS, while P0 and peripheral myelin protein 22 (PMP22) are exclusively found in PNS myelin.