Development of the Myelin Sheath in the PNS
The development of the myelin sheath af-fords an insight into the structure of its spiraling lamellae. The cell body of the Schwanncell (A1) forms a groove into which the axon (A2) becomes embedded. The groove deep-ens and its margins approach each other and finally meet. In this way, a duplication of the cell membrane is formed, the mesaxon (A3), which wraps around the axon like a spiral as the Schwann cell migrates around the encircled axon.
The term mesaxon is based on the term mesenterium, a thin duplication that is formed as a suspension band by the peri-toneum and encloses the intestine. In a sim-ilar way, the Schwann cell forms a duplica-tion and envelops the axon. Like all plasma membranes, the cell membrane of the Schwann cell consists of an outer and an inner dense layer of protein and a light lipid layer between them. Upon membrane du-plication, the two outer protein layers come into apposition first and fuse to form the in-traperiod line (A4). Thus, the six-layeredmembrane duplication becomes the five-layered myelin lamella. With further encir-cling, the inner protein layers of the cell membrane make contact as well and fuse to form the dense major period line (A5). At the end of the process, the start of the duplica-tion lies inside the myelin sheath, the inter-nal mesaxon (AB6), while the end lies out-side, the external mesaxon (7 in A, B).