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).
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