Schwann cell

A freeze fracture electron micrograph shows an axon and its myelin sheath, fractured transversely.  This axon is surrounded by 10-15 wrappings of myelin derived from Schwann cell plasma membrane.  Visible in the axon are numerous small bumps representing microtubules.  Portions of uncompacted Schwann cell cytoplasm are also visible.  60,000x

A freeze fracture electron micrograph shows an axon and its myelin sheath, fractured transversely.  This axon is surrounded by 10-15 wrappings of myelin derived from Schwann cell plasma membrane.  Visible in the axon are numerous small bumps representing microtubules.  Portions of uncompacted Schwann cell cytoplasm are also visible.  60,000x

A freeze fracture electron micrograph shows an axon and its myelin sheath, fractured transversely.  This axon is surrounded by 10-15 wrappings of myelin derived from Schwann cell plasma membrane.  Visible in the axon are numerous small bumps representing microtubules.  Portions of uncompacted Schwann cell cytoplasm are also visible.  60,000x

A freeze fracture electron micrograph shows an axon and its myelin sheath, fractured transversely.  This axon is surrounded by 10-15 wrappings of myelin derived from Schwann cell plasma membrane.  Visible in the axon are numerous small bumps representing microtubules.  Portions of uncompacted Schwann cell cytoplasm are also visible.  60,000x

During myelination, the axon lies in a longitudinal groove of plasma membrane indented into the Schwann cell surface.  One edge of the groove wraps around the axon forming concentric membrane layers, called myelin.  As membranes compact, intervening cytoplasm is eliminated.  Cytoplasm is retained between membranes adjacent to the axon and at the periphery of the myelin.