Schwann cells

At high magnification the myelin sheath displays the wrappings of the Schwann cell plasma membrane.  Myelin is formed from the concentric wrapping of the Schwann cell plasma membrane, leading to a regular pattern of dark and light lines.  100,000x

At high magnification the myelin sheath displays the wrappings of the Schwann cell plasma membrane.  Myelin is formed from the concentric wrapping of the Schwann cell plasma membrane, leading to a regular pattern of dark and light lines.  100,000x

The cytoplasmic surfaces of the Schwann cell plasma membrane closely appose each other, extruding the cytoplasm (compaction) and forming the major dense line.

The intraperiod line results from the juxtaposition of the external surfaces of adjacent Schwann cell membranes.  The intraperiod line is not as electron dense as the major dense line and can appear as a double line.

Schwann cells secrete an external lamina, which is similar in structure to the basal lamina in epithelial tissues.  The external lamina provides anchorage for the Schwann cell during myelination.  Beyond the external lamina is the endoneurium which consists of delicate collagen fibrils.