Electron micrograph of an axon in the process of myelination. At this stage of development, the myelin (M) consists of about six membrane layers. The inner mesaxon (IM) is represented by the apposition of the internal opposing processes of the Schwann cell (SC). Similarly, the outer mesaxon (OM) is represented by the outer fold or lip of the Schwann cell where it meets itself. Another axon (see upper left A) is present that has not yet been enwrapped by a Schwann cell. Other notable features include the Schwann cell basal (external) lamina (BL) and the considerable amount of Schwann cell cytoplasm associated with the myelination process, x50,000. (Courtesy of Dr. Stephen G. Waxman.)
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