Electron micrograph of a mature myelinated axon. The myelin sheath (M) shown here consists of 19 paired layers of Schwann ceil membrane. The pairing of membranes in each layer is due to the extrusion of the Schwann cell cytoplasm. The axon displays an abundance of neurofilaments, most of which have been cross-sectioned, giving the axon a stippled appearance. Also evident in the axon are microtubules (MT) and several mitochondria (Mit). The outer collar of Schwann cell cytoplasm (OCS) is relatively abundant, compared with the inner collar of Schwann cell cytoplasm (ICS). The collagen fibrils (C)
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