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)
constitute the fibrillar component of the endoneurium. BL, basal (external) lamina, x70,000. Inset. Higher magnification of the myelin. The arrow points to cytoplasm within the myelin that would contribute to the appearance of the Schmidt-Lanterman cleft as seen in the light microscrope. It appears as an isolated region here because of the thinness of the section. The intercellular space between axon and Schwann cell is indicated by the arrowhead. A coated vesicle (CV) in an early stage of formation appears in the outer collar of the Schwann cell cytoplasm, x 130,000. (Courtesy of Dr. George D. Pappas.)
node of Ranvier nucleus of Schwann cell outer collar of Schwann
inner collar of Schwann cell cytoplasm of Schwann cell
Schmidt-Lanterman cleft collar of Schwann cell cytoplasm
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