The lateral belts of the myelin sheath wind helically around the paranodal region of the axon. The lateral belt coil leaves an imprint on the axon and thus confers a conspicuous, indented configuration to the freeze-fracture faces of the axolemma. The contact area between the axolemma and the lateral belt membrane is the site of an extensive and unusual cell junction (axo-glial junction). In thin sections the junctional membranes are undulated, the peaks in one membrane mirroring the peaks in the other. The transverse bands (intercellular septa) are in register with the undulations. The intercellular space measures about 30 Å. In freeze-fracture replicas, the undulations are evident as alternating ridges and grooves which run strictly parallel and are oriented at an angle with respect to the helical path of the lateral belt. Both junctional membranes contain parallel rows of intramembrane particles which coincide with the ridges and grooves and, therefore, with the intercellular septa. The center-to-center distance between septa or, equivalently, between adjacent rows of particles measures approximately 250 Å. Although the axo-glial junction possesses structurally symmetrical features, there exist important differences between the two junctional membranes. The intramembrane particles of the glial and the axonal membrane differ in cleaving properties. Furthermore, in some of the fibres the E face of the junctional axolemma displays a crystalline array which is not present in the fracture faces of the glial membrane. The axo-glial junction is limited to the paranodal region, although the inner belt of the myelin sheath may form occasional junctional spots with the internodal region proper of the axolemma. The classification and the presumptive functions of the paranodal axo-glial junction are briefly discussed.
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