THE PARANODAL REGION AND AXO-GLIAL JUNCTION OF MYELINATED FIBERS / 解剖学报

ABSTRACT

By using electron microscopy, the paranodal region and axo-glial junction were examined in optic nerves of rats aged 14 days. The paranodal region was characterized in longitudinal sections by the sequential termination of the myelin lamellae, beginning proximally with the innermost and ending, at the Ranvier node, with the outermost lamella. The termination of each lamella was accom- panied by a separation of the major dense line of the compact myelin and the consequent formation of a "loop" of glial cytoplasm. Each paranodal loop inde- nted the axonal surface as it became junctionally apposed to the axolemma. The periaxonal extracellular space, 10-20nm in width in the internodal region and reduced at the paranodal junction to approximately 3nm, forming an axo-glial junction, which was thought to be held together by dense structure. The parano- dal junction seems to serve strong adhesion between the apposed axonal and glial membranes. Conduction of the nerve impulses in myelinated axons was saltatory. Axons and sheath cells probably maintain vital communication with one another, presumably at the paranodal junctional complex. This communication was viewed as vital to the stability and maintenance of myelin. We found some clear vesi- cles in axoplasm near the Ranvier node and speculated that there were endocyto- sis and exocytosis in paranodal region. This was a direct morphological evidence supporting metabolic coupling between axons and sheeth cells.