Glial membrane specializations in extraparanodal regions

Abstract

Previous freeze-fracture studies of central myelinated nerve fibres have demonstrated a distinctive junction in the paranodal region formed between the terminal loops of the glial cell and the axolemma. This unique junction is characterized by the presence of diagonally oriented rows of particles in the P face and to a lesser extent in the E face of the glial cell and an equivalent pattern in the axolemma. In both, the rows are spaced at 250–300 Å intervals. Although this junction was originally thought to be peculiar to the paranodal region, examples of the same pattern have now been seen in extraparanodal regions in the central nervous system where they appear as circumscribed patches of membrane exhibiting a pattern identical to that in the paranodal glial loops. All examples found were in the immediate vicinity of myelinated nerve fibres and in one case the membrane containing the specialized patch was identified as a lamella of a myelin sheath. These observations constitute evidence that this distinctive membrane specialization is not limited to the paranodal axoglial junction but can also be found in glial membranes not in immediate contact with the specialized membrane of the paranodal axolemma.