Freeze-fracture ultrastructure of the perinodal astrocyte and associated glial junctions.

Freeze-fracture examination of nodes of Ranvier from adult rat optic nerve demonstrates the presence of astrocytic processes at the majority of nodes of Ranvier. Astrocytic processes often run along the entire length of the nodal gap, although they do not necessarily encircle the entire nodal circumference. The E- and P-fracture faces and the cross-fractured cytoplasm of these astrocytes (termed 'perinodal astrocytes') were examined. The cytoplasm of perinodal astrocytes contains 10-nm filaments. The P-faces of perinodal astrocytic membranes are characterized by orthogonal arrays of intramembranous particles ('assemblies'), with a center-to-center periodicity of approximately equal to 6 nm. Complementary orthogonally arranged pits are observed on the E-faces of the astrocytic membranes. The density of these arrays in perinodal astrocytic membranes is similar to that in parenchymal astrocytic membranes, but is substantially lower than that at pericapillary astrocytic membranes. In addition, gap junctions are present between astrocytes, and between astrocytes and paranodal oligodendroglial layers. These findings indicate that astrocytic processes comprise an important structural component of central nodes of Ranvier, and provide a morphological basis for a possible astrocytic role in nodal function.