Regional organization of astrocytic membranes in cerebellar cortex

The internal organization of plasmalemmal membrane, as revealed by freeze-fracture techniques, varies dramatically and predictably over the surface of astrocytes in mouse cerebellar cortex. Assemblies of uniform, small intramembrane particles packed in orthogonal order into square or rectangular aggregates are specialized distribution of intramembrane particles which, in the cerebellar cortex, are found only in astrocytes. The concentration of assemblies is greatest in astrocytic membrane juxtaposed to vascular structures or facing the cerebrospinal fluid at the glial limitans. Many fewer are present in regions of astrocytic membrane apposed to neural structures and virtually none are present on the astrocyte cell body. Corresponding structures have not yet been found in thin-sectioned preparations. While the distribution of assemblies in membranes facing blood and cerebrospinal fluid compartments suggests that they may have a role in transport of some material into or out of those compartments, their function is unknown. A second, distinct specialization of intramembrane structure appears to represent a junction between apposed astrocytic processes. We have provisionally described this as a 'polygonal particle junction', since it appears as large, irregular particles densely packed without obvious order in co-extensive regions of two astrocytic membranes. This junction is regularly present just below the cerebellar surface in the processes of the glial limitans as well as between large, more proximal radial Bergmann fibers, and also occurs occasionally throughout the molecular layer. With tannic acid mordant after aldehyde-osmium fixation or rapid freezing and freeze-substitution, it is possible to demonstrate subtle electron-dense specializations of the astrocytic membranes and extracellular matrix in thin-sections which correspond to the sites of polygonal particle junctions. The function of this astrocytic specialization is also unknown. Cerebellar astrocytes manifest numerous gap junctions as well, whose structure in freeze-fractured and thin-sectioned preparations is quite distinct from that of assemblies or of polygonal particle junctions.