Plasma membrane organization of astrocytes in elasmobranchs with special reference to the brain barrier system

The structural machinery contributing to the blood-brain barrier in elasmobranchs has been examined mainly using freeze-fracture techniques. Capillary endothelial cells, which show local aggregations of pinocytotic vesicles and infrequent fenestrations, are connected by poorly developed tight junctions. Astrocytic processes investing the capillary are linked by well-developed tight junctions between lateral membranes immediately beneath the perivascular space. The tight junctions consist of continuous strands of multiple layers coursing circumferentially around the astrocytic processes parallel to one another as well as to the perivascular space. The presence of intramembrane particles (IMPs) within E-face grooves may result in discontinuities in IMP rows on the P-face. Thus, in compensation for the capillary endothelium, perivascular astrocytes constitute the morphological site of the blood-brain barrier in elasmobranchs. Continuous strands of tight junctions are also detected between astrocytic processes forming the glia limitans at the brain surface. These may act as a barrier between meningeal connective tissue and brain parenchyma. Astrocytic membranes have numerous IMPs of 8-9 nm in diameter on their P-faces. These IMPs are uniformly distributed so that astrocytic membranes are easily distinguished from neuronal membranes even in the neuropil. Ependymal cells also have numerous IMPs in all their membrane domains. Orthogonal arrays are not detected in either astrocytic or ependymal plasma membranes.