The unravelling of the polarized distribution of AQP4 in perivascular astrocytic endfeet has revitalized the interest in the role of astrocytes in controlling water and ion exchange at the brain-blood interface. The importance of the endfeet is based on the premise that they constitute a complete coverage of the vessel wall. Despite a number of studies based on different microscopic techniques this question has yet to be resolved. We have made an electron microscopic 3D reconstruction of perivascular endfeet in CA1 (stratum moleculare) of rat hippocampus. The endfeet interdigitate and overlap, leaving no slits between them. Only in a few sites do processes--tentatively classified as processes of microglia--extend through the perivascular glial sheath to establish direct contact with the endothelial basal lamina. In contrast to the endfoot covering of the endothelial tube, the endfoot covering of the pericyte is incomplete, allowing neuropil elements to touch the basal lamina that enwraps this type of cell. The 3D reconstruction also revealed large bundles of mitochondria in the endfoot processes that came in close apposition to the perivascular endfoot membrane. Our data support the idea that in pathophysiological conditions, the perivascular astrocytic covering may control the exchange of water and solutes between blood and brain and that free diffusion is limited to narrow clefts between overlapping endfeet.
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