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The pia mater becomes tapered and forms a capillary. The Virchow-Robin space (VRS) is the extravascular space around the small arteries and is full of cerebrospinal fluid (CSF). Astrocytes form gap junctions at the outer layer of the VRS, while the endothelial cells from the inner layer of the VRS. Astrocytes are sufficient to express aquaporin-4. The gap junctions facilitate the exchange of CSF and ISF. Substances in the CSF can enter the brain parenchyma through the VRS.
