Perivascular space

A perivascular space, also known as a Virchow–Robin space, is a fluid-filled space surrounding certain blood vessels in several organs, including the brain, potentially having an immunological function, but more broadly a dispersive role for neural and blood-derived messengers. The brain pia mater is reflected from the surface of the brain onto the surface of blood vessels in the subarachnoid space. In the brain, perivascular cuffs are regions of leukocyte aggregation in the perivascular spaces, usually found in patients with viral encephalitis. Perivascular spaces vary in dimension according to the type of blood vessel. In the brain where most capillaries have an imperceptible perivascular space, select structures of the brain, such as the circumventricular organs, are notable for having large perivascular spaces surrounding highly permeable capillaries, as observed by microscopy. The median eminence, a brain structure at the base of the hypothalamus, contains capillaries with wide perivascular spaces. In humans, perivascular spaces surround arteries and veins can usually be seen as areas of dilatation on MRI images. While many normal brains will show a few dilated spaces, an increase in these spaces may correlate with the incidence of several neurodegenerative diseases, making the spaces a topic of research. Perivascular spaces are gaps containing interstitial fluid that span between blood vessels and their host organ, such as the brain, which they penetrate and serve as extravascular channels through which solutes can pass. Like the blood vessels around which they form, perivascular spaces are found in both the brain subarachnoid space and the subpial space. Perivascular spaces surrounding arteries in the cerebral cortex and the basal ganglia are separated from the subpial space by one or two layers of leptomeninges, respectively, as well as the pia mater. By virtue of the leptomeningeal cell layer, the perivascular spaces belonging to the subarachnoid space are continuous with those of the subpial space.