Other regions of the CNS contain many different populations of astroglial cells. Velate astrocytes are found in the cerebellum, where they form a sheath surrounding granule neurones; each velate astrocyte enwraps a single granule neurone. A similar type of astrocyte is also present in the olfactory bulb. Interlaminar astrocytes are specific to the cerebral cortex of higher primates. Their characteristic peculiarity is a very long single process (up to 1 mm) that extends from the soma located within the supragranular layer to cortical layer IV. Tanycytes are specialized astrocytes found in the periventricular organs, the hypophysis and the raphe part of the spinal cord. Astroglial cells in the neuro-hypophysis are known as pituicytes; the processes of these cells surround neuro-secretory axons and axonal endings under resting conditions. Perivascular and marginal astrocytes are localized very close to the pia mater, where they form numerous endfeet with blood vessels; they form the pial and perivascular glia limitans barrier, which assists in isolating the brain parenchyma from the vascular and subarachnoid compartments. Ependymocytes, choroid plexus cells and retinal pigment epithelial cells line the ventricles or the subretinal space.
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