Ependymal Cells

Ependymal cells line the brain ventricles and central canal of the spinal cord (Figs. 15, E, and 19). They arise from the pseudostratified neuroepithelium from which neurons and neuroglial cells originate. That surface is ciliated here and there, and some cilia are seen in the ependyma. They form an apparent simple cuboidal or columnar epithelium with microvilli and occasional cilia, but in fact the bases of certain cells taper into long, slender, outward processes.

In the embryo, some processes reach the surface of the neuraxis, establishing end feet there or on nearby capillaries. Later, they shorten and ultimately disappear, ending somewhere in the neuropil. Where the CNS wall is thin, as near a choroid plexus, they extend the whole way between central canal and pia, expanding into end feet that collectively make up a thin, smooth, external limiting membrane under the pial surface. Shorter processes entangle with those of astrocytes in a dense subependymal layer, the internal limiting membrane, close beneath the ventricular surface.

Modified ependymal cells are found in the choroid plexuses and third ventricle. In the former, they form a simple cuboidal secretory epithelium, with moderately extensive rER, many mitochondria, and irregularly oriented microvilli at their free surfaces, some with bulbous expansions at their tips. In the latter, tanycytes have processes extending into the hypothalamus to end near neurosecretory cells and the capillary plexus of its portal circulation. Their function is uncertain. They may transport hormones in the CSF to these neurons to regulate the release of adenohypophyseal hormones into the portal system or from these neurons into the CSF.

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