Ependymal reactions to injury. A review.

The ependyma reacts to injury with a few stereotypical responses and does not regenerate at any age. Non-neoplastic ependymal cells do not undergo mitotic proliferation and do not re-express fetal cytoskeletal or secretory proteins. Atrophy of ependymal cells accompanies generalized cerebral atrophy. The ependyma may be damaged by stretching during ventricular dilatation, by infarcts of the ventricular wall or by infection and inflammation. Tearing of the epithelium leaves discontinuities that become filled with processes of subventricular astrocytes. In some cases reactive gliosis is minimal, but in most it is extensive and gliotic nodules form beneath intact ependyma and within gaps between ependymal islands. Ependymal rosettes may form in several ways: sequestration of diverticuli from the surface; curling of a torn edge or penetration of an edge into the parenchyma; reactive gliosis overgrowing an ependymal edge; in situ differentiation of ependymal cells from deep neuroepithelial cells. Migration and metaplasia are unlikely mechanisms. Bacterial and fungal ependymitis are highly destructive. Several viruses, especially mumps, selectively infect ependymal cells and are an important cause of acquired aqueductal stenosis without inflammation. Damaged ependyma may not be able to perform its function in the regulation of transport of fluid, ions and small molecules between cerebral parenchyma and ventricular fluid and thus may contribute to hydrocephalus. Damage to the fetal ependyma may result in secondary focal dysplasias of the developing brain.