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The walls of the ventricular system are lined by a single cell layer, the ependyma (C). Each ependymal cell has a basal process, the ependymal fiber, which extends into the brain. The cell surface facing the ventricular lumen often carries several cilia, with the basal bodies, or kinetosomes (C2), lined upbeneath the cell surface.
In the electron-microscopic image, the ventricular surface of the ependymal cells exhibits numerous vesicle-containing pro-trusions (D3). The cilia (D4) contain micro-tubules in the characteristic 9 + 2 arrange-ment: two single microtubules in the center (D5) and nine microtubule doublets (D6) ar-ranged around them. The basal body of each cilia is surrounded by a dense zone (D7) intowhich numerous short rootlets (D8) radiate. A basal foot (D9) is located on one side of the basal body; it may play a role in directing the beat of the cilia. The ependymal cells are interconnected along their lateral surfaces by zonulae adherentes (adherent junctions) (D10) and by zonulae occludentes (tight junctions) (D11); the latter seal the cere-brospinal fluid space against the brain. Neu-ronal processes (D12) run between the ependymal cells. The layer underneath the ependyma consists of radially or horizon-tally running glial fibers (C13) and contains only few cells. Below it lies the subependy-mal cell layer (C14). It contains undifferen-tiated cells in addition to astrocytes. Ac-cording to recent studies, not only glial cells but also neurons are generated here throughout life. Intensive investigations are under way to test whether neuronal stem cells of the subependymal zone can be used for neuronal replacement in various forms of neuronal degeneration.
The structure of the ventricular wall varies widely in different regions. The ependymal cover or the subependymal layer of glial fibers may be completely absent in certain areas. The subependymal glial cell layer is most prominent above the head of the cau-date nucleus and at the base of the anterior horn but is absent above the hippocampus.
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