Section of central canal of the spinal cord, showing ependymal and neuroglial cells
Photomicrograph of normal ependymal cells at 400× magnification in human autopsy tissue
Ependyma is the thin epithelium-like lining of the ventricular system of the brain and the central canal of the spinal cord. Ependyma is one of the four types of neuroglia in the central nervous system (CNS). It is involved in the production of cerebrospinal fluid (CSF).
The ependyma is made up of ependymal cells (ependymocytes), a type of glial cell. These cells line the CSF-filled ventricles in the brain and the central canal of the spinal cord. The cells are ciliated simple columnar epithelium-like cells. Their apical surfaces are covered in a layer of cilia, which circulate CSF around the CNS. Their apical surfaces are also covered with microvilli, which absorb CSF. Ependymal cells also produce CSF. Within the ventricles of the brain, a population of modified ependymal cells and capillaries together form a system called the choroid plexus, which produces the CSF.
Modified tight junctions between ependymal cells control fluid release across the epithelium. This release allows free exchange between CSF and nervous tissue of brain and spinal cord, which is why sampling of CSF (e.g. through a "spinal tap") provides a window to the CNS.
The basal membranes of these cells are characterized by tentacle-like extensions that attach to astrocytes.
Jonas Frisén and his colleagues at the Karolinska Institute in Stockholm provided evidence that ependymal cells act as reservoir cells in the forebrain, which can be activated after stroke and as in vivo and in vitro stem cells in the spinal cord. However, these cells did not self-renew and were subsequently depleted as they generated new neurons, thus failing to satisfy the requirement for stem cells. One study observed that ependymal cells from the lining of the lateral ventricle might be a source for cells which can be transplanted into the cochlea to reverse hearing loss.
Ependyma and neurodegeneration
In 2004, Milan Radojicic proposed the stem cell niche disruption hypothesis, highlighting the role of local ischemia, cerebrospinal fluid dynamics and cytotoxic factors in disrupting the ependymal stromal epithelium, along with periependymal stem-progenitor cells, thereby tipping the balance between injury and self-repair (i.e., neurogenesis and gliogenesis) in the central nervous system toward further degeneration over time.
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