A. Cerebrospinal fluid
Cerebrospinal fluid produced by ependymal cells provides both mechanical protection – padding – and a means to remove metabolic waste products. The brain and spinal column are composed of very soft tissue and need to exist in a wet environment that cushions them within protective bone.
Ependymal cells are cube-shaped epithelial cells that line the surface of the brain’s four interior chambers, the ventricles, and the central canal of the spinal cord. Ependymal cells both secrete cerebrospinal fluid and absorb it.
The surface of the ependymal cell layer that faces the ventricle is covered by cilia and microvilli. Whip like motion of the cilia stir and move the cerebrospinal fluid in the ventricles. Uptake of cerebrospinal fluid by microvilli permits ependymal cells to monitor the quality of the cerebrospinal fluid and provide underlying brain tissue with widespread access to cerebrospinal fluid proteins.
Modified ependymal cells that are continuous with the ventricular ependymal cell layer form the outer covering of a ventricle-wall structure called the choroid plexus. The choroid plexus consists of many capillaries separated from the ventricle by the modified ependymal cell layer. It is the choroid plexus that produces most of the cerebrospinal fluid.
Fluid filters from the choroid plexus capillaries through the ependymal cell layer to become cerebrospinal fluid. To increase the efficiency of this process, choroid plexus ependymal cells actively transport sodium, chloride, and bicarbonate ions into the ventricles to create an osmotic gradient that continuously draws capillary water across the cells and into the cerebrospinal fluid.
There are four choroid plexuses in the human brain – one in each ventricle. The entire volume of cerebrospinal fluid is recycled back into the blood about 4 times per day to remove metabolic waste products and excess neurotransmitters that find their way into the fluid. In humans, about 500 milliliters of cerebrospinal fluid is produced by the choroid plexuses each day.
B. Supporting Adult Neural Stem Cells
For a long time it was thought that nervous system plasticity – the reciprocal interaction between brain structure and brain function – only involved rearrangement of the contacts between pre-existing ‘old’ neurons. That view is changing to include neurogenesis, the birth of new nerves, in the ongoing process of adult brain plasticity.
In rodents and in non-human primates there is a zone of cells below the ependymal epithelial cell layer in the lateral ventricles of adult animals that serves as a reservoir of adult neural stem cells. Neural stem cells are a potential source of new neurons for repair of brain injuries. In humans the existence of this cell layer is of great interest but still controversial, because confirmatory studies of its characteristics using autopsy tissue have produced mixed results.
Studies using adult mice estimate that about 10,000 new neurons are generated daily from sub-ependymal layer neural stem cells. Most of these progenitor cells in mice differentiate into various types of interneurons in the olfactory bulb. Contrary to their name, a portion of the “neural” stem cells of the sub-ependymal layer also develop into oligodendrocytes, brain cells that insulate new neurons.
Ependymal cells lining the ventricles maintain the structural integrity of sub-ependymal layer and possibly provide metabolic support to developing stem cells. In mice the neural stem cells send a long membrane projection between ependymal cells to make contact with the ventricular fluid. Additionally they send a second membrane projection to wrap around a capillary in the vicinity. Because of this anatomical arrangement, it is thought that activation of neural stem cells depends upon a combination of signals from ependymal cells, the cerebrospinal fluid, and blood in the capillaries.
Using the mouse model, studies focused upon the origin of adult neural stem cells of the sub-ependymal layer have also produced a new understanding of the common origin of macroglia and nerves. Macroglia, astrocytes and oligodendrocytes, were long considered specialized supportive cells with an embryonic origin very different from that of neurons. However, new studies demonstrate that both astrocytes and neurons arise from a common stem cell during embryogenesis. Furthermore, one type of adult astrocyte, the radial astrocyte, is the adult neural stem cell. Therefore, macroglia and neurons only represent different stages in development of a single embryonic cell type.
The physiology of brain plasticity is an emerging area in understanding how the brain works and it now includes the concept of neurogenesis. Neurogenesis in the adult brain offers potential hope for future therapies to manage psychiatric disorders and the diseases of human aging such as Alzheimer’s.
C. Protecting Against Viral Infection
Ependymal cells are susceptible to infection by a wide range of common viruses. They act as a first line of viral defense for the brain. In long-lived organisms such as human that are exposed repeatedly to viral infections these cells fight off multiple attacks. It is unknown how repeat viral infection influences the life span of ependymal cells.
Persistent viral infections may be one reason that studies with autopsy material from human brain have failed to discover large populations of ependymal cells lining the lateral ventricles. Similarly, using membrane marker methods, only a small number of mitotically active neuroblasts (developing adult neural stem cells) have been found in the sub-ependymal layer of men and women. However, those neuroblasts possessed the typical migratory morphology and elongated shape with a small body observed in non-human species.
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