Ependyma

The ependyma is a layer of cells along the interior chambers of the brain and the central canal of the spinal cord. It is made up of a type of glial cell, which provides support for the neurons that comprise much of the brain and nervous system. These glial cells are called ependymal cells, and they help to both make and distribute the cerebrospinal fluid (CSF) that cushions and protects the brain and spinal cord.

Background

The brain is the most complex organ in the human body, and the human brain is generally thought to be the most complicated organ of any living creature. In human embryos, the cells that form the brain begin to differentiate from other cells less than three weeks after fertilization of the egg occurs. Initially, the new embryo has three parts known as germ layers, which include the ectoderm, the mesoderm, and the entoderm. The entoderm will help form the digestive and respiratory systems. The mesoderm will form connective and skeletal tissue and the muscular, circulatory, and excretory systems of the body. The ectoderm becomes the skin and the nervous system.

At around seventeen days after fertilization, the cells in the ectoderm begin to form parts of the nervous system and the brain. The ectoderm will then separate into two parts, and one part will form what is called a neural tube that has two areas known as neural crests. These crests, which exist only during embryonic stage, will eventually create several different types of cells in the body.

The neural tube will eventually become the brain and spinal cord. The tube is lined with neuroepithelial cells that will help create many of the nerve cells in the brain, spine, and the rest of the nervous system. In addition, some of these cells will remain in place in the neural tube and eventually form a single layer of epithelial cells lining the four ventricles, or compartments, of the brain and the inside of the spinal cord. These are known as ependymal cells, and the layer they form is known as the ependyma.

The development process in other animals is similar to the process in humans. The time frames differ, but overall brain development follows a similar pattern in all vertebrates. Vertebrates are animals that have a spinal column or backbone.

Overview

The cells that make up the ependyma are cube-shaped and have long finger-like projections known as cilia. The cilia extend into the ventricles of the brain. The ventricles are fluid-filled hollows in the brain that are essentially remnants of the neural tube that formed in the embryo. In certain portions of the ventricles, the ependymal layer is closer to the connective tissue in the brain. As a result, the ependyma here is wrinkled upon itself and encompasses some blood vessels. This area is known as the choroid plexus, and there is one in each of the four ventricles. The choroid plexus is responsible for producing much of the CSF in the brain and spinal cord. Human brains produce an average of five hundred milliliters of CSF per day.

CSF is similar to blood in some ways but significantly different in others. It contains some of the same minerals and nutrients—such as sodium, potassium, calcium, magnesium, glucose, and protein—but in different concentrations than blood. It also contains very few white blood cells.

The production of CSF is one of the most important ways the ependyma helps to protect and support the brain. The tissue that makes up the brain and spinal cord needs to be constantly wet; CSF accomplishes that. The tiny cilia and smaller projections known as microvilli on the surface of the ependymal layer help move the fluid around. They also absorb some of it to help monitor its quality and spread it to the brain tissue that is under the ependyma. The ependyma also helps to filter the CSF to the bloodstream, where it is cycled through to remove waste materials, including excess neurotransmitters.

Another way the ependymal layer helps the brain is by acting as a first line of defense against viral invasions. This is particularly important in species such as humans with long life spans. Researchers are investigating whether the ependyma may be depleted by repeated viral infections; some indications from human autopsies indicate this might be the case.

Researchers are also looking into another possible function for the ependymal layer. Studies in mice have indicated that the ependymal cells may provide support for new stem cells that form in an area beneath the ependymal layer. Stem cells are special because they can develop into different types of cells, much in the same way cells differentiate during embryonic development. It is thought that these cells may develop into new nerve cells, a process known as neurogenesis. If this is found to be the case, it could provide a new avenue of treatment for conditions such as dementia and Alzheimer's disease.

While the ependymal layer is very beneficial to the brain, it can also be the source of some types of brain tumors. These are called ependymomas. They are relatively rare in adults, accounting for less than 3 percent of all adult brain cancers. However, they occur more frequently in children. Ependymoma is the sixth-most common form of childhood brain cancer, and nearly 30 percent of all children under three years of age who develop brain cancer will have an ependymoma. The cause of these tumors is unknown.

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