Oligodendrocytes

Oligodendrocytes are a type of neuroglia, or glial cell. These cells are found in the central nervous system (CNS) of humans and other animals. Oligodendrocytes are best known for creating myelin that surrounds parts of neurons in the CNS. One oligodendrocyte can create myelin for multiple nerve cells. Oligodendrocytes are located in both the gray matter and the white matter of the brain.

Background

Oligodendrocytes are part of the central nervous system. The CNS consists of the brain and the spinal cord. Two types of cells make up the CNS: neurons and neuroglia. Neurons are the cells that send electrical impulses in the CNS. Different types of neurons exist throughout the body, but all have a basic structure. Neurons have cell bodies with dendrites—thread-like projections—that branch out from one side of the neuron. The axon extends from the other side of the neuron. Neurons usually have multiple dendrites, but each neuron has only one axon. Neurons are most often seen as the cells that do most of the communicating in the CNS, although scientists are beginning to understand that neuroglia also help with communication. Neuroglia literally means "nerve glue," as one job of the neuroglia is to hold together and protect the neurons in the nervous system. Neuroglia are best known for sending nutrients to and otherwise supporting neurons. They also help clean up dead cells and other debris from the nervous system. Furthermore, scientists have found that neuroglia play a role in creating synapses, which are structures that help neurons communicate with one another.

Overview

Oligodendrocytes are best known for helping to create myelin in the CNS. Myelin is a material made up mostly of lipids and protein. It is a white substance that helps insulate the axons on neurons. Axons are the parts of neurons that send electrical impulses to other nerves in the body. The myelin and its insulation are important because they help insulate the electrical impulse inside the axon, ensuring the impulse is strong enough to communicate with another neuron when it reaches its destination. Oligodendrocytes are different from neurons. They and some other neuroglia can reproduce. Neurons cannot reproduce, and once a neuron dies, it is gone forever. Oligodendrocytes can reproduce and often do so to mend damage that occurs to myelin. Scientists believe, however, that mature oligodendrocytes do not reproduce.

Myelin also helps protect nerve cells in the peripheral nervous system (PNS). The myelin in the PNS plays a role very similar to that of the myelin in the CNS. Yet, the myelin in the PNS is formed by cells called Schwann cells. The bodies of the Schwann cells wind themselves around the PNS axons to create the myelin. Therefore, the myelin is made from the Schwann cell bodies and does not protrude from the cells' projections. Schwann cells provide only one segment of myelin sheath per cell, whereas each oligodendrocyte can form many segments of myelin with its projections. Each myelin sheath is made up of millions of sections of myelin, which consists of several different oligodendrocytes.

The oligodendrocytes' structure is what allows them to create the myelin to surround axons. Oligodendrocytes have cell bodies with cytoplasm and a nucleus. The nucleus is often dark and elongated. They also have some projections coming out of the cell body. The projections are filled with cytoplasm. Myelin forms at the ends of the projections. The myelin grows a small sheet. The sheet wraps itself many times around a small section of axon. Each projection from an oligodendrocyte forms one section of a myelin sheath. Oligodendrocytes are found in both the gray and white matter of the brain, but they are more prevalent in the white matter.

Although oligodendrocytes are best known for creating myelin sheaths, not all oligodendrocytes form myelin. Some of them act as satellite cells. These satellite cells may or may not have projections coming out from the cell bodies, and they are located in the gray matter of the brain. Scientists do not fully understand the function of these satellites, but they might play a role similar to that of other neuroglia—astrocytes. Astrocytes help regular chemicals and other molecules in the CNS maintain homeostasis, or a state of balance.

Oligodendrocytes are made by the body from precursor stem cells. Stem cells are cells that can be made into many different types of cells, allowing the body to use them wherever they are needed. For example, if a person loses blood because of an injury, the body can create more blood cells using these stem cells. The stem cells in the body transform into different types of precursor stem cells, which will eventually mature into specific mature cells. The precursor cells that help create oligodendrocytes are called oligodendrocyte precursors or glial precursors.

Oligodendrocytes and the myelin they create are very important to the brain. Without oligodendrocytes and without myelin, the brain would not function properly. Some diseases and disorders affect the myelin and the oligodendrocytes in the brain. Multiple sclerosis (MS) is one type of disease that can affect the myelin and oligodendrocytes. MS can cause lesions on the myelin sheath. When a lesion occurs on the myelin sheath, the oligodendrocytes will often try to repair the missing myelin. However, MS can also destroy oligodendrocytes themselves, making it impossible for the myelin sheath to be repaired in some areas. Scientists do not fully understand how MS kills the oligodendrocytes, but they believe that chemical signaling in the brain can cause apoptosis, a process in which the cells kill themselves. Scientists have also found evidence that schizophrenia may have some relation to oligodendrocyte death and altered myelin in the brain. Scientists have found that younger patients with schizophrenia have evidence of myelin damage in the gray matter. They have also found that older patients with schizophrenia have evidence of myelin damage in the white matter. This latter type of damage seemed to be linked to more serious disease symptoms.

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