Oligodendrocytes belong to the glial cell group and are an intrinsic part of the central nervous system, along with astrocytes and neurons. As glial cells, they perform supportive functions for neurons. Some neurological diseases, such as multiple sclerosis, are caused by dysfunction of oligodendrocytes.
What are oligodendrocytes?
Oligodendrocytes are a special type of glial cells. In the central nervous system, they are responsible for forming myelin sheaths to insulate nerve processes (axons). In the past, they were mainly considered to have supporting functions similar to connective tissue. However, unlike connective tissue, oligodendrocytes develop from ectoderm. Today it is known that they have a great influence on the speed of information processing and on the energetic supply of the neurons. In the peripheral nervous system, Schwann cells perform functions similar to those of oligodendrocytes in the CNS. Oligodendrocytes are mainly found in the white matter. The white matter consists of axons surrounded by a myelin sheath. The myelin gives this region of the brain its white color. In contrast, the gray matter consists of the cell nuclei of the neurons. Because there are fewer axons here, the number of oligodendrocytes in the gray matter is also limited.
Anatomy and structure
Oligodendrocytes are cells with small round nuclei. Their nuclei have a high content of heterochromatin, which can be readily detected by various staining techniques. Heterochromatin ensures that the genetic information in oligodendrocytes generally remains inactive. This is to maintain the stability of these cells so that they can perform their supporting function undisturbed. Oligodendrocytes have cell processes that produce myelin. They coat the axons of the nerve cells with their projections and form myelin in the process. With this myelin, they wrap the nerve processes in a spiral. An insulating layer forms around the individual axons. In the process, one oligodendrocyte can produce up to 40 myelin sheaths that wrap around several axons. However, fewer processes originate from oligodendrocytes than from the other glial cells in the brain, the astrocytes. Myelin consists largely of fats and to a lesser extent of certain proteins. It is impermeable to electrical currents and therefore acts like a strong insulating layer. This is how the individual axons are separated from each other. This insulating layer looks similar to the insulation around a cable. At intervals of 0.2 to 1.5 millimeters, the insulating layer is missing in each case. These areas are called Ranvier’s lacing rings. Both the insulation and the formation of insulated sections greatly affect the speed of information transmission.
Function and tasks
The oligodendrocytes effectively insulate the individual nerve cell processes from each other with their myelin sheaths. In addition, at certain intervals in the myelin sheath are short uninsulated sites called Ranvier’s lacing rings. In this way, nerve signals can be transmitted more effectively and quickly. The very act of insulating the axons speeds up signal transmission. Dividing the insulation into sections makes this acceleration even more effective. The signal jumps from lacing ring to lacing ring. Thus, a speed of up to 200 meters per second or 720 Km per hour can be generated. This high speed is what makes it possible for highly complex information processing to emerge in the first place. The same is true for the separate transmission due to the insulation of the nerve cords. Without myelin sheaths, the axons would have to be very thick to achieve high signal velocities. It has already been calculated that without myelin sheaths, our optic nerve alone would have to be as thick as a tree trunk to achieve the same performance. In such complex organisms as vertebrates and especially humans, innumerable nerve impulses are transmitted, which have to be processed for information processing. Without oligodendrocytes, complex information processing and thus the development of intelligence would not be possible. This function of oligodendrocytes has been known for decades. In recent years, however, there has been growing recognition that oligodendrocytes perform even more functions. For example, the axons are very long and the transmission of the signal also costs energy.However, the energy within the axons is not sufficient, especially since no replenishment comes from the cytoplasm of the neuron. According to recent findings, the oligodendrocytes take up additional glucose and even store it as glucogen. When there is an increased energy demand in the axons, the glucose is first converted to lactic acid in the oligodendrocytes. The lactic acid molecules then migrate into the axon via channels in the myelin sheath, where they provide energy for signal transmission.
Diseases
Oligodendrocytes play a major role in the development of neurological diseases such as multiple sclerosis. In multiple sclerosis, destruction of the myelin sheaths occurs, and the insulation of the axons is lost. Signals can no longer be transmitted properly. It is an autoimmune disease, whereby the immune system attacks and destroys the body’s own oligodendrocytes. Multiple sclerosis often occurs in relapses. After each relapse, the body is stimulated again to produce new oligodendrocytes. The disease calms down. If the inflammation and thus the destruction of the oligodendrocytes becomes chronic, the nerve cells also die. Since these cannot regenerate, permanent damage occurs. The question remains, however, why the neurons also perish. Discoveries made in recent years provide an answer. Oligodendrocytes supply the neurons with energy via the axons. When the energy supply ends, the neurons also die.
