Oligodendrocytes: what are they, types and functions
Dec 04, 2021
Neurons are essential for the transmission of electrochemical impulses through the nervous system. However, they cannot fulfill this role on their own: they require the support of glial cells for very important tasks. different, such as the contribution of nutrients, the maintenance of the structure or the acceleration of the driving itself neuronal.
Oligodendrocytes, a type of glial cell that form the myelin sheaths that surround the cells, are especially important in this last function. axons of the central nervous system. In this article we will analyze what are oligodendrocytes and what functions do they perform and we will describe its two main types.
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What are oligodendrocytes?
Oligodendrocytes are a type of glial cell that is found exclusively in the central nervous system, that is, in the brain and in the spinal cord. These cells create myelin sheaths around the axons of neurons., isolating them and increasing the speed of transmission of the electrochemical impulses that run through them.
In some axons the myelin sheaths are separated into sections; unmyelinated spaces are called "nodules of Ranvier". These points allow the saltatory conduction of neural impulses: in Ranvier's nodules the Ion exchanges with the extracellular space regenerate the action potentials, further accelerating the transmission.
Myelination begins to occur before birth but continues for the first three decades of life. Unlike Schwann cells, which serve similar functions in the peripheral nervous system, oligodendrocytes can cover about 50 different axons thanks to their multiple extensions.
This type of glia forms in the ventral ventricular region of the spinal cord during intrauterine development, later than other types of glia. In adults, oligodendrocytes continue to appear from glial progenitor cells, although their number is much lower than that present during the first neurogenesis.
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Glial cells or glia
Glial cells make up half of the nervous system. They fulfill support functions in relation to neurons: they give structure to the networks of neurons, nourish them, maintain the stability of the extracellular environment, regulate the growth of dendrites and axons, repair cell injuries, direct neuronal migration during embryonic development ...
Among the most numerous glia cells we find the astrocytes, which give structure to the blood brain barrier (allowing the contribution of nutrients and the cleaning of waste in the nervous system), the microglia, which performs immune and regenerative functions, and lSchwann cells, responsible for the formation of myelin in the peripheral nervous system.
The set composed of oligodendrocytes and astrocytes, which are also located in the central nervous system, is called “macroglia” due to the remarkable size of these two types of cells compared to the rest of the glia, especially the microglia.
Have been found two main types of oligodendrocytes: interfascicular and satellite. These two glial cell subclasses differ mainly in their functions, although they are very similar at the structural and molecular level.
Interfascicular oligodendrocytes, which are part of the white matter of the brain and give it its characteristic color, are the basic type; When talking about “oligodendrocytes”, the most common is that they are referring to these, since they are the cells that are involved in the formation of myelin sheaths, the main role attributed to oligodendrocytes.
In contrast, satellite oligodendrocytes are included in the gray matter because they are not involved in myelination. They also do not adhere to neurons, so they do not play an isolating role. At the moment it is not known exactly what the functions of these oligodendrocytes are.
Functions of these cells
In this section we will focus on describing the main roles of interfascicular oligodendrocytes which, as we have said, are better known than those of satellites. These functions are mainly associated with the formation of myelin sheaths.
1. Neural transmission acceleration
Myelinated axons send out action potentials at a much higher rate than non-myelinated axons, especially if they contain nodules of Ranvier. An adequate rhythm of neural conduction allows the correct functioning of the muscular and hormonal systems, among other organismic functions, and has also been related to intelligence.
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2. Cell membrane isolation
Myelin sheaths also isolate neuronal axons from the extracellular environment; This function prevents ion seepage through the cell membrane.
3. Structuring the nervous system
Glial cells in general fulfill the important role of maintaining the structure of neuron networks. These cells are not very solid by themselves, so they need the physical support of the glia, including the oligodendrocytes.
4. Support for the development of neurons
Oligodendrocytes produce various neurotrophic factors, proteins that by interacting with neurons favor them to remain active, preventing apoptosis or programmed cell death. They also promote cell differentiation required for the formation of neurons.
5. Extracellular fluid homeostasis
Satellite oligodendrocytes are known not to fulfill the same roles as interfascicular ones because they do not form myelin sheaths. However they are very relevant to maintain the homeostatic balance of the extracellular environment of the neurons next to which they are located; Unlike the inter-fasciculars, the satellites do not join them.