Schwann cells of the peripheral nervous system they collaborate with neurons and play a key role in the conduction of nerve impulses, being the cells responsible for coating axons with an insulating substance that increases the speed at which information is transmitted.
In this article, we will see what Schwann cells are, what their functions are, how they develop and develop and what types of pathologies are linked to them.
What are Schwann cells?
Schwann cells, also called neurolemocytes, are a specific type of glial cell located in the peripheral nervous system. Glial cells are cells of nervous tissue responsible for performing auxiliary and supporting functions of neurons (supporting, nourishing or guiding and controlling neuronal migrations in the early stages of development, among others).
These cells are named after the physician and anatomist Theodor Schwann (1810-1882), the father of the cell theory which postulated that all living things are composed of cells and products made by them, a theory that marked a paradigm shift. in the way we conceive of life in the mid-19th century.
Schwann cells maintain a close relationship with neurons from their origin in embryonic tissue, satisfying a key role in the proper guidance and control of axonal growth. So let’s see what functions these cells perform.
Functions of these cells
Schwann cells perform the same functions in the peripheral nervous system (PNS) as other types of glial cells in the central nervous system (CNS). One of the main tasks of this type of cell is to serve as a support and guide in the regeneration processes of the peripheral nervous system after injury or axonal injury.
These cells appear unique in their ability to stimulate the growth and regeneration of peripheral nerves.
Schwann cells located in axonal endings and in synaptic buttons of neuromuscular junctions, they provide physiological support to maintain ionic homeostasis of synapses (Self-regulation and maintenance of constancy in its composition and properties).
Another of the fundamental tasks of these cells is to form a myelin sheath around the axons of the PNS, a function that their homologous cells, the oligodendrocytes, perform in the CNS.
Unlike the latter, which can form myelin sheaths on several different axons (extensions of the neuron responsible for conducting nerve impulses), Schwann cells can only form a segment of myelin on a single axon, a mechanism that facilitates the spread of nerve impulses faster.
The myelin sheath
Neurons of the peripheral nervous system transmit nerve impulses more or less rapidly depending on whether or not their axon is covered with the myelin sheath, an insulating layer made up of proteins and fats. This sheath is not continuous, as Schwann cells only cover 100 micrometers in length at a time, leaving tiny cracks between the sheath and the sheath, called Ranvier’s nodules.
These nodules facilitate the transmission of nerve impulse or action potential, allowing the electrical activity that passes through axons to be maintained at an appropriate rate until it reaches the cell body or soma of the body. neuron. This activity occurs “by hopping”, hence the name of neuronal conduction by hopping.
Although the nature and origin of the factors involved in the proliferation processes (the increase in the number of cells due to their growth and multiplication) are still unknown, it is known that Schwann cells proliferate during development. peripheral nerves in essentially three contexts:
1. During normal development of the peripheral nerve
Next to the rest of the cells.
2. After a nerve injury
In other words, that is to say by mechanical trauma, neurotoxins or disease that damage myelin.
3. In Schwann cell tumors
This way, it can proliferate anywhere in the peripheral nervous system, As with neurofibromatosis or acoustic fibroids.
The development of Schwann cells begins in a first embryonic and neonatal phase of rapid proliferation, followed by the interruption of proliferation and its final differentiation. In their normal development, these cell types go through two basic stages: migration and myelination.
In their migration phase, Schwann cells are long, bipolar, with a composition rich in microfilaments and no basal lamina or myelin covering them. They are placed on the nerve, on the axons in their final position, So that they divide into small groups of several axons surrounded by one or two Schwann cells.
Thereafter, the cells continue to proliferate and the number of axons contained in each of them decreases. Simultaneously, axons of larger diameter begin to secrete from their counterparts and isolate themselves in a single Schwann cell.
In this phase, the connective tissue spaces of the nerve have already developed better and the cell he is already able to fix the basal lamina. The future maturation and myelination of cells will depend on the correct coupling of this basal lamina.
Pathologies involving this type of cells
The functionality and survival of Schwann cells in the peripheral nervous system can be compromised by multiple factors of various origins: infectious, immune, toxic, traumatic and tumor.
Among the most common infectious factors are Hansen’s bacillus and Klebs-Löffler’s bacillus.. Although the alterations these microorganisms cause in Schwann cells are not yet fully understood and are still the subject of studies and research, there have been indications that a Hansen’s bacillus infection could stop the proliferation of these cells and the myelination of axons.
One of the most common metabolic disorders is diabetic neuropathy., Wherein the Schwann cells exhibit an excessive accumulation of lipid bodies in their cytoplasm. This accumulation seems to reflect an alteration in lipid metabolism, leading to demyelination, without knowing whether it is primary or secondary to axonal alteration.
Tumor factors that affect Schwann cells are usually benign in nature and are classified into four groups: schwannomas, neurofibromas, plexiform fibroids, and malignant fibroids. In addition, there are a large number of metabolic immune disorders which modify Schwann cells, causing demyelination processes generally secondary to axonal injury.
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