The Schwann cells or neurolemocitos are a specific type of glial cells of the nervous system of the brain. These cells are located in the peripheral nervous system and their main function is to accompany neurons during their growth and development.
Schwann cells are characterized by covering the processes of neurons; that is, they are located around the axons, forming an insulating myelin sheath in the outer layer of neurons.
Source: OpenStax / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
Schwann cells present their analogue within the central nervous system, oligodendrocytes. While Schwann cells are part of the peripheral nervous system and are located outside the axons, oligodendrocytes belong to the central nervous system and cover the axons with their cytoplasm.
At present, multiple conditions have been described that can alter the functioning of this type of cells, the best known being multiple sclerosis.
Characteristics of Schwann cells
Schwann cells are a type of cell that was first described in 1938 by Theodor Schwann.
These cells constitute the glia of the peripheral nervous system and are characterized by surrounding the axons of the nerve. In some cases, this action is carried out by wrapping the axons through their own cytoplasm, and in other cases it develops through the elaboration of a myelin sheath.
Schwann cells fulfill multiple functions within the peripheral nervous system and are important for the achievement of optimal brain function. Its main function lies in axonal metabolic protection and support. Likewise, they also contribute to nerve conduction processes.
The development of Schwann cells, as with most cells of the peripheral nervous system, derives from a transient embryonic structure of the neural crest.
However, today it is unknown in what embryonic stage the cells of the neural crest begin to differentiate and constitute what is known as Schwann cells.
Structure
From bottom to top: axon, Schwann cells, satellite cells, and peripheral ganglionic neuron body (unipolar cell)
The main property of Schwann cells is that they contain myelin (a multilaminar structure that is formed by the plasma membranes that surround the axons).
Depending on the diameter of the axon in which the Schwann cells are attached, they can develop different functions and activities.
For example, when these types of cells accompany small diameter (narrow) nerve axons, a layer of myelin develops that can be lodged in different axons.
In contrast, when Schwann cells coat larger diameter axons, circular bands without myelin are observed known as nodes of Ranvier. In this case, myelin is made up of concentric layers of the cell membrane that spirally surround the axon of difference.
Finally, it should be noted that Schwann cells can be found in the axonal terminals and synaptic buttons of the neuromuscular junctions, where they provide physiological support for the maintenance of ionic homeostasis of the synapse.
Proliferation
The proliferation of Schwann cells during the development of the peripheral nervous system is intense. Certain studies suggest that such proliferation is dependent on a mitogenic signal provided by the growing axon.
In this sense, the proliferation of these substances of the peripheral nervous system takes place in three main contexts.
- During the normal development of the peripheral nervous system.
- After nerve injury due to mechanical trauma from neuro-toxins or demyelinating diseases.
- In the cases of Schwann cell tumors such as those seen in the case of neurofibromatosis and acoustic fibromas.
Development
The development of Schwann cells is characterized by presenting an embryonic and a neonatal phase of rapid proliferation and their final differentiation. This developmental process is very common among cells of the peripheral nervous system.
In this sense, the normal development of Schwann cells has two main stages: the migratory stage and the myelinating stage.
During the migratory phase, these cells are characterized by being long, bipolar and with a composition rich in micro-filaments, but with the absence of a basal myelin lamina.
Subsequently, the cells continue to proliferate and the number of axons per cell decreases.
Simultaneously, the larger diameter axons begin to segregate from their peers. At this stage, the connective tissue spaces in the nerve have already developed better and the basal myelin sheets are beginning to be seen.
Features
Schwann cells act in the peripheral nervous system as electrical insulators through myelin. This insulator is responsible for wrapping the axon and causing an electrical signal that runs through it without losing intensity.
In this sense, Schwann cells give rise to the so-called saltatory conduction of myelin-containing neurons.
On the other hand, these types of cells also help guide the growth of axons and are basic elements in the regeneration of certain lesions. Especially, they are vital substances in the regeneration of brain damage caused by neuropraxia and axonotmesis.
Related diseases
The vitality and functionality of Schwann cells can be seen affected through multiple factors of diverse origin. In fact, infectious, immune, traumatic, toxic or tumor problems can affect the activity of this type of cells of the peripheral nervous system.
Among the infectious factors, Mycobacterium leprae and Cornynebacterium diphtheriae stand out, microorganisms that cause alterations in Schwann cells.
Diabetic neuropathy stands out among the metabolic alterations. The tumor pathologies that affect this type of cells are
- During the normal development of the peripheral system.
- After nerve injury due to mechanical trauma from neuro-toxins or demyelinating diseases.
- Plexiform fibromas.
- Malignant fibroids.
Finally, the loss or demyelination of the neuron can generate pathologies that affect the central nervous system, as happens with multiple sclerosis.
References
- Bunge MB, WilliarnsAK, WoodPM.NeuronSchwann cei interaction in basal lamina formation. Dev. Biol.. 1982; 92: 449.
- Gould RM. Metabolic Organizatlon of the rnyeinating schwann Cells. Ann. NY Acad. Sci. 1990; 605: 44.
- Jessen KR, and Mirsky R. Schwann cell precursors and their deveioprnent. Glia. 1991: 4: 185.
- Birdi T Jand Anthia NH. Effectofthe M.ieprae infected Schwann ceils and their supernatant on lymphocyte neuroglia interaction. JNeuroimmunol. 1989,22: 149-155.