Schwann cells: what they are, characteristics and functions.

A type of glial cell that envelops the axons of neurons in nerves.

The Schwann cells of the peripheral nervous system collaborate with neurons and play a fundamental role in nerve impulse conduction, as they are the cells responsible for coating the axons with an insulating substance that enhances the speed at which information is transmitted.

In this article we are going to see what Schwann cells are, what their functions are, how they grow and develop and what type of pathologies are related to them.

What are Schwann cells?

Schwann cells, also called neurolemocytes, are a specific type of glial cell. a specific type of glial cell located in the peripheral located in the peripheral nervous system. Glial cells are cells of the nervous tissue responsible for performing auxiliary and support functions of neurons (support, nutrition or guidance and control of neuronal migrations in the early stages of development, among others).

These cells are named after the physician and anatomist Theodor Schwann (1810-1882), father of the cellular theory that postulated that all living beings are composed of cells and products made by them, a theory that marked a paradigm shift in the way life was conceived in the 19th century.

Schwann cells maintain a close relationship with neurons since their origin in the embryonic tissue, playing a fundamental role in the guidance and a fundamental role in the guidance and proper control of axon growth.. Let's see, next, what functions these cells fulfill.

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 cell type is to act as a support and guide in the regeneration processes of the peripheral nervous system after axonal injury or damage.

These cells appear to be unique in their ability to stimulate peripheral nerve growth and regeneration..

Schwann cells, which are located in axon terminals and synaptic boutons at neuromuscular junctions, physiological support to maintain the ionic homeostasis of synapses (autoregulation and maintenance of the (self-regulation and maintenance of constancy in the composition and properties of the synapses).

Another of the fundamental tasks performed by these cells is to form a myelin sheath around the axons of the PNS, a function performed in the CNS by their counterpart cells, the oligodendrocytes.

Unlike the latter, which can form myelin sheaths on several different axons (extensions of the neuron responsible for conducting the nerve impulse), Schwann cells can only form a myelin segment on a single axon, a mechanism that facilitates the nerve impulse to propagate faster. faster.

The myelin sheath

The neurons of the peripheral nervous system transmit nerve impulses more or less rapidly depending on whether or not their axon is covered by the myelin sheath, an insulating layer composed of proteins and fats. This sheath is not continuous, because Schwann cells cover only 100 micrometers in length at a time, leaving tiny indentations between sheath and sheath, known as nodes of Ranvier..

These nodes facilitate the transmission of the nerve impulse or action potential, allowing the electrical activity passing through the axons to be maintained at an appropriate speed until it reaches the cell body or soma of the neuron. This activity occurs "in jumps", hence it is known as neuronal jumping conduction.

Proliferation

Although the nature and origin of the factors involved in the processes of proliferation (the increase in the number of cells as a result of their growth and multiplication) are still unknown, it is known that Schwann cells proliferate during the development of peripheral nerves in basically three contexts:

1. During normal peripheral nerve development 2.

Together with the rest of the cells.

2. After a nerve injury

I.e, mechanical trauma, neurotoxins or diseases that cause damage to the myelin.

3. In Schwann cell tumors.

In this way, can proliferate anywhere in the peripheral nervous system, such as in neurofibrotic fibrosis.as occurs with neurofibromatosis or acoustic fibromas.

Development

The development of Schwann cells begins in a first embryonic and neonatal phase of rapid proliferation, followed by the interruption of proliferation and their final differentiation. In their normal development, this cell type undergoes two fundamental stages: migration and myelination..

In their migration phase, Schwann cells are long, bipolar, with a microfilament-rich composition and no basal lamina or myelin to coat them. They are placed in the nerve, on the axons in their final position.The cells then divide into small groups of several axons surrounded by one or two Schwann cells.

Subsequently, the cells continue to proliferate and the number of axons contained in each cell decreases. Simultaneously, the axons of larger diameter begin to segregate from their fellow axons and to isolate into a single Schwann cell.

At this stage, the connective tissue spaces of the nerve are already better developed and the cell is able to assemble a single Schwann cell. is already able to assemble basal lamina.. Future maturation and myelination of the cells will depend on the correct assembly of this basal lamina.

Pathologies involving this type of cells

The functionality and survival of Schwann cells as part of the peripheral nervous system can be compromised by multiple factors of diverse origin: infectious, immune, toxic, traumatic and tumor.

The most common infectious factors include Hansen's bacillus and Klebs-Löffler bacillus.. Although the alterations that these microorganisms cause in Schwann cells are not yet fully understood and are still the subject of study and research, there are indications that a Hansen's bacillus infection could stop the proliferation of these cells and the myelination of axons.

Among the most common metabolic disturbances is diabetic neuropathy, in which Schlumberger's Schwanger cells are the most common.in which Schwann cells show an excessive accumulation of lipid bodies in their cytoplasm. This accumulation seems to reflect an alteration of lipid metabolism, resulting in demyelination, without knowing whether it is primary or secondary to an axonal alteration.

Tumorigenic factors affecting Schwann cells are usually benign in nature and are classified into four groups: Schwannomas, neurofibromas, plexiform fibromas and malignant fibromas. In addition, there are a large number of metabolic immune alterations that modify Schwann cells, causing demyelinating processes that are usually secondary to axonal lesions.

Bibliographic references:

• Bhatheja K, Field J. Schwann cells: origins and role in axonal maintenance and regeneration. The International Journal of Biochemistry and Cell Biology 2006, 38: 1995-1999.
• Kessen KR, and Mirsky R. Schwann cell precursors and their development. Glia. 1991:4:185.
• Perdomo S, Spinel C. The Schwann cell. Acta Biológica Colombiana 2004; 9: 25-34.