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Schwann cells or neurolemmocytes (named after German physiologist Theodor Schwann) are the principal
glia Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They maintain homeostasis, form mye ...
of the peripheral nervous system (PNS). Glial cells function to support
neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
and in the PNS, also include
satellite cells Myosatellite cells, also known as satellite cells, muscle stem cells or MuSCs, are small multipotent cells with very little cytoplasm found in mature muscle. Satellite cells are precursors to skeletal muscle cells, able to give rise to satellite ...
,
olfactory ensheathing cells Olfactory ensheathing cells (OECs), also known as olfactory ensheathing glia or olfactory ensheathing glial cells, are a type of macroglia ( radial glia) found in the nervous system. They are also known as olfactory Schwann cells, because they en ...
, enteric glia and glia that reside at sensory nerve endings, such as the
Pacinian corpuscle Pacinian corpuscle or lamellar corpuscle or Vater-Pacini corpuscle; is one of the four major types of mechanoreceptors (specialized nerve ending with adventitious tissue for mechanical sensation) found in mammalian skin. This type of mechanorecept ...
. The two types of Schwann cells are myelinating and nonmyelinating. Myelinating Schwann cells wrap around axons of motor and sensory neurons to form the myelin sheath. The Schwann cell promoter is present in the downstream region of the human dystrophin gene that gives shortened transcript that are again synthesized in a tissue-specific manner. During the development of the PNS, the regulatory mechanisms of myelination are controlled by feedforward interaction of specific genes, influencing transcriptional cascades and shaping the morphology of the myelinated nerve fibers. Schwann cells are involved in many important aspects of peripheral nerve biology—the conduction of nervous impulses along
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...
s, nerve development and regeneration, trophic support for neurons, production of the nerve extracellular matrix, modulation of neuromuscular synaptic activity, and presentation of
antigen In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...
s to
T-lymphocytes A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell r ...
.
Charcot–Marie–Tooth disease Charcot–Marie–Tooth disease (CMT) is a hereditary motor and sensory neuropathy of the peripheral nervous system characterized by progressive loss of muscle tissue and touch sensation across various parts of the body. This disease is the mo ...
, Guillain–Barré syndrome (acute inflammatory demyelinating polyradiculopathy type), schwannomatosis,
chronic inflammatory demyelinating polyneuropathy Chronic inflammatory demyelinating polyneuropathy (CIDP) is an acquired autoimmune disease of the peripheral nervous system characterized by progressive weakness and impaired sensory function in the legs and arms. The disorder is sometimes called ...
, and
leprosy Leprosy, also known as Hansen's disease (HD), is a long-term infection by the bacteria '' Mycobacterium leprae'' or '' Mycobacterium lepromatosis''. Infection can lead to damage of the nerves, respiratory tract, skin, and eyes. This nerve dam ...
are all neuropathies involving Schwann cells.


Structure

Schwann cells are a variety of
glial cell Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They maintain homeostasis, form mye ...
s that keep peripheral nerve fibres (both myelinated and unmyelinated) alive. In myelinated axons, Schwann cells form the myelin sheath. The sheath is not continuous. Individual myelinating Schwann cells cover about 1 mm of an axon—equating to about 1000 Schwann cells along a 1-m length of the axon. The gaps between adjacent Schwann cells are called
nodes of Ranvier In neuroscience and anatomy, nodes of Ranvier ( ), also known as myelin-sheath gaps, occur along a myelinated axon where the axolemma is exposed to the extracellular space. Nodes of Ranvier are uninsulated and highly enriched in ion channels, al ...
. 9-O-Acetyl GD3 ganglioside is an acetylated glycolipid which is found in the cell membranes of many types of vertebrate cells. During peripheral
nerve regeneration Neuroregeneration refers to the regrowth or repair of nervous tissues, cells or cell products. Such mechanisms may include generation of new neurons, glia, axons, myelin, or synapses. Neuroregeneration differs between the peripheral nervous syst ...
, 9-O-acetyl GD3 is expressed by Schwann cells.


Function

The vertebrate nervous system relies on the myelin sheath for insulation and as a method of decreasing membrane
capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...
in the axon. The
action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
jumps from node to node, in a process called
saltatory conduction In neuroscience, saltatory conduction () is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials. The uninsulated nodes of Ranvier are th ...
, which can increase
conduction Conductor or conduction may refer to: Music * Conductor (music), a person who leads a musical ensemble, such as an orchestra. * ''Conductor'' (album), an album by indie rock band The Comas * Conduction, a type of structured free improvisation ...
velocity up to 10 times, without an increase in axonal diameter. In this sense, Schwann cells are the PNS's analogues of the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
's oligodendrocytes. However, unlike oligodendrocytes, each myelinating Schwann cell provides insulation to only one axon (see image). This arrangement permits saltatory conduction of action potentials with repropagation at the nodes of Ranvier. In this way, myelination greatly increases speed of conduction and saves energy. Nonmyelinating Schwann cells are involved in maintenance of axons and are crucial for neuronal survival. Some group around smaller axons
External image here
and form Remak bundles. Myelinating Schwann cells begin to form the myelin sheath in mammals during fetal development and work by spiraling around the axon, sometimes with as many as 100 revolutions. A well-developed Schwann cell is shaped like a rolled-up sheet of paper, with layers of myelin between each coil. The inner layers of the wrapping, which are predominantly membrane material, form the myelin sheath, while the outermost layer of nucleated
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. Th ...
forms the
neurilemma Neurilemma (also known as neurolemma, sheath of Schwann, or Schwann's sheath) is the outermost nucleated cytoplasmic layer of Schwann cells (also called neurilemmocytes) that surrounds the axon of the neuron. It forms the outermost layer of the ner ...
. Only a small volume of residual cytoplasm allows communication between the inner and outer layers. This is seen
histologically Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...
as the Schmidt-Lantermann incisure.


Regeneration

Schwann cells are known for their roles in supporting
nerve regeneration Neuroregeneration refers to the regrowth or repair of nervous tissues, cells or cell products. Such mechanisms may include generation of new neurons, glia, axons, myelin, or synapses. Neuroregeneration differs between the peripheral nervous syst ...
. Nerves in the PNS consist of many axons myelinated by Schwann cells. If damage occurs to a nerve, the Schwann cells aid in digestion of its axons ( phagocytosis). Following this process, the Schwann cells can guide regeneration by forming a type of tunnel that leads toward the target neurons. This tunnel is known as
band of Büngner Band or BAND may refer to: Places *Bánd, a village in Hungary * Band, Iran, a village in Urmia County, West Azerbaijan Province, Iran * Band, Mureș, a commune in Romania * Band-e Majid Khan, a village in Bukan County, West Azerbaijan Province, ...
, a guidance track for the regenerating axons, which behaves like an endoneural tube. The stump of the damaged axon is able to sprout, and those sprouts that grow through the Schwann-cell "tunnel" do so at the rate around 1 mm/day in good conditions. The rate of regeneration decreases with time. Successful axons can, therefore, reconnect with the muscles or organs they previously controlled with the help of Schwann cells, but specificity is not maintained and errors are frequent, especially when long distances are involved. Because of their ability to impact regeneration of axons, Schwann cells have been connected to
preferential motor reinnervation Preferential motor reinnervation (PMR) refers to the tendency of a regenerating axon in the peripheral nervous system (PNS) to reinnervate a motor pathway as opposed to a somatosensory pathway.Brushart, M. E. (1993). Motor Axons Preferentially Re ...
, as well. If Schwann cells are prevented from associating with axons, the axons die. Regenerating axons will not reach any target unless Schwann cells are there to support them and
guide A guide is a person who leads travelers, sportspeople, or tourists through unknown or unfamiliar locations. The term can also be applied to a person who leads others to more abstract goals such as knowledge or wisdom. Travel and recreation Exp ...
them. They have been shown to be in advance of the
growth cone A growth cone is a large actin-supported extension of a developing or regenerating neurite seeking its synaptic target. It is the growth cone that drives axon growth. Their existence was originally proposed by Spanish histologist Santiago Ram� ...
s. Schwann cells are essential for the maintenance of healthy axons. They produce a variety of factors, including
neurotrophin Neurotrophins are a family of proteins that induce the survival, development, and function of neurons. They belong to a class of growth factors, secreted proteins that can signal particular cells to survive, differentiate, or grow. Growth facto ...
s, and also transfer essential molecules across to axons.


Genetics


Schwann cell formation


Sox10

SOX10 is a transcription factor active during embryonic development and abundant evidence indicates that it is essential for the generation of glial lineages from trunk crest cells. When SOX10 is inactivated in mice, satellite glia and Schwann cell precursors fail to develop, though neurons are generated normally without issue. In the absence of SOX10, neural crest cells survive and are free to generate neurons, but glial specification is blocked. SOX10 might influence early glial precursors to respond to neuregulin 1 (see below).


Neuregulin 1

Neuregulin 1 (NRG1) acts in a number of ways to both promote the formation and ensure the survival of immature Schwann cells. During embryonic development, NRG1 inhibits the formation of neurons from neural crest cells, instead contributing to neural crest cells being led down a path to gliogenesis. NRG1 signaling is not, however, required for glial differentiation from the neural crest. NRG1 plays important roles in the development of neural crest derivatives. It is required for neural crest cells to migrate past the site of dorsal root ganglia to find the ventral regions of sympathetic gangliogenesis. It is also an essential axon-derived survival factor and a mitogen for Schwann cell precursors. It is found in the dorsal root ganglion and motor neurons at the point in time that Schwann cell precursors begin to populate spinal nerves and therefore influences Schwann cell survival. In embryonic nerves, the transmembrane III isoform likely is the primary variant of NRG1 responsible for survival signals. In mice that lack the transmembrane III isoform, Schwann cell precursors are eventually eliminated from spinal nerves.


Formation of myelin sheath


P0

Myelin protein zero Myelin protein zero (P0, MPZ) is a single membrane glycoprotein which in humans is encoded by the ''MPZ'' gene. P0 is a major structural component of the myelin sheath in the peripheral nervous system (PNS). Myelin protein zero is expressed by ...
(P0) is a cell-adhesion molecule belonging to the immunoglobulin superfamily and is the major component of peripheral myelin, constituting over 50% of the total protein in the sheath. P0 has been shown to be essential for the formation of compact myelin, as P0 null mutant (P0-) mice showed severely aberrant peripheral myelination. Although myelination of large caliber axons was initiated in P0- mice, the resulting myelin layers were very thin and poorly compacted. Unexpectedly, P0- mice also showed degeneration of both axons and their surround myelin sheaths, suggesting that P0 plays a role in maintaining the structural integrity of both myelin formation and the axon with which it’s associated. P0- mice developed behavioral deficits around 2 weeks of age when mice began to show signs of slight trembling. Gross incoordination also arose as the animals developed, while trembling became more severe and some older mice developed convulsing behaviors. Despite the array of impaired motor behavior, no paralysis was observed in these animals. P0 is also an important gene expressed early within the Schwann cell lineage, expressed in Schwann cell precursors after differentiating from migrating neural crest cells within the developing embryo.


Krox-20

Several important transcription factors are also expressed and involved at various stages in development changing the features on the Schwann cells from an immature to mature state. One indispensable transcription factor expressed during the myelination process is Krox-20. It is a general zinc-finger transcription factor and is expressed in the rhombomeres 3 and 5. Krox-20 is considered one of the master regulators of PNS myelination and is important in driving transcription of specific structural proteins in the myelin. It has been shown to control a set of genes responsible for interfering with this feature in the axon changing it from a pro-myelinating to myelinating state. In this way, in Krox-20 double knock out mice, it has been recorded that hindbrain segmentation is affected as well as myelination of Schwann cell associated axons. Indeed, in these mice, the Schwann cells are not able to perform their myelination properly as they only wrap their cytoplasmic processes one and half turn around the axon and despite the fact that they still express the early myelin marker, late myelin gene products are absent. In addition, recent studies have also proven the importance of this transcription factor in maintaining the myelination phenotype (and requires the co-expression of Sox 10) as its inactivation leads to dedifferentiation of the Schwann cells.


Clinical significance

Charcot–Marie–Tooth disease Charcot–Marie–Tooth disease (CMT) is a hereditary motor and sensory neuropathy of the peripheral nervous system characterized by progressive loss of muscle tissue and touch sensation across various parts of the body. This disease is the mo ...
(CMT), Guillain–Barré syndrome (GBS, acute inflammatory demyelinating polyradiculopathy type), schwannomatosis, and
chronic inflammatory demyelinating polyneuropathy Chronic inflammatory demyelinating polyneuropathy (CIDP) is an acquired autoimmune disease of the peripheral nervous system characterized by progressive weakness and impaired sensory function in the legs and arms. The disorder is sometimes called ...
(CIDP),
leprosy Leprosy, also known as Hansen's disease (HD), is a long-term infection by the bacteria '' Mycobacterium leprae'' or '' Mycobacterium lepromatosis''. Infection can lead to damage of the nerves, respiratory tract, skin, and eyes. This nerve dam ...
, and
Zika Virus ''Zika virus'' (ZIKV; pronounced or ) is a member of the virus family ''Flaviviridae''. It is spread by daytime-active ''Aedes'' mosquitoes, such as '' A. aegypti'' and '' A. albopictus''. Its name comes from the Ziika Forest of Uganda, wh ...
are all neuropathies involving Schwann cells.


Transplantation

A number of experimental studies since 2001 have implanted Schwann cells in an attempt to induce
remyelination Remyelination is the process of propagating oligodendrocyte precursor cells to form oligodendrocytes to create new myelin sheaths on demyelinated axons in the CNS. This is a process naturally regulated in the body and tends to be very efficient i ...
in multiple sclerosis-afflicted patients. In the past two decades, many studies have demonstrated positive results and potential for Schwann cell transplantation as a therapy for spinal cord injury, both in aiding regrowth and myelination of damaged CNS axons. Schwann cell transplants in combination with other therapies such as Chondroitinase ABC have also been shown to be effective in functional recovery from spinal cord injury.


See also

*
Electrophysiology Electrophysiology (from Greek , ''ēlektron'', "amber" etymology of "electron"">Electron#Etymology">etymology of "electron" , ''physis'', "nature, origin"; and , ''-logia'') is the branch of physiology that studies the electrical properties of bi ...
* Hodgkin–Huxley model *
Mesaxon In neurobiology, a mesaxon is a pair of parallel plasma membranes of a Schwann cell. It marks the point of edge-to-edge contact by the Schwann cell encircling the axon. A single Schwann cell of the peripheral nervous system will wrap around and sup ...
*
Neurotransmission Neurotransmission (Latin: ''transmissio'' "passage, crossing" from ''transmittere'' "send, let through") is the process by which signaling molecules called neurotransmitters are released by the axon terminal of a neuron (the presynaptic neuron), ...
*
Olfactory ensheathing cell Olfactory ensheathing cells (OECs), also known as olfactory ensheathing glia or olfactory ensheathing glial cells, are a type of macroglia ( radial glia) found in the nervous system. They are also known as olfactory Schwann cells, because they en ...
*
Schwannoma A schwannoma (or neurilemmoma) is a usually benign nerve sheath tumor composed of Schwann cells, which normally produce the insulating myelin sheath covering peripheral nerves. Schwannomas are homogeneous tumors, consisting only of Schwann cells. ...
* List of human cell types derived from the germ layers


References


External links


Diagram at clc.uc.edu
* —"Ultrastructure of the Cell: myelinated axon and Schwann cell"
Cell Centered Database—Schwann cell
{{DEFAULTSORT:Schwann Cell Glial cells Neurophysiology