Named after the
GermanGermany , officially the Federal Republic of Germany , is a country in Central Europe. It is bordered to the north by the North Sea, Denmark, and the Baltic Sea; to the east by Poland and the Czech Republic; to the south by Austria and Switzerland; and to the west by France, Luxembourg, Belgium,...
physiologist
Theodor Schwann----Theodor Schwann was a German physiologist. His many contributions to biology include the development of cell theory, the discovery of Schwann cells in the peripheral nervous system, the discovery and study of pepsin, the discovery of the organic nature of yeast, and the invention of the term...
,
Schwann cells (also referred to as
neurolemnocytes) are a variety of
glial cellGlial cells, commonly called neuroglia or simply glia , are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system...
that keep peripheral nerve fibres (both myelinated and unmyelinated) alive. In myelinated axons, Schwann cells form the myelin sheath (see below). The sheath is not continuous. Individual myelinating Schwann cells cover about 100 micrometre of an axon. The end result is a string of Schwann cells along the length of the axon, much like a string of sausages.
Named after the
GermanGermany , officially the Federal Republic of Germany , is a country in Central Europe. It is bordered to the north by the North Sea, Denmark, and the Baltic Sea; to the east by Poland and the Czech Republic; to the south by Austria and Switzerland; and to the west by France, Luxembourg, Belgium,...
physiologist
Theodor Schwann----Theodor Schwann was a German physiologist. His many contributions to biology include the development of cell theory, the discovery of Schwann cells in the peripheral nervous system, the discovery and study of pepsin, the discovery of the organic nature of yeast, and the invention of the term...
,
Schwann cells (also referred to as
neurolemnocytes) are a variety of
glial cellGlial cells, commonly called neuroglia or simply glia , are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system...
that keep peripheral nerve fibres (both myelinated and unmyelinated) alive. In myelinated axons, Schwann cells form the myelin sheath (see below). The sheath is not continuous. Individual myelinating Schwann cells cover about 100 micrometre of an axon. The end result is a string of Schwann cells along the length of the axon, much like a string of sausages. The gaps between adjacent Schwann cells are called the nodes of Ranvier (see below).
The vertebrate nervous system relies on the myelin sheath for insulation and as a method of decreasing membrane capacitance in the axon. The action potential jumps from node to node, in a process called
saltatory conductionSaltatory 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 without needing to increase the diameter of an axon.-Mechanism:Because the cytoplasm of the axon is electrically...
, which can increase conduction velocity up to X10, without an increase in axonal diameter. In this sense, Schwann cells are the peripheral nervous system's analogues of the
central nervous systemThe central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of bilaterian animals—that is, all animals more advanced than sponges or jellyfish. In vertebrates, the central nervous system is enclosed in the meninges. It contains...
oligodendrocyteOligodendrocytes , or oligodendroglia , are a variety of neuroglia. Their main function is the insulation of the axons exclusively in the central nervous system of the higher vertebrates, a function performed by Schwann cells in the peripheral nervous system...
s. However, unlike oligodendrocytes, each myelinating Schwann cell provides insulation to only one axon (see image). This arrangement permits saltatory conduction of
action potentialsAn action potential is a transient alteration of the transmembrane voltage across an excitable membrane generated by the activity of voltage-gated ion channels embedded in the membrane. Action potentials play multiple roles in several types of excitable cells such as neurons, myocytes, and...
with
repropagationPropagation can refer to:*Reproduction, and other forms of multiplication or increase**Plant propagation, the production of more plants by seeds, cuttings, grafting or other methods**Propagation of schema, in artificial reproduction...
at the
Nodes of RanvierNodes of Ranvier are the gaps formed between the myelin sheaths generated by different cells. A myelin sheath is a many-layered coating, largely composed of a fatty substance called myelin, that wraps around the axon of a neuron and very efficiently insulates it...
, the gaps between myelinated segments. In this way, myelination greatly increases speed of conduction and saves energy
Non-myelinating Schwann cells are involved in maintenance of axons and are crucial for neuronal survival. Some group around smaller axons 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
myelinMyelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system...
in between each coil. The inner layers of the wrapping, which are predominantly
membraneThe cell membrane is the biological membrane separating the interior of a cell from the outside environment....
material, form the myelin sheath while the outermost layer of nucleated
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
forms the
neurolemmaNeurolemma is the outermost nucleated cytoplasmic layer of Schwann cells that surrounds the axon of the neuron. It forms the outermost layer of the nerve fiber in the peripheral nervous system....
. Only a small volume of residual cytoplasm communicates the inner from the outer layers. This is seen histologically as the Schmidt-Lantermann Incisure. Since each Schwann cell can cover about a
millimeterConversion of units refers to conversion factors between different units of measurement for the same quantity.- Rounding of results :The process of making a conversion cannot produce a more precise result than the original quoted figure. Appropriate rounding of results is normally performed after...
(0.04
inchAn inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...
es) along the axon, hundreds and often thousands are needed to completely cover an axon, which can sometimes span the length of the body.
A number of experimental studies since 2001 have implanted Schwann cells in an attempt to induce remyelination in
multiple sclerosisMultiple sclerosis is an idiopathic disease of suspected autoimmune cause, in which the body's immune response attacks a person's central nervous system , leading to demyelination. Disease onset usually occurs in young adults, and it is more common in females...
-afflicted patients . Indeed, Schwann cells are known for their roles in supporting
nerve regenerationNeuroregeneration refers to the regrowth or repair of nervous tissues, cells or cell products. Such mechanisms may include remyelination, generation of new neurons, glia, axons, myelin or synapses...
. . Nerves in the PNS consist of many axons myelinated by Schwann cells. If damage occurs to a nerve, the Schwann cells will aid in digestion of its axons
phagocytosisPhagocytosis is the cellular process of phagocytes and protists of engulfing solid particles by the cell membrane to form an internal phagosome...
. Following this process, the Schwann cells can guide regeneration by forming a type of tunnel that leads toward the target neurons. 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 of approximately 1mm/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, however, specificity is not maintained and errors are frequent, especially when long distances are involved.
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 them. They have been shown to be in advance of the growth cones.
Schwann cells are absolutely essential for the maintenance of healthy axons. They produce a variety of factors, including neurotrophins, and also transfer essential molecules across to axons.
Schwann cell lineage
Schwann cells are of
neural crestThe neural crest, a transient component of the ectoderm, is located in between the neural tube and the epidermis of an embryo during neural tube formation...
origin. During mouse embryonic development, neural crest cells first differentiate into Schwann Cell Precursors (SCPs) at around embryonic day (E) 12-13. These precursor cells subsequently differentiate into immature Schwann cells at approximately E15-16, persisting until birth. The postnatal fate of the immature Schwann cell depends on its random association with axons. In a process called radial sorting, whereby Schwann cells segregate axons by extending processes into axon bundles, the Schwann cells that happen to associate with a large diameter axon (>1μm) will develop into myelinating Schwann cells. Small diameter axons become entrenched in the invaginations of non-meylinating Schwann cells, also called Remak bundles. A further class of non-myelinating Schwann cell, the terminal (or perisynaptic) Schwann cell exists at the neuromuscular junction, in close proximity to the neuron-muscle synapse. The transition from immature Schwann cell to myelinating/non-myelinating Schwann cell is reversible. When the nerve is injured, Schwann cells can dedifferentiate and re-enter the cell cycle in order to proliferate and aid nerve regeneration.
Histology
Schwann cells appear under a light microscope when immunostained with an anti-S-100 antibody .
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