Schwann cells or neurolemmocytes are the principle glia of the peripheral nervous system.
Glial cells function to support neurons and, in the PNS, also include satellite cells,
olfactory ensheathing cells, enteric glia and glia that reside at sensory nerve endings,
such as the Pacinian corpuscle.
There are two types of Schwann cell, myelinating and nonmyelinating.
Myelinating Schwann cells wrap around axons of motor and sensory neurons to form the myelin
sheath.
Schwann cells are involved in many important aspects of peripheral nerve biology—the
conduction of nervous impulses along axons, nerve development and regeneration, trophic
support for neurons, production of the nerve extracellular matrix, modulation of neuromuscular
synaptic activity, and presentation of antigens to T-lymphocytes.
Charcot-Marie-Tooth disease, Guillain-Barré syndrome, schwannomatosis, acute inflammatory
demyelinating polyradiculopathy, and chronic inflammatory demyelinating polyneuropathy,
and leprosy are all neuropathies involving Schwann cells.
Description
Named after the German physiologist Theodor Schwann, Schwann cells are a variety of glial
cell that keep peripheral nerve fibres alive.
In myelinated axons, Schwann cells form the myelin sheath.
The sheath is not continuous.
Individual myelinating Schwann cells cover about 100 micrometres of an axon—equating
to approximately 10,000 Schwann cells along a 1 metre length of the axon—which can be
up to a metre or more in length.
The gaps between adjacent Schwann cells are called nodes of Ranvier.
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 conduction,
which can increase conduction velocity up to ten times, without an increase in axonal
diameter.
In this sense, Schwann cells are the peripheral nervous system's analogues of the central
nervous system's oligodendrocytes.
However, unlike oligodendrocytes, each myelinating Schwann cell provides insulation to only one
axon.
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.
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
myelin in 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 forms the neurolemma.
Only a small volume of residual cytoplasm communicates the inner from the outer layers.
This is seen histologically as the Schmidt-Lantermann incisure.
Schwann cell transplantation and regeneration A number of experimental studies since 2001
have implanted Schwann cells in an attempt to induce remyelination 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.
Indeed, Schwann cells are known for their roles in supporting nerve regeneration.
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.
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.
Because of their ability to impact regeneration of axons, Schwann cells have been connected
to preferential motor reinnervation 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 them.
They have been shown to be in advance of the growth cones.
Schwann cells are essential for the maintenance of healthy axons.
They produce a variety of factors, including neurotrophins, and also transfer essential
molecules across to axons.
Ganglioside 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, 9-O-acetyl GD3 is expressed by schwann cells.
See also Electrophysiology
Hodgkin-Huxley Model Neurotransmission
Olfactory ensheathing cells Schwannoma
References
External links Diagram at clc.uc.edu
Histology image: 21301loa - Histology Learning System at Boston University—"Ultrastructure
of the Cell: myelinated axon and Schwann cell" Cell Centered Database - Schwann cell