Schwann cells or neurolemmocytes are the principal glia of the peripheral nervous system (PNS). They were named after physiologist Theodor Schwann.
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. 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 peripheral nervous system, the regulatory mechanisms of myelination are controlled via 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 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, acute inflammatory demyelinating polyradiculopathy type, schwannomatosis, and chronic inflammatory demyelinating polyneuropathy (CIDP), and leprosy are all neuropathies involving Schwann cells.
Function Of Schwann Cells
Portrait of Theodor Schwann. Wellcome Images
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, neurolemmocytes 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.
Schwann cell myelinating axons. Credit: Dr David Furness, Wellcome Images
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 neurilemma. Only a small volume of residual cytoplasm allows communication between the inner and outer layers. This is seen histologically as the Schmidt-Lantermann incisure.
If damage occurs to a nerve, theneurolemmocytes will 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.
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.