Introduction[edit | edit source]
Schwann cells (SCs) are a type of glial cell that surrounds neurons, keeping them alive and sometimes covering them with a myelin sheath, and are the major glial cell type in the peripheral nervous system. They play essential roles in the development, maintenance, function, and regeneration of peripheral nerves. 
Myelin is a fatty substance produced by glial cells that is non-conductive. Myelinating a nerve cell results in better conduction of nerve impulses. It has been shown in studies that myelinated nerve cells can transfer signals up to 10X faster than unsheathed nerves. 
Classes[edit | edit source]
Two major classes exist of SCs, based on myelin structure:
- Myelinating SCs provide the myelin ensheathment of all large-diameter peripheral axons. Each myelinating SC associates with a single axon. Transmit electrical signals at the highest speed.
- Satellite glial cells (nonmyelin-forming SCs): Ensheath the somata of neuron bodies in sensory, sympathetic and parasympathetic ganglia. They are thought to have a similar role to astrocytes in the central nervous system (CNS). They supply nutrients to the surrounding neurons and also have some structural function. Satellite glial cells bundle the axons close together by surrounding them, keeping them from touching each other by squeezing its cytoplasm between the axons
Both types of Schwann cells are vital in the maintenance and regeneration of axons of the neurons within the PNS. Schwann cells originate from the neural crest, which is a group of embryonic cells.
Structure and Formation[edit | edit source]
Each Schwann cell makes up a single myelin sheath on a peripheral axon, with each ensuing myelin sheath made by a different Schwann cell, such that numerous Schwann cells are needed to myelinate the length of an axon.
Myelin sheath formation: Schwann cell wraps its plasma membrane concentrically around the inner axon, the nucleus remaining fixed whilst the inner turn of the glial cell membrane spirals around the axon to add membrane layers the myelin sheath. The plasma membrane of Schwann cells has an extremely high lipid content, and cholesterol is particularly important for assembling the myelin sheath. The compact myelin sheath insulates the axon segment, significantly reducing membrane capacitance and increasing conduction velocity.
Both types of Schwann cells are covered by a basal lamina. The outside of the lamina is surrounded by a layer of connective tissue namely the endoneurium, which contains blood vessels, fibroblasts, and macrophages. Whereas, the inner surface of the lamina faces the Schwann cell plasma membrane.
Axon Regeneration[edit | edit source]
Schwann cells play a vital role in axon regeneration. An injury to the axon may lead to cell death and axonal degeneration. When injured, Schwann cells and macrophages are enlisted to the injury site to remove dead cells and promote axonal regeneration. In addition, Schwann cells induce axon regeneration and nerve cell survival by increasing the expression of a wide variety of growth factors. They also secrete extracellular matrix molecules, such as laminin and collagen, and cell adhesion molecules to support the regeneration process.
The Schwann cells arrange a regeneration pathway along a tube of the basal lamina for the stump of the axon. The sprout of the damaged axon can then grow through this tube which helps to stimulate and guide its regeneration.
Disorders[edit | edit source]
Impaired Schwann cell functioning is mainly associated with demyelinating diseases examples include:
- Multiple sclerosis
- Charcot-Marie-Tooth disease
- Diabetic neuropathy
- Guillain-Barre Syndrome
- Neoplastic diseases eg schwannomas, neurofibromas, and malignant peripheral nerve sheath tumors.
The Future[edit | edit source]
Demyelination occurs in several central nervous system (CNS) disorders, including multiple sclerosis, viral infection and spinal cord injury and can result in severe functional impairment. There is great interest in developing therapies promoting repair in CNS demyelinating diseases and trauma. Cell replacement therapy is one approach for myelin repair, and experimental transplantation has provided convincing evidence of the repair potential of grafted myelin-forming cells. .
SCs have potential for SCI treatment because of their capacity to promote axon regrowth and myelination within the injured spinal cord. Numerous studies on SC therapy for SCI have reported encouraging results in animal models, and some SC therapies have been the subjects of phase I clinical trials.
References[edit | edit source]
- Lavdas AA, Matsas R. Schwann cell morphology. Available: https://www.sciencedirect.com/science/article/pii/B9780128012383047589 (accessed 10.5.2022)
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- Fu H, Hu D, Chen J, Wang Q, Zhang Y, Qi C, Yu T. Repair of the Injured Spinal Cord by Schwann Cell Transplantation. Frontiers in Neuroscience. 2022;16.Available:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8891437/ (accessed 10.5.2022)