Schwann cell
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Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemnocytes) are a variety of glial cell 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 conduction, 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 system 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, the gaps between myelinated segments. In this way, myelination greatly increases speed of conduction and saves energy [1]
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. [2]
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. Since each Schwann cell can cover about a millimeter (0.04 inches) 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 sclerosis-afflicted patients [3]. Indeed, Schwann cells are known for their roles in supporting nerve regeneration. [4]. 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 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. [5] 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.
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[edit] Schwann cell lineage
Schwann cells are of neural crest 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.[6]
[edit] Histology
Schwann cells appear under a light microscope when immunostained with an anti-S-100 antibody [7].
[edit] References
- ^ Kalat, James W. Biological Psychology, 9th ed. USA: Thompson Learning, 2007.
- ^ http://focus.hms.harvard.edu/2003/Oct24_2003/neurology.html
- ^ http://www.findarticles.com/p/articles/mi_m0850/is_4_19/ai_79957646
- ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=16807057&query_hl=3&itool=pubmed_docsum
- ^ Carlson, Neil R. Physiology of Behavior, 9th ed. USA: Pearson Education, Inc., 2007.
- ^ Jessen, K.R. & Mirsky, R. The origin and development of glial cells in peripheral nerves. Nat. Rev. Neurosci 6, 671- 682(2005).
- ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11436989&dopt=Citation
[edit] External links
- Diagram at clc.uc.edu
- Histology at BU 21301loa - "Ultrastructure of the Cell: myelinated axon and Schwann cell"
- Cell Centered Database - Schwann cell
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