Schwann cell
Named after the
German physiologist
Theodor Schwann,
Schwann cells are a variety of
neuroglia that mainly provide
myelin insulation to
axons in the
peripheral nervous system of jawed vertebrates. The vertebrate nervous system relies on this myelin sheath for insulation and as a method of decreasing membrane capacitance in the axon, thus allowing for saltatory conduction to occur and for an increase in impulse speed, without an increase in axonal diameter. 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 cells are the peripheral nervous system's analogues of the
central nervous system oligodendrocytes.
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 a body. The gaps between the Schwann cell covered segments are the
Nodes of Ranvier, important sites of ionic and other exchanges of the axon with the extracellular liquid. Unlike
oligodendrocytes, myelinating Schwann cells provide insulation to only one axon (see image).This arrangement permits
saltatory conduction of
action potentials which greatly speeds it and saves energy.
A number of experimental studies since 2001 have implanted Schwann cells in an attempt to induce remyelination in
multiple sclerosis-afflicted patients.[
1]
Schwann cells appear under a light microscope when immunostained with an anti-S-100 antibody ([
2]
*
Diagram at clc.uc.edu
