Schwann cell

Structure of a typical neuron

Schwann cell
Dendrite Soma Axon Nucleus Node of Ranvier Axon Terminal Schwann cell Myelin sheath

Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemmocytes) are a variety of glial cell that mainly provide myelin insulation to axons in the peripheral nervous system (PNS) 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. Myelination allows for saltatory conduction to occur and for an increase in impulse speed, 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.

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 (due to asexual reproduction) 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.

A number of experimental studies since 2001 have implanted Schwann cells in an attempt to induce remyelination in multiple sclerosis-afflicted patients. 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 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 an increased rate of about 3-4 mm/day. Successful axons can therefore reconnect with the muscles or organs they previously controlled with the help of Schwann cells.

Histology

Schwann cells appear under a light microscope when immunostained with an anti-S-100 antibody. They are of neural crest origin.