Schwann cell
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Schwann cells are a variety of neuroglia that wrap around axons in the peripheral nervous system, forming the myelin sheath. The nervous system depends crucially on this sheath for insulation and an increase in impulse speed. They are the periferal nervous system 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. This arrangement permits saltatory conduction of action potentials which greatly speeds it and saves energy.
The cells are named after the German physiologist Theodor Schwann, who discovered them.pt:Célula de Schwann de:Schwannsche Zelle