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.
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. 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 cells are named after the German physiologist Theodor Schwann, who discovered them.