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How nerve cells become myelinated.

Glial cells (called oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system) produce an insulating material called myelin which wraps around nerve axons. Because of myelin, electrical signals can move along nerve axons at high speed. Myelination of axons starts late in embryonic development and continues through post-natal life. It requires as yet poorly understood communication between axons and glial cells. The molecules and mechanisms involved are being investigated with the hope of contributing to therapies for neural disorders that are caused by mistakes in myelination.

Insulating nerve axons with myelin is a complicated business. Early in life, Schwann cells meet up with bundles of axons growing out from the spinal cord towards the muscles and organs of the body. The clever Schwann cell is able to pick out an individual axon that requires myelination and pair with it. How does this happen? The Schwann cell is told by the axon to stretch out alongside the axon and to start making myelin. At the same time, the contact with the axon stimulates the Schwann cell to make daughter cells which can interact with new regions of the growing axon. While producing myelin the Schwann cell winds a sheath-like protrusion around the axon yielding multiple layers. Finally these layers are compacted resulting in a tightly packed insulating coat around the axon. (figure 4) Bigger axons have thicker myelin sheaths. The axon regulates the thickness of the sheath but the process is still poorly understood.


Figure 4.
Several myelinated axons.
Note the different sizes of the myelin coats.


Oligodendrocytes in the central nervous system (brain and spinal cord) do a similar job to the Schwann cells. In contrast, however, they are able to myelinate several axons at once. The reason for this difference is not clear.

The peripheral nervous system also differs from the central nervous system in that it is much better at repairing damaged nerves. Repair in the peripheral nervous system is aided by Schwann cells. So, understanding how Schwann cells do this in the periphery, might suggest ways of improving repair in the central nervous system.

All this myelination starts in the embryo, but is not complete until one or two years after birth.


Here is a test used to check for incomplete myelination in babies.. Softly rub the sole of a baby’s foot from the heel towards the large toe. The baby’s toes will fan out, with the large toe pointing up. In contrast, when you do the same to an older child or an adult, his toes will curve inwards. Regeneration of damaged nerves in the periphery of adults is in some ways similar to what happens in the embryo as its nervous system grows. So understanding how nerves grow in the embryo gives us another chance of finding strategies for mending brain, spinal cord and peripheral nerve damage.

Although we can broadly describe how nerves become myelinated, many more molecular details are needed. Scientists are trying to fill in these molecular gaps in our knowledge hoping it will lead to better therapies for neural disorders.