Explain how myelination of a nerve fiber occurs? What cells do it?

Neurons are supported both structurally and functionally by supporting cells, which are called neuroglia. These cells are ten times more numerous than neurons and serve a variety of functions, including supplying the neurons with nutrients, removing wastes from neurons, guiding axon migration, and providing immune functions. Two of the most important kinds of neuroglia in vertebrates are Schwann cells and oligodendrocytes, which produce myelin sheaths that surround the axons of many neurons.

Myelination, which increases the rate of impulse conduction up to a hundredfold, permits the close packing of neurons characteristic of vertebrate brains. During development, these cells wrap themselves around each axon several times to form the myelin sheath, an insulating covering consisting of multiple layers of membrane. Axons that have myelin sheaths are said to be myelinated, and those that don't are unmyelinated. The myelin sheath is interrupted at intervals of 1 to 2 mm by small gaps known as nodes of Ranvier.