DEVELOPMENT OF MYELINATION AND AXON ENSHEATHMENT
Myelination requires a cooperative interaction between the neuron and its myelinating support cell. Unmyelinated peripheral axons are invested with a single layer of Schwann cell cytoplasm.
When a peripheral axon at least 1 to 2 µm in diameter triggers myelination, a Schwann cell wraps many layers of tightly packed cell membrane around a single segment of that axon. In the CNS, an oligodendroglia cell extends several arms of cytoplasm, which then wrap multiple layers of tightly packed membrane around a single segment of each of several axons (or occasionally two autonomic preganglionic axons). Although myelination is a process that occurs most intensely during development, Schwann cells may remyelinate peripheral axons following injury, and oligodendroglial cells may proliferate and remyelinate injured or demyelinated central axons in diseases such as multiple sclerosis.