Regenerating nerve cell. Coloured transmission electron micrograph (TEM) of a section through a nerve axon (pink) regenerating within a Schwann cell (green). The Schwann cell's nucleus is blue. An axon is a nerve fibre that passes information between nerve cells. It is usually surrounded by a myelin sheath, a fatty insulating layer formed by Schwann cells, but this cell is yet to regenerate its sheath. Schwann cells are only found around axons in the peripheral nervous system, and not the central nervous system (CNS). It is only cells of the peripheral nervous system that are able to regenerate after injury.