Myelinated nerve tissue. Transmission electron micrograph (TEM) of a section through myelinated nerve fibres (axons). Each axon is coated with many layers of myelin, a lipid (fat)-rich layer of concentric plasma membranes that insulates the nerve and enhances the speed of nerve impulses. The cells that synthesise the myelin sheaths are called Schwann cells. Each group of axons is supported externally by connective tissue cells. Magnification: x11,000, when printed 10 centimetres wide.