Peripheral nerve. Scanning electron micrograph (SEM) of a freeze fracture through part of a peripheral nerve, showing Schwann cells and myelinated and unmyelinated nerve fibres (axons). Myelin (red) is a fatty insulating sheath composed of Schwann cells that surrounds the axons (brown) and increases the speed of the nerve signal. Schwann cell cytoplasm (green) surrounds several unmyelinated axons. Nuclei (pale brown) of the Schwann cells are also visible. Connective tissue (blue) forms part of the endoneurium. Magnification: x8, 000 when printed 10 centimetres wide.