Nerve capillary. Coloured transmission electron micrograph (TEM) of a section through a capillary in peripheral nerve tissue, showing red blood cells within the lumen (inner space) of the capillary. Capillaries are the smallest blood vessels in the body, only a few micrometres across, just large enough for the passage of red blood cells. The capillary lining consists of endothelial cells. Here, a pericyte can be seen surrounding the capillary. Pericytes are contractile cells that wrap around the endothelial cells of capillaries and venules. Myelin (dark rings) is an insulating fatty layer that surrounds the myelinated nerve fibres, increasing the speed at which nerve impulses travel. It is formed when Schwann cells wrap around the fibre, depositing layers of myelin between each coil. Magnification: x2000 when printed 10 centimetres wide.