Sperm tails. Coloured transmission electron micrograph (TEM) of cross sections through sperm tails. A sperm's tail's ability to move is partly due to the arrangement of microtubules (part of the cell's cytoskeleton, purple rings) within the tail. The microtubules are arranged in a central axoneme with two central single microtubules and nine peripheral double microtubules. Surrounding the microtubules are dense fibres (purple, solid), which provide rigidity and protection to the tail. Next is a layer of mitochondria (brown) which provide the energy for movement. Finally the tail is wrapped in a protective outer sheath (yellow). Magnification: x20,000 when printed 10 centimetres wide.