Color enhanced transmission electron microscope of a cross section of human spermatozoa. A sperm's tail's ability to move is partly due to the arrangement of microtubules (part of the cell's cytoskeleton) 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 fibers, which provide rigidity and protection to the tail. Next is a layer of mitochondria which provide the energy for movement. Finally the tail is wrapped in a protective outer sheath.