Sperm cells. Coloured transmission electron micrograph (TEM) of a section through the testis, showing sperm cells. The heads of the sperm, which contain the genetic material, are red. The central part of the cells, which contain the mitochondria that provide the energy for the cell, are purple. The ends of the tails are yellow. The tail's motility is aided by the arrangement of microtubules (part of the cell cytoskeleton, dark yellow and dark purple) within the tail. The microtubules are arranged in a central axoneme, with a central pair of microtubules surrounded by nine other pairs in a ring.