Sperm cell fertilising an egg cell, coloured ESEM (environmental scanning electron micrograph). The sperm cell (brown) has a tail and a head, and the head is trying to penetrate the surface of the egg cell (blue). Once a sperm cell has fertilised the egg cell, rapid chemical changes make the outer layer (zona pellucida) of the egg cell thicken, preventing other sperm cells entering. The head of the successful sperm cell releases genetic material that mixes with the genetic material in the egg cell. The resulting zygote then has the full set of genes required to develop into a new human. The sperm cell (spermatozoon, male) and the egg cell (ovum, female) are the human gametes (reproductive cells). Magnification: x4200 when printed 10cm wide.