Human egg cell and sperm cells, coloured ESEM (environmental scanning electron micrograph). There are three sperm cells (brown) attempting to fertilise the egg cell (ovum, blue). Once one of the sperm cells has fertilised the egg cell, rapid chemical changes make the outer layer (zona pellucida) of the egg cell thicken, preventing any of the other sperm cells entering it. 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: x680 when printed 10cm wide.