Sperm production. Coloured scanning electron micrograph (SEM) of sperm cells (blue tails) that are developing inside a seminiferous tubule, the site of sperm production in the human testis. The developing heads of the sperm cells (green/red, across bottom) are embedded in a layer of Sertoli cells (red) that nourish the developing sperm. It is the heads that contain the genetic material used to fertilise the female egg cell. The process of sperm cell formation is called spermatogenesis. Magnification: x3750 when printed 10cm high.