Sperm production. Coloured scanning electron micrograph (SEM) of a freeze-fracture through a seminiferous tubule in the testis, showing the tails (blue) of multiple sperm cells (spermatozoa). The seminiferous tubules are the site of spermatogenesis (sperm production). During production and maturation, the head of each sperm cell is buried in the surrounding Sertoli cells (green), which nourish the developing sperm. A connective tissue sheath (brown) surrounds each individual tubule. Magnification: x350 when printed at 10 centimetres across.