Sperm production site. Coloured scanning electron micrograph (SEM) of a seminiferous tubule that has been freeze fractured. The fracture plane (centre left) has cut through the tubule, which runs off to far right. Seminiferous tubules produce sperm (male reproductive cells) when sexual maturity is attained. In human males, the seminiferous tubules are found in the testes. The developing sperm form in the outer layer of the tubule, maturing as they move inwards towards the centre, finally becoming fully motile (free-moving). Magnification unknown.