Testis. Coloured scanning electron micrograph (SEM) of a freeze fractured seminiferous tubule, the site of sperm production in the testes. This tubule contains a swirl of the tails of forming sperm cells at its centre. The tubule is lined with a stratified epithelium containing two types of cell: Sertoli cells, which nourish developing sperm, and spermatogenic cells, from which the sperm cells form in a process known as spermatogenesis. After this, the sperm cells mature into their mobile form in a process known as spermiogenesis. Magnification: x360 when printed 10 centimetres wide.