Sperm production. Coloured scanning electron micrograph (SEM) of a longitudinal section down a freeze-fractured seminiferous tubule, the site of sperm production in the human testis. This tubule contains a swirl of the tails (blue) of developing sperm cells at its centre. The tubule is lined with a stratified epithelium containing two types of cell: Sertoli cells (red), which nourish developing sperm, and spermatogenic cells (green), 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.