Apoptosis cell death. Transmission electron micrograph (TEM) of a section through the epithelium of a seminiferous tubule in a human testis (testicle), showing five germ cells going through apoptosis (black and white) surrounded by healthy germ cells (grey). This is the site of spermatogenesis, the production and maturation of spermatozoa (sperm cells). Apoptosis is the process of programmed cell death and is usually a rapid (4-6 hours) physiological process used to regulate tissue function, for example in the constant turnover of epithelial cells of the gut, or as here in the seminiferous epithelium. Death of germ cells is a common event in normal spermatogenesis. Magnification: x8300, when printed 10 centimetres wide.