The two testes (testicles) are oval shaped glands that sit in a pouch of skin (the scrotum) behind the penis. In adult men, each testis is between 12-25 ml in size, growing from 1-3 ml in prepubertal children. It is normal for the size of each testis to differ slightly and for one to sit lower than the other. Each testis is attached to the body by the spermatic cord, which contains nerves, blood vessels and the vas deferens (which carries sperm from the testes to the urethra).
The location of the testes and scrotum on the outside of the body means that the testes can be kept cooler than normal body temperature. This is important for sperm production.
The testes are made up of 200-300 compartments called lobules. Each lobule contains several coiled structures called seminiferous tubules, where sperm are made. The seminiferous tubules then release the sperm into a series of ducts where they mature and pass to the urethra. Around the seminiferous tubules are the specialised cells that produce testosterone, called the Leydig cells.