Alternative names

Testicles; testis (singular), testes (plural)

Coloured scanning electron micrograph of sperm cells that are developing inside a seminiferous tubule. The developing heads of the sperm cells are embedded in a layer of Sertoli cells (red) that nourish the developing sperm. Magnification: x3750

Where are the testes?

The testes, also known as testicles or male gonads, lie behind the penis in a pouch of skin called the scrotum. The testes move freely in the scrotum but each testis is attached to the body wall by a thin cord called the spermatic cord, which passes through a cavity in the pelvis and into the abdomen. The cord contains the nerves and blood vessels for the testes as well as the vas deferens, which carries the sperm from the testes to the urethra; the urethra is the passageway for sperm to the outside of the body at ejaculation.

The testes lie outside of the body and are maintained at a temperature about 2^oC lower than the body's core temperature. This is because sperm production and quality is optimal at this lower temperature.

What do the testes do?

The testes have two functions – to produce sperm and to produce hormones, particularly testosterone.

Sperm are produced in the seminiferous tubules of the testes. There are about 700 of these tubes in each testis, each is the length of an arm and the width of a few hairs; the whole collection of tubules if laid end to end is longer than a football pitch! Once the sperm are produced in the seminiferous tubules, they pass into the epididymis (a long coiled tube in which sperm mature as they are conveyed along it). They are then ready to be released at ejaculation via the vas deferens.

What hormones do the testes produce?

The main hormone secreted by the testes is testosterone, an androgenic hormone. Testosterone is secreted by cells that lie between the seminiferous tubules, known as the Leydig cells. The testes also produce inhibin B and anti-Müllerian hormone from Sertoli cells, and insulin-like peptide 3 (INSL3) and oestradiol from the Leydig cells.

Testosterone is important in the first stages of developing the male reproductive organs in a foetus. It also causes the development of male characteristics such as growth of facial hair, deepening of the voice and the growth spurt that takes place during puberty. Testosterone is important in maintaining these secondary male characteristics throughout a man’s life. From puberty onwards, testosterone provides the main stimulus for sperm production

What could go wrong with the testes?

Many things can wrong with the testes; these can be grouped into physical injury and diseases or conditions that affect the function of the testes:

1. Physical injury – the testes lie outside of the body and are not protected by muscle and bone so any physical shock (trauma) to the testes can cause severe pain, bruising and swelling. Usually this is not serious, but very rarely a severe trauma can cause blood to leak into the scrotum; this is called testicular rupture. Surgery may be needed to repair the rupture.
   
   Another rare form of injury is a ‘twisted testicle’ or testicular torsion. This is when the spermatic cord becomes twisted, usually after an injury to the testis or after strenuous activity. It happens more often in teenage boys. This torsion cuts off the blood supply to the testis. This is a medical emergency and surgery is needed to untwist the cord, restore the blood supply and save the testis.
    
2. Diseases and conditions that affect the function of the testes – there are many reasons for testicular dysfunction caused by diseases and conditions:
   • Cryptorchidism – the failure of one or both testes to drop down into the scrotum before birth. This can vary in severity, with testes being felt in the inguinal canal, just above the scrotum, or can be severe where the testes remain in the abdomen and require imaging or surgery to be found. This can harm normal development and function of the testes and lead to infertility. Infants are often treated with an operation to bring the undescended testis into the scrotum (orchidopexy).
     
     
   • Epididymitis – an infection of the epididymis caused by a general infection, tuberculosis, or sexually transmitted illnesses such as Chlamydia. Epididymitis can be treated with antibiotics.
      
   • Testicular cancer – abnormal growth of cells within the testis. The growth can disrupt normal function of one or both testes. It is most common in young men. Urgent medical treatment is required.
      
   • Klinefelter's syndrome – this is a genetic condition which stops the testes from developing normally. As a result, low levels of testosterone are produced and released.

Other factors that can affect the function of the testes are radiation and chemotherapy (used in the treatment of cancer), certain drugs, and disorders of the pituitary gland that stop signals from the hormone (endocrine) system that trigger production of testosterone from the testes.