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The testicles are the male sex organs. They are located in the scrotum just below the groin and just behind the penis. They are primarily responsible for the production of sperm cells for reproduction, and release the male sex hormone testosterone and smaller quantities of the female sex hormone estrogen. Because sperm production cannot take place at the normal body temperature of 37C, the testicles must be kept away from the body in order to keep them cooler.

In normal male development in the womb, the testicles drop from the rest of the body. The testosterone produced at this time also causes the development of the penis. In puberty, the testicles grow larger in response to the increased prouction of testosterone. In rare cases, the testicles may not develop, showing the symptom of hypogonadism. This may be due to several diseases or disorders, the primary of which is the lack of human growth hormone from a pituitary disorder.

Trauma to the testicles is incredibly painful. Because the testicles are the male equivalent of the ovaries, which are deep in the abdomen, the nerve endings for the testicles are also perceived by the brain as if they are deep in the abdomen as well. As such, they are far more sensitive to pain than any other part of the body that is on the outside of the body. The consequences of a miswiring where pain is perceived in the testicles is graphically depicted in the episode Painless

Testicle at Wikipedia