Testicles (Testes)

The testicles are paired organs, normally five centimeters (2 inches) long, located within the scrotum (a multi-layered muscular structure that protects the testes and helps with temperature regulation). The testicles have two very important functions, and each of them is essential for normal male fertility: the first is the production of testosterone and the second is the production of sperm. Sperm only start their development in the testes. From there they move on to the epididymis where they mature and are stored.

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Epidydimis

The epididymis is a long (about 6 meters) tube that sits on the back of the testis. Here sperm undergo their final development and maturation and are stored until they are made available for ejaculation. From the epididymis, sperm proceed into the vas deferens, also known as the spermatic cord.

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Vas Deferens

The vas deferens is also a long, tube-like structure that connects the epididymis (where the sperm is stored) to the urethra (the tube that expels sperm). Although the urethra also expels urine, there is a valve that controls the flow of sperm versus urine. During ejaculation, the sperm flows out of the testicles, through the vas deferens and into the urethra. Seminal fluid is added by the prostate (a gland located in front of the rectum just below the bladder, whose main purpose is to produce fluid for semen) and other glands as the sperm flow outside the body through the penis.

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Normal Male Rreproductive Physiology

The production of sperm is a very complicated process that begins at puberty and lasts, in healthy males, until death. The production of sperm begins in the testicles. The function of the testicles is controlled by several hormones. These hormones are regulated by the hypothalamus and the pituitary gland located in the brain.

The hypothalamus regulates the hormonal activity of the pituitary gland by secreting gonadotropin-releasing hormone (GnRH). This hormone controls the production of the gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary. LH triggers the production of testosterone (a hormone needed for sperm production). FSH also triggers hormones that help with sperm production.

The sperm is released into the seminiferous tubules where they leave the testes and enter the epididymis. In the epididymis, the sperm mature and become capable of fertilization. The entire process of sperm production, beginning with a primary spermatocyte, takes about 74 days. After ejaculation, the sperm can live for about 72  hours in the female reproductive tract.

During sexual activity, motile sperm are ejaculated into the female reproductive tract and begin their journey through the cervix and uterus to the fallopian tubes, the site of fertilization.

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