Testosterone is an androgen, or male hormone. It's produced by the testes in response to hormones produced in and secreted by the brain, and is responsible for male sexual characteristics, appearance and behaviors. Though the hormone is made by the testes, it also has an effect on the testes.
Testicular Growth
A baby boy is born with testes, but they're not functional or capable of producing sperm until he reaches puberty. At that time, changes occurring throughout the body cause the testicles to change anatomically and physiologically in such a way that the boy matures sexually. Testosterone has a number of effects upon pubescent testicles, explains Dr. Lauralee Sherwood in her book, "Human Physiology." One of the most obvious of these, at least to the pubescent boy, is that his testicles grow significantly in size and weight. While testicular size isn't determined purely by the amount of testosterone secreted, generally men who secrete more testosterone will grow larger testicles.
Spermatogenesis
Another effect of testosterone on the testes is that it leads to spermatogenesis, or sperm production. As testosterone levels increase during puberty, a boy's testicles become capable of producing sperm for the first time. After puberty, a healthy man's body maintains a relatively constant testosterone level in the blood throughout the remainder of his reproductive life, meaning that sperm production is relatively constant, explains Dr. Gary Thibodeau in his book, "Anatomy and Physiology." The hormones FSH and LH, or follicle stimulating and luteinizing hormone, work together to induce both testosterone production and sperm production in the testes. As the hormones cause the testes to produce more testosterone, the testosterone causes sperm-producing cells to generate sperm.
Feedback Inhibition
An interesting and counterintuitive effect of testosterone upon the testes is that it limits testosterone production. This is because a man's body, to function optimally, must maintain nearly constant levels of testosterone throughout reproductive life. As such, testosterone acts as part of a feedback inhibition loop that helps turn off testosterone production if it gets too high. As Dr. Sherwood explains, as testosterone causes the testes to generate sperm, the sperm begin to produce a hormone called inhibin. Inhibit signals the brain to release smaller quantities of LH and FSH, which means that the testes then produce less sperm, and smaller amounts of testosterone. As such, testosterone levels stay constant and don't get abnormally high or low.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
