Testosterone is an androgen, or male hormone, produced by the Leydig cells in the testes. It's made by chemically modifying cholesterol, and has a number of effects on the male body, both anatomical and physiological. While testosterone is actually produced in small quantities by women, in whom it is responsible for the sex drive, its far more concentrated in men, and its major and best-known effects are observed in males.
Development of Reproductive System
One of the earliest effects of testosterone on men takes place at only a few weeks of gestational age, while an embryo is still in the uterus. While genetic material determines a baby's gender from the moment of conception, early embryos look identical, and possess undifferentiated gonads and reproductive ducts and tissues. The gonads of baby boys eventually differentiate into testes, explains Dr. Lauralee Sherwood in her book, "Human Physiology." As the testes begin to secrete testosterone, the hormone causes female reproductive duct progenitors, called Mullerian ducts, to wither. Male duct progenitors, called Wolffian ducts, develop into the ducts of the male reproductive system. The external genitals also differentiate at this point under the influence of testosterone, with tissue migrating and taking shape as the penis and scrotum.
Effect on Brain
Testosterone has many effects on the male brain--some are prenatal and others occur at puberty. In their book, "You: Having A Baby," Drs. Michael Roizen and Mehmet Oz note that early testosterone production by a male embryo has a profound effect on the brain. The hormone causes atrophy of communication centers in the brain, and proliferation of aggression centers. This predisposes males to less potential for development of communication skills, and increased potential for development of aggressive behavior as adults, though influences throughout life continue to affect brain development, and may either augment or ameliorate early effects of testosterone. At puberty, testosterone again begins to affect the brain, heightening sex drive and increasing instances of aggression.
Physical Effects
There are many physical effects of testosterone upon a pubescent male. The hormone causes the skeletal and muscular systems to become dense in comparison to those of women, giving men a thicker, heavier body. Body hair proliferates under the hormone's influence, and the voice box thickens, deepening the voice. In his textbook "Anatomy and Physiology," Dr. Gary Thibodeau notes that sperm production, too, is testosterone dependent, since the hormone affects levels of other body hormones, most notably from the hypothalamus and pituitary gland, that lead to sperm production.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "You: Having A Baby"; Michael Roizen, M.D. and Mehmet Oz, M.D.; 2009
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
