Testosterone is one of the hormones most directly responsible for the male sex drive, reproductive function and secondary sex characteristics. Not only does the hormone produce typical anatomical features of mature males such as heavy musculature and body hair, it is also responsible for characteristic male aggression. The hormone is produced in response to a hormone axis involving several different body organs.
Cholesterol
The direct chemical source of testosterone in the male body is cholesterol, the molecule from which testosterone is biosynthesized, or made, by the cells. In their book "Biochemistry," Drs. Reginald Garrett and Charles Grisham note that cholesterol serves as the chemical precursor for all the steroid hormones, including estrogen, progesterone, and testosterone, among others. Enzymes control each step of the chemical modification of cholesterol into other steroid molecules, and the last enzyme responsible for the transformation to testosterone is under the control of cells in the testes.
Leydig Cells
The direct physical source of testosterone in the male body are the Leydig cells of the testes, which are located in the spaces between seminiferous tubules, or the tubes in which sperm grow. Leydig cells make the enzyme that converts the immediate biochemical precursor of testosterone into testosterone, and operate in response to hormones from the brain, explains Dr. Lauralee Sherwood in her book, "Human Physiology." The testosterone made in the testes is released into the bloodstream, and circulates throughout the human body.
Hypothalamic-Pituitary-Testicular Axis
The ultimate source of testosterone in the male is the brain, without which perfectly functional testes would fail to produce any hormone whatsoever. Through an interaction between two crucial brain structures and the testes themselves, the brain sends the gonads messages that help initiate and regulate testosterone production, explains Dr. Gary Thibodeau in his book, "Anatomy and Physiology." The hypothalamus, located near the middle of the brain, initiates testosterone production by releasing gonadotropin-releasing hormone, or GnRH. This hormone travels through a direct vascular connection to another brain structure, called the anterior pituitary, found just below the hypothalamus. The anterior pituitary responds to GnRH by releasing hormones including luteinizing hormone, or LH, into the bloodstream. When LH arrives at the testes, it binds to receptors and signals the cells to begin making the enzyme responsible for the ultimate chemical step in testosterone production.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
