Testosterone is a hormone made in both males and females, although it is found in significantly higher quantities in men. Because it is responsible for sexual development and sperm production in men, it is known as a male sex hormone. Testosterone is made in the highest quantities in the male testes through a series of reactions involving a number of specialized enzymes, and there are two different synthesis pathways that produce this hormone. Understanding these is important for the treatment of conditions such as infertility in men.
Testosterone as a Hormone
Testosterone is an important sex hormone produced in the greatest quantities in males, although females also produce small amounts. It is a steroid hormone produced in the testes of males and its functions can be subdivided into androgenic, or sexual development functions, and anabolic, or functions related to the growth of tissues such as muscle and bone.
Testosterone Synthesis Pathway
Testosterone, like all steroid hormones, is produced from the precursor molecule cholesterol, a fatty substance derived from the foods you eat, such as red meat, cheese and eggs. In the synthesis of testosterone, cholesterol is turned into testosterone via a complex biosynthetic pathway that includes several intermediate molecules. These intermediates also can be used in your body to perform other functions. For example, the steroid synthesis pathway that produces testosterone also can produce female sex hormones, the stress hormone cortisol and the fluid regulation hormone aldosterone, if the correct enzymes are present.
Testosterone Synthesis in the Testes
The exact reaction pathway that takes place predominantly in the testes for the production of testosterone is as follows: cholesterol to pregnenolone, to progesterone, to 17-beta-hydroxyprogesterone, to androstenedione, which is finally converted to testosterone.
All of these reactions require extremely specific enzymes to catalyze, or speed up, the reaction. In both the male testes and the female ovaries, the enzymes for this reaction are present, but in the ovaries there is an additional enzyme that converts testosterone into estrogen.
Testosterone Synthesis in Other Organs
These processes occur in specialized cells within the testes known as the Leydig cells, and 95 percent of the circulating testosterone is produced by the testes. Leydig cells are responsible for producing testosterone for the Sertoli cells, which also are located in the testes, and which require this hormone for the production of sperm. Testosterone also is produced by the zona reticularis in the adrenal gland, where a slightly different synthetic route occurs to produce testosterone from cholesterol: cholesterol to pregnenolone, to 17-a-hydroxypregnenolone, to 5-dehydroepiandosterone or DHEA, to androstenediol, to testosterone.
Testosterone Regulation
Testosterone production is regulated by two other hormones that act on the cells in the testes. These are luteinizing hormone, or LH, and follicle stimulating hormone, FSH, both of which also are found in large quantities in females. LH and FSH are produced by the pituitary gland in the brain, which in turn is regulated by another part of the brain, the hypothalamus. Between these three glands reciprocal feedback loops exist that allow hormone levels to be tightly controlled. In this mechanism, LH stimulates an increase in the number of Leydig cells and also an increase in the production of testosterone from these cells.
What Does Testosterone Do?
Testosterone is produced during puberty to cause sexual development, including the growth of facial and body hair, the deepening of the voice and the production of sperm. It also causes a marked increased in muscle mass due to the anabolic effects of testosterone, to the point that synthetic testosterone is often taken by people wanting to increase their muscle bulk. Testosterone also is thought to have other roles in your body that are currently poorly understood, such as brain development, immune system activity and emotional responses.
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