Androgen, also called androgenic hormone or testoid, is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This includes the activity of the accessory male sex organs and development of male secondary sex characteristics. Androgens were first discovered in 1936. Androgens are also the original anabolic steroids and the precursor of all estrogens, which are stress hormones. The primary and most well-known androgen is testosterone. Dihydrotestosterone (DHT) and androstenedione are less known generally, but are of equal importance in male development. DHT in the embryo life causes differentiation of penis, scrotum and prostate. Later in life DHT contributes to male balding, prostate growth and sebaceous gland activity
A subset of androgens, adrenal androgens, includes any of the 19-carbon steroids synthesized by the adrenal cortex, the inner-most layer of the adrenal cortex (zonula reticularis—innermost region of the adrenal cortex), that function as weak steroids or steroid precursors, including dehydroepiandrosterone (DHEA), dehydroepiandrosterone sulfate (DHEA-S), and androstenedione.
Besides testosterone, other androgens include:
- Dehydroepiandrosterone (DHEA) is a steroid hormone produced in the adrenal cortex from cholesterol. It is the primary precursor of natural estrogens. DHEA is also called dehydroisoandrosterone or dehydroandrosterone.
- Androstenedione (Andro) is an androgenic steroid produced by the testes, adrenal cortex, and ovaries. While androstenediones are converted metabolically to testosterone and other androgens, they are also the parent structure of estrone. Use of androstenedione as an athletic or bodybuilding supplement has been banned by the International Olympic Committee, as well as other sporting organizations.
- Androstenediol is the steroid metabolite thought to act as the main regulator of gonadotropin secretion.
- Androsterone is a chemical byproduct created during the breakdown of androgens, or derived from progesterone, that also exerts minor masculinising effects, but with one-seventh the intensity of testosterone. It is found in approximately equal amounts in the plasma and urine of both males and females.
- Dihydrotestosterone (DHT) is a metabolite of testosterone, and a more potent androgen than testosterone in that it binds more strongly to androgen receptors. It is produced in the adrenal cortex.
Development of the male 
Testes formation 
During mammalian development, the gonads are at first capable of becoming either ovaries or testes. In humans, starting at about week 4, the gonadal rudiments are present within the intermediate mesoderm adjacent to the developing kidneys. At about week 6, epithelial sex cords develop within the forming testes and incorporate the germ cells as they migrate into the gonads. In males, certain Y chromosome genes, particularly SRY, control development of the male phenotype, including conversion of the early bipotential gonad into testes. In males, the sex cords fully invade the developing gonads.
Androgen production 
The mesoderm-derived epithelial cells of the sex cords in developing testes become the Sertoli cells, which will function to support sperm cell formation. A minor population of nonepithelial cells appear between the tubules by week 8 of human fetal development. These are Leydig cells. Soon after they differentiate, Leydig cells begin to produce androgens.
Androgen effects 
The androgens function as paracrine hormones required by the Sertoli cells to support sperm production. They are also required for masculinization of the developing male fetus (including penis and scrotum formation). Under the influence of androgens, remnants of the mesonephron, the Wolffian ducts, develop into the epididymis, vas deferens and seminal vesicles. This action of androgens is supported by a hormone from Sertoli cells, Müllerian inhibitory hormone (MIH), which prevents the embryonic Müllerian ducts from developing into fallopian tubes and other female reproductive tract tissues in male embryos. MIH and androgens cooperate to allow for the normal movement of testes into the scrotum.
Early regulation 
Before the production of the pituitary hormone luteinizing hormone (LH) by the embryo starting at about weeks 11–12, human chorionic gonadotrophin (hCG) promotes the differentiation of Leydig cells and their production of androgens at week 8. Androgen action in target tissues often involves conversion of testosterone to 5α-dihydrotestosterone (DHT).
During puberty, androgen, LH and follicle stimulating hormone (FSH) production increase and the sex cords hollow out, forming the seminiferous tubules, and the germ cells start to differentiate into sperm. Throughout adulthood, androgens and FSH cooperatively act on Sertoli cells in the testes to support sperm production. Exogenous androgen supplements can be used as a male contraceptive. Elevated androgen levels caused by use of androgen supplements can inhibit production of LH and block production of endogenous androgens by Leydig cells. Without the locally high levels of androgens in testes due to androgen production by Leydig cells, the seminiferous tubules can degenerate, resulting in infertility. For this reason, many transdermal androgen patches are applied to the scrotum.
Inhibition of fat deposition 
Males typically have less body fat than females. Recent results indicate androgens inhibit the ability of some fat cells to store lipids by blocking a signal transduction pathway that normally supports adipocyte function. Also, androgens, but not estrogens, increase beta adrenergic receptors while decreasing alpha adrenergic receptors- which results in increased levels of epinephrine/ norepinephrine due to lack of alpha-2 receptor negative feedback and decreased fat accumulation due to epinephrine/ norepinephrine then acting on lipolysis-inducing beta receptors.
Muscle mass 
Males typically have more skeletal muscle mass than females. Androgens promote the enlargement of skeletal muscle cells and probably act in a coordinated manner to function by acting on several cell types in skeletal muscle tissue. One cell type conveys hormone signals to generating muscle, the myoblast. Higher androgen levels lead to increased expression of androgen receptor. Fusion of myoblasts generates myotubes, in a process linked to androgen receptor levels.
Circulating levels of androgens can influence human behavior because some neurons are sensitive to steroid hormones. Androgen levels have been implicated in the regulation of human aggression and libido. Indeed, androgens are capable of altering the structure of the brain in several species, including mice, rats, and primates, producing sex differences. Numerous reports have shown androgens alone are capable of altering the structure of the brain, but identification of which alterations in neuroanatomy stem from androgens or estrogens is difficult, because of their potential for conversion.
Insensitivity to androgen in humans 
Relative potency of natural androgens (%) 
Determined by consideration of all biological assay methods (circa 1970). Data from Steroid Biochemistry and Pharmacology by Briggs and Brotherton, Academic Press.
5alpha-dihydrotestosterone (DHT) was 2.4 times more potent than testosterone at maintaining normal prostate weight and duct lumen mass (this is a measure of epithelial cell function stimulation). Whereas DHT was equally potent as testosterone at preventing prostate cell death after castration.
- Chemical or surgical ablation can be used as an effective therapy in prostate cancer. See androgen deprivation therapy.
- Yolk androgen levels in certain birds have been positively correlated to social dominance later in life. See American Coot.
See also 
- Scott F. Gilbert; with a chapter on plant development by Susan R. Singer (2000). In Scott F. Gilbert. Developmental Biology (6th ed.). Sunderland, Massachusetts: Sinauer Associates. ISBN 0-87893-243-7.[page needed]
- Stephen Nussey and Saffron Whitehead (2001). In Saffron A. Whitehead and Stephen Nussey. Endocrinology: an integrated approach. Oxford: British Institute of Organ Studies. ISBN 1-85996-252-1.[page needed]
- Singh R, Artaza JN, Taylor WE, et al. (January 2006). "Testosterone inhibits adipogenic differentiation in 3T3-L1 cells: nuclear translocation of androgen receptor complex with beta-catenin and T-cell factor 4 may bypass canonical Wnt signaling to down-regulate adipogenic transcription factors". Endocrinology 147 (1): 141–54. doi:10.1210/en.2004-1649. PMID 16210377.
- Sinha-Hikim I, Taylor WE, Gonzalez-Cadavid NF, Zheng W, Bhasin S (October 2004). "Androgen receptor in human skeletal muscle and cultured muscle satellite cells: up-regulation by androgen treatment". The Journal of Clinical Endocrinology and Metabolism 89 (10): 5245–55. doi:10.1210/jc.2004-0084. PMID 15472231.
- Vlahopoulos S, Zimmer WE, Jenster G, et al. (March 2005). "Recruitment of the androgen receptor via serum response factor facilitates expression of a myogenic gene". The Journal of Biological Chemistry 280 (9): 7786–92. doi:10.1074/jbc.M413992200. PMID 15623502.
- Cooke B, Hegstrom CD, Villeneuve LS, Breedlove SM (October 1998). "Sexual differentiation of the vertebrate brain: principles and mechanisms". Frontiers in Neuroendocrinology 19 (4): 323–62. doi:10.1006/frne.1998.0171. PMID 9799588.
- Zuloaga DG, Puts DA, Jordan CL, Breedlove SM (May 2008). "The role of androgen receptors in the masculinization of brain and behavior: what we've learned from the testicular feminization mutation". Hormones and Behavior 53 (5): 613–26. doi:10.1016/j.yhbeh.2008.01.013. PMC 2706155. PMID 18374335.
- Wright AS, Thomas LN, Douglas RC, Lazier CB, Rittmaster RS (December 1996). "Relative potency of testosterone and dihydrotestosterone in preventing atrophy and apoptosis in the prostate of the castrated rat". J. Clin. Invest. 98 (11): 2558–63. doi:10.1172/JCI119074. PMC 507713. PMID 8958218.