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Like other steroid hormones testosterone is derived from cholesterol. The largest amounts of testosterone are produced by testes, but it is also synthesized in smaller quantities by the theca cell s of the ovaries, the zona reticulosa of the adrenal cortex, and the placenta. Substantial amounts of the testosterone in women are also produced from estradiol by reverse aromatization in the liver, adipose cells, and other peripheral tissues.
In the testes testosterone is produced by the Leydig cells. Due to dual function of the male gonad testosterone directly influences spermatogenesis. Like most hormones, testosterone is supplied to target tissues in the blood where much of it is transported bound to a specific plasma protein, sex hormone binding globulin (SHBG).
Effects of testosterone in humans and other vertebrates occurs by way of two main mechanisms: by activation of the androgen receptor (directly or as DHT), and by conversion to estradiol and activation of certain estrogen receptors.
Free testosterone (T) is transported into the cytoplasmCytoplasm is the viscid, semifluid matter contained within the plasma membrane of a cell that helps to hold the cell together. In contrast to the protoplasm, however, the cytoplasm does not include the cell nucleus. The watery or aqueous component of the of target tissue cells, where it can bind to the androgen receptor, or can be reduced to 5α- dihydrotestosteroneDihydrotestosterone is the hormone formed in the prostate gland, testes, hair follicles, and adrenal glands when the enzyme 5-alpha reductase acts on testosterone. Traditionally considered a "male hormone", DHT is also produced by females in smaller quant (DHT) by the cytoplasmic enzyme 5α-reductase. DHT binds to the same androgen receptor even more strongly than T, so that its androgenic potency is about 2.5 times that of T. The T-receptor or DHT-receptor complex undergoes a structural change that allows it to move into the nucleusIn chemistry and physics, the nucleus atomic nucleus is the collection of protons and neutrons in the center of an atom that carries the bulk of the atom's mass and positive charge. In cell biology, the nucleus cell nucleus is the membrane-bound subcellul and bind directly to specific nucleotideA nucleotide is an organic molecule consisting of a nitrogenous heterocyclic base (a purine or a pyrimidine), a pentose sugar ( deoxyribose in DNA or ribose in RNA), and a phosphate or polyphosphate group. A nucleoside is similar, except that it contains sequences of the chromosomal2) Centromere. The point where the two chromatids touch, and where the microtubules attach. 3) Short arm. 4) Long arm. A chromosome is, minimally, a very long, continuous piece of DNA, which contains many genes, regulatory elements and other intervening n DNA. The areas of binding are called hormone response elements (HREs), and influence transcriptional activity of certain geneDNA and to a chromosome (right). Introns are regions often found in eukaryote genes which are removed in the splicing process: only the exons encode the protein. This diagram labels a region of only 40 or so bases as a gene. In reality many genes are muchs, producing the androgen effects.
Androgen receptors occur in many different vertebrate body system tissues, and both males and females respond similarly to similar levels. Greatly differing amounts of testosterone prenatally, at puberty, and throughout life account for a large share of biological differences between males and females.
The bones and the brain are two important tissues in humans where the primary effect of testosterone is by way of aromatization to estradiol. In the bones, estradiol accelerates maturation of cartilage into bone, leading to closure of the epiphyses and conclusion of growth. In the central nervous system, testosterone is aromatized to estradiol. Estradiol rather than testosterone serves as the most important feedback signal to the hypothalamus (especially affecting LH secretion). In many mammals, prenatal or perinatal "masculinization" the sexually dimorphic areas of the brain by estradiol derived from testosterone programs later male sexual behavior.