Testis, androgens

Androgens Testis Testosterone Development of male secondary sex characteristics... 

Female sexual development and behaviour in mammals occurs by default and requires no ovarian secretion, and it is only in genetic males that the testis can secrete hormones which destroy this female pattern and superimpose that of the male. Sexual differentiation is not so well defined in fish, and larval exposure to both synthetic estrogens and androgens is widely used in aquaculture to produce monosex cultures. Endocrine disruption of sexual differentiation in fish may therefore reflect both the complexity and diversity of such processes between different species. Some care is required in use of the terms hermaphrodite and sex-reversal since a true hermaphrodite has both functional testes and ovaries and a sex-reversed fish is fully functional as its final sex—both produce the appropriate viable gametes. Such functional sex-reversal is not possible in mammals, but in some species of fish it is the normal developmental pattern. In most of the cases of hermaphroditism or sex-reversal reported in the non-scientific press, there is evidence only for a few ovarian follicles within a functional testis. This may be considered as feminisation or a form of intersex, and is very clearly endocrine disruption, but it is certainly neither sex-reversal nor hermaphroditism. In some cases the terms have even been used to infer induction of a single female characteristic such as production of yolk-protein by males. 

Table 1 Sites of androgen and oestrogen reeeptors and aromatase aetivity in the testis and assoeiated duets during fetal/perinatal life in the rat ...

Testiculat androgens are synthesized in the interstitial tissue by the Leydig cells. The immediate precursor of the gonadal steroids, as for the adrenal steroids, is cholesterol. The rate-limiting step, as in the adrenal, is delivery of cholesterol to the inner membrane of the mitochondria by the transport protein StAR. Once in the proper location, cholesterol is acted upon by the side chain cleavage enzyme P450scc. The conversion of cholesterol to pregnenolone is identical in adrenal, ovary, and testis. In the latter two tissues, however, the reaction is promoted by LH rather than ACTH. 

Androgens are masculinizing substances. The endogenous male gonadal hormone is the steroid testosterone from the interstitial Leydig cells of the testis. 

Pharmacology Testosterone, produced by the Leydig cells of the testis, is the primary natural androgen. In women, small amounts are synthesized by the ovary and adrenal cortex. 

It is a natural androgen secreted by testis. The secretion is regulated by LH hormone secreted by pituitary gland. 

THE TESTIS (ANDROGENS ANABOLIC STEROIDS, ANTIANDROGENS, MALE CONTRACEPTION)... 

In humans, the most important androgen secreted by the testis is testosterone. The pathways of synthesis of testosterone in the testes are similar to those previously described for the adrenal and ovary (Figures 39-1 and 40-2). 

In men, approximately 8 mg of testosterone is produced daily. About 95% is produced by the Leydig cells and only 5% by the adrenals. The testis also secretes small amounts of another potent androgen, dihydrotestosterone, as well as androstenedione and dehydroepiandrosterone, which are weak androgens. Pregnenolone and progesterone and their 17-hydroxylated derivatives are also released in small amounts. Plasma levels of testosterone in males are about 0.6 mcg/dL after puberty and appear to decline after age 50. Testosterone is also present in the plasma of women in concentrations of approximately 0.03 mcg/dL and is derived in approximately equal parts from the ovaries and adrenals and by the peripheral conversion of other hormones. 

Control of androgen secretion and activity and some sites of action of antiandrogens (1), competitive inhibition of GnRH receptors (2), stimulation (+, pulsatile administration) or inhibition via desensitization of GnRH receptors (-, continuous administration) (3), decreased synthesis of testosterone in the testis (4), decreased synthesis of dihydrotestosterone by inhibition of 5a-reductase (5), competition for binding to cytosol androgen receptors. 

FSH plays a key role in spermatogenesis it binds G-protein-coupled membrane-bound receptors on Sertoli cells and induces the Sertoli cells to proliferate during pre- and postnatal development. FSH also induces Sertoli cells to produce androgen binding protein (ABP), TGF-beta 1, MIS and other important signalling compounds. Sertoli cells also play an important role in spermiation and are the source of the seminiferous tubule fluid that provides nutrients to sperm cells as they travel from the testis to the epididymis. 

General anabolic effects stimulates release of insulinlike growth factor-I Stimulates milk synthesis Ovary luteinization, progesterone synthesis testis interstitial cell development, androgen synthesis... 

Cyproterone inhibits the action of androgens and gossypol prevents spermatogenesis without altering the other endocrine functions of the testis (Figure 61.9). 

The Testis (Androgens Anabolic Steroids, Antiandrogens, Male Contraception)... 

Fig. 10. Pathways of androgen biosynthesis in rat testis. A — B —> C and a —> b - c are the A5 and xlJ pathways, respectively, for testosterone biosynthesis. Enzymes A.a. 17-hydroxylase B.b, C-17,20-lyase C,c, 17j3-HSD. Reaction c is reversible.

Abbreviations ABP, androgen-binding protein 5q-DHT. 5a-dihydrotestosterone 5/3-DHT, 5/3-dihy-drotestosterone EGF, epidermal growth factor FSH. folitropin KAP, kidney androgen-induced protein LH, lutropin NGF, nerve growth factor SGF, testis-specific growth factor. 

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