Testis hormones

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. 

Figure 1 The major hormones involved in growth and funetion of the fetal/neonatal testis illustrating how exogenous, environmental oestrogens eould disrupt the normal balanee of these meehanisms.

Testosterone, the principal male sex steroid hormone, is synthesized in five steps from cholesterol, as shown below. In the last step, five isozymes catalyze the 17/3-hydroxysteroid dehydrogenase reactions that interconvert 4-androstenedione and testosterone. Defects in the synthesis or action of testosterone can impair the development of the male phenotype during embryogenesis and cause the disorders of human sexuality termed male pseudohermaphroditism. Specifically, mutations in isozyme 3 of the 17/3-hydroxysteroid dehydrogenase in the fetal testis impair the for-... 

AQP10 has only been identified in the small intestine so far and is thought to play a role in hormonal secretion. AQP11 is expressed in kidney, liver, testis and brain, but no function has been found so far. AQP12 has been identified in pancreatic acinar cells, where it is thought to facilitate the release of digestive enzymes into the pancreatic duct. 

In addition, several other organs, like the heart, ovaty, amnion, chorion, decidua, testis, epididymis and prostate, have been reported to synthesize OT, suggesting a paracrine role for this hormone in these tissues. Ectopic AVP production by lung cancer cells or other neoplasms has been described in humans, leading to the syndrome of inappropriate antidiuretic hormone secretion. 

The reproductive system includes the ovaries in the female and testes in the male, together with the ducts and tubes in which the ovum or spermatozoa travel and meet in the fallopian tube, and the environment in which the foetus develops. Since both ovary and testis produce hormones, there is overlap with the endocrine system (Chapters 12 and 19). 

This process is important for proliferating cells, in which cholesterol is reqnired for formation of new membranes, and for some endocrine cells (in the testis, ovary and adrenal cortex) for the synthesis of steroid hormones. 

Testis Testosterone/androstenediol Lactate dehydrogenase Oestradiol Male steroid hormones Not known Possibly encourages contraction of smooth muscle in walls of the vagina and uterus... 

Figure 19.11 Hormones secreted by the hypothalamus, anterior pituitary, ovary and testis and feedback regulation. GnRH is gonadotrophin-releasing hormone the gonadotrophins are follicle-stimulating hormone (FSH) and luteinising hormone (LH). The effect of these hormones on activities in the ovary and testes is shown. FSH stimulates synthesis and secretion of oestradiol from follicle, and spermatogenesis in testis. LH stimulates synthesis and secretion of progesterone from corpus luteum and synthesis and secretion of testosterone by the Leydig cells.

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

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