Secretion of testosterone

Testosterone, the major androgenic hormone, accounts for 95% of the androgen concentration. The primary source of testosterone is the testes however, 3% to 5% of the testosterone concentration is derived from direct adrenal cortical secretion of testosterone or C-19 steroids such as androstenedione.17,18... 

The advantage of this division of labour is not obvious but one possibility is that secretion of testosterone by the cells not only provides a precursor for oestradiol but provides a hormone that is secreted into the bloodstream to influence, perhaps surprisingly, sexual activity in women, particularly sexual arousal and interest. 

Figure 19.8 A brief summary of the pathways for formation and secretion of oestradiol and progesterone within the cells of the follicle. Cholesterol is taken up by thecal cells in a complex with low density lipoprotein. In the thecal cells, cholesterol is converted to testosterone which is released to be taken up by granulosa cells where it is converted into oestradiol. For synthesis of progesterone in the granulosa cells, cholesterol is synthesised de novo within the cells from acetyl-CoA. In the follicle the enzyme aromatase, which produces the aromab c ring in the female sex hormones, is restricted to the granulosa cells. The reacrions that are stimulated by LH and FSH increase synthesis and, therefore, secretion of testosterone and increased synthesis of oestrogens and progesterone.

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.

The explanation for this phenomenon is that in the prepubertal phase the amount of testosterone that is secreted is not sufficient to cause development of the male genitalia, which is normally stimulated by dihydrotestosterone. However, at puberty the secretion of testosterone increases. The increase is large in this syndrome, because lack of dihydrotestosterone decreases the extent of the feedback inhibition of the hormone secretions by the pituitary so that more gonadotrophins are released which stimulate, markedly, the rate of testosterone secretion and hence its concentration in the blood. At these high levels, testosterone has sufficient androgenic effects to stimulate development of the external genitalia. 

Referred to as anabolic steroids, such compounds used in excess are not without side effects. In men, excessive use leads to a decrease in the secretion of testosterone, to testicular atrophy, and sometimes to breast enlargement (gynecomastia) if some of the excess androgen is converted into estrogen. In women, excess testosterone causes a decrease in ovulation and estrogen secretion it also causes breast regression and growth of facial hair. 

Secretion of testosterone is fairly constant and consistent in adult men, although higher levels occur in the morning and lower levels in late evening. This circadian rhythm is not accompanied by changes in the levels of LH thus, it appears to be an intrinsic testicular rhythm. Testosterone levels decline by 10-15% between the ages of 30 and 70 years, accompanied by reduction in tissue responsiveness to androgenic stimulation. 

The testes secrete testosterone and manufacture spermatoztta. Before puberty, gonadotrophin and testosterone concentrations in plasma are very low. The development of the l.eydig cells and their secretion of testosterone is influenced by LH. whereas Sertoli cell function is influenced by FSH. Stimulation and maintenance of spermatogenesis require that both FSH and LH be present (Fig. I). Testosterone is responsible for growth and function of the prostate and epididymis, and for the development of the male secondary sex characteristics such as hair growth, deep voice and characteristic musculature. 

The normal prostate is dependent upon the testicular synthesis and secretion of testosterone to maintain cellular integrity and functional activity of the... 

Both LH and FSH are referred to as gonadotropins, beoause they aot on the male and female gonads, which results in the production of the sex steroids testosterone and estradiol, respectively. In the female, FSH and LH act in concert in regulating ovarian function egg maturation, ovulation, and transformation of the ruptured follicle into the corpus luteum. In males, spermatogenesis is dependent on these two hormones. Specifically, FSH in females facilitates the maturation of ovarian folliole oells and their secretion of estradiol, whereas in males, it stimulates the maturation of sperm in the testes. In females, LH promotes ovulation, formation of the oorpus luteum, and progesterone secretion in males, it enables the secretion of testosterone from the testes. 

Follicle-stimulating hormone 204 Induces the secretion of testosterone and dihydrotestosterone. 

Testosterone is the principal hormone of the androgen group of steroids. Secretion of testosterone... 

During development, a portion of the mesonephric kidney develops into two different ducts, the Wolffian and Mullerian ducts. The Wolffian duct, under the influence of testosterone, differentiates into the vas deferens, the epididymis, and the seminal vesicle. A second pair of embryonic ducts, the Mullerian ducts, develops alongside the Wolffian ducts. In males, the Mullerian ducts are suppressed due to the action of Miillerian-inhibiting substance (MIS), a hormone secreted by the testes. Maleness depends upon the secretion of testosterone from the testis, and in the absence of testosterone a male will develop a female phenotype. The SRY gene apparently activates the synthesis of MIS, which in turn assures that the Mullerian ducts will atrophy and that the mammal develops as a male. 

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