Endocrine Functions of the Testes

Testosterone is by far the major androgen secreted by the adult testis. This hormone and its derivatives (the 5a-reduced androgens) are responsible for initiation and maintenance of the body changes associated with puberty in males. Androgens are sometimes called anabolic steroids because they stimulate the growth of muscles and other structures (table 20.4). Increased testosterone secretion during puberty is also required for growth of the accessory sex organs— primarily the seminal vesicles and prostate. Removal of androgens by castration results in atrophy of these organs.

Androgens stimulate growth of the larynx (causing a lowering of the voice) and promote hemoglobin synthesis (males have higher hemoglobin levels than females) and bone growth. The effect of androgens on bone growth is self-limiting, however,

5a-DHT

O OH

17b Estradiol

Estradiol-17b

HO OH

Estradiol-17b

5a-Androstane-3^,17^-diol

5a-Androstane-3a,17b-diol

■ Figure 20.14 Derivatives of testosterone. Testosterone secreted by the interstitial (Leydig) cells of the testes can be converted into active metabolites in the brain and other target organs. These active metabolites include DHT and other 5a-reduced androgens and estradiol.

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Chapter Twenty

Table 20.4 Actions of Androgens in the Male

Category

Action

Sex Determination

Growth and development of wolffian ducts into epididymis, ductus deferens, seminal vesicles, and ejaculatory ducts

Development of urogenital sinus into prostate

Development of male external genitalia (penis and scrotum)

Spermatogenesis

At puberty: Completion of meiotic division and early maturation of spermatids

After puberty: Maintenance of spermatogenesis

Secondary Sex Characteristics

Growth and maintenance of accessory sex organs

Growth of penis

Growth of facial and axillary hair

Body growth

Anabolic Effects

Protein synthesis and muscle growth

Growth of bones

Growth of other organs (including larynx)

Erythropoiesis (red blood cell formation)

because they ultimately cause replacement of cartilage by bone in the epiphyseal discs, thus "sealing" the discs and preventing further lengthening of the bones (as described in chapter 19).

Although androgens are by far the major endocrine products of the testes, there is evidence that both Sertoli cells and Leydig cells secrete small amounts of estradiol. Further, receptors for estradiol are found in Sertoli and Leydig cells, as well as in the cells lining the male reproductive tract (efferent ductules and epi-didymis) and accessory sex organs (prostate and seminal vesicles). Estrogen receptors have also been located in the developing sperm cells (spermatocytes and spermatids, described in the next section) of many species, including humans. This suggests a role for estrogens in spermatogenesis, and indeed knockout mice (chapter 3) missing an estrogen receptor gene are infertile. Further, men with a congenital deficiency in aromatase—the enzyme that converts an-drogens to estrogens (fig. 20.14)—are also infertile.

Estradiol, either secreted by the testes or produced locally as a paracrine regulator, may be responsible for a number of effects in men that have previously been attributed to androgens. For example, the importance of the conversion of testosterone into estradiol in the brain for negative feedback control was described earlier. Estrogen also may be responsible for sealing of the epi-physeal plates of cartilage; this is suggested by observations that men who lack the ability to produce estrogen or who lack estrogen receptors (due to rare genetic defects only recently discovered) maintain their epiphyseal plates and continue to grow.

The two compartments of the testes interact with each other in paracrine fashion (fig. 20.15). Paracrine regulation, as described in chapter 11, refers to chemical regulation that occurs among tissues within an organ. Testosterone from the Leydig cells is metabolized by the tubules into other active androgens and is required for spermatogenesis, for example. The tubules also secrete products that might influence Leydig cell function. Such interactions are suggested by evidence that exposure of pubertal male rats to FSH augments the responsiveness of the Leydig cells to LH. Since FSH can directly stimulate only the Sertoli cells of the tubules, the FSH-induced enhancement of LH responsiveness must be mediated by products secreted from the Sertoli cells.

Inhibin secreted by the Sertoli cells in response to FSH can facilitate the Leydig cells' response to LH, as measured by the

Human Physiology Endorphin

■ Figure 20.15 Interactions between the two compartments of the testes. Testosterone secreted by the interstitial (Leydig) cells stimulates spermatogenesis in the tubules. Leydig cells may also secrete ACTH, MSH, and P-endorphin. Secretion of inhibin by the tubules may affect the sensitivity of the Leydig cells to LH stimulation.

amount of testosterone secreted. Further, it has been shown that the Leydig cells are capable of producing a family of polypeptides previously associated only with the pituitary gland—ACTH, MSH, and P-endorphin. Experiments suggest that ACTH and MSH can stimulate Sertoli cell function, whereas P-endorphin can inhibit Sertoli function. The physiological significance of these fascinating paracrine interactions between the two compartments of the testes remains to be demonstrated.

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Responses

  • Aamu
    What is the endocrine function of the interstitial cells of leydig of the testes?
    5 years ago
  • sven barth
    How would the removal of the testes affect endocrine function?
    5 years ago
  • germana
    What are the endocrine functions of the testes?
    5 years ago
  • anja eberhardt
    How would removal of the testes effect endocrine function in the reproductive function?
    4 years ago
  • satu
    What are the endocrine secretion and functions of testes?
    4 years ago
  • rose
    What are the endocrine and exocrine functions of the testes?
    4 years ago
  • Savanna
    What are the endocrine products of testis?
    4 years ago
  • willie
    What is the endocrine product of the testes?
    4 years ago
  • alexander
    What are the ndocrine functions of the testes?
    4 years ago
  • KIDANE
    How would removal of the testes affect endocrine function and reproductive function?
    4 years ago
  • wilimar
    What is the target organ of testes?
    4 years ago
  • arabella
    What is the endocrine product of the testis?
    4 years ago
  • Sabrina
    Why is the removal of androgens results in atrophy?
    10 months ago

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