Control Of Testicular Function

Physiological activity of the testis is governed by two pituitary gonadotropic hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH) (see Chapter 38). The same gonadotropic hormones are

^ spermatozoa

FIGURE 3 The formation of mammalian germ cells. Each primary spermatogonia ultimately gives rise to 64 sperm cells. Cytokinesis is incomplete in all but the earliest spermatogonial divisions, resulting in expanding clones of germ cells that remain joined by intercellular bridges. (From Fawcett DW, A Textbook of histology, 11th ed., Philadelphia: W.B. Saunders, 1986, p. 815. With permission.)

^ spermatozoa

FIGURE 3 The formation of mammalian germ cells. Each primary spermatogonia ultimately gives rise to 64 sperm cells. Cytokinesis is incomplete in all but the earliest spermatogonial divisions, resulting in expanding clones of germ cells that remain joined by intercellular bridges. (From Fawcett DW, A Textbook of histology, 11th ed., Philadelphia: W.B. Saunders, 1986, p. 815. With permission.)

late spermatid microtubules rough reticulum late spermatid microtubules rough reticulum

spermatid spermatocyte junction complex lipid nucleus

FIGURE 4 Ultrastructure of the Sertoli cell and its relation to the germ cells. The spermatocytes and early spermatids occupy niches in the sides of the columnar supporting cell, whereas late spermatids reside in deep recesses in its apex. (From Fawcett DW, A Textbook of histology, 11th ed., Philadelphia: W.B. Saunders, 1986, p. 834. With permission.)

spermatid golgi complex smooth reticulum rs _t?

spermatocyte junction complex lipid nucleus

FIGURE 4 Ultrastructure of the Sertoli cell and its relation to the germ cells. The spermatocytes and early spermatids occupy niches in the sides of the columnar supporting cell, whereas late spermatids reside in deep recesses in its apex. (From Fawcett DW, A Textbook of histology, 11th ed., Philadelphia: W.B. Saunders, 1986, p. 834. With permission.)

produced in pituitary glands of men and women, but, because the physiology of these hormones has been studied more extensively in women, the names that have been adopted for them describe their activity in the ovary (see Chapters 46 and 47). FSH and LH are closely related glycoprotein hormones that consist of a common a subunit and unique p subunits that confer FSH or LH specificity. The three subunits are the products of three genes that are regulated independently. Both gonado-tropins are synthesized and secreted by a single class of pituitary cells, the gonadotropes. Their sites of stimulation of testicular function, however, are discrete, as LH acts on the Leydig cells and FSH acts on the Sertoli cells in the germinal epithelium.

Leydig Cells

The principal role of Leydig cells is synthesis and secretion of testosterone in response to stimulation by LH. Testosterone is an important paracrine regulator of intratesticular functions as well as a hormonal regulator of a variety of extratesticular cells. In addition to stimulating steroidogenesis, LH controls the availability of its own receptors (downregulation) and governs growth and differentiation of Leydig cells. After hypophysec-tomy of experimental animals, Leydig cells atrophy and lose their extensive smooth endoplasmic reticulum where the bulk of testosterone synthesis takes place. LH restores them to normal and can produce frank hypertrophy if given in excess. Leydig cells, which are abundant in newborn baby boys, regress and die shortly after birth. Secretion of LH at the onset of puberty causes dormant Leydig cell precursors to proliferate and differentiate into mature steroidogenic cells. In the fetus, growth and development of Leydig cells depend initially on the placental hormone, chorionic gonadotropin, which is present in high concentrations and which stimulates LH receptors, and later on LH secreted by the fetal pituitary gland.

The LH receptor is a member of the superfamily of receptors that are coupled to heterotrimeric G-proteins. LH stimulates the formation of cyclic adenosine monophosphate (cAMP) (see Chapter 2), which in turn activates protein kinase A and the subsequent phosphor-ylation of proteins that promote steroidogenesis, gene transcription, and other cellular functions. As with the adrenal cortex (see Chapter 40), the initial step in the synthesis of testosterone is the conversion of cholesterol to pregnenolone. This reaction requires mobilization of cholesterol from storage droplets and its translocation from cytosol to the intramitochondrial compartment, where cleavage of the side chain occurs. Access of cholesterol to the P450scc enzyme in the mitochondria is governed by a yet to be defined action of the steroid acute regulatory protein (StAR) the expression and phosphor-ylation of which are accelerated by cAMP. Stored cholesterol may derive either from de novo synthesis within the Leydig cell or from circulating cholesterol, which enters the cell by receptor-mediated endocytosis of low-density lipoproteins. The biochemical pathway for testosterone biosynthesis is shown in Fig. 5. Neither LH nor cAMP appears to accelerate the activity of any of the four enzymes responsible for conversion of the 21-carbon pregnenolone to testosterone. It may be recalled that the rate of steroid hormone secretion, as well as synthesis, is determined by the rate of conversion of cholesterol to pregnenolone. However, in maintaining the functional integrity of the Leydig cells, LH maintains the levels of all of the steroid-transforming enzymes. Transcription of the gene that encodes P450c17, the enzyme responsible for the two-step conversion of 21-carbon steroids to 19-carbon steroids, appears to be especially sensitive to cAMP. Testosterone released from Leydig cells may diffuse into nearby capillaries for transport in the general circulation or may diffuse to nearby seminiferous tubules, where it performs its essential role in spermatogenesis.

The testes also secrete small amounts of estradiol and some androstenedione, which serves as a precursor for extratesticular synthesis of estrogens. The Leydig cell is the chief source of testicular estrogens, but immature Sertoli cells have the capacity to convert testosterone to estradiol. In addition, developing sperm cells express the

21W\23

Get Rid of Gallstones Naturally

Get Rid of Gallstones Naturally

One of the main home remedies that you need to follow to prevent gallstones is a healthy lifestyle. You need to maintain a healthy body weight to prevent gallstones. The following are the best home remedies that will help you to treat and prevent gallstones.

Get My Free Ebook


Post a comment