Gametogenesis in men extends from fetal life to active reproductive life, and involves reserve stem cells and stem cell spermatogonia (Figure 1.1). Several endocrine glands are involved in the reproductive physiology of men (Figures 1.2 and 1.3; Table 1.1). The hypothalamus produces gonadotropin-releasing hormone (GnRH), which stimulates the release of follicle stimulating hormone/luteinizing hormone (FSH/LH). These pituitary peptides stimulate steroidogenesis in the testes (Figure 1.4; Table 1.2), which is under neurological control (Figures 1.5 and 1.6). There is a functional organization of the hypothalamic-pituitary-gonadal axis, which involves feed-forward and feedback loops. In particular, prostaglandin biosynthesis has a major role in various aspects of reproductive physiology and pathology of reproduction in men. Extensive studies have been conducted on the molecular parameters of reproductive mechanisms (Figure 1.7). The clinical/physiological interactions between these hormones and various growth factors are summarized in Tables 1.3 and 1.4.
Hormones or reproduction may be classified according to either their biochemical structure or mode of action. The biochemical structure of hormones includes lipoproteins, polypeptides, steroids, fatty acids and amines. Polypeptide hormones (FSH, LH, oxytocin) have a molecular weight of 30070 000 Da. Steroids (estrogens, testosterone) are derived from cholesterol and have a molecular weight of 300-400 Da. Fatty acids are derived from arachidonic acid and have a molecular weight of about 400 Da, while amines are derived from tyrosine.
HYPOTHALAMUS
The hypothalamus consists of the region of the third ventricle, extending from the optic chiasma to the mammillary bodies. There are neural connections between the hypothalamus and the posterior lobe through the hypothalamic hyperphysical tract, and vascular connections between the hypothalamus and the anterior lobe. Arterial blood enters the pituitary by way of the superior hyperphysical artery and inferior hyperphysical artery. The superior hyperphysical artery forms capillary loops at the median eminence and pars nervosa. From these capillaries, blood flows into the hypothalamohypophyseal portal system. Part of the venous outflow from the anterior pituitary occurs by retrograde back flow, which exposes the hypothalamus to high concentrations of anterior pituitary hormones. This blood flow provides the pituitary gland with the negative-feedback (shortloop) mechanism of regulating the functions of the hypothalamus.
PITUITARY GLAND
The pituitary gland, located in the sella turcica, is divided into anterior, intermediate and posterior lobes. The anterior pituitary has five different cell types secreting six hormones. By cell type, the soma- totropes secrete growth hormone, corticotropes secrete adenocorticotropic hormone, mammotropes secrete prolactin, thyrotropes secrete thyroid-stimulating hormone and gonadotropes secrete FSH/LH.
Neural communication involves neurotransmitters released at synaptic junctions from nerve cells that act across narrow synaptic clefts.
Testosterone, produced by the testes, has effects on most tissues throughout the body, including the brain. Within the brain, testosterone has a prominent role in the organization, programming and activation of neural circuits. In particular, testosterone exerts a negative-feedback effect on GnRH secretion by the brain, which in turn affects the secretion of LH from the pituitary gland. The action of testosterone on the brain also affects behavior, including reproductive behavior.
Figure l.l Comparison of gametogenesis in male and female mammals from fetal life to active sexual life.Ao, reserve stem cells; As, stem cell spermatogonia (adapted from Hafez and Hafez, 2000; Hafez et al., 2003)
ANTI-MULLERIAN HORMONE (AMH)
Anti-Mullerian hormone/Mullerian inhibiting substance (AMH/MIS) has a key role in the formation of the urogenital system. Embryos of both sexes possess two pairs of genital ducts. In males, the Wolffian or mesonephric ducts give rise to the epididymides, vas deferentia and seminal vesicles, whereas in females the Mullerian or paramesonephric ducts give rise to the oviducts, uterus and upper portion of the vagina. For normal male development, the Mullerian ducts must regress. In contrast, females must retain their Mullerian ducts; Wolffian ducts degenerate in the absence of testicular androgens. The regression of the Mullerian ducts is induced by AMH/MIS, a member of the transforming growth factor (TGF)-P super family of growth and differentiation factors.
Figure 1.2 Some endocrine glands involved in the reproductive physiology of men. (a) The pituitary gland and the brain. The pituitary has an anterior portion (1), which produces several hormones that affect other endocrine glands, and a posterior portion (2) that is attached to the hypothalamus by a stalk. Blood vessels (3) carry hormones to the pituitary and away from it (4). Hormones of the posterior pituitary are involved in blood vessel tone and water metabolism. (b) The testis (1) is the site of sperm and hormone production. Male germ cells originating in tubules of the testis pass into the epididymis (2) and the seminal duct or vas deferens (3). Hormone-producing tissue (4) produces the principal masculinizing hormone, testosterone, which enters the bloodstream. Sperm and seminal fluids are transported into the posterior urethra. (c) The pancreas (1) produces digestive enzymes, which drain from collecting channels into the duodenum (2), in proximity to the bile duct outlets (3) from the gallbladder (4).The pancreas also produces a hormone, insulin, in islet cells that are independent of the enzyme-producing structures. Insulin enters directly into the bloodstream. (d) The adrenal glands (1,2), kidneys (3) and ureters (4). (e) The thyroid gland, showing lobes (1-3), trachea (4) and (5, 6). (f) The thyroid (4,5), parathyroid (6), trachea (2) and musculature (1)
Figure 1.3 Thyroid gland. (a) Colloid goiter, frequently called ‘simple goiter’, is a soft, diffuse, symmetrical enlargement of the thyroid gland due to large deposits of thyroid hormone of poor quality, as may be caused by deficiency of iodine in the diet.The diagram shows an external view of the left lobe (1) and a cross-section of the right lobe (2) of a thyroid with colloid goiter. (b) Nodular, or adenomatous, goiter is so-called because of masses that may distort the surface (1) and interior (2) of the thyroid gland. Often there are no constitutional symptoms and only mild disfigurement, but it is possible for some adenomas to become toxic and produce symptoms of hyperthyroidism
Table 1.1 Hormones secreted by reproductive organs
|
Hormone |
Structure and source Principal functions |
|
|
Estrogen |
18-carbon steroid, secreted by the theca interna of the ovarian follicle |
Promotes sexual behavior; stimulates development of secondary sex characteristics, anabolic effects |
|
Progesterone |
21-carbon steroid, secreted by the corpus luteum |
Acts synergistically with estrogen in promoting estrous behavior |
|
Testosterone |
19-carbon steroid, secreted by Leydig cells in the testis |
Develops and maintains accessory sex glands; stimulates secondary sexual characteristics, sexual behavior, spermatogenesis; possesses anabolic effects |
|
Relaxin |
Polypeptide hormone with a and P subunits secreted by the corpus luteum |
Dilates cervix; causes uterine contractions |
|
Prostaglandin |
F2a 20-carbon unsaturated fatty acid, secreted by almost all body tissues |
Causes uterine contractions assisting sperm transport in the female tract and parturition |
|
Activins |
Proteins, found in follicular fluid in female and rete testis fluid in male |
Cause regression of the corpus luteum (luteolytic); stimulate FSH secretion |
|
Inhibins |
Proteins, found in Sertoli cells in male and the granulosa cells in female |
Inhibit release of FSH to a level which maintains species specific number of ovulations |
|
Follistatin |
Protein, found in ovarian follicular fluid in the female |
Modulates the secretion of FSH |
During male development, Sertoli cells of the testes secrete AMH, which signals (through its type II receptor expressed in mesenchymal cells adjacent to the Mullerian duct) the epithelium to induce regression. In male fetuses, AMH expression reaches peak levels during the period of Mullerian duct regression. However, expression continues after Mullerian duct regression is complete, but at reduced levels, declining to very low levels after puberty. In contrast, the ovaries do not synthesize AMH during fetal stages, creating a permissive environment for female reproductive tract differentiation. It is localized to the granulosa cells of preantral and small and large antral follicles, but is not detected in primordial follicles, atretic follicles and corpora lutea. AMH has inhibitory effects on granulosa cell proliferation, aromatase activity and expression of the LH receptor. AMH is an indirect regulator of primordial follicle recruitment, via a decrease in FSH and an increase in inhibin (Pask et al, 2004).
Figure 1.4 FSH steroid/gonadal peptide regulation of testicular function. The hypothalamus produces GnRH, which stimulates the release of LH/ FSH. These pituitary peptides stimulate steroidogenesis. Estradiol/progesterone regulate pituitary/hypothalamic signals. Pituitary peptides (activin, inhibin and follistatin) regulate pituitary release of FSH alone. scg, soluble guanylyl cyclase
Table 1.2 Altogether, the pituitary gland produces a number of hormones of diverse effects. Disorders of the pituitary gland are those of too much or too little hormone production
|
Syndrome |
Hormone involved |
|
|
Acromegaly |
Growth stimulating hormone (somatotropin) |
too much |
|
Gigantism |
Growth stimulating hormone (somatotropin) |
too much |
|
too early |
||
|
Dwarfism |
Growth stimulating hormone (somatotropin) |
too little |
|
Thyrotoxicosis |
Thyroid stimulating hormone |
too much |
|
Masked myxedema |
Thyroid stimulating hormone |
too little |
|
Cushing’s disease |
Adrenal cortex stimulating hormone (ACTH) |
too much |
|
(of pituitary origin) |
||
|
Adrenal insufficiency (of pituitary origin) |
too little |
|
|
Sexual precocity |
Ovary and testis stimulating hormones |
too much |
|
Sexual infantilism |
(gonadotropins) |
too early |
|
Panhypopituitarism |
All pituitary hormones |
too little |
|
Syndrome of persistent |
Prolactin |
too much |
|
lactation |
||
|
Diabetes insipidus |
Antidiuretic hormone |
too little |
Figure 1.5 Endocrine control of testicular and neurological regulation of male sexual function (courtesy Professor A. A. Acosta)
FSH/LH/ACTIVIN
LH and FSH are produced in pituitary gonadotrophs, and are composed of two non-covalently linked subunits, a and p. The a-subunit, called lipoprotein hormone a-subunit, is common to LH and FSH, and the P-subunit is specific to each hormone. The synthesis of each P-subunit is the rate-limiting step in LH and FSH production, and is regulated by the complex interaction of multiple factors, including activin and GnRH.
Activin, a member of the TGF-P family of growth factors, was originally identified as a factor in ovarian fluid that stimulated the secretion of FSH from pituitary gonadotrophs. Activin increases FSH-P subunit synthesis at the transcriptional level. The expression of activin is not limited to the ovary; it is also secreted within the pituitary gland, where it exerts an autocrine/paracrine effect. Activin physiologically regulates the production of LH as well as FSH. GnRH activates the LH-P promoter via the mitogen-activated protein (MAP) kinase pathway; activin A-induced activation of the LH-P promoter is independent of the MAP kinase pathway (Yamada et al., 2004).
RECOMBINANT FSH
The introduction of recombinant human FSH (rFSH) is a milestone for fertility treatment. Compared with its older relatives, urinary human menopausal gonadotropin and urinary FSH (uFSH), rFSH has the clear advantages of a high level of purity, batch-to- batch consistency and better availability; however, rFSH remains much more expensive than the older products. The conclusion from a meta-analysis of 12 randomized trials was that rFSH is superior to uFSH in terms of clinical pregnancy rate per cycles started. Recombinant FSH was also more effective than uFSH in inducing multifollicular development. The higher effectiveness of rFSH has been demonstrated in studies that compared treatment with the same IU dose, and in studies that compared treatment with 150IU of rFSH versus 225IU of uFSH. The higher effectiveness of rFSH is probably due to a more basic hormone profile, compared with uFSH; the basic isohormones exhibit higher in vitro bioactivity than the acidic isohormones. A prospective randomized controlled trial was performed in humans to compare the three common IVF protocols: a short protocol and two long protocols in which GnRH agonist is started at either the early follicular phase or the mid-luteal phase. The two long protocols had the same implantation and pregnancy rates, which were better than with the short protocol (Ravhon, 2002).
Figure 1.6 (a) Endocrine-neuroendocrine relationship between the hypothalamus, pituitary gland and gonad (ovary-testis). Hypothalamic neurosecretory materials (GnRH) are transported by the portal blood capillaries to the cells of the anterior pituitary. FSH and LH stimulate the testis. (b) Hypothalamus, anterior pituitary and testis inter-relationships. Solid arrows indicate stimulatory effects, dashed arrows indicate inhibitory effects. (c) Schematic drawing of hypothalamic nuclei and the pituitary. AH,adenohypophysis; AHA, anterior hypothalamic area; ARC, arcuate nucleus; DHA, dorsal hypothalamic area; DMN, dorsal medial nucleus; ME, median eminence; MB, mammillary body; NH, neurohypophysis; OC, optic chiasma; PHA, posterior hypothalamic area; PM, premammillary nucleus; PON, preoptic nucleus; PT, pars tuberalis; PVN, paraventricular nucleus; SCN, suprachiasmatic nucleus; SON, supraoptic nucleus;VMN, ventromedial nucleus. (d) Hypothalamic-pituitary-gonadal complex. Hypothalamic nerve cells release neurohormones into the portal vessels for transport to the anterior pituitary via the hypothalamohypophyseal vessels. Solid particles in nerve cells represent neurohormones. Blood is transported by the retrograde venous system back to the hypothalamus
Figure 1.7 (a) Interstitial cell (TEM). ER, endoplasmic reticulum; G, Golgi complex; Gr, spherical granule; M, mitochondria; En, endothelium. (b) Anterior lobe of the pituitary (TEM): somatotrophs and gonadotrophs. ER, endoplasmic reticulum; G, Golgi complex; Gr, cytoplasm granules; M, mitochondria
LEPTIN
The peptide leptin is produced primarily by adipocytes, and achieves hormonal status by virtue of its secretion into the bloodstream. Its role in reproduction includes important effects on the hypothalamus to induce release of LH-releasing hormone, thereby triggering gonadotropin release and leading to development of the reproductive tract and induction of puberty.
Steroidogenesis depends on the supply of its precursor, cholesterol, derived from intracellular and extracellular sources. Intracellular levels are tightly controlled by regulation of the uptake, storage and synthesis by a unique family of transcription factors known as the sterol regulatory element-binding proteins (SREBPs). These transcription factors are localized to the endoplasmic reticulum. Upon depletion of cholesterol, the membrane-bound proteins are cleaved by proteases, releasing a 68-kDa transcription regulator. The mature SREBPs enter the nucleus, where they bind to sterol regulatory sites located in the promoter regions of genes involved in cholesterol homeostasis and transport. Circulating leptin has an effect on the gene expression profile and phenotype of white adipose tissue.
Table 1.3 Differential diagnosis of azoospermia/endocrine profile
MELATONIN (N-ACETYL- 5-METHOXYTRYPTAMINE)
Melatonin, the principal hormone of the pineal gland, is also found in plants, but at much lower concentrations than in animals. This hormone is involved in setting the timing (entertainment) of circadian rhythms, as well as regulating seasonal responses to changes in day length in seasonally breeding mammals (sheep, goats, horses, camels) - so called photoperiodic responses. Photoperiodic responses include changes in reproductive status, behavior and body weight. Melatonin is synthesized endogenously by the pinealocytes of the pineal gland. The essential amino acid L-tryptophan is a precursor in the synthesis of melatonin.
Melatonin synthesis displays a circadian rhythm that is reflected in serum melatonin levels. The rhythm is generated by a circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN clock is set to the 24-hour day by the natural light-dark cycle. Light signals through a direct retinal pathway to the SCN. The SCN clock sends circadian signals over a neural pathway to the pineal gland, which drives rhythmic melatonin synthesis. The neural input to the gland is norepinephrine, and the output is melatonin. Melatonin may be indicated for some forms of insomnia and other sleep disturbances, and can abolish some of the symptoms.
The effects of hormones are typically mediated through receptors. Two forms of high-affinity melatonin receptors and one form of a low-affinity receptor have been identified. The high-affinity ML1 receptors are designated Mella and Mellb. The low-affinity receptor is designated ML2. Melatonin has anti- apoptotic activity in the thymus, as well as in cultured dexamethasone-treated thymocytes (a standard model for the study of apoptosis). Its anti-apoptotic activity is thought to occur via down-regulation of the glucocorticoid receptor.
HORMONE RECEPTORS/ REPRODUCTION
GnRH is a hypothalamic decapitate, which has a central role in the regulation of reproduction by stimulating the synthesis/secretion of pituitary LH/ FSH. This effect is mediated through high-affinity receptors belonging to the G-protein-coupled seven transmembrane receptor family. GnRH receptors are also found in the testes, ovary, brain, adrenal gland and placenta, as well as hormone-dependent cancers, in addition to the pituitary. Analysis of the primary structure of the GnRH receptor has been performed to permit comparative analysis of such receptors and to evaluate the second messenger transduction mechanisms of the GnRH receptor, through which the effects of GnRH are mediated.
GnRH receptors of various species have been cloned and sequenced. The mammalian GnRH receptor encodes a single polypeptide of 327/328 amino acids, and the non-mammalian receptor encodes a protein of 379 amino acids. The predicted mammalian/non-mammalian receptor proteins contain seven transmembrane domains and belong to the G-protein-coupled receptor family. The mammalian GnRH receptor is 38% homologous to the nonmammalian GnRH receptor at the amino acid level. When expressed in COS-7 cells or HEK293 cells, the isolated GnRH receptor cDNA invokes GnRH- activated second messenger systems. The gene encoding the GnRH receptor is composed of three exons and two introns, and contains multiple transcriptional start sites and regulatory sequences in the 5' flanking region. Studies suggest that idiopathic hypogonadotropic hypogonadism may be the result of single or multiple mutations in the GnRH receptor gene. Future studies should lead to a better understanding of the mechanisms involved in the transcriptional regulation of the GnRH receptor gene in the pituitary, extra-pituitary tissues and cancers.
Table 1.4 Growth factors in andrology (data from Tanji et al., 2001)
|
Growth factor |
Physiological role in andrology |
Reference |
|
HGF |
Multifunctional growth factor induces mitogenesis, dissociation and motility of epithelial and endothelial cells in vitro, and acts as a morphogenetic factor for tubular epithelium. Expressed in the mesenchyme, whereas the HGF receptor, c-MET, is expressed in epithelium. Expression of c-MET is unregulated by androgen deprivation. |
Humphrey et al., 1995 Pisters et al., 1995 |
|
KGF |
Secreted by fibroblasts and specifically promotes keratinocyte proliferation. In prostate, it is expressed and secreted by normal stromal cells. Only epithelial cells express the BEK/FGFR-2 receptor that binds KGF, suggesting that this growth factor controls epithelial cell proliferation in a paracrine manner. It is a potent mitogen for normal prostatic epithelial cells in vitro and promotes the growth of these cells under serum-free conditions. Prostatic stromal cell-derived KGF exhibits the properties of an andromedin, which may indirectly mediate control of epithelial cell growth and function by androgen. |
Cunha et al., 1995 Finch et al., 1989 Story et al., 1992 Sugimura et al., 1996 Yan et al., 1992 |
|
IGFs |
Polypeptide with amino acid sequence and functional homology to insulin; proteins are produced by a variety of tissues, and the regulation of their production and function is extremely complex.There are 2 IGF peptides (IGF-1 and IGF-2),2 cell surface receptors and at least 6 specific high-affinity binding proteins that modulate IGF actions.The mitogenic effect of IGFs is due to their ability to facilitate the transfer of cells from the G1 phase to the S phase in the cell cycle. Produced only by the stromal cells, and normal epithelial cells, particularly basal cells, express IGF-1 receptors, suggesting a paracrine pathway. Have important mitogenic effects in the prostate and are essential for the development of the prostate. Expression of IGF regulated with other growth factor pathways, EGF, FGF and TGF-β. |
Alarid et al., 1994 Cohen et al., 1994 Fiorelli et al., 1991 Stile et al, 1979 |
|
TGF-β |
Multifunctional polypeptides regulate cellular differentiation and functions. At least 5 isoforms of TGF-β have been identified, but only types 1,2 and 3 are present in mammalian cells.The polypeptides contain 112 amino acids and share about 80% sequence homology. TGF-β predominates in all tissues, whereas the expression of TGF-β2 and -β3 is more tissue restricted.TGF-β1 is expressed in smooth muscle cells, which are located adjacent to epithelial cells. Play dual roles of both stimulator and inhibitor in male reproductive organs. TGF-βs induce proliferation of mesenchymal cells and inhibit the growth of epithelial cells and inhibit DHT-dependent epithelial branching morphogenesis of seminal vesicles. TGF-β1, -β2 and -β3 are important for fetal development and are expressed at high levels in 17-day murine urogenital sinus mesenchyme, but not to play a role in regulating cell growth through the antiproliferative effects of EGF/TGF-β. On epithelial cells can counterbalance the mitogenic effects of various growth factors, thus having a role in growth regulation; associated with castration-induced prostate cell apoptosis. Normal prostate tissues have TGF-β receptors and generally respond to TGF-β through reduction in proliferation.TGF-β receptors are predominantly expressed in epithelium.Three types of receptors identified according to molecular weight. Only type I and II receptors have a direct role in TGF-β signal transduction.Type II receptors bind to TGF-β and then recruit type I receptor. |
Cunha et al., 1995 Graycar et al., 1989 Kondaiah et al., 1990 Massague et al., 1990 Schurrmans et al., 1988 Story et al., 1996 Tanji et al., 1994 Timme et al., 1986 Wrana et al., 1994 |
|
EGF TGF-a |
Related polypeptides, which consist of 53 and 50 amino acids, respectively, share about 35% sequence homology and bind to same cell surface receptor. They have roles in embryogenesis, cell differentiation and angiogenesis. EGF is secreted by both normal and malignant cells, while TGF-a is produced by tumor cells. Human prostate epithelial cells require EGF in serum-free medium for growth in primary culture, and normal human fetal prostatic fibroblasts can replicate in response to this growth factor. Regulated by androgens and may be an important factor in an autocrine or paracrine mode of regulation. Exert their effects through their receptor (EGF-R), present in epithelial cells of male sex accessory organs.Their ability to interact with the same receptor is accounted for by their sequence homology.The differences in action of the 2 ligands may be due to different conformational changes induced within the receptor by the binding of each ligand. Androgens down-regulate EGF-Rs in prostate.TGF-a plays a role in prostatic differentiation and maintenance of epithelial integrity rather than proliferation.TGF-a expression occurs predominantly in stroma, whereas its receptor is expressed by epithelial cells, suggesting a paracrine mode of regulation. |
Cohen et al., 1994 Lee et al., 1985 Nishi et al., 1996 Rappolee et al., 1988 St Arnaud et al., 1988 Traish and Wotiz, 1987 Ullrich et al., 1984 |
Sperm capacitation/acrosome reaction/fertiliza- tion involve sperm receptors on plasma lemma, oocyte receptors located on the zona pellucida (ZP3; ultrastructural interactions involve ZP2) and inner acrosomal membrane receptors. Immunochemical parameters, using monoclonal antibodies to acrosin, exhibit an abnormal pattern of weak fluorescence in the post-nuclear region.
FUTURE RESEARCH IN ENDOCRINOLOGY
The Center for Scientific Review of the National Institutes of Health reviews individual studies in the following research areas of endocrinology, metabolism, nutrition and reproductive sciences:
(1) Pituitary, thyroid and adrenal physiology and pathophysiology
(2) Cancers of the endocrine system
(3) Growth, development and disorders of endocrine organs and their products
(4) Neuroendocrinology
(5) Reproductive neuroendocrinology, including development of the hypothalamic-pituitary- gonadal axis and mechanisms underlying the biorhythms of reproductive hormones
(6) Hormone interactions with other organ systems and tissues
(7) Hormones and immunobiology
(8) Pediatric and developmental endocrinology
(9) Endocrinology of aging
(10) Endocrine pharmacology and toxicology, including the effects of endocrine disrupters and xenobiotics
(11) Endocrine-related disorders of the male and female reproductive systems
(12) Hormones, stress and the autonomic systems
(13) Hormone-based therapies
(14) Comparative endocrinology
(15) Animal models of endocrine disorders