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Perspective: Male Reproduction

Department of Obstetrics and Gyneacology (I.H.) University of Aberdeen Aberdeen AB24 2ZD, Scotland, United Kingdom and Department of Physiology (A.B.) Southern Illinois University of Medicine Carbondale, Illinois 62901-6512

Address all correspondence and requests for reprints to: Prof. Ilpo Huhtaniemi, Department of Obstetrics and Gynaecology, University of Aberdeen, Aberdeen AB24 2ZD, Scotland, United Kingdom. E-mail: ilpo.huhtaniemi{at}utu.fi

	Introduction

Top Introduction Human mutations and genetically... The hypothalamic-pituitary... The GH, insulin-like growth... The inhibin/activin-follistatin... Spermatogenesis Estrogens and testicular... Paracrine/autocrine regulation... Testicular apoptosis Aging and male endocrine... Clinical relevance of the... References

 
Besides unraveling the basic mechanisms underlying male reproductive functions, studies of testicular function are clinically important for the specialty called andrology. Andrology covers all physiological and pathophysiological functions specific for the male gender, ranging from conception until senescence. The main challenges of clinical andrology entail improvement of diagnostics and treatment of male infertility, development of male-specific methods of fertility control, combating sexually transmitted diseases, and improving quality of life of aging males. Better basic understanding of male reproductive endocrinology is essential for improving our chances to tackle these challenges. As in all fields of biomedical research, the major recent advances have been made using the versatile methodology of molecular biology. This short review describes some of the recently emerged concepts of testicular endocrine regulation and function, as well as the approaches that have enabled this progress. The topics selected represent the subjective views of the authors. We admit that the scope chosen by someone else could have been different, and apologize to those whose findings and concepts were not included, mainly because of space limitations. For the same reason, the main emphasis of this review will be in testicular function.

	Human mutations and genetically modified animal models

Top Introduction Human mutations and genetically... The hypothalamic-pituitary... The GH, insulin-like growth... The inhibin/activin-follistatin... Spermatogenesis Estrogens and testicular... Paracrine/autocrine regulation... Testicular apoptosis Aging and male endocrine... Clinical relevance of the... References

 
The specific role of a gene product can be resolved by overexpressing or disrupting its gene in genetically modified animals, usually mice. Novel functions for specific genes have been found, and useful models for the study of molecular pathogenesis of human diseases are available. Also male reproductive endocrinology has greatly benefited from this in vivo application of molecular biology.

	The hypothalamic-pituitary-testicular (HPT) axis

Top Introduction Human mutations and genetically... The hypothalamic-pituitary... The GH, insulin-like growth... The inhibin/activin-follistatin... Spermatogenesis Estrogens and testicular... Paracrine/autocrine regulation... Testicular apoptosis Aging and male endocrine... Clinical relevance of the... References

 
Mutations of gonadotropin receptors have recently been discovered in human patients. Although very rare, these findings have clarified the molecular pathogenesis of some disturbances in the reproductive function (1). An activating mutation of the LH receptor (R) gene causes the male-limited, early-onset, gonadotropin-independent precocious puberty (testotoxoicosis) but, interestingly, women with a similar mutation seem to have no alterations of the phenotype. If the activating LHR mutation activates the inositol trisphosphate cycle, as was found with one particular mutation, the patient presented with testicular Leydig cell adenomas (2). This finding emphasizes the potential role of gonadotropins as tumor promoters, thus supporting a theory that has been put forward on the basis of some clinical finding on gonadal malignancies. Similar findings have been made on the role of gonadotropins in gonadal tumorigenesis of gene-modified mice, e.g. inhibin- knockout (KO) mice with high FSH levels (3), LH overexpressing mice (4, 5), and those expressing the viral oncogene SV40 T-antigen (6). Such models offer good opportunities for further exploration of this potentially important but poorly characterized action of gonadotropins.

Inactivating mutations of the human LHR disrupt male sex differentiation (1), which ranges, depending on severity of the receptor inactivation, from mild undervirilization to total lack of genital masculinization. The phenotype in females is milder, characterized only by anovulatory infertility. The phenotype of a single male with inactivating mutation of the LHß gene differs from the respective receptor mutation: the subject was normally masculinized at birth, but failed to undergo sexual maturation (7). This difference can be explained by the stimulatory role of human CG (hCG) on his testicular testosterone in utero, which naturally is not possible if the receptor is defective. The very recently developed mouse KO model for the LHR (LuRKO mouse) allows interesting comparison between LH/hCG effect on sexual differentiation in mice and men (8, 9). As in the male patient with inactivated LHß, the male LuRKO mice were normally masculinized at birth but failed to show postnatal sexual maturation. Unlike in humans, functional LHR is thus not necessary in the mouse fetus for sufficient testicular androgen production to induce masculinization. Other blood-borne or paracrine factors can apparently maintain sufficient androgen production in the absence of LH action. It is intriguing that such a profound difference prevails in the hormonal regulation of masculinization between man and rodents.

Inactivating FSHß subunit and FSHR mutations have been detected in humans (1), and the respective KO mouse models have been developed (3, 10, 11). There is good agreement between the mouse and human phenotypes of the inactivating FSH ligand and receptor mutations in females; all are infertile because of arrested follicular development. A discrepancy prevails in males; while men with FSHR inactivation (11A ), as well as FRHß and FSHR KO mice (4, 10, 11) present with suppressed testicular size in the face of qualitatively normal spermatogenesis, and are fertile or subfertile, the two men so far described with inactivating FSHß mutation are azoospermic (12, 13). The small number of these cases in comparison to the other models shifts the balance toward the contention that FSH action per se is not necessary for spermatogenesis. This finding is of practical importance, indicating that a male contraceptive strategy based on elimination of FSH action may not be feasible. Interestingly, no clear activating mutations of the human FSHR have been discovered in either sex, although large numbers of patients with the expected phenotypes (e.g. premature ovarian failure or macro-orchia) have been screened. It is possible that our educated guesses of phenotypes of such cases are incorrect, or the mutations in the candidate syndromes have to be looked for in genes participating in the postreceptor cascade of FSH signal transduction. Therefore, genetically modified animal models for activating gonadotropin receptor mutations would be of great help in predicting the human phenotypes.

All in all, the severity of the phenotypes of human mutations and genetically modified mice emphasize the sexual dichotomy between LH and FSH; the former is critically the male gonadotropin, and the latter the female one.

	The GH, insulin-like growth factor (IGF)-1, and PRL effects

Top Introduction Human mutations and genetically... The hypothalamic-pituitary... The GH, insulin-like growth... The inhibin/activin-follistatin... Spermatogenesis Estrogens and testicular... Paracrine/autocrine regulation... Testicular apoptosis Aging and male endocrine... Clinical relevance of the... References

 
There is considerable evidence that the somatotropic axis, GH, and IGF-I, as well as PRL, are involved in control of the HPT axis, and that locally produced IGF-I participates in paracrine interactions between different cell types in the testis. Targeted gene disruption and transgenic technology are used to study the actions of PRL, GH, and IGF-I in the male. The weight of the accessory reproductive glands was reduced in PRL-KO mice (14) and increased in transgenic mice overexpressing PRL (15). Both PRL-KO and PRLR-KO males are fertile, although reproductive development of PRLR-KO male mice may be delayed (14, 16). The very "mild" reproductive phenotype of PRL-KO and PRLR-KO male mice might reflect availability of redundant regulatory mechanisms, most likely through GH and IGF-I.

KO of IGF-I and IGF-IR genes causes severe developmental abnormalities, often leading to early mortality. In the surviving IGF-KO animals, the male reproductive system is usually infantile (17). In contrast, GHR/GH binding protein KO mice are usually fertile but mimic many symptoms of Laron dwarfism in the human, including a delay in sexual maturation (18 ; Keene, D. and A. Bartke, unpublished). GHR-KO males have reduced levels of LHR and PRLR in the testis and reduced responsiveness to GnRH or LH stimulation (19 ; Chandrashekar, V., M. L. Dufau, and A. Bartke, unpublished). Elevation of plasma PRL levels in these animals (19) is of interest in that it might represent a mechanism of physiological compensation for the loss of GH signaling. It is hoped that studies in this novel model of GH resistance will allow clear separation of the role of GH-dependent and GH-independent IGF-I production in the regulation of male sexual maturation and adult reproductive functions. However, production of animals with testis-specific KO of IGF-I and IGF-IR is needed to evaluate the physiological significance of IGF-I in the paracrine control of somatic and germ cells in the testis.

Transgenic male mice overexpressing GH are usually fertile. However, overexpression of human GH is associated with hyperplasia and hypertrophy of seminal vesicles, loss of androgen receptors from these organs, elevation of plasma LH, and premature loss of reproductive competence. Many of these effects are presumably due to the combined somatogenic and lactogenic activity of human GH in rodents because they are either milder or absent in transgenic males overexpressing bovine GH, which is purely somatogenic, or overexpressing endogenous GH driven by MT-hGHRH transgene (20).

	The inhibin/activin-follistatin system

Top Introduction Human mutations and genetically... The hypothalamic-pituitary... The GH, insulin-like growth... The inhibin/activin-follistatin... Spermatogenesis Estrogens and testicular... Paracrine/autocrine regulation... Testicular apoptosis Aging and male endocrine... Clinical relevance of the... References

 
In clinical studies, the dimeric inhibin assays have established inhibin B as the testis-specific form of this hormone, and it serves as a good positive marker for spermatogenesis and Sertoli cell function (21). In the adult testis, paracrine signals from germ cells are important for Sertoli cell inhibin production. A considerable amount of basic research has been devoted to the study of the role of inhibins, activins, and the activin-binding molecule follistatin in the paracrine regulation of testicular germinal and somatic cell development and functions. While inhibin seems to have a dual, endocrine (regulation of FSH secretion in pituitary) and para/autocrine role in testicular function, the activin actions seem to be of the local type in the testis and numerous extragonadal sites (22). The testicular effects of the activins are inhibitory, stimulatory, age and cell specific, and often opposite to those of inhibin. Follistatin adds another dimension of complexity to these effects. An important part to our knowledge about the physiology and pathophysiology of inhibin peptides has emerged from the gene-modified mouse models for inhibin, activin, activin receptors and follistatin produced by Matzuk and co-workers (23). Clear testicular phenotypes were found with many of the models, in support of important role of these peptides in testicular function, including the function of inhibin as tumor suppressor. Recent studies from their laboratory address the importance spatiotemporal expression of the different inhibin peptides and their receptors using insertional mutagenesis (24). In general, a topical question to be answered with cell-specific KO models, using the Cre/loxP technique, is the functional importance of the numerous putative para/autocrine local factors within the testis, including the inhibin peptides.

While the activin receptors and mechanisms of activin action are relatively well delineated, the receptor for inhibin, and its ultimate mechanism of action still remain elusive. One theory is that inhibin in fact is an inhibitor of activin action, although mounting evidence indicates that it also interacts with specific membrane-binding proteins that are likely to be the putative inhibin receptors. Some molecules with inhibin binding activity, as well as their interactions with the known activin receptor ActRIIA, have recently been identified: one is a 120-kDa membrane-anchored proteoglycan, p120 (25); the other is the type III TGFß receptor, betaglycan (26). It is possible that inhibin acts, in part, through inhibition of activin action by forming inhibitory complexes with type II activin receptors, while some of the inhibin actions could be independent using different receptor molecules and signaling mechanisms. The final answer to this conundrum may be near.

	Spermatogenesis

Top Introduction Human mutations and genetically... The hypothalamic-pituitary... The GH, insulin-like growth... The inhibin/activin-follistatin... Spermatogenesis Estrogens and testicular... Paracrine/autocrine regulation... Testicular apoptosis Aging and male endocrine... Clinical relevance of the... References

 
It seems to be almost a universal phenomenon that transgenes show unexpected expression in some of the numerous cell types of the testis, a finding that agrees with the low endogenous expression of so many genes in the testis. The functional significance of this phenomenon is not understood. Another general feature of transgenesis is that quite often, an unexpected side finding in a study exploring a nonreproductive phenomenon, is the infertility of male transgenic or KO mice. This failure of the experimental model can be turned into virtue, if the model elucidates the basic mechanisms of spermatogenesis. There are numerous studies where KO or overexpression of specific growth factors affects some aspects of spermatogenesis (27, 28, 29, 30, 31).

If we exit the realm of endocrine and paracrine regulatory events, KO mice have been developed with primary defects at every stage of spermatogenesis (32), thus creating a database that can be used for decoding the network of genetic hierarchy that is responsible for spermatogenesis. Besides function of gene products in vivo, the models have elucidated promoter and untranslated sequences involved in male germ cell differentiation. The exploitation of these models to explore the basic mechanisms of spermatogenesis is just in its initial phase. It has been somewhat puzzling that the KO of some genes known to be involved in spermatogenesis has not caused infertility, and that silencing of some genes not known to be involved in testicular function have caused infertility (32). Besides unraveling the basic mechanisms of spermatogenesis, the novel animal models may offer new strategies for male contraception. If, for instance, a gene product crucial for spermatogenesis or sperm maturation can be specifically inactivated using a pharmacological inactivator, a specific male contraceptive method may be within reach.

In the study of spermatogenesis, another recent methodological breakthrough is the technique of germ cell transplantation, which has vast prospects of application (33). These include at least the possibilities to investigate fundamental aspects of spermatogenesis, the clinical prospects to treat male infertility, and the possibilities to genetically manipulate spermatogonial stem cells to develop transgenic animals. It is possible that the pending questions about the physiological significance of paracrine regulation of spermatogenesis will be solved with this novel approach. The power of this technique is perhaps best illustrated by the recent demonstration that in heterospecific germ cell transplantation between the rat and the mouse, duration of spermatogenesis is determined by the transplanted germ cells rather than by the resident Sertoli cells (34). This result could not have been predicted from previous studies of Sertoli cells-germ cells interactions. Finally, the study of spermatogenesis in lower animal species, e.g. Drosophila and C. elegans, have opened up new avenues into studies of mammalian spermatogenesis.

	Estrogens and testicular function

Top Introduction Human mutations and genetically... The hypothalamic-pituitary... The GH, insulin-like growth... The inhibin/activin-follistatin... Spermatogenesis Estrogens and testicular... Paracrine/autocrine regulation... Testicular apoptosis Aging and male endocrine... Clinical relevance of the... References

 
It is very well established that the principal female sex steroid, estradiol (E₂) is produced by the testis, that E₂ and other estrogens are present in the circulation of a normal adult male, and that conversion of testosterone to E₂ is important in androgen signaling particularly in the brain. Production of E₂ by the Leydig cells in response to LH and by the immature rat Sertoli cells in response to FSH have been studied in considerable detail. In recent years there has been a resurgence of interest in the study of the role of estrogens in the male. This stemmed primarily from the following developments:

1) Discovery of the second type of estrogen receptor, named estrogen receptor ß (ERß) and demonstration that its distribution in the testis and in the male reproductive tract differs from that of the long-known ER (35).

2) Development of mice with targeted disruption of ER or ERß (36), which allowed studies of the physiological role of estrogen signaling in the male.

3) Suggestion that reduction in sperm counts and increase in the incidence of testicular tumors in men in some industrialized areas may be due to estrogenic substances in the environment (37).

Studies in male mice, rats and monkeys revealed the presence of ER in the Leydig cells, rete testis, efferent ducts and epididymis, as well as in the pituitary, with some species differences (36). Expression of ER was particularly intense in the efferent ducts that transport spermatozoa suspended in the fluid secreted by the Sertoli cells from the testis to the epididymis. In contrast, ERß was widely expressed in the testis, including Sertoli cells, Leydig cells, and germ cells, as well as in the epididymis, prostate, and seminal vesicles.

Male ERKO mice with targeted disruption of ER have increased plasma testosterone levels but are infertile due to distension of the rete testis and excurrent ducts and progressive loss of testicular weight, reflecting damage to seminiferous epithelium and exfoliation of germ cells (36). Studies in these animals revealed a previously unsuspected important physiological role of endogenous estrogen: the control of fluid reabsorption in the efferent ducts and the initial segment of the epididymis (36). Male BERKO mice with selective disruption of ERß are fertile and exhibit no major alterations in the function of the male reproductive system (36). This suggests that the effects of endogenous estrogens on the development and function of the male reproductive system identified by treatment with aromatase inhibitors (38) or KO of the aromatase gene (39) are due primarily, if not exclusively, to signaling via ER. However, several weak estrogens, including plant estrogens and environmental pollutants, were recently shown to bind to ERß with greater affinity than to ER (40). There is also evidence for the inhibitory effects of liganded ERß on the signaling via ER. These findings are potentially very important because they may explain the apparent paradox of significant biological effects of phytoestrogens and environmental estrogenic pollutants that are very weakly estrogenic and are usually ingested in very limited amounts. Current progress in elucidating the actions of estrogens and antiestrogens at the molecular level and in development and study of selective ER modulators (SERMS) raises hope for resolving at least some of the controversies surrounding the possible impact of environmental estrogens on male reproductive health. Of particular concern is the suspected importance of prenatal exposure to environmental estrogens and other endocrine disruptors on development of the male reproductive tract and adult reproductive function (37).

	Paracrine/autocrine regulation of testicular function

Top Introduction Human mutations and genetically... The hypothalamic-pituitary... The GH, insulin-like growth... The inhibin/activin-follistatin... Spermatogenesis Estrogens and testicular... Paracrine/autocrine regulation... Testicular apoptosis Aging and male endocrine... Clinical relevance of the... References

 
For the last 15 yr or so, a deluge of information has emerged on paracrine and autocrine effects within the testis of almost any bioactive molecule detected somewhere in the body (41, 42). These effects have invariably been demonstrated in vitro, often in cocultures of two testicular cell compartments. The confusing picture about importance of these findings has been boosted by the findings that a surprisingly large number of genes are expressed somewhere in the testis, often having their transcripts differently processed from those observed in other organs. This appears to have resulted in stagnation of the research of para/autocrine regulation of the testis, and a novel intratesticular expression or novel in vitro effects of molecule X no longer receive the enthusiasm it would have received some years ago. It is clear that new methodological breakthroughs are needed to make progress, and such methods are now available. The cell-specific and/or inducible gene KO techniques are the likely approach to demonstrate whether the intratesticular production and action of a given molecule is physiologically important or whether it simply represents biological redundancy.

	Testicular apoptosis

Top Introduction Human mutations and genetically... The hypothalamic-pituitary... The GH, insulin-like growth... The inhibin/activin-follistatin... Spermatogenesis Estrogens and testicular... Paracrine/autocrine regulation... Testicular apoptosis Aging and male endocrine... Clinical relevance of the... References

 
It is very well documented that both natural and experimentally induced fluctuations in production of spermatozoa can almost always be traced to alterations in the proportion of germ cells that survive particular stages of spermatogenesis. Loss of germ cells in the testis, similarly to the situation in the ovary, occurs primarily, and perhaps exclusively, via programmed cell death, apoptosis (43, 44). Germ cell apoptosis in the testis increases in response to seasonal or experimentally induced hormonal deprivation, elevated temperature, ischemia, or toxicants known to suppress sperm production, with both H-P-T axis and somatotropic axis (GH and IGF-I) acting as germ cell survival factors. Testicular apoptosis involves the Fas system. Sertoli cells produce Fas ligand (FasL), a proapoptotic factor that binds to Fas, a transmembrane receptor protein on germ cells (45). A recent analysis of the available data led to the conclusion that survival of germ cells may reflect balance of prosurvival and proapoptotic signals (e.g. FasL) originating from the Sertoli cells and concomitant changes in Fas protein in germ cells (45, 46). Studies in mice with targeted disruption of genes involved in the regulation of apoptosis provided evidence that this process is essential for normal sperm production and male fertility (47, 48, 49). Further studies of the Fas system and the regulation of pro- and anti-apoptotic signals in the testis should produce major advances in the understanding of paracrine interactions between the Sertoli cells and the germ cells and may provide new directions for the treatment of male infertility and for development of novel approaches to contraception.

It should be mentioned that apoptotic cell death is also importantly involved in regression of male accessory reproductive glands in response to androgen withdrawal and in the loss of Leydig cells in response to toxicants (46).

	Aging and male endocrine function

Top Introduction Human mutations and genetically... The hypothalamic-pituitary... The GH, insulin-like growth... The inhibin/activin-follistatin... Spermatogenesis Estrogens and testicular... Paracrine/autocrine regulation... Testicular apoptosis Aging and male endocrine... Clinical relevance of the... References

 
The present understanding of the effects of aging on the endocrine function of the testis is derived primarily from the studies in men and in laboratory rats. As recently as 10 yr ago, the mechanisms of reproductive aging in these two species were believed to be fundamentally different. In healthy men, there is a relatively small but detectable gradual decline in testosterone production by the testis, accompanied by increase in testosterone binding globulin levels, leading to significant decline in the levels of free testosterone in peripheral circulation (50). There is also a decline in the amplitude of the diurnal rhythms of plasma testosterone levels (51). These changes are occurring in the presence of increasing gonadotropin levels and thus were assumed to represent a primary defect in function of the Leydig cell (50). In contrast, in male rats, age-related decline in plasma testosterone levels were ascribed to alterations in catecholaminergic transmission in the hypothalamus and decline in GnRH and gonadotropin secretion, and thus representing secondary rather than primary Leydig cell failure (52). In one strain of rats commonly used for aging research, Fischer 344, these changes are amplified by development of Leydig cell adenomas which secrete progesterone and, therefore, suppress gonadotropin release (53).

Research conducted during the last decade resulted in the determination that male reproductive aging is far more complex. Application of novel statistical methods to the analysis of gonadotropin and testosterone levels in serially collected blood samples from young and elderly men revealed numerous age-related changes in function of the HPT axis. These alterations included reduced amplitude of LH pulses apparently due to reduced LHRH stimulatory input, increased LH pulse frequency reflecting reduced negative feedback, and evidence for disrupted temporal relationships between LH and testosterone pulses (54). Synchrony between LH pulses and oscillations in the levels of FSH and PRL was also disrupted in healthy older men (54). These findings indicate existence of primary changes at both hypothalamic and testicular levels in the aging human male.

Many of the recent studies of reproductive aging in the male rat used the Brown Norway rat, a long-lived strain with very low incidence of reproductive tumors or age-related obesity. In old males from this strain, plasma testosterone levels are greatly reduced and there are various indications of primary Leydig cell failure. For example, Zirkin and his colleagues (55) treated old rats with ethane 1, 2-dimethane sulfonate (EDS), a Leydig cell toxin and examined the animals after their testes were repopulated by newly produced Leydig cells. These "young" Leydig cells restored high plasma testosterone levels even though they were functioning under the influence of an old hypothalamic-pituitary system and in the environment of an old testis. Detailed study of LH pulses and hypothalamic control of LH release in Brown Norway rat (56) provided evidence for abnormalities that resemble those in aging men. Thus, reproductive aging in both species apparently involves changes both in the Leydig cells and in the hypothalamic-pituitary control of their function.

Many recent and current studies of male reproductive aging address a very important issue of androgen replacement in the aging males. Interest in this area coincides with development of several novel transdermal and depot injection systems of delivery of testosterone. There is considerable evidence that testosterone therapy can improve libido, potency, muscle function and bone mineral density as well as various aspects of cognitive function in hypogonadal men and in elderly men. However, the opinions concerning risk:benefit ratio of androgen therapy in healthy middle aged and elderly subjects differ widely (57) and much additional work will be needed to achieve consensus in this area. Likewise, the causality between the aging-related decline of testosterone levels and the symptoms of aging remains to be demonstrated.

	Clinical relevance of the information obtained and future perspectives

Top Introduction Human mutations and genetically... The hypothalamic-pituitary... The GH, insulin-like growth... The inhibin/activin-follistatin... Spermatogenesis Estrogens and testicular... Paracrine/autocrine regulation... Testicular apoptosis Aging and male endocrine... Clinical relevance of the... References

 
It is intriguing to speculate how much the novel information on the genetic causes of male infertility will help in the diagnostics and treatment of male infertility. A rare mutation as cause of infertility can be diagnosed, but if there is no specific treatment available, the mere knowledge of pathogenesis may turn out to be very costly for the health care systems. The Y chromosome deletions (58) may be common enough to be diagnosed in men from couples undergoing intracytoplasmic sperm injection (ICSI), to predict whether the father’s infertility is likely to be inherited by the son. Anything less frequent, although possible to diagnose using molecular biological methods, may be too costly for health care systems. What is reassuring is the realistic expectation that the proportion of idiopathic male infertility will be shrinking through advances in molecular diagnostics.

While the role of the classical endocrine regulatory systems in maintaining testicular function has been quite well delineated with an array of gene-modified animals, we still know relatively little about the intracellular signaling systems involved. It is likely that the next generation of transgenics and KOs will concentrate on functions at this level of the hormonal signaling cascade and on detecting pathogenesis of syndromes at this level. For instance, the CREM system seems to be the master switch in effects of FSH on spermatogensis (59), and the C/EBP ß is involved in the downstream cascade of LH action (60). It is likely that novel mutations at the postreceptor level of regulation of gonadal function will be identified as underlying pathogenesis in men with apparent disturbance of gonadotropin action, but without defects in the cognate ligand or receptor.

Our concepts about estrogen action underwent quite dramatic revision upon the recent discovery of another estrogen receptor, ERß. The possibility of existence of another androgen receptor has remained elusive. We still do not understand why androgen action within the testis requires incomparably higher androgen concentrations, if after all the testicular and extratesticular androgen effects are regulated by the same receptor molecule. Determining whether there is another intratesticular androgen receptor, akin to the ERß, remains a challenge for future research.

In andrology, similar to other fields of biomedical research, novel techniques hold promise of progress at an unprecedented pace. Subtractive hybridization and DNA array analyses, combined with advances in cloning individual genes and mapping the whole genome and with the emerging field of proteomics, should make it possible to discover novel regulatory mechanisms and pathways and to elucidate mechanisms of action of hormones, local factors, drugs, and environmental substances that influence testicular function. The issues of statistical analysis and experimental verification of the enormous amounts of new data that can be generated by microarray procedures are certain to be resolved, and this and other powerful novel methods will find their place in both research and diagnostics.

Received February 28, 2001.

	References

Top Introduction Human mutations and genetically... The hypothalamic-pituitary... The GH, insulin-like growth... The inhibin/activin-follistatin... Spermatogenesis Estrogens and testicular... Paracrine/autocrine regulation... Testicular apoptosis Aging and male endocrine... Clinical relevance of the... References

 

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