Hum. Reprod. Advance Access originally published online on May 15, 2007
Human Reproduction 2007 22(7):1885-1892; doi:10.1093/humrep/dem091
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Human fetal testis: source of estrogen and target of estrogen action
1 Univ Paris-Sud, UMR-S 693 Le kremlin Bicêtre Cedex F-94276, France 2 Inserm, U693, Faculté de Médecine Paris Sud, 63 rue Gabriel Péri, Le Kremlin Bicêtre Cedex F-94276, France 3 Centre de Recherches Biomédicales des Cordeliers, Univ Pierre et Marie Curie F-75006, France 4 Department of Pathology, University of Bari, Bari, Italy 5 AP-HP, Service de Génétique Moléculaire, Pharmacogénétique et Hormonologie, CHU Bicêtre F-94276, France 6 AP-HP, Service d'endocrinologie, CHU Bicêtre F-94276, France
7 Correspondance address. Tel: +00 33 1 49 59 67 05; Fax: +00 33 1 49 59 67 32; E-mail: geri.meduri{at}u-psud.fr
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Abstract |
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BACKGROUND: Estrogens are involved in masculine fertility and spermatogenesis. However, little is known about estrogen involvement in human testicular organogenesis. Therefore the aim of this study was to investigate the cellular sources and targets of estrogens and their variations in the human testis during fetal development. Expression profiles of aromatase (CYP19) and estrogen receptors (ER)
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were analysed in human fetal testes at various gestational stages by immunohistochemistry and quantitative RT–PCR. METHODS: Fifty-four archival paraffin-embedded and four frozen fetal testes were studied by immunohistochemistry and real-time PCR. Tissue quality was confirmed by histology and expression of specific functional markers: androgenic enzymes for Leydig cells, anti-Müllerian hormone for Sertoli cells and Steel factor receptor for germ cells.
RESULTS: We demonstrate that the human fetal testes express aromatase and ER simultaneously in Sertoli, Leydig and germ cells but are devoid of ER. Quantification of positive cells indicates a window of protein expression, especially between 13 and 22–24 weeks. Quantitative RT–PCR confirmed that the human fetal testis expresses CYP19 and ER but not ER mRNA.
CONCLUSIONS: Our findings suggest that locally produced estrogens influence human testicular development through autocrine and paracrine mechanisms, most notably during the period of maximal testicular susceptibility to endocrine disruptors.
Key words: Aromatase/estrogen receptors/human fetal testis/steroidogenesis
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Introduction |
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Estrogens have been traditionally considered female hormones but are physiologically important in both sexes and target several male organs (Nilsson and Gustafsson, 2002
; Hess, 2003). Estrogen action is mainly mediated by activation of two specific receptors (estrogen receptors, ER) in target cells, ER and ER, which are highly homologous ligand-inducible transcription factors regulating the expression of specific genes. The synthesis of estrogens from androgenic precursors is catalyzed by the aromatase enzyme complex situated in the endoplasmic reticulum of estrogen-producing cells. The human aromatase is the product of the 18-exons CYP19 gene localized in the 15q21.1 region. CYP19 gene expression in the human is regulated by different tissue-specific promoters upstream of unique first exon encoding 5'-untranslated regions of the aromatase mRNAs, but the 503 amino acids translated protein is the same in all tissues. In the human male the enzyme is expressed in testis, adipose tissue, skin and brain, and also in various fetal tissues (Simpson et al., 1994). Estrogen concentrations in the rete testis fluid of various adult male species including the human are higher than in plasma, indicating the occurrence of testicular estrogen synthesis (O'Donnell et al., 2001; Hess, 2003). Mature Leydig cells are the main source of testicular estrogens in mammals (Hess, 2003), while Sertoli cells of adult men do not express aromatase in significant quantities (Brodie et al., 2001; Turner et al., 2002). Conversely, in neonatal and prepubertal animals, immature Sertoli cells are a source of estrogens; Sertoli cells aromatase expression then declines during maturation (Payne and Youngblood, 1995; Sharpe et al., 2003). Recently, the ability of germ cells and sperm to produce aromatase has been demonstrated in various species including the human (Carreau et al., 2003; Bourguiba et al., 2003). These cells are an additional source of estrogens in the adult testis (Carreau et al., 2003).
Besides being capable of estrogen synthesis, adult testes are also targets of estrogen action. The presence of ER has been well documented in adult testes. ER have been detected in rat and mouse Leydig, Sertoli and germ cells at various stages of maturation. The presence of ER has been reported in rodent Leydig cells (Fisher et al., 1997). ER, but not ER have been described in Sertoli, Leydig and germ cells of mature non-human primates (Sierens et al., 2005), and in the adult man (O'Donnell et al., 2001; Hess, 2003). The results of immunolocalization studies of ER in the adult human testis are conflicting. This receptor, if present, seems restricted to Leydig cells (Pelletier and El-Alfy, 2000).
Few studies are available on ER expression in the fetal testis. In the rodent, ER is expressed in fetal Leydig, Sertoli and germ cells, and ER in interstitial cells. In rare studies concerning the human fetal testis, ER is detected in germ, Sertoli and interstitial cells between 13 and 20 weeks of development, but ER is absent (Takeyama et al., 2001; Gaskell et al., 2003; Sierens et al., 2005).
While estrogen effects on female reproduction are traditionally well known, their actions on the testis have been only recently elucidated after studies on genetically modified mice lacking functional ER (ERKO) or ER (ERKO) (Korach, 1994) or with disruption of the CYP19 gene (ArKO) (Fisher et al., 1998; Robertson et al., 1999). Case studies of men with inactivating mutations of ER or CYP19 genes also suggest a role of estrogens in human masculine fertility (Smith et al., 1994; Morishima et al., 1995; Carani et al., 1997). Few data exist on estrogenic effects on human masculine gonadogenesis. The ability of the human fetal testis to aromatize androgenic precursors is also unclear. We thus decided to examine the expression of both ER and aromatase proteins and mRNAs in the human fetal testis up to the final stages of development to determine whether the human fetal testis is capable of estrogen production as the adult gonad. We also wished to identify the cellular targets of the locally produced hormones and the temporal relationship between susceptibility to hormone action, estrogen production and stages of organogenesis. The use of immunohistochemistry with specific antibodies allowed us to provide evidence for the capacity of estrogen synthesis of the human fetal gonad and to identify the cellular subpopulations involved. We also detected the cellular targets of estrogens and their variations during testicular development. Our immunohistochemical results on protein expression were confirmed by detection of the corresponding RNA messengers by real-time PCR.
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Materials and Methods |
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Tissues
Fifty-four archival formol-fixed paraffin-embedded fetal testes were selected from the organ bank of the Pathology Institute of Bari's University after approval of the local ethical committee. Testes from fetuses with abnormal karyotypes or macroscopic or histological abnormalities were excluded. Normal adult human testes, cycling ovaries, endometria and porcine ovaries were used as positive controls.
Snap-frozen testes from 13, 24, 25 and 35 weeks fetuses obtained from the tissue bank of the Pathology Institute of Bari's University were used for real-time quantitative RT–PCR. Normal frozen human placenta, adult testes and cycling endometria were used as controls. The human samples were from a declared tissue collection according to French bioethical law n° 2004-800.
Antibodies
Eight different antibodies were used: polyclonal anti-P450scc (P450 side chain cleavage; Hanukoglu et al., 1990), anti-3HSD (3-beta-hydroxysteroid-dehydrogenase; Dupont et al., 1990), anti-P450c17 (P450c17 alpha hydroxylase; Kominami et al., 1983), anti-aromatase (Osawa et al., 1987) anti-Anti-Müllerian Hormone (AMH) (Kind gifts respectively of Dr I Hanukoglu, College of Judea and Samaria, Israel; Professor Van-Luu The, Laval University, Canada, Professor S Takemori, Hiroshima University, Japan: Professor Y Osawa, Michigan University, USA and Dr N Di Clemente INSERM U 493 Clamart, France) and anti-c-kit (DakoCytomation, Glostrup, Danemark) at the 1:8000, 1:3000, 1:8000, 1:700, 1:4000 and 1:200 dilutions respectively, and monoclonal anti-ER (clone 1D5 DakoCytomation Glostrup, Danemark) and anti-ER directed against the 1 isoform (MCA 1974, Serotec Ltd, Oxford, UK) at 1:20 and 1:40 dilutions, respectively.
Immunohistochemistry
Briefly, 5µm-thick tissue sections were deparaffinized and rehydrated by successive baths of toluene and graded alcohols and subjected to 15 min microwave antigen retrieval in citrate buffer (pH 6). After cooling and 15 min pre-incubation with a blocking serum (DakoCytomation Glostrup, Danemark), slides were incubated with the primary antibodies overnight at 4°C in a humid chamber. Bound immunoglobulins were revealed with a streptavidin-biotin-peroxydase-aminoethylcarbazole kit (LSAB+, DakoCytomation Glostrup, Danemark) according to the manufacturer's instructions.
Counting of immunostained cells
At least six 40 x power fields were digitized for each sample with an 18.2 Spot software camera (Diagnostic Instruments Inc, MI, USA). The digital images were transferred onto a computer screen and a grid was superimposed using Adobe Photoshop CS2 software. The percentage of stained cells and the intensity of staining of each testicular subpopulation were evaluated blindly by two independent observers.
Real-time RT–PCR
Specific gene expression was quantified by real-time PCR. Total RNA was isolated from frozen samples by TRIZOL reagent (Invitrogen Cergy Pontoise, France) according to the manufacturer's recommendations. RNA concentration was determined by measuring absorbance at A260 and A280 with a ratio of 1.8 or higher. The integrity of RNA was visualized on agarose gel. Total RNA (2 µg) was subjected to DNase I treatment (DNase I Amplification Grade procedure; Invitrogen Cergy Pontoise, France) and then reverse transcribed with 200 units of reverse transcriptase using random hexamer primers (Superscript kit; Invitrogen Cergy Pontoise, France). PCRs were performed with 200 ng cDNA in the presence of qPCRTM Mastermix Plus for SybrTM Green I (Eurogentec, Seraing, Belgium) containing 300 nM of specific primers (Table 1). Real-time PCR was carried out on a ABI 7000 Sequence Detector (Applied Biosystems, Foster City, CA, USA). For standards preparation, amplicons were subcloned into pGEMT-easy plasmid (Invitrogen Cergy Pontoise, France) and sequenced to confirm the identity of each fragment. Standard curves were generated using serial dilutions of linearized standard plasmids. All samples and standards were amplified in duplicate. Amplification of ribosomal 18S was used as internal control for data normalization. Results are mean ± SEM of at least two independent analyses of at least two different reverse transcribed samples. Relative expression of a given gene is expressed as the ratio: attomoles of specific gene/femtomole of 18S.
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Verification of the integrity of tissue samples
The selected fetal testes had a normal morphology with seminiferous tubules constituted by Sertoli and germ cells separated by interstitial tissue including fetal Leydig cells with round nuclei and abundant cytoplasm and peritubular elongated Leydig cell precursors.
As a further validation of the integrity of the fetal testes, each sample was studied by immunocytochemistry with well-characterized antibodies recognizing specific markers of Leydig, Sertoli and germ cells.
The immunoexpression pattern of the steroidogenic enzymes involved in androgen synthesis P450scc, 3HSD and P450c17 (Murray et al., 2000), were used to examine the fetal Leydig cell population. Fetal Sertoli cells integrity was estimated by expression of AMH, a functional marker of Sertoli cell differentiation responsible for Mullerian ducts regression and produced from the 8th week of gestation (Rajpert-De Meyts et al., 1999). C-kit, the Steel factor receptor was used to evaluate germ cells (Robinson et al., 2001). All markers were evaluated semiquantitatively. Samples lacking one or more markers or with an abnormal quantity of immunopositive cells for the fetal age (Murray et al., 2000; Habert et al., 2001) were excluded from the study.
Immunolabelling for the steroidogenic enzymes P450scc (Fig. 1A–D), 3HSD (Fig. 1E–H), P450c17 (Fig. 2A–D) were always localized in the cytoplasm of interstitial cells. Two types of immunopositive cells could be distinguished: polygonal, with round nuclei and abundant cytoplasm corresponding to fetal Leydig cells and peritubular, elongated Leydig cell precursors with smaller nuclei and scanty cytoplasm. Immunolabelling intensity was constant throughout fetal testicular development but the number of immunopositive cells varied (Fig. 3A). At the 14th week of fetal life, 50% of interstitial cells were labelled (Fig. 2A); the number of labelled cells peaked around the 18–19th week (Fig. 2B) and decreased afterwards. The labelled interstitial cells never completely disappeared, however, at 28–35 weeks of age only few immunopositive cells could be observed in a reduced interstitial space between the seminiferous tubules (Fig. 2D). The staining intensity was the same.
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AMH expression was localized to the cytoplasm of intratubular Sertoli cells and constantly found at all fetal ages examined (Figs 2E–H and 3). Germ and Sertoli cells represented about 25 and 65% of the intratubular population, respectively. The counting was performed according to nuclear morphology as the proximity of the cells made it difficult to discriminate the two populations by AMH expression only. C-kit immunoexpression was limited to the membrane of germ cells (Fig. 1I–L). About 25% of the germ cells were immunonegative. This marker was constantly expressed during testicular development, however the number of C-kit positive cells was more abundant between 18 and 22 weeks of age (Fig. 1J and K) and decreased at the end of term (Fig. 1L).
Aromatase expression
Aromatase immunolabelling was localized to the cytoplasm of both interstitial and intratubular cells. Labelled interstitial cells (Figs 2I–L and 3) had round nuclei and polygonal shape or an elongated appearance. The enzyme could be localized in 50% of the interstitial cells of fetal testes from the 13–14th week of life (Figs 2I and 3A). Interstitial aromatase expression increased gradually: 65% of interstitial cells were labelled up to the 18–20th week of fetal life (Figs 2J and 3A). The number of immunopositive cells started decreasing afterwards. At 22 weeks of fetal development, only 50% of the interstitial cells were immunopositive (Figs 2K and 3A), while at 35 weeks aromatase expression ceased almost completely (Fig. 2L). In the intratubular spaces, both Sertoli and germ cells were immunolabelled (Figs 2I-L and 3B), the enzyme was expressed in 50% of the intratubular cells at 14 weeks and in the majority of intratubular cells at 17–18 weeks, then decreased gradually between 18 and 22 weeks (Fig. 2J–K), and more abruptly afterwards. No immunopositive cells were visible at 35 weeks of age (Figs 2L and 3B).
ER and ER expression
ER immunolabelling was localized to the nuclei of both interstitial and intratubular cells (Figs 2M–P and 3). In the interstitium, cells with abundant clear cytoplasm and round nuclei and peritubular cells with an elongated nucleus and scanty cytoplasm were immunopositive. In the intratubular spaces, both Sertoli and germ cells expressed the receptor. The expression (staining intensity and percentage of labelled cells) was almost constant between 14 and 22 weeks of fetal life, and involved about 50% of the interstitial (Fig. 3A) and intratubular cells (Fig. 3B). After 22 weeks of age the immunolabelling declined progressively to disappear completely near term.
In sharp contrast with ER, immunolabelling for ER was constantly negative in every cell of all fetal testes examined from the 13th to the 35th week of development (data not shown).
CYP19, ER and ER mRNA
In order to get a confirmation of the presence of CYP19 and ER mRNA during fetal testis development, mRNA levels of these genes have been determined in four fetal testes at 12, 24, 25 and 35 weeks of age and in two adult (young and aged) testes using real-time PCR (Fig. 4).
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Setting up of standard plasmids allowed us to generate standard curves yielding correlation coefficients > 0.99 and efficiencies of at least 0.91 in all experiments. When real-time PCR products were visualized on 3% agarose gel, 150 bp amplicons for CYP19 (Fig. 4A) and ER
(Fig. 4C) were detected in all the samples of fetal and adult testes.
In the human fetal testes analysed, the relative expression level of CYP19 mRNA was in the same range as that of the aged adult testis but lower than that of the young adult testis and relatively invariable at any age examined (Fig. 4B). ER expression was more pronounced in the fetal gonads of 24 and 25 weeks gestational age than in the 12 and 35 weeks old testes (Fig. 4D).
No transcript for ER was found in any of the gonads studied while the messenger was highly expressed in the endometrium used as positive control (data not shown). These results indicate that ER messengers are not expressed in the human fetal testis, consistent with the absence of immunodetectable ER.
Although the number of our samples is limited due to the rarity of available frozen fetal testicular samples of good quality, our results clearly demonstrate that CYP19 and ER but not ER mRNAs are expressed in the human fetal testis.
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Discussion |
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We demonstrate for the first time by immunohistochemistry, and confirm by quantitative RT–PCR, the presence of protein and mRNA of aromatase in the human fetal testis. Aromatase immunoexpression is observed in fetal Leydig, Sertoli and germ cells from the 13th week of fetal life, and decreases after the 22nd week. The mRNA is found from the 12th to 35th week. We also extend previous reports on ER
localization in the human fetal testis between 13 and 20 weeks (Takeyama et al., 2001; Gaskell et al., 2003), including in our study stages of fetal development up to the 35th week of gestation. The presence of the receptor protein is observed from the 14th to the 22nd week of gestation, while the mRNA is detected in samples from the 12th to the 35th week. ER protein and transcript are not observed in all fetal testes examined.
Our results confirm that the human fetal testis is a target of estrogen action through the preferential activation of the receptor particularly between the 13th and 22nd week. The immunoexpression of both aromatase and ER follows an almost parallel course. In line with previous reports (Murray et al., 2000), we detected a maximal immunoexpression of the enzymes involved in androgen synthesis in Leydig cells between 13 and 22 weeks of gestation. The contemporary high expression of aromatase suggests a significant local estrogen production through aromatization of the available elevated quantity of androgenic precursors. We also observed a simultaneous abundant expression of ER in human fetal germ, Sertoli and Leydig cells. Taken together, these findings suggest direct autocrine and paracrine effects of locally produced estrogens on the human fetal testis mediated through activation of the receptor, especially between 13 and 22 weeks of development.
The window of expression of aromatase suggests a direct or indirect regulation of this enzyme by gonadotropic hormones. FSH has been shown to regulate aromatase levels in the testis of the prepubertal mouse and rat (Dorrington et al., 1978) while FSH and cyclic AMP stimulate aromatase activity in the fetal rat testis (Weniger and Zeis, 1988). LH-induced testosterone enhances CYP19 gene expression in Leydig and germ cells (Bourguiba et al., 2003).
Estrogen action on the adult testis has been the subject of many studies on laboratory animals and some reports about human patients. Studies of genetically modified rodents have demonstrated a positive effect of endogenous estrogens on testicular function. ERKO, but not ERKO, males are infertile through a defect of efferent ducts fluid reabsorption (Korach, 1994; Hess et al., 1997). ArKO males, although initially fertile, develop progressive infertility with spermatogenesis arrest (Fisher et al., 1998). While phenotypes of ERKO and ArKO males are indicative of critical roles of estrogens on fertility in adult rodents, mediated through ER activation, the functions of estrogens on human masculine fertility are not completely elucidated. The limited observations on testicular function in adult humans with a disruption of the ER gene (Smith et al., 1994) or incapable of estrogen synthesis secondary to congenital aromatase deficiency (Morishima et al., 1995; Carani et al., 1997), have not given clear-cut indications. The available data however suggest that estrogens are required for normal fertility in man.
Reports about the actions of endogenous estrogens on the fetal testis are rare. So far most studies on the estrogenic effects on testicular organogenesis are based on the administration of large doses of exogenous estrogens or estrogen-like compounds to animals and show a deleterious effect on fertility, with a decrease of testicular growth and sperm production. Some hypotheses can be formulated about the physiological roles of endogenous estrogens on human testicular development. During testicular organogenesis the number of fetal Leydig, Sertoli and germ cells is strictly regulated through a balance of proliferative and apoptotic stimuli (Habert et al., 2001). Proliferation in the human fetal testis is strong at 13–14 weeks and declines at 18–19 weeks (Murray et al., 2000) while apoptotic cells are rare at week 16, increase at mid-gestation and persist until term (Ketola et al., 2003).
Human Leydig cells start differentiating from mesenchymal precursors at 8 weeks of age, proliferate between the 12th and 18 th week and decrease by apoptosis after the 19–20th week of fetal life till only rare cells remain at the end of term (Murray et al., 2000; Habert et al., 2001). Estrogens block the proliferation of precursor Leydig cells (Abney and Myers, 1991) and modulate Leydig cell steroidogenesis (Hsueh et al., 1978). Between the 13th and 22nd week of gestation locally produced estrogens might thus participate to a regulatory loop controlling precursor Leydig cell proliferation and differentiation thereby regulating testosterone synthesis, possibly through changes in Leydig cell responsiveness to LH (Huhtaniemi et al., 1980).
Sertoli cell growth in the human follows two proliferation waves, during testicular development and before puberty (Plant and Marshall, 2001). The number of Sertoli cells in the human fetal testis decreases with gestational age (Helal et al., 2002). After a period of proliferation, Sertoli cells divide less and, from the second trimester to term, decrease by apoptosis (Ketola et al., 2003). While high doses of estrogens are deleterious, low doses increase Sertoli cell proliferation (Atanassova et al., 2005). Estrogens also enhance immature Sertoli cell biosynthesis of adhesion proteins such as N-Cadherin (MacCalman et al., 1997) and increase ER expression in the mouse Sertoli cell line SK11 (Sneddon et al., 2005). Thus estrogens could induce fetal Sertoli cell proliferation, at the same time upregulating ER expression and enhancing Sertoli-germ cell adhesion. The cessation of estrogen production in the human fetal testis coincides with the observed decrease in Sertoli cells.
The effects of estrogens on spermatogenesis have been reported in several contradictory studies. In cultures of whole testis explants estrogens exert deleterious effects on perinatal mouse germ cells through ER activation (Delbes et al., 2004). However, estrogens induce proliferation of immature rat testis gonocytes (Li et al., 1997), at low concentrations inhibit adult human male germ cell apoptosis (Huhtaniemi et al., 1980) and act synergistically with FSH to increase spermatogenesis in the immature rat testis (Kula et al., 2001). Endogenous estrogens could therefore directly stimulate gonocyte proliferation in association with gonadotropins or local factors. At cessation of local estrogen effects, germ cell apoptosis would increase and proliferation decrease. In mice germ cell apoptosis is a requirement for fertility: failure to maintain a proper ratio between Sertoli and germ cells causes infertility (Rodriguez et al., 1997; Russell et al., 2002)
A higher level of complexity of estrogenic effects on fetal germ cells is added by previous findings on the differential distribution in the human fetal testis of two spliced variants of ER: 1 and 2 with different hormone affinity; the ER2 isoform seems to act as a suppressor of ER and ER1-induced transactivation. The ER2 isoform predominates in fetal gonocytes between 13 and 18 weeks and could prevent estrogen action on fetal germ cells (Gaskell et al., 2003). Further studies about the presence of the variant forms in the human fetal testis at late stages of development and their relationship to aromatase expression are warranted to clarify this topic. Recent studies have also reported that in different tissues ER does not necessarily require estrogen as a ligand but can be activated, through binding to an ERE, by the short lived androgen metabolite 5-alpha-androstane-3-beta-17-beta-diol (3-beta-diol) (Weihua et al., 2002; Pak et al., 2005; Picciarelli-Lima et al., 2006).
This alternative role of ER as a mediator of androgenic action suggests a possible synergic effect of locally produced estrogens and androgens on testicular organogenesis mediated by this receptor between 13–14 and 22–25 weeks. This hypothesis would confirm the particular importance of the ER rather than the ER in the human masculine gonad.
In conclusion, our results demonstrate an important intratesticular production of aromatase during fetal life and confirm the importance of estrogens on masculine fertility starting early during organogenesis. The main cellular subpopulations of the human fetal testis are both source and target of estrogenic action through ER activation especially between 13 and 22–24 weeks of development. The period of maximal estrogen synthesis and expression of the receptor suggests a modulating action on the effects of gonadotropins and locally produced hormones and growth factors on the proliferation and differentiation of Leydig, Sertoli and germ cells. Local estrogen action on the developing human testis could be a prerequisite for normal gonadal function in man.
The exclusive expression of ER in the developing human testis throughout gestation underlines an important difference between rodents, in whom ER is needed for testicular development, and man, and suggests caution in extrapolating results in rodent models to human pathophysiology. A better comprehension of hormone effects on human testicular development could elucidate some antenatal causes of masculine infertility. The discovery of a window of maximal testicular susceptibility to estrogens could also have important implications in the studies of the effects of endocrine disruptors on human testicular development and function.
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Submitted on January 4, 2007; resubmitted on March 1, 2007; accepted on March 7, 2007.
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