Effects of thyroid hormone on Leydig cell regeneration in the adult rat following ethane dimethane sulphonate treatment

Environmental Exposure to 6:2 Polyfluoroalkyl Phosphate Diester and Impaired Testicular Function in Men

6:2 polyfluoroalkyl phosphate diester (6:2 diPAP) has been demonstrated to disrupt reproductive endocrine functions using experimental studies. However, evidence from humans is not available yet. This cross-sectional study aims to assess the relationship between 6:2 diPAP exposure and the testicular function among adult men. A total of 902 men seeking preconception care were included. Plasma 6:2 diPAP concentrations were determined, while the testicular function was assessed via semen quality and reproductive hormones in serum. The association was assessed by multiple linear regression. Stratified analyses by age and body mass index (BMI) were conducted to assess the potential effect modification by these two variables. Regression analyses revealed that 6:2 diPAP exposure was significantly inversely associated with androgens [i.e., total testosterone (TT) and free androgen index (FAI)], markers of testosterone production potential [i.e., TT/luteinizing hormone (LH) and FAI/LH], estradiol, and insulin-like factor 3, a biomarker of Leydig cell function. These associations were robust in sensitivity analyses. However, age and BMI did not modify these associations, and no association was observed between 6:2 diPAP and semen quality. Our study suggests that exposure to 6:2 diPAP may inhibit androgen synthesis and impair Leydig cell function in adult men.

Differentiation of human adipose derived stem cells into Leydig-like cells with molecular compounds

Leydig cells (LCs) are the primary source of testosterone in the testis, and testosterone deficiency caused by LC functional degeneration can lead to male reproductive dysfunction. LC replacement transplantation is a very promising approach for this disease therapy. Here, we report that human adipose derived stem cells (ADSCs) can be differentiated into Leydig-like cells using a novel differentiation method based on molecular compounds. The isolated human ADSCs expressed positive CD29, CD44, CD59 and CD105, negative CD34, CD45 and HLA-DR using flow cytometry, and had the capacity of adipogenic and osteogenic differentiation. ADSCs derived Leydig-like cells (ADSC-LCs) acquired testosterone synthesis capabilities, and positively expressed LC lineage-specific markers LHCGR, STAR, SCARB1, SF-1, CYP11A1, CYP17A1, HSD3B1 and HSD17B3 as well as negatively expressed ADSC specific markers CD29, CD44, CD59 and CD105. When ADSC-LCs labelled with lipophilic red dye (PKH26) were injected into rat testes which were selectively eliminated endogenous LCs using ethylene dimethanesulfonate (EDS, 75 mg/kg), the transplanted ADSC-LCs could survive and function in the interstitium of testes, and accelerate the recovery of blood testosterone levels and testis weights. These results demonstrated that ADSCs could be differentiated into Leydig-like cells by few defined molecular compounds, which might lay the foundation for further clinical application of ADSC-LC transplantation therapy.

Differentiation of human induced pluripotent stem cells into Leydig-like cells with molecular compounds

Leydig cells (LCs) play crucial roles in producing testosterone, which is critical in the regulation of male reproduction and development. Low levels of testosterone will lead to male hypogonadism. LC transplantation is a promising alternative therapy for male hypogonadism. However, the source of LCs limits this strategy for clinical applications. Thus far, others have reported that LCs can be derived from stem cells by gene transfection, but the safe and effective induction method has not yet been reported. Here, we report that Leydig-like cells can be derived from human induced pluripotent stem cells (iPSCs) using a novel differentiation protocol based on molecular compounds. The iPSCs-derived Leydig-like cells (iPSC-LCs) acquired testosterone synthesis capabilities, had the similar gene expression profiles with LCs, and positively expressed Leydig cell lineage-specific protein markers LHCGR, STAR, SCARB1, SF-1, CYP11A1, HSD3B1, and HSD17B3 as well as negatively expressed iPSC-specific markers NANOG, OCT4, and SOX2. When iPSC-LCs labeled with lipophilic red dye (PKH26) were transplanted into rat testes that were selectively eliminated endogenous LCs using EDS (75 mg/kg), the transplanted iPSC-LCs could survive and function in the interstitium of testes, and accelerate the recovery of serum testosterone levels and testis weights. Collectively, these findings demonstrated that the iPSCs were able to be differentiated into Leydig-like cells by few defined molecular compounds, which may lay the safer groundwork for further clinical application of iPSC-LCs for hypogonadism.

Transplantation of CD51+ Stem Leydig Cells: A New Strategy for the Treatment of Testosterone Deficiency

Stem Leydig cell (SLC) transplantation could provide a new strategy for treating the testosterone deficiency. Our previous study demonstrated that CD51 (also called integrin αv) might be a putative cell surface marker for SLCs, but the physiological function and efficacy of CD51⁺ SLCs treatment remain unclear. Here, we explore the potential therapeutic benefits of CD51⁺ SLCs transplantation and whether these transplanted cells can be regulated by the hypothalamic-pituitary-gonadal (HPG) axis. CD51⁺ cells were isolated from the testes of 12-weeks-old C57BL/6 mice, and we showed that such cells expressed SLC markers and that they were capable of self-renewal, extensive proliferation, and differentiation into multiple mesenchymal cell lineages and LCs in vitro. As a specific cytotoxin that eliminates Leydig cells (LCs) in adult rats, ethane dimethanesulfonate (EDS) was used to ablate LCs before the SLC transplantation. After being transplanted into the testes of EDS-treated rats, the CD51⁺ cells differentiated into mature LCs, and the recipient rats showed a partial recovery of testosterone production and spermatogenesis. Notably, a testosterone analysis revealed a circadian rhythm of testosterone secretion in cell-transplanted rats, and these testosterone secretions could be suppressed by decapeptyl (a luteinizing hormone-releasing hormone agonist), suggesting that the transplanted cells might be regulated by the HPG axis. This study is the first to demonstrate that CD51⁺ SLCs can restore the neuroendocrine regulation of testicular function by physiologically recovering the expected episodic changes in diurnal testosterone serum levels and that SLC transplantation may provide a new tool for the studies of testosterone deficiency treatment. Stem Cells 2017;35:1222-1232.

Differentiation of human umbilical cord mesenchymal stem cells into steroidogenic cells in vitro

Although previous studies have shown that stem cells can be differentiated into Leydig cells by gene transfection, a simple, safe and effective induction method has not yet been reported. Therefore, the present study investigated novel methods for the induction of human umbilical cord mesenchymal stem cell (HUMSC) differentiation into Leydig-like, steroidogenic cells. HUMSCs were acquired using the tissue block culture attachment method, and the expression of MSC surface markers was evaluated by flow cytometry. Leydig cells were obtained by enzymatic digestion and identified by lineage-specific markers via immunofluorescence. Third-passage HUMSCs were cultured with differentiation-inducing medium (DIM) or Leydig cell-conditioned medium (LC-CM), and HUMSCs before induction were used as the control group. Following the induction of HUMSCs, Leydig cell lineage-specific markers (CYP11A1, CYP17A1 and 3β-HSD) were positively identified using immunofluorescence analysis. Additionally, reverse transcription-quantitative polymerase chain reaction and western blot analysis were performed to evaluate the expression levels of these genes and enzymes. In contrast, the control group cells did not show the characteristics of Leydig cells. Collectively, these results indicate that, under in vitro conditions, LC-CM can achieve a comparable effect to that of DIM on inducing HUMSCs differentiation into steroidogenic cells.

Expression of dominant-negative thyroid hormone receptor alpha1 in Leydig and Sertoli cells demonstrates no additional defect compared with expression in Sertoli cells only

BACKGROUND

In the testis, thyroid hormone (T3) regulates the number of gametes produced through its action on Sertoli cell proliferation. However, the role of T3 in the regulation of steroidogenesis is still controversial.

METHODS

The TRαAMI knock-in allele allows the generation of transgenic mice expressing a dominant-negative TRα1 (thyroid receptor α1) isoform restricted to specific target cells after Cre-loxP recombination. Here, we introduced this mutant allele in both Sertoli and Leydig cells using a novel aromatase-iCre (ARO-iCre) line that expresses Cre recombinase under control of the human Cyp19(IIa)/aromatase promoter.

FINDINGS

We showed that loxP recombination induced by this ARO-iCre is restricted to male and female gonads, and is effective in Sertoli and Leydig cells, but not in germ cells. We compared this model with the previous introduction of TRαAMI specifically in Sertoli cells in order to investigate T3 regulation of steroidogenesis. We demonstrated that TRαAMI-ARO males exhibited increased testis weight, increased sperm reserve in adulthood correlated to an increased proliferative index at P3 in vivo, and a loss of T3-response in vitro. Nevertheless, TRαAMI-ARO males showed normal fertility. This phenotype is similar to TRαAMI-SC males. Importantly, plasma testosterone and luteinizing hormone levels, as well as mRNA levels of steroidogenesis enzymes StAR, Cyp11a1 and Cyp17a1 were not affected in TRαAMI-ARO.

CONCLUSIONS/SIGNIFICANCE

We concluded that the presence of a mutant TRαAMI allele in both Leydig and Sertoli cells does not accentuate the phenotype in comparison with its presence in Sertoli cells only. This suggests that direct T3 regulation of steroidogenesis through TRα1 is moderate in Leydig cells, and that Sertoli cells are the main target of T3 action in the testis.

Morphological and functional maturation of Leydig cells: from rodent models to primates

BACKGROUND

Leydig cells (LC) are the sites of testicular androgen production. Development of LC occurs in the testes of most mammalian species as two distinct growth phases, i.e. as fetal and pubertal/adult populations. In primates there are indications of a third neonatal growth phase. LC androgen production begins in embryonic life and is crucial for the intrauterine masculinization of the male fetal genital tract and brain, and continues until birth after which it rapidly declines. A short post-natal phase of LC activity in primates (including human) termed 'mini-puberty' precedes the period of juvenile quiescence. The adult population of LC evolves, depending on species, in mid- to late-prepuberty upon reawakening of the hypothalamic-pituitary-testicular axis, and these cells are responsible for testicular androgen production in adult life, which continues with a slight gradual decline until senescence. This review is an updated comparative analysis of the functional and morphological maturation of LC in model species with special reference to rodents and primates.

METHODS

Pubmed, Scopus, Web of Science and Google Scholar databases were searched between December 2012 and October 2014. Studies published in languages other than English or German were excluded, as were data in abstract form only. Studies available on primates were primarily examined and compared with available data from specific animal models with emphasis on rodents.

RESULTS

Expression of different marker genes in rodents provides evidence that at least two distinct progenitor lineages give rise to the fetal LC (FLC) population, one arising from the coelomic epithelium and the other from specialized vascular-associated cells along the gonad-mesonephros border. There is general agreement that the formation and functioning of the FLC population in rodents is gonadotrophin-responsive but not gonadotrophin-dependent. In contrast, although there is in primates some controversy on the role of gonadotrophins in the formation of the FLC population, there is consensus about the essential role of gonadotrophins in testosterone production. Like the FLC population, adult Leydig cells (ALC) in rodents arise from stem cells, which have their origin in the fetal testis. In contrast, in primates the ALC population is thought to originate from FLC, which undergo several cycles of regression and redifferentiation before giving rise to the mature ALC population, as well as from differentiation of stem cells/precursor cells. Despite this difference in origin, both in primates and rodents the formation of the mature and functionally active ALC population is critically dependent on the pituitary gonadotrophin, LH. From studies on rodents considerable knowledge has emerged on factors that are involved besides LH in the regulation of this developmental process. Whether the same factors also play a role in the development of the mature primate LC population awaits further investigation.

CONCLUSION

Distinct populations of LC develop along the life span of males, including fetal, neonatal (primates) and ALC. Despite differences in the LC lineages of rodents and primates, the end product is a mature population of LC with the main function to provide androgens necessary for the maintenance of spermatogenesis and extra-gonadal androgen actions.

Thyroid and male reproduction

Male reproduction is governed by the classical hypothalamo-hypophyseal testicular axis: Hypothalamic gonadotropin releasing hormone (GnRH), pituitary luteinizing hormone (LH) and follicle stimulating hormone (FSH) and the gonadal steroid, principally, testosterone. Thyroid hormones have been shown to exert a modulatory influence on this axis and consequently the sexual and spermatogenic function of man. This review will examine the modulatory influence of thyroid hormones on male reproduction.

Thyroid hormone stimulates the proliferation of Sertoli cells and single type A spermatogonia in adult zebrafish (Danio rerio) testis

Thyroid hormones participate in regulating growth and homeostatic processes in vertebrates, including development and adult functioning of the reproductive system. Here we report a new stimulatory role of thyroid hormone on the proliferation of Sertoli cells (SCs) and single, type A undifferentiated spermatogonia (A(und)) in adult zebrafish testes. A role for T3 in zebrafish testis is suggested by in situ hybridization studies, which localized thyroid receptor α (thrα) in SCs and the β (thrβ) mRNA in Sertoli and Leydig cells. Using a primary zebrafish testis tissue culture system, the effect of T3 on steroid release, spermatogenesis, and the expression of selected genes was evaluated. Basal steroid release and Leydig cell gene expression did not change in response to T3. However, in the presence of FSH, T3 potentiated gonadotropin-stimulated androgen release as well as androgen receptor (ar) and 17α-hydroxylase/17,20 lyase (cyp17a1) gene expression. Moreover, T3 alone stimulated the proliferation of both SCs and A(und), potentially resulting in newly formed spermatogonial cysts. Additional tissue culture studies demonstrated that Igf3, a new, gonad-specific member of the IGF family, mediated the stimulatory effect of T3 on the proliferation of A(und) and SCs. Finally, T3 induced changes in connexin 43 mRNA levels in the testis, a known T3-responsive gene. Taken together, our studies suggest that T3 expands the population of SCs and A(und) involving Igf signaling and potentiates gonadotropin-stimulated testicular androgen production as well as androgen sensitivity.

Brief maternal exposure of rats to the xenobiotics dibutyl phthalate or diethylstilbestrol alters adult-type Leydig cell development in male offspring

Maternal exposure to estrogenic xenobiotics or phthalates has been implicated in the distortion of early male reproductive development, referred to in humans as the testicular dysgenesis syndrome. It is not known, however, whether such early gestational and/or lactational exposure can influence the later adult-type Leydig cell phenotype. In this study, Sprague-Dawley rats were exposed to dibutyl phthalate (DBP; from gestational day (GD) 14.5 to postnatal day (PND) 6) or diethylstilbestrol (DES; from GD14.5 to GD16.5) during a short gestational/lactational window, and male offspring subsequently analysed for various postnatal testicular parameters. All offspring remained in good health throughout the study. Maternal xenobiotic treatment appeared to modify specific Leydig cell gene expression in male offspring, particularly during the dynamic phase of mid-puberty, with serum INSL3 concentrations showing that these compounds led to a faster attainment of peak values, and a modest acceleration of the pubertal trajectory. Part of this effect appeared to be due to a treatment-specific impact on Leydig cell proliferation during puberty for both xenobiotics. Taken together, these results support the notion that maternal exposure to certain xenobiotics can also influence the development of the adult-type Leydig cell population, possibly through an effect on the Leydig stem cell population.

The endocrine disruptors dibutyl phthalate (DBP) and diethylstilbestrol (DES) influence Leydig cell regeneration following ethane dimethane sulphonate treatment of adult male rats

The manner by which endocrine-disrupting xenobiotics, such as phthalates, can induce changes in the development of the male reproductive system still remains largely unknown. Herein, we have explored the application of ethane dimethane sulphonate (EDS) to eliminate adult-type Leydig cells in the mature rat testis, leading to their regeneration from resident stem cells, as a novel system to investigate the effects of dibutyl phthalate (DBP) and diethylstilbestrol (DES) on adult-type Leydig cell differentiation. The advantage of this model is that one can study adult-type Leydig cell differentiation in vivo divorced from the concomitant endocrine development of puberty. In these preliminary studies, we show that both DBP and/or DES, given for 2 or 4 days following EDS application, indeed affect Leydig cell differentiation in the adult testis, largely by increasing early Leydig cell proliferation and possibly thereby delaying early differentiation. In particular, on day 27 post-EDS, a time-point when the differentiation trajectory appears to be most discriminating, we observe that both DBP and/or DES cause a fourfold increase in Leydig cell density, and a significant increase in the expression of the Leydig cell-specific marker transcripts INSL3, LH receptor, Cyp17a1 and Cyp 11a1. In conclusion, both DBP and DES are able to affect adult-type Leydig cells during their differentiation to cause a significant perturbation in their ultimate functional capacity.

PROLONGED AND TRANSIENT NEONATAL HYPOTHYROIDISM ON LEYDIG CELL DIFFERENTIATION IN THE POSTNATAL RAT TESTIS

Hypothyroidism arrests the differentiation of adult Leydig cells (ALC) in the neonatal rat testis, and transient neonatal hypothyroidism produces a two-fold increase in the ALC numbers in the adult rat testis. We investigated 1) whether prolonged hypothyroidism beyond the neonatal period could continue to arrest the differentiation of the ALC, and 2) to understand how a two-fold increase in the number of ALC is produced in adult rats subjected to transient neonatal hypothyroidism. Three groups of Sprague Dawley rats were used; control, PTU-water group (transiently hypothyroid; added 0.1% propyl thiouracil/PTU to drinking water of lactating mothers at parturition until weaning of pups at day 21, pups were fed regular water thereafter), and PTU group (prolonged hypothyroid; mothers were fed 0.1% PTU in drinking water from parturition until pups were sacrificed at days 28 and 40 (pups had access to solid food after 21 days). Findings showed that PTU treatment continued to arrest ALC differentiation. Withdrawal of the PTU treatment at 21 days resulted in ALC differentiation by two-fold in number in PTU-water rats. Findings on luteinizing hormone (LH)-stimulated androgen secretory capacity per testis in vitro agreed with the morphological data. These results confirmed that 1) thyroid hormone is crucial to the onset of ALC differentiation in the postnatal rat testis, 2) increased numbers of mesenchymal cells present in the hypothyroid testes differentiate into ALC upon withdrawal of the PTU treatment to produce a two-fold number of ALC in adult rats subjected to transient neonatal hypothyroidism (i.e., PTU-water treatment), and 3) numbers of ALC and mesenchymal cells increase with age at a rate of 2:1 during the process of ALC differentiation in testes of control and PTU-water rats.

Androgen receptor roles in spermatogenesis and fertility: lessons from testicular cell-specific androgen receptor knockout mice

Androgens are critical steroid hormones that determine the expression of the male phenotype, including the outward development of secondary sex characteristics as well as the initiation and maintenance of spermatogenesis. Their actions are mediated by the androgen receptor (AR), a member of the nuclear receptor superfamily. AR functions as a ligand-dependent transcription factor, regulating expression of an array of androgen-responsive genes. Androgen and the AR play important roles in male spermatogenesis and fertility. The recent generation and characterization of male total and conditional AR knockout mice from different laboratories demonstrated the necessity of AR signaling for both external and internal male phenotype development. As expected, the male total AR knockout mice exhibited female-typical external appearance (including a vagina with a blind end and a clitoris-like phallus), the testis was located abdominally, and germ cell development was severely disrupted, which was similar to a human complete androgen insensitivity syndrome or testicular feminization mouse. However, the process of spermatogenesis is highly dependent on autocrine and paracrine communication among testicular cell types, and the disruption of AR throughout an experimental animal cannot answer the question about how AR in each type of testicular cell can play roles in the process of spermatogenesis. In this review, we provide new insights by comparing the results of cell-specific AR knockout in germ cells, peritubular myoid cells, Leydig cells, and Sertoli cells mouse models that were generated by different laboratories to see the consequent defects in spermatogenesis due to AR loss in different testicular cell types in spermatogenesis. Briefly, this review summarizes these results as follows: 1) the impact of lacking AR in Sertoli cells mainly affects Sertoli cell functions to support and nurture germ cells, leading to spermatogenesis arrest at the diplotene primary spermatocyte stage prior to the accomplishment of first meiotic division; 2) the impact of lacking AR in Leydig cells mainly affects steroidogenic functions leading to arrest of spermatogenesis at the round spermatid stage; 3) the impact of lacking AR in the smooth muscle cells and peritubular myoid cells in mice results in similar fertility despite decreased sperm output as compared to wild-type controls; and 4) the deletion of AR gene in mouse germ cells does not affect spermatogenesis and male fertility. This review tries to clarify the useful information regarding how androgen/AR functions in individual cells of the testis. The future studies of detailed molecular mechanisms in these in vivo animals with cell-specific AR knockout could possibly lead to useful insights for improvements in the treatment of male infertility, hypogonadism, and testicular dysgenesis syndrome, and in attempts to create safe as well as effective male contraceptive methods.

Chronic hypothyroidism only marginally affects adult-type Leydig cell regeneration after EDS administration

Chronic prenatally induced dietary hypothyroidism delays adult-type Leydig cell development, but does not block this process. Using a chemical model to induce hypothyroidism, it was suggested that development of a new population of Leydig cells was completely inhibited following the addition of the cytotoxic compound ethane-1,2-dimethyl sulphonate (EDS). In this study, we used a dietary approach to induce hypothyroidism and reinvestigated the regeneration of the Leydig cell population following EDS administration. Eighty-four day old euthyroid and chronically hypothyroid rats received an injection of EDS and were killed directly before or at regular intervals up to 77 days after EDS. In some control and hypothyroid animals, the first progenitor-type Leydig cells were observed at day 12 after EDS. At day 16, Leydig cell progenitors were present in all rats. The percentage of proliferating Leydig cells peaked in the euthyroid animals at day 21 after EDS. In the hypothyroid testis such a peak was not observed, although the percentage of proliferating regenerating Leydig cells was significantly higher from days 35 to 56 compared with the controls. This suggested that the wave of Leydig cell proliferation was delayed in the hypothyroid animals as compared with the euthyroid controls. On the day of EDS injection, the Leydig/Sertoli cell ratio was 37% lower in the hypothyroid rats compared with the controls. The Leydig/Sertoli cell ratio remained lower in the EDS-treated hypothyroid animals compared with the controls at all time points investigated. At day 77 after EDS, the Leydig cell population had returned to its pre-treatment size in both groups. Plasma testosterone production was reduced to below detectable levels immediately after EDS injection, and started to increase again on day 16, reaching pre-treatment values on day 21 in both groups. Taken together, severely reduced thyroid hormone levels did not block the regeneration of the adult-type Leydig cell population following EDS, as has been suggested previously.

Leydig cell re-generation and expression of cell signaling molecules in the germ cell-free testis

Leydig cells in the rat testis can be specifically ablated with ethane dimethane sulfonate (EDS) and will subsequently re-generate. In this study, we have characterized Leydig cell re-generation and expression of selected cell-signaling molecules in a germ cell-free model of EDS action. This model offers the advantage that re-generation occurs on a stable background without confounding changes from the regressing and repopulating germ cell population. Adult rats were treated with busulfan to remove the germ cell population and Leydig cells were then ablated with EDS. Testicular testosterone levels declined markedly within 24 h of EDS treatment and started to recover after 8 days. After EDS treatment there were marked declines in levels of Leydig cell-specific mRNA transcripts coding for steroidogenic enzymes cytochrome P450 11a1 (Cyp11a1), cytochrome P450 17a1 (Cyp17a1), 3beta-hydroxysteroid dehydrogenase type 1 (Hsd3b1), 17beta-hydroxysteroid dehydrogenase type 3 (Hsd17b3) and the LH receptor. Levels of all transcripts recovered within 20 days of EDS treatment apart from Hsd17b3, which remained undetectable up to 20 days. Immunohistochemical localization of CYP11A1 during the phase of early Leydig cell re-generation showed that the Leydig cell precursors are spindle-shaped peritubular cells. Studies on factors which may be involved in Leydig cell re-generation showed there were significant but transient increases in platelet-derived growth factor A (Pdgfa), leukemia inhibitory factor (Lif), and neurofilament heavy polypeptide (Nefh) after EDS, while desert hedgehog (Dhh) levels declined sharply but recovered by 3 days. This study shows that the Leydig cell precursors are peritubular cells and that expression of Pdgfa and Lif is increased at the start of the re-generation process when precursor proliferation is likely to be taking place.

The role of thyroid hormone in testicular development and function

Thyroid hormone is a critical regulator of growth, development, and metabolism in virtually all tissues, and altered thyroid status affects many organs and systems. Although for many years testis has been regarded as a thyroid hormone unresponsive organ, it is now evident that thyroid hormone plays an important role in testicular development and function. A considerable amount of data show that thyroid hormone influences steroidogenesis as well as spermatogenesis. The involvement of tri-iodothyronine (T(3)) in the control of Sertoli cell proliferation and functional maturation is widely accepted, as well as its role in postnatal Leydig cell differentiation and steroidogenesis. The presence of thyroid hormone receptors in testicular cells throughout development and in adulthood implies that T(3) may act directly on these cells to bring about its effects. Several recent studies have employed different methodologies and techniques in an attempt to understand the mechanisms underlying thyroid hormone effects on testicular cells. The current review aims at presenting an updated picture of the recent advances made regarding the role of thyroid hormones in male gonadal function.

Alteration in testicular cell components following transiently induced ischaemia in prepubertal bulls

The aim of the present study was to evaluate transient testicular ischaemia (induced using elastrator bands) in Jersey calves on testicular morphology and development. Treatments (at 27 +/- 5 days of age) consisted of control (0 h banding) and banding for 2, 4 or 8 h (n = 4 in each group). After castration (at 60 +/- 5 days of age), the right testis was used for calculation of cell components per testis according to the point-counting method. Bodyweight (59.8 +/- 6.2 kg) and scrotal circumference (SC) at banding (9.1 +/- 0.2 cm) did not differ between groups. Fresh testis weight, scrotal temperature immediately before band removal and daily SC growth were decreased in ischaemic (4 and 8 h) testes compared with controls (P < 0.05). In addition, the number of Sertoli and Leydig cells was significantly reduced in the 8 h ischaemic treatment group (P < 0.05). Transiently induced ischaemia significantly decreased the number of germ cells in the 8 h ischaemic treatment group (13 +/- 5 x 10(6) cells) compared with the 0, 2 and 4 h ischaemic treatment groups (38 +/- 6, 32 +/- 6 and 33 +/- 5 x 10(6) cells, respectively; P < 0.05). These results suggest that transiently induced ischaemia for 8 h significantly decreases the number of germ, Sertoli and Leydig cells in prepubertal testis.

Essential roles of COUP-TFII in Leydig cell differentiation and male fertility

Chicken Ovalbumin Upstream Promoter-Transcription Factor II (COUP-TFII; also known as NR2F2), is an orphan nuclear receptor of the steroid/thyroid hormone receptor superfamily. COUP-TFII-null mice die during the early embryonic development due to angiogenesis and cardiovascular defects. To circumvent the early embryonic lethality and investigate the physiological function of COUP-TFII, we knocked out COUP-TFII gene in a time-specific manner by using a tamoxifen inducible Cre recombinase. The ablation of COUP-TFII during pre-pubertal stages of male development results in infertility, hypogonadism and spermatogenetic arrest. Homozygous adult male mutants are defective in testosterone synthesis, and administration of testosterone could largely rescue the mutant defects. Notably, the rescued results also provide the evidence that the major function of adult Leydig cell is to synthesize testosterone. Further phenotypic analysis reveals that Leydig cell differentiation is arrested at the progenitor cell stage in the testes of null mice. The failure of testosterone to resumption of Leydig cell maturation in the null mice indicates that COUP-TFII itself is essential for this process. In addition, we identify that COUP-TFII plays roles in progenitor Leydig cell formation and early testis organogenesis, as demonstrated by the ablation of COUP-TFII at E18.5. On the other hand, when COUP-TFII is deleted in the adult stage after Leydig cells are well differentiated, there are no obvious defects in reproduction and Leydig cell function. Taken together, these results indicate that COUP-TFII plays a major role in differentiation, but not the maintenance of Leydig cells.

Thyroid hormone receptor alpha 1-beta 1 expression in epididymal epithelium from euthyroid and hypothyroid rats

The objectives of the present work were to assess whether epithelial cells from the different segments of epididymis express TR alpha 1-beta 1 isoforms, to depict its subcellular immunolocalization and to evaluate changes in their expression in rats experimentally submitted to a hypothyroid state by injection of 131I. In euthyroid and hypothyroid groups, TR protein was expressed in epididymal epithelial cells, mainly in the cytoplasmic compartment while only a few one showed a staining in the nucleus as well. A similar TR immunostaining pattern was detected in the different segments of the epididymis. In hypothyroid rats, the number of TR-immunoreactive epithelial cells as well as the intensity of the cytoplasmic staining significantly increased in all sections analyzed. In consonance to the immunocytochemical analysis, the expression of TR alpha 1-beta 1 isoforms, assessed by Western blot revealed significantly higher levels of TR in cytosol compared to the nuclear fractions. Furthermore, TR expression of both alpha 1 and beta 1 isoforms and their mRNA levels were increased by the hypothyroid state. The immuno-electron-microscopy showed specific reaction for TR in principal cells associated with eucromatin, cytosolic matrix and mitochondria. The differences in expression levels assessed in control and thyroidectomized rats ascertain a specific function of TH on this organ.

Differential expression and regulation of integral membrane protein 2b in rat male reproductive tissues

AIM

To examine the expression and regulation of integral membrane protein 2b (Itm2b) in rat male reproductive tissues during sexual maturation and under different treatments by in situ hybridization.

METHODS

Testis, epididymis, and vas deferens were collected on days 1-70 to examine Itm2b expression during sexual maturation. To further examine the regulation of Itm2b, adult rats underwent surgical castration and cryptorchidism. Ethylene dimethane sulfonate and busulfan treatments were carried out to test the regulation of Itm2b after destruction of Leydig cells and germ cells.

RESULTS

In testis, Itm2b expression was moderately detected in the adluminal area of seminiferous cords on days 1-10, and detected at a low level in the spermatogonia on days 20 and 30. The Itm2b level was markedly increased in Leydig cells from day 20 to day 70. In epididymis and vas deferens, Itm2b was detected from neonate to adults, and the signal gradually increased in accordance with sexual maturation. Itm2b expression was significantly downregulated in epididymis and vas deferens of castrated rats, and strongly stimulated when castrated rats were treated with testosterone. Cryptorchidism led to a significant decline of Itm2b expression in testis and caput epididymis. Itm2b expression in epididymis and vas deferens was significantly decreased after the Leydig cells were destroyed by ethylene dimethane sulfonate. Busulfan treatment produced no obvious change in Itm2b expression in epididymis or vas deferens.

CONCLUSION

Our data suggested that Itm2b expression is upregulated by testosterone and might play a role in rat male reproduction.

Effect of methimazole-induced hypothyroidism on adrenal and gonadal functions in male Japanese quail (Coturnix japonica)

To investigate the effect of hypothyroidism on gonadal and adrenal functions in male Japanese quail (Coturnix japonica), hypothyroidism was induced in male adult Japanese quail by daily administration of 2-Mercapto-1-methylimidazole (methimazole) in their drinking water. Four weeks after methimazole treatment, the Japanese quail were sacrificed, and the plasma concentrations of free triiodothyronine (FT3), free thyroxine (FT4), total T3 (TT3), total T4 (TT4), corticosterone, testosterone, LH and immunoreactive (ir) inhibins were measured by radioimmunoassay, the testes and adrenal glands were removed and weighed and the thyroid glands and testes were fixed in 4% paraformaldehyde for histological observation. The results showed that the hypothyroidism induced by methimazole caused a significant decrease in body and testes weight; the plasma levels of FT3, FT4 and TT4 significantly decreased, and the hypothyroid quail possessed a greater number of small follicles and more follicular epithelial cells in the thyroid gland. In addition, hypothyroidism resulted in a significant decrease in the plasma concentrations of corticosterone, LH, testosterone and ir-inhibin. Furthermore, no spermatogenesis was found in the seminiferous tubules of the methimazole treatment groups. These results clearly demonstrate that hypothyroidism caused both gonadal and adrenal disturbances in the adult male Japanese quail.

Expression of insulin-like peptide 3 in the postnatal rat Leydig cell lineage: timing and effects of triiodothyronine-treatment

Fetal (FLC) and adult Leydig cells (ALC) secrete insulin-like peptide 3 (INSL3), which is linked to cryptorchidism in the newborn rat. Its gene regulation appears to be independent of that for most steroidogenic enzymes, and may thus be a marker for other aspects of ALC differentiation. Our study examined the following on INSL3 peptide expression in ALC lineage (i) timing, (ii) which cell stage, and (iii) effects of triiodothyronine (T3). Male Sprague-Dawley (SD) rats of postnatal days (pd) 1, 5, 7-21, 28, 40, 60, and 90 were used for the objectives (i) and (ii). For the objective (iii), control and T3-treated (daily T3 SC, 50 mug/kg bw) SD rats of pd7-16 and 21 were used. INSL3 was immunolocalized in Bouin's-fixed testes. FLC were positive and mesenchymal and Leydig progenitor cells were negative for INSL3 at tested ages. INSL3 in ALC lineage was first detected in newly formed ALC on pd16, although they were present from pd10. The intensity of INSL3 label was greater in ALC of pd40-90. ALC were present in T3-treated testes at pd9, but INSL3 first detected in them was on pd12. While INSL3 in FLC regulates testicular descent, INSL3 in ALC still has no well-defined function. However, its pattern of expression correlates temporally with the development of steroidogenic function and spermatogenesis. Thus, the delay between ALC differentiation and INSL3 expression in them implies that INSL3 in ALC is associated with maturation. The advancement of INSL3 expression in the ALC of T3-treated rats implies that this function is established earlier with T3-treatment.

Changes in the testis seminiferous tubules and interstitium in prepubertal bull calves immunised against inhibin at the time of gonadotropin administration

The aim of the present study was to evaluate the effects of gonadotropin administration at initiation of inhibin passive immunisation in Jersey bull calves (age 27 � 5 days) on testicular morphology and development. Primary treatments consisted of control (keyhole limpet haemocyanin, KLH; n = 9) or immunisation against inhibin (INH; n = 9). Subsets of calves were randomly assigned within primary treatments (TRT) to receive saline ( n = 3 per TRT), follicle-stimulating hormone (FSH; n = 3 per TRT) or gonadotrophin-releasing hormone (GnRH, n = 3 per TRT). The right testis was removed (age 118 � 5 days) to determine volumes of testicular components and cell numbers per testis using stereology. Data were analysed using the MIXED procedure of the SAS program. Antibody titres against inhibin were increased in INH bulls compared with KLH bulls (P < 0.05). In addition, a significant immunisation � hormone treatment interaction was noted for the number of germ cells. Administration of FSH at the time of initial immunisation against inhibin significantly increased the number of germ cells (92.2 � 9 � 106 cells) compared with INH+saline bulls (54.9 � 10 � 106 cells), with INH+GnRH bulls having an intermediate number of cells (64.5 � 9 � 106 cells; P < 0.05). These results suggest that gonadotropin administration at the time of inhibin immunisation increases the number of germ cells in the testis.

Histological changes of the testis and epididymis in adult rats as a result of Ley dig cell destruction after ethane dimethane sulfonate treatment: a morphometric study

AIM

To quantitatively study the histological changes of the testis and epididymis as a result of a drastic reduction of testosterone secretion.

METHODS

Fourteen adult Sprague-Dawley rats were injected intraperitoneally with ethane dimethane sulfonate (EDS, 75 mg/kg) and the same number of animals were injected with normal saline as a control. At days 7 and 12 (after treatment), respectively, half of the animals from each group were killed. The testes and epididymides were removed and tissue blocks embedded in methacrylate resin. The cell number per testis was estimated using the stereological optical disector and some other parameters were obtained using other morphometric methods.

RESULTS

The EDS treatment resulted in an almost complete elimination of Leydig cells but had no effect on the numbers of Sertoli cells per testis. At day 7 after EDS treatment, many elongated spermatids were retained in the seminiferous epithelium and many round spermatids could be seen in the epididymal ducts. At day 12, a looser arrangement of spermatids and spermatocytes became evident, with apparent narrow empty spaces being formed between germ cells in an approximately radial direction towards the tubule lumen; the numbers (per testis) of non-type B spermatogonia and spermatocytes were similar to controls, whereas that of type B spermatogonia increased by 59%, and that of early round, elongating and late elongated spermatids decreased by 37%, 72% and 52%, respectively.

CONCLUSION

The primary spermatogenic lesions following EDS administration were (i) spermiation failure and (ii) detachment of spermatids and spermatocytes associated with impairment in spermiogenesis and meiosis.

Development and function of the adult generation of Leydig cells in mice with Sertoli cell-selective or total ablation of the androgen receptor

It is established that androgens and unidentified Sertoli cell (SC)-derived factors can influence the development of adult Leydig cells (LC) in rodents, but the mechanisms are unclear. We evaluated adult LC development and function in SC-selective androgen receptor (AR) knockout (SCARKO) and complete AR knockout (ARKO) mice. In controls, LC number increased 26-fold and LC size increased by approximately 2-fold between 12 and 140 d of age. LC number in SCARKOs was normal on d 12, but was reduced by more than 40% at later ages, although LC were larger and contained more lipid droplets and mitochondria than control LC by adulthood. ARKO LC number was reduced by up to 83% at all ages compared with controls, and LC size did not increase beyond d 12. Serum LH and testosterone levels and seminal vesicle weights were comparable in adult SCARKOs and controls, whereas LH levels were elevated 8-fold in ARKOs, although testosterone levels appeared normal. Immunohistochemistry and quantitative PCR for LC-specific markers indicated steroidogenic function per LC was probably increased in SCARKOs and reduced in ARKOs. In SCARKOs, insulin-like factor-3 and estrogen sulfotransferase (EST) mRNA expression were unchanged and increased 3-fold, respectively, compared with controls, whereas the expression of both was reduced more than 90% in ARKOs. Changes in EST expression, coupled with reduced platelet-derived growth factor-A expression, are potential causes of altered LC number and function in SCARKOs. These results show that loss of androgen action on SC has major consequences for LC development, and this could be mediated indirectly via platelet-derived growth factor-A and/or estrogens/EST.

Hormonal regulation of Leydig cell proliferation and differentiation in rodent testis: a dynamic interplay between gonadotrophins and testicular factors

Studies over the last few decades have documented that LH is the principal regulator of Leydig cell function. Recent studies indicate that locally produced intratesticular factors are equally important in modulating Leydig cell development and function. In the present review, results of studies on Leydig development and function with rodent models, in conjunction with recent advances in our understanding, are discussed. Studies on Leydig cell development revealed that there are two different waves of proliferation: the first one is independent of LH and the other is dependent on LH. In addition to LH, FSH plays a major role in Leydig cell development and function by modulating the production of Sertoli cell-derived factors. Studies directed towards understanding the oestrogen-mediated inhibition of Leydig cell proliferation revealed that collagen IV-mediated signalling is involved in Leydig cell proliferation and 17beta-oestradiol inhibits this event. Leydig cell proliferation and differentiation is associated with changes in gene expression. Research in this area has identified several genes that are involved in Leydig cell proliferation and differentiation; the possible role of these genes in the context of Leydig cell development are discussed in this review.

Immunohistochemical Study of Intermediate Filaments and Neuroendocrine Marker Expression in Leydig Cells of Laboratory Rodents

The aims of this study were to detect the expression of intermediate filaments and to verify the existence of marker substances for neuronal and neuroendocrine cells within the interstitial Leydig cells of laboratory rodent's testes, such as it has been described in other species. Adult male rats, mice, gerbils, Syrian hamsters and guinea-pigs were used and the localization of the different markers was achieved by the streptoavidin-peroxidase immunohistochemical method. The present study demonstrates in all rodents studied a similar pattern of localization in Leydig cells of intermediate filaments (vimentin, cytokeratin, neurofilament 200 kD and glial fibrillary acidic protein) and other marker substances (S-100, CgA, substance P and neurone-specific enolase), which are typical of neuroendocrine (APUD cells or paraneurones) and glial cells. The expression of these substances, related to neurotransmitters or neurohomones and other proteins characteristic of neuroendocrine cells, could suggest that it is a neural crest derived cell. Although this study provides more evidences about the immunoexpression of neuronal and glial markers in Leydig cells, this fact cannot be related directly to their embryological origin, because the current data support the hypothesis of a mesenchymal origin of the Leydig cells.

Male reproductive function in relation with thyroid alterations

Despite the high prevalence of thyroid diseases in the general population, the impact of the latter on male reproductive function has been the subject of only a few well-controlled clinical studies. Hyperthyroidism appears to cause alterations in the sex steroid hormone metabolism as well as in spermatogenesis and fertility. Sperm motility is mainly affected. These abnormalities reverse after restoration of euthyroidism. The effects of hypothyroidism on male reproduction appear to be more subtle than those of hyperthyroidism and reversible. Severe juvenile hypothyroidism may be associated with precocious puberty. Hypothyroidism in adults is associated with disturbances in the sex steroid hormone metabolism as well as infertility, although available data concerning the latter are scarce. Radioiodine ((131)I) treatment for dfferentiated thyroid cancer may cause transient impairment of testicular function Gonadal damage may be cumulative in those requiring multiple administrations and sperm banking should be considered in such patients.

Expression and Regulation of Lipocalin-Type Prostaglandin D Synthase in Rat Testis and Epididymis1

Lipocalin-type prostaglandin D synthase (L-PGDS), a bifunctional protein, is expressed in the male reproductive organs of many species. However, the expression and regulation of L-PGDS in rat are still uncertain. The present study investigated the regionalization and regulation of L-PGDS expression in rat testis and epididymis by in situ hybridization and immunohistochemistry under the conditions of sexual maturation, castration, and ethylene dimethane sulfonate (EDS) treatments. In sexually mature rats, L-PGDS mRNA was weakly expressed only in the testicular peritubular cells, whereas L-PGDS immunostaining was highly detected in the Leydig cells by Day 70 postpartum. During sexual maturation, L-PGDS mRNA expression was highly detected in the caput, corpus, and cauda of the epididymis 70 days after birth. Compared with normal L-PGDS expression in adult epididymis, both L-PGDS mRNA expression and protein immunostaining were significantly reduced in the caput, corpus, and cauda epididymis after castration. Testosterone propionate treatment induced a significant increase of L-PGDS expression in the epididymis of castrated rats. Compared with adult rat epididymis, L-PGDS mRNA and protein expression was down-regulated after EDS treatment. Testosterone propionate treatment could induce an increase of L-PGDS mRNA and protein expression in the epididymis of EDS-treated rats. In conclusion, both castration and EDS treatments caused a significant decrease of L-PGDS expression in the epididymis, whereas testosterone propionate treatment could induce an increase of L-PGDS expression in the epididymis of both castrated and EDS-treated rats, indicating that L-PGDS expression in the rat epididymis can be up-regulated by testosterone.

Thyroid hormone, glucocorticoids, and prolactin at the nexus of physiology, reproduction, and toxicology

A symposium at the 2003 Annual Meeting of the Society of Toxicology brought together an expert group of endocrinologists to review how non-reproductive hormones can affect the endocrine system. This publication captures the essence of those presentations. Paul Cooke and Denise Holsberger recapitulate the evidence of how thyroid hormones affect male and female reproduction, and reproductive development. Ray Witorsch summarizes the many effects of glucocorticoids on the reproductive system. Finally, Paul Sylvester reviews the mechanism of action of prolactin, and reminds us that this ancient hormone has many functions beyond lactation.

The interrelationships between thyroid dysfunction and hypogonadism in men and boys

Thyroid hormone deficiency affects all tissues of the body, including multiple endocrine changes that alter growth hormone, corticotrophin, glucocorticoids, and gonadal function. Primary hypothyroidism is associated with hypogonadotropic hypogonadism, which is reversible with thyroid hormone replacement therapy. In male children follicle-stimulating hormone (FSH) is elevated and associated with testicular enlargement without virilization. Men with primary hypothyroidism have subnormal responses of luteinizing hormone (LH) to gonadotropin-releasing hormone (GnRH) administration and normal response to human chorionic gonadotropin (hCG). Free testosterone concentrations are reduced in men with primary hypothyroidism and thyroid hormone replacement normalizes free testosterone concentrations. In men with primary hypothyroidism, prolactin is not consistently elevated (except in men and children with longstanding severe primary hypothyroidism), but prolactin declines following thyroid hormone replacement therapy. Thyroid hormone is known to affect sex hormone-binding hormonal globulin (SHBG) concentrations. Men with hyperthyroidism have elevated concentrations of testosterone and SHBG. Thyroid hormone therapy in normal men may also duplicate this elevation. In addition estradiol elevations are observed in men with hyperthyroidism, and gynecomastia is common in them as well. In contrast to patients with primary hypothyroidism, men with hyperthyroidism exhibit hyperresponsiveness of LH to GnRH administration and subnormal responses to hCG. Radioactive iodine therapy (RAI) of men treated for thyroid cancer produces a dose-dependent impairment of spermatogenesis and elevation of FSH up to approximately 2 years. Permanent testicular germ cell damage may occur in men treated with high doses of RAI. RAI commonly increases serum concentrations of FSH and LH while reducing inhibin B levels without affecting serum concentrations of testosterone. Thus, radioiodine therapy transiently impairs both germinal and Leydig cell function that usually recover by 18 months posttherapy.

Cell biology of Leydig cells in the testis

This article reviews results on differentiation, structure, and regulation of Leydig cells in the testes of rodents and men. Two different populations-fetal and adult Leydig cells-can be recognized in rodents. The cells in these two populations are different in ultrastructure, life span, capacity for androgen synthesis, and mechanisms of regulation. A brief survey on the origin, ontogenesis, characterization of precursors, ultrastructure, and functional markers of fetal and adult Leydig cells is presented, followed by an analysis of genes in Leydig cells and the role of luteinizing hormone and its receptor, steroidogenic acute regulatory protein, hydroxysteroid dehydrogenases, androgen and its receptor, anti-Müllerian hormone, estrogens, and thyroid hormones. Various growth factors modulate Leydig cell differentiation, regeneration, and steroidogenic capacity, for example, interleukin 1alpha, transforming growth factor beta, inhibin, insulin-like growth factors I and II, vascular endothelial growth factor, and relaxin-like growth factor. Retinol and retinoic acid increase basal testosterone secretion in adult Leydig cells, but decrease it in fetal Leydig cells. Resident macrophages in the interstitial tissue of the testis are important for differentiation and function of Leydig cells. Apoptosis of Leydig cells is involved in the regulation of Leydig cell number and can be induced by cytotoxins. Characteristics of aging Leydig cells in rodents seem to be species specific. 11beta-hydroxysteroid dehydrogenase protects testosterone synthesis in the Leydig cells of stressed rats. Last, the following aspects of human Leydig cells are briefly described: origin, differentiation, triphasic development, aging changes, pathological changes, and gene mutations leading to infertility.

Reciprocal interaction between seasonal testis and thyroid activity in Zembra Island wild rabbits (Oryctolagus cuniculus): effects of castration, thyroidectomy, temperature, and photoperiod

The reproductive cycle of wild rabbits (Oryctolagus cuniculus) living in Zembra Island (North Tunisia) is dependent on an external factor, the photoperiod: the gonads are inhibited by long days and stimulated by short days or melatonin implants. Here we studied the role of an internal factor, thyroid hormones and the possible thyroid-gonadal interrelationships, in animals captured on Zembra Island and maintained in natural conditions of photoperiod and temperature. We determined the seasonal profile of the thyroid and testis cycles and investigated the effects of castration and thyroidectomy on the seasonal testosterone and thyroxine cycles. Plasma thyroxine and testosterone levels followed similar, parallel seasonal patterns, with a peak in autumn (October) and low values from January to August. In thyroidectomized animals, plasma testosterone levels, although significantly higher than those in controls (P < 0.001), remained low throughout the 13 mo of the experiment, and no testicular reactivation was observed in the fall. In castrated animals, despite the increase in thyroxine concentration in the 3 mo following castration (P < 0.01), plasma thyroxine levels remained low during the 2 yr of the study. We then investigated the combined effects of long days (16L:8D) and moderately high temperature (25 degrees C) on these two endocrine axes. In constant gonado-inhibiting conditions (16L:8D), whether the temperature was kept constantly high or allowed to fluctuate naturally, no reactivation of the thyroid and testicular axes was observed in the fall. In control animals, the peaks of testosterone and thyroxine concentrations observed in September were larger (P < 0.001) than those in animals subjected to the same natural photoperiod conditions but with constantly high temperature. The lower level of autumnal testis stimulation (P < 0.001) in animals maintained in conditions of constant high temperature (25 degrees C) may be attributed to the low thyroxine levels induced by high temperature. These results clearly confirm that the thyroid and testicular cycles display similar seasonal variations and show that the thyroid and gonadal axes are strictly interdependent. This study provides the first demonstration, for a given species, that the seasonal reactivation of gonad activity is controlled by the thyroid, and thyroid activity is controlled by the gonads.

Müllerian-inhibiting substance inhibits rat Leydig cell regeneration after ethylene dimethanesulphonate ablation

The postnatal development of Leydig cell precursors is postulated to be controlled by Sertoli cell secreted factors, which may have a determinative influence on Leydig cell number and function in sexually mature animals. One such hormone, Mullerian inhibiting substance (MIS), has been shown to inhibit DNA synthesis and steroidogenesis in primary Leydig cells and Leydig cell tumor lines. To further delineate the effects of MIS on Leydig cell proliferation and steroidogenesis, we employed the established ethylene dimethanesulphonate (EDS) model of Leydig cell regeneration. Following EDS ablation of differentiated Leydig cells in young adult rats, recombinant MIS or vehicle was delivered by intratesticular injection for 4 days (Days 11-14 after EDS). On Days 15 and 35 after EDS (1 and 21 days post-MIS injections), endocrine function was assessed and testes were collected for stereology, immunohistochemistry, and assessment of proliferation and steroidogenesis. Although serum testosterone and luteinizing hormone (LH) were no different, intratesticular testosterone was higher on Day 35 in MIS-treated animals. At both time points, intratesticular 5alpha-androstan-3alpha,17beta-diol concentrations were much higher than that of testosterone. MIS-treated animals had fewer mesenchymal precursors on Day 15 and fewer differentiated Leydig cells on Day 35 with decreased numbers of BrdU+ nuclei. Apoptotic interstitial cells were observed only in the MIS-treated testes, not in the vehicle-treated group on Day 15. These data suggest that MIS inhibits regeneration of Leydig cells in EDS-treated rats by enhancing apoptotic cell death as well as by decreasing proliferative capacity.

Thyroid hormones: their role in testicular steroidogenesis

Thyroid hormones are important for growth and development of many tissues. Altered thyroid hormone status causes testicular abnormalities. For instance, juvenile hypothyroidism/neonatal transient hypothyroidism induces macroorchidism, increases testicular cell number (Sertoli, Leydig, and germ cells) and daily sperm production. Triiodothyronine (T3) receptors have been identified in sperm, developing germ cells, Sertoli, Leydig, and peritubular cells. T3 stimulates Sertoli cell lactate secretion as well as mRNA expression of inhibin-alpha, androgen receptor, IGF-I, and IGFBP-4. It also inhibits Sertoli cell mRNA expression of Müllerian inhibiting substance (MIS), aromatase, estradiol receptor, and androgen binding protein (ABP) and ABP secretion. T3 directly increases Leydig cell LH receptor numbers and mRNA levels of steroidogenic enzymes and steroidogenic acute regulatory protein. It stimulates basal and LH-induced secretion of progesterone, testosterone, and estradiol by Leydig cells. Steroidogenic factor-1 acts as a mediator for T3-induced Leydig cell steroidogenesis. Although the role of T3 on sperm, germ, and peritubular cells has not yet been completely studied, it is clear that T3 directly regulates Sertoli and Leydig cell functions. Further studies are required to elucidate the direct effect of T3 on sperm, germ, and peritubular cells.

Effects of ethane dimethane sulfonate on the functional structure of the adult rat testis

In ethane dimethane sulfonate (EDS)-treated adult Sprague Dawley rats, Leydig cells (LC) were not present up to 14 days but seen at 21 days. They increased in number thereafter and reached the values of age-matching controls (i.e., 150-day-old untreated) at day 60. Mesenchymal cell number per testis also increased and reached a peak at day 21, and remained at a higher (p<.05) value than the controls at days 28-60. LC were smaller at day 21, but were larger at days 28-60 (compared to untreated 90- and 150-day-old rats) and secreted more testosterone at day 60 compared to both control groups. Testes of treated rats had greater numbers of macrophages (except at day 28) and they were smaller than those in untreated rats and 60-day EDS rats. Immunolabeling studies on 3beta-HSD, 11beta-HSD1, and LH receptor activity and androgen data agreed with morphological findings. The relationship between mesenchymal and LC numbers during LC differentiation following EDS treatment is reminiscent of this process in prepubertal testis. The presence of increased numbers of macrophages in treated testes agreed with the role of macrophages on LC differentiation. The absence of aging signs in LC of 60-day treated rats who were 150 days of age can be attributed at least in part to their newly differentiated status in older rats (i.e., equivalent to pubertal LC and not to aged LC). Larger LC observed in EDS rats at days 28-60 and their increased testosterone secretory capacity at day 60 (compared to controls) are attributed to elevated plasma LH levels and locally produced factors in EDS rats.

Effect of neonatal treatment of rats with potent or weak (environmental) oestrogens, or with a GnRH antagonist, on Leydig cell development and function through puberty into adulthood

This study addressed whether reduced Sertoli cell number or manipulation of the neonatal hormone environment has an influence on final Leydig cell number per testis in the rat, by applying neonatal treatments known to affect these parameters, namely administration of a GnRH antagonist (GnRHa) or diethylstilboestrol (DES, in doses of 10, 1 or 0.1 microg per injection). The effect of treatment with either of two 'environmental oestrogens', bisphenol-A (Bis-A) or octylphenol (OP), was also evaluated. Leydig (3beta-hydroxysteroid dehydrogenase immunopositive) cell development and function (plasma testosterone levels) were studied through puberty into adulthood. Treatment with GnRHa impaired testis growth, Leydig cell (nuclear) volume per testis and testosterone levels during puberty, when compared with controls, but final Leydig cell volume/number in adulthood was comparable with controls. As adult testis weight was reduced by 45% in GnRHa-treated rats, the percentage Leydig cell volume per testis was approximately double (p < 0.01) that in controls, and also at day 35. Testosterone levels in adulthood in GnRHa-treated rats were lower (p < 0.01) than in controls but were within the lower end of the normal range. Treatment with DES caused largely dose-dependent suppression of testis growth, Leydig cell (nuclear) volume per testis and testosterone levels up to day 35. Although by adulthood, Leydig cell volume/number per testis was comparable with controls in DES-treated rats, testosterone levels remained grossly subnormal. Neonatal treatment with either Bis-A or OP had little consistent effect on any of the parameters studied except that both treatments significantly elevated testosterone levels on day 18, as did treatment with DES-0.1 microg. The present findings are interpreted in the context of what is known about the hormonal regulation of Leydig cell development. These lead to the conclusion that final Leydig cell number per testis is not determined by the number of Sertoli cells per testis and appears not to be influenced in any major way by gonadotrophins, androgens or oestrogens in the first 2 weeks of postnatal life. This implies that adult Leydig cell number may be determined prior to birth.

Changes in the testis interstitium of Brown Norway rats with aging and effects of luteinizing and thyroid hormones on the aged testes in enhancing the steroidogenic potential

We tested the possibility of using LH and thyroxine (T(4)) to restore the testicular steroidogenic ability in aged Brown Norway rats. Three-, 6-, 12- (n = 8 per group), and 18-mo-old (n = 32; 3M, 6M, 12M, and 18M, respectively) rats were used. The 18M rats were divided into four groups (n = 8 per group) and implanted subdermally with Alzet mini-osmotic pumps containing saline (control), LH (24 microg/day), T(4) (5 microg/day), and LH+T(4) (24+5 microg/day), respectively, for 4 wk (to 19 mo [19M] of age). Testis volume and absolute volumes of many testicular components were unchanged with advancing age and treatments, except for the blood vessels (occasional thickening), lymphatic space (increased), and Leydig cells (decreased with age but increased to the 3M level with LH and to the 12M level with both T(4) and LH+T(4), respectively). The number of Leydig and connective tissue cells per testis was unchanged with aging and treatments. The number of macrophages was significantly higher in treated rats. The average volume of a Leydig cell was significantly decreased in 12M and 19M control rats. However, LH and LH+T(4) restored it to the 3M level, and T(4) restored to the 12M level. The steroidogenic ability of Leydig cells in vitro decreased when aging from the 3M to the 19M level, LH and T(4) enhanced it to the 12M level, and LH+T(4) raised it to the 3M level. Serum LH was unchanged from 3M to 12M rats, significantly reduced in 19M control rats, and raised above the 3M values with both LH and LH+T(4) treatment and above the 19M (control) values with T(4) treatment; the latter values were lower than the 3M level. Serum T(4) and tri-iodothyronine (T(3)) were highest in 3M and 6M rats and declined in 12M and 19M control rats; the latter group had the lowest levels. In all treated groups, T(4) and T(3) levels were significantly above those of 19M control rats but were lower than those of 3M through 12M rats. Serum testosterone was unchanged from 3M to 12M rats but was reduced in 19M control rats. Both LH and T(4) significantly raised these values above the 19M control levels, but they were still lower than the 3M through 12M levels. Additionally, LH+T(4) significantly raised the serum testosterone levels to those of 12M rats, but these values were significantly lower than those of 3M and 6M rats. These findings show that with 24+5-microg dose of LH+T(4) per day for 4 wk, a 100% recovery of the average volume of a Leydig cell and its steroidogenic ability in vitro and a 73% and 300% restoration of serum testosterone levels compared to 3M and 19M control rats, respectively, could be achieved in aged Brown Norway rats. A 100% reversibility (compared to 3M rats) in serum testosterone levels appears to be possible with adjustments in the LH and T(4) doses in the LH+T(4) treatment.

Type II iodothyronine deiodinase is preferentially expressed in rainbow trout (Oncorhynchus mykiss) liver and gonads

It is well admitted that thyroid hormones (TH) play a role in the development of vertebrates. The major secretory product of the thyroid is a pro-hormone, T(4), which is activated in peripheral tissues by outer ring deiodination to T(3). We have isolated from rainbow trout testis, a full length cDNA encoding type II iodothyronine deiodinase (rtD2). The cDNA was 2410 nucleotides long and coded for a polypeptide of 264 amino acids including a selenocysteine residue. The predicted molecular weight of rtD2 was 29.3 kDa and the isoelectric point 8.71. The deduced amino acids sequence showed 80% identity with Fundulus heteroclitus D2 (fhD2) but only 68-69% identity with rat, mouse, and human D2. The 3' UTR contained a putative selenocysteine insertion sequence (SECIS) similar to that described in human cDNA. The rtD2 gene was isolated and the gene structure was similar to that described in human with two exons separated by a large intron. We studied rtD2 gene expression by Northern blot analysis using total RNA extracted from testis, ovary, and other tissues. We found a high expression of a 3 kb transcript in liver and in gonads. A lower expression was also detected in posterior kidney. In testis, rtD2 mRNA expression was dependent on spermatogenic stages: it increased at the onset of spermatogenesis. Our results show that the structural characteristics of the D2 protein and gene have been highly conserved during evolution. The rtD2 mRNA expression in the gonads suggests that rtD2 may be a key factor regulating local supply of active T(3) during rainbow trout gametogenesis.

Differentiation of the adult Leydig cell population in the postnatal testis

Five main cell types are present in the Leydig cell lineage, namely the mesenchymal precursor cells, progenitor cells, newly formed adult Leydig cells, immature Leydig cells, and mature Leydig cells. Peritubular mesenchymal cells are the precursors to Leydig cells at the onset of Leydig cell differentiation in the prepubertal rat as well as in the adult rat during repopulation of the testis interstitium after ethane dimethane sulfonate (EDS) treatment. Leydig cell differentiation cannot be viewed as a simple process with two distinct phases as previously reported, simply because precursor cell differentiation and Leydig cell mitosis occur concurrently. During development, mesenchymal and Leydig cell numbers increase linearly with an approximate ratio of 1:2, respectively. The onset of precursor cell differentiation into progenitor cells is independent of LH; however, LH is essential for the later stages in the Leydig cell lineage to induce cell proliferation, hypertrophy, and establish the full organelle complement required for the steroidogenic function. Testosterone and estrogen are inhibitory to the onset of precursor cell differentiation, and these hormones produced by the mature Leydig cells may be of importance to inhibit further differentiation of precursor cells to Leydig cells in the adult testis to maintain a constant number of Leydig cells. Once the progenitor cells are formed, androgens are essential for the progenitor cells to differentiate into mature adult Leydig cells. Although early studies have suggested that FSH is required for the differentiation of Leydig cells, more recent studies have shown that FSH is not required in this process. Anti-Müllerian hormone has been suggested as a negative regulator in Leydig cell differentiation, and this concept needs to be further explored to confirm its validity. Insulin-like growth factor I (IGF-I) induces proliferation of immature Leydig cells and is associated with the promotion of the maturation of the immature Leydig cells into mature adult Leydig cells. Transforming growth factor alpha (TGFalpha) is a mitogen for mesenchymal precursor cells. Moreover, both TGFalpha and TGFbeta (to a lesser extent than TGFalpha) stimulate mitosis in Leydig cells in the presence of LH (or hCG). Platelet-derived growth factor-A is an essential factor for the differentiation of adult Leydig cells; however, details of its participation are still not known. Some cytokines secreted by the testicular macrophages are mitogenic to Leydig cells. Moreover, retarded or absence of Leydig cell development has been observed in experimental models with impaired macrophage function. Thyroid hormone is critical to trigger the onset of mesenchymal precursor cell differentiation into Leydig progenitor cells, proliferation of mesenchymal precursors, acceleration of the differentiation of mesenchymal cells into Leydig cell progenitors, and enhance the proliferation of newly formed Leydig cells in the neonatal and EDS-treated adult rat testes.