Deciphering the Roles & Regulation of Estradiol Signaling during Female Mini-Puberty: Insights from Mouse Models

Mini-puberty of infancy is a short developmental phase occurring in humans and other mammals after birth. In females, it corresponds to transient and robust activation of the hypothalamo-pituitary-ovarian (HPO) axis revealed by high levels of gonadotropin hormones, follicular growth, and increased estradiol production by the ovary. The roles of estradiol signaling during this intriguing developmental phase are not yet well known, but accumulating data support the idea that it aids in the implementation of reproductive function. This review aims to provide in-depth information on HPO activity during this particular developmental phase in several mammal species, including humans, and to propose emerging hypotheses on the putative effect of estradiol signaling on the development and function of organs involved in female reproduction.

Sex Maintenance in Mammals

The crucial event in mammalian sexual differentiation occurs at the embryonic stage of sex determination, when the bipotential gonads differentiate as either testes or ovaries, according to the sex chromosome constitution of the embryo, XY or XX, respectively. Once differentiated, testes produce sexual hormones that induce the subsequent differentiation of the male reproductive tract. On the other hand, the lack of masculinizing hormones in XX embryos permits the formation of the female reproductive tract. It was long assumed that once the gonad is differentiated, this developmental decision is irreversible. However, several findings in the last decade have shown that this is not the case and that a continuous sex maintenance is needed. Deletion of Foxl2 in the adult ovary lead to ovary-to-testis transdifferentiation and deletion of either Dmrt1 or Sox9/Sox8 in the adult testis induces the opposite process. In both cases, mutant gonads were genetically reprogrammed, showing that both the male program in ovaries and the female program in testes must be actively repressed throughout the individual's life. In addition to these transcription factors, other genes and molecular pathways have also been shown to be involved in this antagonism. The aim of this review is to provide an overview of the genetic basis of sex maintenance once the gonad is already differentiated.

Sox8 and Sox9 act redundantly for ovarian-to-testicular fate reprogramming in the absence of R-spondin1 in mouse sex reversals

In mammals, testicular differentiation is initiated by transcription factors SRY and SOX9 in XY gonads, and ovarian differentiation involves R-spondin1 (RSPO1) mediated activation of WNT/β-catenin signaling in XX gonads. Accordingly, the absence of RSPO1/Rspo1 in XX humans and mice leads to testicular differentiation and female-to-male sex reversal in a manner that does not requireSry or Sox9 in mice. Here we show that an alternate testis-differentiating factor exists and that this factor is Sox8. Specifically, genetic ablation of Sox8 and Sox9 prevents ovarian-to-testicular reprogramming observed in XX Rspo1 loss-of-function mice. Consequently, Rspo1 Sox8 Sox9 triple mutant gonads developed as atrophied ovaries. Thus, SOX8 alone can compensate for the loss of SOX9 for Sertoli cell differentiation during female-to-male sex reversal.

In humans, mice and other mammals, genetic sex is determined by the combination of sex chromosomes that each individual inherits. Individuals with two X chromosomes (XX) are said to be chromosomally female, while individuals with one X and one Y chromosome (XY) are chromosomally males.

One of the major differences between XX and XY individuals is that they have different types of gonads (the organs that make egg cells or sperm). In mice, for example, before males are born, a gene called Sox9 triggers a cascade of events that result in the gonads developing into testes. In females, on the other hand, another gene called Rspo1 stimulates the gonads to develop into ovaries.

Loss of Sox9 in XY embryos, or Rspo1 in XX embryos, leads to mice developing physical characteristics that do not match their genetic sex, a phenomenon known as sex reversal. For example, in XX female mice lacking Rspo1, cells in the gonads reprogram into testis cells known as Sertoli cells just before birth and form male structures known as testis cords. The gonads of female mice missing both Sox9 and Rspo1 (referred to as “double mutants”) also develop Sertoli cells and testis cords, suggesting another gene may compensate for the loss of Sox9.

Previous studies suggest that a gene known as Sox8, which is closely related to Sox9, may be able to drive sex reversal in female mice. However, it was not clear whether Sox8 is able to stimulate testis to form in female mice in the absence of Sox9.

To address this question, Richardson et al. studied mutant female mice lacking Rspo1, Sox8 and Sox9, known as “triple mutants”. Just before birth, the gonads in the triple mutant mice showed some characteristics of sex reversal but lacked the Sertoli cells found in the double mutant mice. After the mice were born, the gonads of the triple mutant mice developed as rudimentary ovaries without testis cords, unlike the more testis-like gonads found in the double mutant mice.

The findings of Richardson et al. show that Sox8 is able to trigger sex reversal in female mice in the absence of Rspo1 and Sox9. Differences in sexual development in humans affect the appearance of individuals and often cause infertility. Identifying Sox8 and other similar genes in mice may one day help to diagnose people with such conditions and lead to the development of new therapies.

Analysis of DNA methylation differences in gonads of the large yellow croaker

DNA methylation is an essential epigenetic modification that significantly regulates gene expression during development and differentiation. In this study, genome-wide methylation analysis of different gonads of the large yellow croaker was performed using whole-genome bisulfite sequencing (WGBS), which has characterized DNA methylation patterns in gonad tissue and identified candidate regions for future studies. Clustering analysis revealed that male and neomale methylation patterns were close compared to female. Based on KEGG pathway analysis of differentially methylated genes, we obtained signaling pathways related to gonadal development. We further investigated the methylation status of previously reported sex determination genes, and found that these genes showed different methylation status in three types of gonads, which may provide important clues to reveal the sex determination genes in the large yellow croaker. Furthermore, combined with transcriptome analysis, we found 7 sex-related genes in three comparison groups where expression negatively correlated with methylation.

Mouse Gonad Development in the Absence of the Pro-Ovary Factor WNT4 and the Pro-Testis Factor SOX9

The transcription factors SRY and SOX9 and RSPO1/WNT4/β-Catenin signaling act as antagonistic pathways to drive testis and ovary development respectively, from a common gonadal primordium in mouse embryos. In this work, we took advantage of a double knockout mouse model to study gonadal development when Sox9 and Wnt4 are both mutated. We show that the XX gonad mutant for Wnt4 or for both Wnt4 and Sox9 develop as ovotestes, demonstrating that ectopic SOX9 function is not required for the partial female-to-male sex reversal caused by a Wnt4 mutation. Sox9 deletion in XY gonads leads to ovarian development accompanied by ectopic WNT/β-catenin signaling. In XY Sox9 mutant gonads, SRY-positive supporting precursors adopt a female-like identity and develop as pre-granulosa-like cells. This phenotype cannot be fully prevented by the deletion of Wnt4 or Rspo1, indicating that SOX9 is required for the early determination of the male supporting cell identity independently of repressing RSPO1/WNT4/β-Catenin signaling. However, in XY Sox9 Wnt4 double mutant gonads, pre-granulosa cells are not maintained, as they prematurely differentiate as mature granulosa cells and then trans-differentiate into Sertoli-like cells. Together, our results reveal the dynamics of the specific and independent actions of SOX9 and WNT4 during gonadal differentiation: SOX9 is essential in the testis for early specification of male-supporting cells whereas WNT4 functions in the ovary to maintain female-supporting cell identity and inhibit male-specific vascular and steroidogenic cell differentiation.

Dissecting Cell Lineage Specification and Sex Fate Determination in Gonadal Somatic Cells Using Single-Cell Transcriptomics

Sex determination is a unique process that allows the study of multipotent progenitors and their acquisition of sex-specific fates during differentiation of the gonad into a testis or an ovary. Using time series single-cell RNA sequencing (scRNA-seq) on ovarian Nr5a1-GFP⁺ somatic cells during sex determination, we identified a single population of early progenitors giving rise to both pre-granulosa cells and potential steroidogenic precursor cells. By comparing time series single-cell RNA sequencing of XX and XY somatic cells, we provide evidence that gonadal supporting cells are specified from these early progenitors by a non-sex-specific transcriptomic program before pre-granulosa and Sertoli cells acquire their sex-specific identity. In XX and XY steroidogenic precursors, similar transcriptomic profiles underlie the acquisition of cell fate but with XX cells exhibiting a relative delay. Our data provide an important resource, at single-cell resolution, for further interrogation of the molecular and cellular basis of mammalian sex determination.

Effects of Co-Administration of Bone Marrow Stromal Cells and L-Carnitine on The Recovery of Damaged Ovaries by Performing Chemotherapy Model in Rat

Background

L-carnitine (Lc) as a type of flavonoid antioxidants and bone marrow stromal cells (BMSCs) as a type of mesenchymal stem cells may recover damaged ovaries. It seems that Lc has favorable effects on differentiation, increasing lifespan and decreasing apoptosis in BMSCs. The aim of this study was to investigate effects of co-administration of BMSC+Lc on damaged ovaries after creating a chemotherapy model with cyclophosphamide in rats.

Materials and Methods

In this experimental study, cyclophosphamide was intraperitoneally (IP) injected to forty female wistar rats for 14 days, in terms of chemotherapy-induced ovarian destruction. The rats were then randomly divided into four groups: control, Lc, BMSCs and co-administration of BMSC+Lc. Injection of BMSCs into bilateral ovaries and intraperitoneal injection of Lc were performed individually and together. Four weeks later, levels of serum estradiol (E2) and follicle-stimulating hormone (FSH) using enzyme-linked immunosorbent assay (ELISA) kit, number of ovarian follicles at different stages using hematoxylin and eosin (H and E) staining and expression of ovarian Bcl-2 and Bax proteins using western blot were assessed.

Results

Co-administration of BMSC+Lc increased E2 and decreased FSH levels compared to the control group (P<0.001). The number of follicles was higher in the co-administrated group compared to the control group (P<0.001). Co-administration of BMSC+Lc increased Bcl-2 protein level, decreased Bax protein level and increased Bcl-2/Bax ratio (P<0.001).

Conclusion

The effect of co-administration of BMSC+Lc is probably more effective than the effect of their separate administration on the recovery of damaged ovaries by chemotherapy.

Genetic Control of Gonadal Sex Determination and Development

Sex determination is the process by which the bipotential gonads develop as either testes or ovaries. With two distinct potential outcomes, the gonadal primordium offers a unique model for the study of cell fate specification and how distinct cell populations diverge from multipotent progenitors. This review focuses on recent advances in our understanding of the genetic programs and epigenetic mechanisms that regulate gonadal sex determination and the regulation of cell fate commitment in the bipotential gonads. We rely primarily on mouse data to illuminate the complex and dynamic genetic programs controlling cell fate decision and sex-specific cell differentiation during gonadal formation and gonadal sex determination.

Molecular and genetic characterization of partial masculinization in embryonic ovaries grafted into male nude mice

In most of mammalian embryos, gonadal sex differentiation occurs inside the maternal uterus before birth. In several fetal ovarian grafting experiments using male host mice, an experimental switch from the maternal intrauterine to male-host environment gradually induces partial masculinization of the grafted ovaries even under the wild-type genotype. However, either host-derived factors causing or molecular basis underlying this masculinization of the fetal ovaries are not clear. Here, we demonstrate that ectopic appearance of SOX9-positive Sertoli cell-like cells in grafted ovaries was mediated by the testosterone derived from the male host. Neither Sox8 nor Amh activity in the ovarian tissues is essential for such ectopic appearance of SOX9-positive cells. The transcriptome analyses of the grafted ovaries during this masculinization process showed early downregulation of pro-ovarian genes such as Irx3, Nr0b1/Dax1, Emx2, and Fez1/Lzts1 by days 7-10 post-transplantation, and subsequent upregulation of several pro-testis genes, such as Bhlhe40, Egr1/2, Nr4a2, and Zc3h12c by day 20, leading to a partial sex reversal with altered expression profiles in one-third of the total numbers of the sex-dimorphic pre-granulosa and Sertoli cell-specific genes at 12.5 dpc. Our data imply that the paternal testosterone exposure is partially responsible for the sex-reversal expression profiles of certain pro-ovarian and pro-testis genes in the fetal ovaries in a temporally dependent manner.

Bone Marrow Stromal Cells with the Granulocyte Colony-Stimulating Factor in the Management of Chemotherapy-Induced Ovarian Failure in a Rat Model

BACKGROUND

Bone marrow stromal cells (BMSCs), as a type of mesenchymal stem cells, and the granulocyte colony-stimulating factor (G-CSF), as a type of growth factor, may recover damaged ovaries. The aim of the present study was to investigate the effects of the coadministration of BMSCs and the G-CSF on damaged ovaries after creating a chemotherapy model with cyclophosphamide (CTX) in rats.

METHODS

The present study was performed in Semnan, Iran, in the late 2016 and the early 2017. BMSCs were cultured and were confirmed using the CD markers of stromal cells. Forty female Wistar rats were randomly divided into 4 groups. The rats were injected intraperitoneally with CTX for 14 days to induce chemotherapy and ovarian destruction. Then, the BMSCs were injected into bilateral ovaries and the G-CSF was injected intraperitoneally, individually and together. Four weeks later, the number of ovarian follicles using H&E staining, the number of apoptotic granulosa cells using the TUNEL assay, the number of produced oocytes from the ovaries, and the levels of serum E2 and FSH using an ELISA reader were assessed. Statistical analysis was done using one-way ANOVA with SPSS, version 16.0.

RESULTS

The results showed that the effects of the coadministration of 2×10⁶ BMSCs and 70 µg/kg of the G-CSF were significantly more favorable than those in the control group (P<0.001), the BMSC group (P=0.016), and the G-CSF group (P<0.001) on the recovery of damaged ovaries.

CONCLUSION

The efficacy of the coadministration of BMSCs and the G-CSF in the recovery of ovaries damaged by chemotherapy was high by comparison with the administration of either of them separately.

At the Crossroads of Fate-Somatic Cell Lineage Specification in the Fetal Gonad

The reproductive endocrine systems are vastly different between males and females. This sexual dimorphism of the endocrine milieu originates from sex-specific differentiation of the somatic cells in the gonads during fetal life. Most gonadal somatic cells arise from the adrenogonadal primordium. After separation of the adrenal and gonadal primordia, the gonadal somatic cells initiate sex-specific differentiation during gonadal sex determination with the specification of the supporting cell lineages: Sertoli cells in the testis vs granulosa cells in the ovary. The supporting cell lineages then facilitate the differentiation of the steroidogenic cell lineages, Leydig cells in the testis and theca cells in the ovary. Proper differentiation of these cell types defines the somatic cell environment that is essential for germ cell development, hormone production, and establishment of the reproductive tracts. Impairment of lineage specification and function of gonadal somatic cells can lead to disorders of sexual development (DSDs) in humans. Human DSDs and processes for gonadal development have been successfully modeled using genetically modified mouse models. In this review, we focus on the fate decision processes from the initial stage of formation of the adrenogonadal primordium in the embryo to the maintenance of the somatic cell identities in the gonads when they become fully differentiated in adulthood.

CRISPR/Cas9-mediated knock-in of the murine Y chromosomal Sry gene

Mammalian zygote-mediated genome editing via the clustered regularly interspaced short palindromic repeats/CRISPR-associated endonuclease 9 (CRISPR/Cas9) system is widely used to generate genome-modified animals. This system allows for the production of loss-of-function mutations in various Y chromosome genes, including Sry, in mice. Here, we report the establishment of a CRISPR-Cas9-mediated knock-in line of Flag-tag sequences into the Sry locus at the C-terminal coding end of the Y chromosome (Y^Sry-flag). In the F1 and successive generations, all male pups carrying the Y^Sry-flag chromosome had normal testis differentiation and proper spermatogenesis at maturity, enabling complete fertility and the production of viable offspring. To our knowledge, this study is the first to produce a stable Sry knock-in line at the C-terminal region, highlighting a novel approach for examining the significance of amino acid changes at the naive Sry locus in mammals.

Effect of Transplantation of Bone Marrow Stromal Cell- Conditioned Medium on Ovarian Function, Morphology and Cell Death in Cyclophosphamide-Treated Rats

Objective

Although stem cell transplantation has beneficial effects on tissue regeneration, but there are still problems such as high cost and safety issues. Since stem cell therapy is largely dependent on paracrine activity, in this study, utilization of transplantation of bone marrow stromal cells (BMSCs)-secretome instead of the cells, into damaged ovaries was evaluated to overcome the limitations of stem cell transplantation.

Materials and Methods

In this experimental study, BMSCs were cultured and 25-fold concentrated conditioned medium (CM) from BMSCs was prepared. Female rats were injected intraperitoneally with cyclophosphamide (CTX) for 14 days. Then, BMSCs and CM were individually transplanted into bilateral ovaries, and the ovaries were excised after four weeks of treatment. The follicle count was performed using hematoxylin and eosin (H&E) staining and the apoptotic cells were counted using TUNEL assay. Ovarian function was evaluated by monitoring the ability of ovulation and the levels of serum estradiol (E₂) and follicle-stimulating hormone (FSH).

Results

Evaluation of the ovarian function and structure showed that results of secretome transplantation were almost similar to those of BMSCs transplantation and there was no significant differences between them.

Conclusion

BMSCs-secretome is likely responsible for the therapeutic paracrine effect of BMSCs. Stem cell- secretome is expected to overcome the limitations of stem cell transplantation and become the basis of a novel therapy for ovarian damage.

Testis Determination Requires a Specific FGFR2 Isoform to Repress FOXL2

Male sex determination in mammals relies on sex determining region Y-mediated upregulation of sex determining region-box 9 (SOX9) expression in XY gonads, whereas Wnt family member (WNT)/R-spondin 1 signaling and forkhead box L2 (FOXL2) drive female sex determination in XX gonads. Fibroblast growth factor (FGF) 9 signaling ensures sustained SOX9 expression through repression of one of the ovarian pathways (WNT signaling), whereas the significance of FGF-mediated repression of the FOXL2 pathway has not been studied. Previously, we demonstrated that FGFR2 is the receptor for FGF9 in the XY gonad. Whether a specific isoform (FGFR2b or FGFR2c) is required was puzzling. Here, we show that FGFR2c is required for male sex determination. Initially, in developing mouse embryos at 12.5 to 13.5 days postcoitum (dpc), XY Fgfr2c-/- gonads appear as ovotestes, with SOX9 and FOXL2 expression predominantly localized to the posterior and anterior gonadal poles, respectively. However, by 15.5 dpc, XY Fgfr2c-/- gonads show complete male-to-female sex reversal, evident by the lack of SOX9 and ectopic expression of FOXL2 throughout the gonads. Furthermore, ablation of the Foxl2 gene leads to partial or complete rescue of gonadal sex reversal in XY Fgfr2c-/- mice. Together with previous findings, our data suggest that testis determination involves FGFR2c-mediated repression of both the WNT4- and FOXL2-driven ovarian-determining pathways.

Oleic acid induces down-regulation of the granulosa cell identity marker FOXL2, and up-regulation of the Sertoli cell marker SOX9 in bovine granulosa cells

During negative energy balance, the concentration of different fatty acids, especially of oleic acid (OA) increases in the follicular fluid of cattle. Previously, we showed that OA induced morphological, physiological and molecular changes in cultured bovine granulosa cells. In our present study we analyzed effects of OA on the expression of markers for granulosa and Sertoli cell identity, FOXL2 and SOX9, respectively, in addition to effects on the FOXL2 regulated genes ESR2, FST, PTGS2 and PPARG. The results showed that OA down-regulated FOXL2, ESR2, FST and PPARG but up-regulated PTGS2 and SOX9. From these data we conclude that OA can compromise granulosa cell functionality and may initiate trans-differentiation processes in bovine granulosa cells. This novel mechanism may be causally involved in postpartum fertility problems of lactating dairy cows.

Defects in the first wave of folliculogenesis in mouse XO ovaries

In mouse ovaries, the first wave of folliculogenesis perinatally starts near the medullary region, which directs the initial round of follicular growth soon after birth. At the same time, cortical primordial follicles start forming in the ovarian surface region, and then some are cyclically recruited for the second and subsequent rounds of follicular growth. Recent studies suggest different dynamics between the first and subsequent waves of follicular growth in postnatal ovaries. However, the phenotypic differences between these phases remain unclear. Here, we show direct evidence that XO female mice, a murine model for Turner Syndrome, lack the first wave of folliculogenesis. Our histopathological analyses of XX and XO littermates revealed a lack of anti-Müllerian hormone (AMH)-positive primary follicles in the XO ovaries by 4 days post partum (dpp). This loss of first follicles was also confirmed by histological bioassay for SRY-dependent SOX9 inducibility, a specific marker for the first follicular granulosa cells. In contrast, cortical primordial follicles formed properly in XO ovaries, and some of them formed primary and secondary follicles in the subcortical region by 7 dpp. They rapidly developed into late antral follicles, showing similarities to XX littermate ovaries by 21 dpp. These results suggest distinct X-monosomy effects between the first and subsequent waves of follicular growth, highlighting the high susceptibility to elimination of XO oocytes in the first wave of mammalian folliculogenesis.

Sexual Fate Change of XX Germ Cells Caused by the Deletion of SMAD4 and STRA8 Independent of Somatic Sex Reprogramming

The differential programming of sperm and eggs in gonads is a fundamental topic in reproductive biology. Although the sexual fate of germ cells is believed to be determined by signaling factors from sexually differentiated somatic cells in fetal gonads, the molecular mechanism that determines germ cell fate is poorly understood. Herein, we show that mothers against decapentaplegic homolog 4 (SMAD4) in germ cells is required for female-type differentiation. Germ cells in Smad4-deficient ovaries respond to retinoic acid signaling but fail to undergo meiotic prophase I, which coincides with the weaker expression of genes required for follicular formation, indicating that SMAD4 signaling is essential for oocyte differentiation and meiotic progression. Intriguingly, germline-specific deletion of Smad4 in Stra8-null female germ cells resulted in the up-regulation of genes required for male gonocyte differentiation, including Nanos2 and PLZF, suggesting the initiation of male-type differentiation in ovaries. Moreover, our transcriptome analyses of mutant ovaries revealed that the sex change phenotype is achieved without global gene expression changes in somatic cells. Our results demonstrate that SMAD4 and STRA8 are essential factors that regulate the female fate of germ cells.

Double ablation of SMAD4 and STRA8 causes female-to-male switching of XX germ cells without affecting somatic cell fate. This suggests that SMAD4 and STRA8 are essential intrinsic factors that determine the female fate of germ cells, collaborating to suppress expression of male genes.

Mammalian sex depends on a male-specific gene, sex-determining region Y (SRY), which is located on the Y chromosome. Individuals lacking this gene will develop as female. Accordingly, germ cell fate also changes from male to female in the absence of SRY. Therefore, it is thought that somatic cells regulate germ cells to become sperm or oocytes. However, it is largely unknown what factor is responsible for sexual fate determination in germ cells. In fetal ovaries, retinoic acid (RA) initiates STRA8 expression in germ cells and induces meiosis. Female germ cells without STRA8 fail to enter meiosis but still progress to oogenesis and form oocyte-like cells, indicating that RA is not the regulator of oogenesis. Here, we found that female germ cells lacking both SMAD4 and STRA8 (but not a single knockout) develop as male gonocyte-like cells in ovaries, indicating that these two factors work as female germ cell determinants. To our surprise, the sexual fate switch observed in the double knockout ovary is not accompanied by gene expression changes in somatic cells, revealing the unexpected finding that somatic factors controlled by SRY are dispensable for the upregulation of male-specific genes in germ cells.

Geography of follicle formation in the embryonic mouse ovary impacts activation pattern during the first wave of folliculogenesis

During embryonic development, mouse female germ cells enter meiosis in an anterior-to-posterior wave believed to be driven by retinoic acid. It has been proposed that ovarian follicle formation and activation follow the same general wave of meiotic progression; however, the precise anatomic specification of these processes has not been delineated. Here, we created a mouse line using Mvh, Gdf9, and Zp3 promoters to drive distinct temporal expression of three fluorescent proteins in the oocytes and to identify where the first follicle cohort develops. The fluorescent profile revealed that the first growing follicles consistently appeared in a specific region of the ovary, the anterior-dorsal region, which led us to analyze if meiotic onset occurred earlier in the dorsal ovarian region. Surprisingly, in addition to the anterior-to-posterior wave, we observed an early meiotic entry in the ventral region of the ovary. This additional anatomic stratification of meiosis contrasts with the localization of the initial follicle formation and activation in the dorsal region of the ovary. Therefore, our study suggests that the specification of cortical and medullar areas in the ventral and dorsal regions on the ovary, rather than the onset of meiosis, impacts where the first follicle activation event occurs.

SMAD2 and p38 signaling pathways act in concert to determine XY primordial germ cell fate in mice

The sex of primordial germ cells (PGCs) is determined in developing gonads on the basis of cues from somatic cells. In XY gonads, sex-determining region Y (SRY) triggers fibroblast growth factor 9 (FGF9) expression in somatic cells. FGF signaling, together with downstream nodal/activin signaling, promotes male differentiation in XY germ cells by suppressing retinoic acid (RA)-dependent meiotic entry and inducing male-specific genes. However, the mechanism by which nodal/activin signaling regulates XY PGC fate is unknown. We uncovered the roles of SMAD2/3 and p38 MAPK, the putative downstream factors of nodal/activin signaling, in PGC sexual fate decision. We found that conditional deletion of Smad2, but not Smad3, from XY PGCs led to a loss of male-specific gene expression. Moreover, suppression of RA signaling did not rescue male-specific gene expression in Smad2-mutant testes, indicating that SMAD2 signaling promotes male differentiation in a RA-independent manner. By contrast, we found that p38 signaling has an important role in the suppression of RA signaling. The Smad2 deletion did not disrupt the p38 signaling pathway even though Nodal expression was significantly reduced, suggesting that p38 was not regulated by nodal signaling in XY PGCs. Additionally, the inhibition of p38 signaling in the Smad2-mutant testes severely impeded XY PGC differentiation and induced meiosis. In conclusion, we propose a model in which p38 and SMAD2 signaling coordinate to determine the sexual fate of XY PGCs.

A novel Amh-Treck transgenic mouse line allows toxin-dependent loss of supporting cells in gonads

Cell ablation technology is useful for studying specific cell lineages in a developing organ in vivo. Herein, we established a novel anti-Müllerian hormone (AMH)-toxin receptor-mediated cell knockout (Treck) mouse line, in which the diphtheria toxin (DT) receptor was specifically activated in Sertoli and granulosa cells in postnatal testes and ovaries respectively. In the postnatal testes of Amh-Treck transgenic (Tg) male mice, DT injection induced a specific loss of the Sertoli cells in a dose-dependent manner, as well as the specific degeneration of granulosa cells in the primary and secondary follicles caused by DT injection in Tg females. In the testes with depletion of Sertoli cell, germ cells appeared to survive for only several days after DT treatment and rapidly underwent cell degeneration, which led to the accumulation of a large amount of cell debris within the seminiferous tubules by day 10 after DT treatment. Transplantation of exogenous healthy Sertoli cells following DT treatment rescued the germ cell loss in the transplantation sites of the seminiferous epithelia, leading to a partial recovery of the spermatogenesis. These results provide not only in vivo evidence of the crucial role of Sertoli cells in the maintenance of germ cells, but also show that the Amh-Treck Tg line is a useful in vivo model of the function of the supporting cell lineage in developing mammalian gonads.

Development of mammalian ovary

Pre-natal and early post-natal ovarian development has become a field of increasing importance over recent years. The full effects of perturbations of ovarian development on adult fertility, through environmental changes or genetic anomalies, are only now being truly appreciated. Mitigation of these perturbations requires an understanding of the processes involved in the development of the ovary. Herein, we review some recent findings from mice, sheep, and cattle on the key events involved in ovarian development. We discuss the key process of germ cell migration, ovigerous cord formation, meiosis, and follicle formation and activation. We also review the key contributions of mesonephric cells to ovarian development and propose roles for these cells. Finally, we discuss polycystic ovary syndrome, premature ovarian failure, and pre-natal undernutrition; three key areas in which perturbations to ovarian development appear to have major effects on post-natal fertility.