SRY, SOX9, and DAX1 expression patterns during human sex determination and gonadal development

Etiology of Male Infertility: an Update

Spermatogenesis is a complex process of germ cell division and differentiation that involves extensive cross-talk between the developing germ cells and the somatic testicular cells. Defective endocrine signaling and/or intrinsic defects within the testes can adversely affect spermatogenic progression, leading to subfertility/infertility. In recent years, male infertility has been recognized as a global public health concern, and research over the last few decades has elucidated the complex etiology of male infertility. Congenital reproductive abnormalities, genetic mutations, and endocrine/metabolic dysfunction have been demonstrated to be involved in infertility/subfertility in males. Furthermore, acquired factors like exposure to environmental toxicants and lifestyle-related disorders such as illicit use of psychoactive drugs have been shown to adversely affect spermatogenesis. Despite the large body of available scientific literature on the etiology of male infertility, a substantial proportion of infertility cases are idiopathic in nature, with no known cause. The inability to treat such idiopathic cases stems from poor knowledge about the complex regulation of spermatogenesis. Emerging scientific evidence indicates that defective functioning of testicular Sertoli cells (Sc) may be an underlying cause of infertility/subfertility in males. Sc plays an indispensable role in regulating spermatogenesis, and impaired functional maturation of Sc has been shown to affect fertility in animal models as well as humans, suggesting abnormal Sc as a potential underlying cause of reproductive insufficiency/failure in such cases of unexplained infertility. This review summarizes the major causes of infertility/subfertility in males, with an emphasis on infertility due to dysregulated Sc function.

A conserved NR5A1-responsive enhancer regulates SRY in testis-determination

The Y-linked SRY gene initiates mammalian testis-determination. However, how the expression of SRY is regulated remains elusive. Here, we demonstrate that a conserved steroidogenic factor-1 (SF-1)/NR5A1 binding enhancer is required for appropriate SRY expression to initiate testis-determination in humans. Comparative sequence analysis of SRY 5' regions in mammals identified an evolutionary conserved SF-1/NR5A1-binding motif within a 250 bp region of open chromatin located 5 kilobases upstream of the SRY transcription start site. Genomic analysis of 46,XY individuals with disrupted testis-determination, including a large multigenerational family, identified unique single-base substitutions of highly conserved residues within the SF-1/NR5A1-binding element. In silico modelling and in vitro assays demonstrate the enhancer properties of the NR5A1 motif. Deletion of this hemizygous element by genome-editing, in a novel in vitro cellular model recapitulating human Sertoli cell formation, resulted in a significant reduction in expression of SRY. Therefore, human NR5A1 acts as a regulatory switch between testis and ovary development by upregulating SRY expression, a role that may predate the eutherian radiation. We show that disruption of an enhancer can phenocopy variants in the coding regions of SRY that cause human testis dysgenesis. Since disease causing variants in enhancers are currently rare, the regulation of gene expression in testis-determination offers a paradigm to define enhancer activity in a key developmental process.

Isolation and In Vitro Culture of Germ Cells and Sertoli Cells from Human Fetal Testis

In the human fetal testis, fetal germ cells (FGCs) are progressively surrounded by supporting Sertoli cells inside seminiferous cords. During the second trimester, the FGCs develop asynchronously and can be observed in several stages of development. However, the mechanism that regulates the transition between the different developmental stages as well as the formation of spermatogonia is currently not well understood. For this, it is necessary to develop suitable isolation protocols and a platform for in vitro culture of FGCs of different stages. Here, we report a method to isolate distinct populations of FGCs and Sertoli cells from second trimester human testis using a panel of conjugated antibodies for THY1, PDPN, ALPL, KIT, and SUSD2 for fluorescence-activated cell sorting (FACS) followed by in vitro culture up to 7 days. This platform provides the base for cellular and molecular characterization of the different testicular cell populations to investigate the transition between FGCs and spermatogonia and shed some light on crucial processes of early human gametogenesis unknown until now.

The developmental dynamics of the human male germline

Male germ cells undergo a complex sequence of developmental events throughout fetal and postnatal life that culminate in the formation of haploid gametes: the spermatozoa. Errors in these processes result in infertility and congenital abnormalities in offspring. Male germ cell development starts when pluripotent cells undergo specification to sexually uncommitted primordial germ cells, which act as precursors of both oocytes and spermatozoa. Male-specific development subsequently occurs in the fetal testes, resulting in the formation of spermatogonial stem cells: the foundational stem cells responsible for lifelong generation of spermatozoa. Although deciphering such developmental processes is challenging in humans, recent studies using various models and single-cell sequencing approaches have shed new insight into human male germ cell development. Here, we provide an overview of cellular, signaling and epigenetic cascades of events accompanying male gametogenesis, highlighting conserved features and the differences between humans and other model organisms.

Variants in SART3 cause a spliceosomopathy characterised by failure of testis development and neuronal defects

Squamous cell carcinoma antigen recognized by T cells 3 (SART3) is an RNA-binding protein with numerous biological functions including recycling small nuclear RNAs to the spliceosome. Here, we identify recessive variants in SART3 in nine individuals presenting with intellectual disability, global developmental delay and a subset of brain anomalies, together with gonadal dysgenesis in 46,XY individuals. Knockdown of the Drosophila orthologue of SART3 reveals a conserved role in testicular and neuronal development. Human induced pluripotent stem cells carrying patient variants in SART3 show disruption to multiple signalling pathways, upregulation of spliceosome components and demonstrate aberrant gonadal and neuronal differentiation in vitro. Collectively, these findings suggest that bi-allelic SART3 variants underlie a spliceosomopathy which we tentatively propose be termed INDYGON syndrome (Intellectual disability, Neurodevelopmental defects and Developmental delay with 46,XY GONadal dysgenesis). Our findings will enable additional diagnoses and improved outcomes for individuals born with this condition.

Gene expression of male pathway genes sox9 and amh during early sex differentiation in a reptile departs from the classical amniote model

BACKGROUND

Sex determination is the process whereby the bipotential embryonic gonads become committed to differentiate into testes or ovaries. In genetic sex determination (GSD), the sex determining trigger is encoded by a gene on the sex chromosomes, which activates a network of downstream genes; in mammals these include SOX9, AMH and DMRT1 in the male pathway, and FOXL2 in the female pathway. Although mammalian and avian GSD systems have been well studied, few data are available for reptilian GSD systems.

RESULTS

We conducted an unbiased transcriptome-wide analysis of gonad development throughout differentiation in central bearded dragon (Pogona vitticeps) embryos with GSD. We found that sex differentiation of transcriptomic profiles occurs at a very early stage, before the gonad consolidates as a body distinct from the gonad-kidney complex. The male pathway genes dmrt1 and amh and the female pathway gene foxl2 play a key role in early sex differentiation in P. vitticeps, but the central player of the mammalian male trajectory, sox9, is not differentially expressed in P. vitticeps at the bipotential stage. The most striking difference from GSD systems of other amniotes is the high expression of the male pathway genes amh and sox9 in female gonads during development. We propose that a default male trajectory progresses if not repressed by a W-linked dominant gene that tips the balance of gene expression towards the female trajectory. Further, weighted gene expression correlation network analysis revealed novel candidates for male and female sex differentiation.

CONCLUSION

Our data reveal that interpretation of putative mechanisms of GSD in reptiles cannot solely depend on lessons drawn from mammals.

Nuclear Receptor Gene Variants Underlying Disorders/Differences of Sex Development through Abnormal Testicular Development

Gonadal development is the first step in human reproduction. Aberrant gonadal development during the fetal period is a major cause of disorders/differences of sex development (DSD). To date, pathogenic variants of three nuclear receptor genes (NR5A1, NR0B1, and NR2F2) have been reported to cause DSD via atypical testicular development. In this review article, we describe the clinical significance of the NR5A1 variants as the cause of DSD and introduce novel findings from recent studies. NR5A1 variants are associated with 46,XY DSD and 46,XX testicular/ovotesticular DSD. Notably, both 46,XX DSD and 46,XY DSD caused by the NR5A1 variants show remarkable phenotypic variability, to which digenic/oligogenic inheritances potentially contribute. Additionally, we discuss the roles of NR0B1 and NR2F2 in the etiology of DSD. NR0B1 acts as an anti-testicular gene. Duplications containing NR0B1 result in 46,XY DSD, whereas deletions encompassing NR0B1 can underlie 46,XX testicular/ovotesticular DSD. NR2F2 has recently been reported as a causative gene for 46,XX testicular/ovotesticular DSD and possibly for 46,XY DSD, although the role of NR2F2 in gonadal development is unclear. The knowledge about these three nuclear receptors provides novel insights into the molecular networks involved in the gonadal development in human fetuses.

Genetic control of typical and atypical sex development

Sex development relies on the sex-specific action of gene networks to differentiate the bipotential gonads of the growing fetus into testis or ovaries, followed by the differentiation of internal and external genitalia depending on the presence or absence of hormones. Differences in sex development (DSD) arise from congenital alterations during any of these processes, and are classified depending on sex chromosomal constitution as sex chromosome DSD, 46,XY DSD or 46,XX DSD. Understanding the genetics and embryology of typical and atypical sex development is essential for diagnosing, treating and managing DSD. Advances have been made in understanding the genetic causes of DSD over the past 10 years, especially for 46,XY DSD. Additional information is required to better understand ovarian and female development and to identify further genetic causes of 46,XX DSD, besides congenital adrenal hyperplasia. Ongoing research is focused on the discovery of further genes related to typical and atypical sex development and, therefore, on improving diagnosis of DSD.

Development of the human ovary: Fetal through pubertal ovarian morphology, folliculogenesis and expression of cellular differentiation markers

A definition of normal human fetal and early postnatal ovarian development is critical to the ability to accurately diagnose the presence or absence of functional ovarian tissue in clinical specimens. Through assembling an extensive histologic and immunohistochemical developmental ontogeny of human ovarian specimens from 8 weeks of gestation through 16 years of postnatal, we present a comprehensive immunohistochemical mapping of normal protein expression patterns in the early fetal through post-pubertal human ovary and detail a specific expression-based definition of the early stages of follicular development. Normal fetal and postnatal ovarian tissue is defined by the presence of follicular structures and characteristic immunohistochemical staining patterns, including granulosa cells expressing Forkhead Box Protein L2 (FOXL2). However, the current standard array of immunohistochemical markers poorly defines ovarian stromal tissue, and additional work is needed to identify new markers to advance our ability to accurately identify ovarian stromal components in gonadal specimens from patients with disorders of sexual differentiation.

Ontogeny of mouse Sertoli, Leydig and peritubular myoid cells from embryonic day 10 to adulthood

We present a comprehensive description of the differentiating somatic cell types (Sertoli, Leydig, and peritubular myoid cells) of the mouse testis from embryonic day 10.5 (E10.5) to adulthood, postnatal day 60 (P60). Immunohistochemistry was used to analyze expression of: Sox9 (a Sertoli cell marker), 3βHSD-1 (a fetal Leydig cell marker), 3βHSD-6 (an adult Leydig cell marker), α-actin (a peritubular myoid cell marker), and androgen receptor (a marker of all three somatic cell types). The temporal-spatial expression of these markers was used to interrogate findings of earlier experimental studies on the origin of Sertoli, Leydig and peritubular myoid cells, as well as extend previous descriptive studies across a broader developmental period (E10.5-P60). Such comparisons demonstrate inconsistencies that require further examination and raise questions regarding conservation of developmental mechanisms across higher vertebrate species.

Overview of single-cell RNA sequencing analysis and its application to spermatogenesis research

Background

Single-cell transcriptomics allows parallel analysis of multiple cell types in tissues. Because testes comprise somatic cells and germ cells at various stages of spermatogenesis, single-cell RNA sequencing is a powerful tool for investigating the complex process of spermatogenesis. However, single-cell RNA sequencing analysis needs extensive knowledge of experimental technologies and bioinformatics, making it difficult for many, particularly experimental biologists and clinicians, to use it.

Methods

Aiming to make single-cell RNA sequencing analysis familiar, this review article presents an overview of experimental and computational methods for single-cell RNA sequencing analysis with a history of transcriptomics. In addition, combining the PubMed search and manual curation, this review also provides a summary of recent novel insights into human and mouse spermatogenesis obtained using single-cell RNA sequencing analyses.

Main Findings

Single-cell RNA sequencing identified mesenchymal cells and type II innate lymphoid cells as novel testicular cell types in the adult mouse testes, as well as detailed subtypes of germ cells. This review outlines recent discoveries into germ cell development and subtypes, somatic cell development, and cell-cell interactions.

Conclusion

The findings on spermatogenesis obtained using single-cell RNA sequencing may contribute to a deeper understanding of spermatogenesis and provide new directions for male fertility therapy.

Fetal germ cell development in humans, a link with infertility

Gametes are cells that have the unique ability to give rise to new individuals as well as transmit (epi)genetic information across generations. Generation of functionally competent gametes, oocytes and sperm cells, depends to some extent on several fundamental processes that occur during fetal development. Direct studies on human fetal germ cells remain hindered by ethical considerations and inaccessibility to human fetal material. Therefore, the majority of our current knowledge of germ cell development still comes from an invaluable body of research performed using different mammalian species. During the last decade, our understanding of human fetal germ cells has increased due to the successful use of human pluripotent stem cells to model aspects of human early gametogenesis and advancements on single-cell omics. Together, this has contributed to determine the cell types and associated molecular signatures in the developing human gonads. In this review, we will put in perspective the knowledge obtained from several mammalian models (mouse, monkey, pig). Moreover, we will discuss the main events during human fetal (female) early gametogenesis and how the dysregulation of this highly complex and lengthy process can link to infertility later in life.

Germline stem cells in human

The germline cells are essential for the propagation of human beings, thus essential for the survival of mankind. The germline stem cells, as a unique cell type, generate various states of germ stem cells and then differentiate into specialized cells, spermatozoa and ova, for producing offspring, while self-renew to generate more stem cells. Abnormal development of germline stem cells often causes severe diseases in humans, including infertility and cancer. Primordial germ cells (PGCs) first emerge during early embryonic development, migrate into the gentile ridge, and then join in the formation of gonads. In males, they differentiate into spermatogonial stem cells, which give rise to spermatozoa via meiosis from the onset of puberty, while in females, the female germline stem cells (FGSCs) retain stemness in the ovary and initiate meiosis to generate oocytes. Primordial germ cell-like cells (PGCLCs) can be induced in vitro from embryonic stem cells or induced pluripotent stem cells. In this review, we focus on current advances in these embryonic and adult germline stem cells, and the induced PGCLCs in humans, provide an overview of molecular mechanisms underlying the development and differentiation of the germline stem cells and outline their physiological functions, pathological implications, and clinical applications.

Early Gonadal Development and Sex Determination in Mammal

Sex determination is crucial for the transmission of genetic information through generations. In mammal, this process is primarily regulated by an antagonistic network of sex-related genes beginning in embryonic development and continuing throughout life. Nonetheless, abnormal expression of these sex-related genes will lead to reproductive organ and germline abnormalities, resulting in disorders of sex development (DSD) and infertility. On the other hand, it is possible to predetermine the sex of animal offspring by artificially regulating sex-related gene expression, a recent research hotspot. In this paper, we reviewed recent research that has improved our understanding of the mechanisms underlying the development of the gonad and primordial germ cells (PGCs), progenitors of the germline, to provide new directions for the treatment of DSD and infertility, both of which involve manipulating the sex ratio of livestock offspring.

Single-cell roadmap of human gonadal development

Gonadal development is a complex process that involves sex determination followed by divergent maturation into either testes or ovaries¹. Historically, limited tissue accessibility, a lack of reliable in vitro models and critical differences between humans and mice have hampered our knowledge of human gonadogenesis, despite its importance in gonadal conditions and infertility. Here, we generated a comprehensive map of first- and second-trimester human gonads using a combination of single-cell and spatial transcriptomics, chromatin accessibility assays and fluorescent microscopy. We extracted human-specific regulatory programmes that control the development of germline and somatic cell lineages by profiling equivalent developmental stages in mice. In both species, we define the somatic cell states present at the time of sex specification, including the bipotent early supporting population that, in males, upregulates the testis-determining factor SRY and sPAX8s, a gonadal lineage located at the gonadal-mesonephric interface. In females, we resolve the cellular and molecular events that give rise to the first and second waves of granulosa cells that compartmentalize the developing ovary to modulate germ cell differentiation. In males, we identify human SIGLEC15⁺ and TREM2⁺ fetal testicular macrophages, which signal to somatic cells outside and inside the developing testis cords, respectively. This study provides a comprehensive spatiotemporal map of human and mouse gonadal differentiation, which can guide in vitro gonadogenesis.

Deciphering Sex-Specific Differentiation of Human Fetal Gonads: Insight From Experimental Models

Sex-specific gonadal differentiation is initiated by the expression of SRY in male foetuses. This promotes a signalling pathway directing testicular development, while in female foetuses the absence of SRY and expression of pro-ovarian factors promote ovarian development. Importantly, in addition to the initiation of a sex-specific signalling cascade the opposite pathway is simultaneously inhibited. The somatic cell populations within the gonads dictates this differentiation as well as the development of secondary sex characteristics via secretion of endocrine factors and steroid hormones. Opposing pathways SOX9/FGF9 (testis) and WNT4/RSPO1 (ovary) controls the development and differentiation of the bipotential mouse gonad and even though sex-specific gonadal differentiation is largely considered to be conserved between mice and humans, recent studies have identified several differences. Hence, the signalling pathways promoting early mouse gonad differentiation cannot be directly transferred to human development thus highlighting the importance of also examining this signalling in human fetal gonads. This review focus on the current understanding of regulatory mechanisms governing human gonadal sex differentiation by combining knowledge of these processes from studies in mice, information from patients with differences of sex development and insight from manipulation of selected signalling pathways in ex vivo culture models of human fetal gonads.

Mapping Human Reproduction with Single-Cell Genomics

The trillions of cells in the human body develop as a result of the fusion of two extremely specialized cells: an oocyte and a sperm. This process is essential for the continuation of our species, as it ensures that parental genetic information is mixed and passed on from generation to generation. In addition to producing oocytes, the female reproductive system must provide the environment for the appropriate development of the fetus until birth. New genomic and computational tools offer unique opportunities to study the tight spatiotemporal regulatory mechanisms that are required for the cycle of human reproduction. This review explores how single-cell technologies have been used to build cellular atlases of the human reproductive system across the life span and how these maps have proven useful to better understand reproductive pathologies and dissect the heterogeneity of in vitro model systems. Expected final online publication date for the Annual Review of Genomics and Human Genetics, Volume 23 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Human SRY Expression at the Sex-determining Period is Insufficient to Drive Testis Development in Mice

The sex-determining region of the Y chromosome, Sry/SRY, is an initiation factor for testis development in both humans and mice. Although the functional compatibility between murine SRY and human SRY was previously examined in transgenic mice, their equivalency remains inconclusive. Because molecular interaction and timeline of mammalian sex determination were mostly described in murine experiments, we generated a mouse model in which Sry was substituted with human SRY to verify the compatibility. The mouse model had the human SRY open reading frame at the locus of murine Sry exon 1-Sry(SRY) mice-and was generated using the CRISPR/Cas9 system. The reproductive system of the mice was analyzed. The expression of human SRY in the fetal gonadal ridge of Sry(SRY) mice was detected. The external and internal genitalia of adult Sry(SRY) mice were similar to those of wild-type females, without any significant difference in anogenital distance. Sry(SRY) mice obtained gonads, which were morphologically considered as ovaries. Histological analysis revealed that the cortical regions of gonads from adult Sry(SRY) mice contained few follicles. We successfully replaced genes on the Y chromosome with targeted genome editing using the CRISPR/Cas9 system. Because the Sry(SRY) XY mice did not develop testis, we concluded that human SRY was insufficient to drive testis development in mouse embryos. The difference in response elements and lack of glutamine-rich domains may have invalidated human SRY function in mice. Signal transduction between Sry/SRY expression and Sox9/SOX9 activation is possibly organized in a species-specific manner.

Adrenal hyperplasias in childhood: An update

Pediatric adrenocortical hyperplasias are rare; they usually present with Cushing syndrome (CS); of them, isolated micronodular adrenal disease and its variant, primary pigmented adrenocortical disease are the most commonly encountered. Most cases are due to defects in the cyclic AMP/protein kinase A (cAMP/PKA) pathway, although a few cases remain without an identified genetic defect. Another cause of adrenal hyperplasia in childhood is congenital adrenal hyperplasia, a group of autosomal recessive disorders that affect steroidogenic enzymes in the adrenal cortex. Clinical presentation varies and depends on the extent of the underlying enzymatic defect. The most common form is due to 21-hydroxylase deficiency; it accounts for more than 90% of the cases. In this article, we discuss the genetic etiology of adrenal hyperplasias in childhood.

Environmental Impacts on Male Reproductive Development: Lessons from Experimental Models

BACKGROUND

Male reproductive development in mammals can be divided into a gonadal formation phase followed by a hormone-driven differentiation phase. Failure of these processes may result in Differences in Sex Development (DSD), which may include abnormalities of the male reproductive tract, including cryptorchidism, hypospadias, infertility, and testicular germ cell cancer (TGCC). These disorders are also considered to be part of a testicular dysgenesis syndrome (TDS) in males. Whilst DSDs are considered to result primarily from genetic abnormalities, the development of TDS disorders is frequently associated with environmental factors.

SUMMARY

In this review, we will discuss the development of the male reproductive system in relation to DSD and TDS. We will also describe the experimental systems, including studies involving animals and human tissues or cells that can be used to investigate the role of environmental factors in inducing male reproductive disorders. We will discuss recent studies investigating the impact of environmental chemicals (e.g., phthalates and bisphenols), lifestyle factors (e.g., smoking) and pharmaceuticals (e.g., analgesics) on foetal testis development. Finally, we will describe the evidence, involving experimental and epidemiologic approaches, for a role of environmental factors in the development of specific male reproductive disorders, including cryptorchidism, hypospadias, and TGCC. Key Messages: Environmental exposures can impact the development and function of the male reproductive system in humans. Epidemiology studies and experimental approaches using human tissues are important to translate findings from animal studies and account for species differences in response to environmental exposures.

nr0b1 (DAX1) loss of function in zebrafish causes hypothalamic defects via abnormal progenitor proliferation and differentiation

The nuclear receptor DAX-1 (encoded by the NR0B1 gene) is presented in the hypothalamic tissues in humans and other vertebrates. Human patients with NR0B1 mutations often have hypothalamic-pituitary defects, but the involvement of NR0B1 in hypothalamic development and function is not well understood. Here, we report the disruption of the nr0b1 gene in zebrafish causes abnormal expression of gonadotropins, a reduction in fertilization rate, and an increase in post-fasting food intake, which is indicative of abnormal hypothalamic functions. We find that loss of nr0b1 increases the number of prodynorphin (pdyn)-expressing neurons but decreases the number of pro-opiomelanocortin (pomcb)-expressing neurons in the zebrafish hypothalamic arcuate region (ARC). Further examination reveals that the proliferation of progenitor cells is reduced in the hypothalamus of nr0b1 mutant embryos accompanying with the decreased expression of genes in the Notch signaling pathway. Additionally, the inhibition of Notch signaling in wild-type embryos increases the number of pdyn neurons, mimicking the nr0b1 mutant phenotype. In contrast, ectopic activation of Notch signaling in nr0b1 mutant embryos decreases the number of pdyn neurons. Taken together, our results suggest that nr0b1 regulates neural progenitor proliferation and maintenance to ensure normal hypothalamic neuron development.

Self-organising human gonads generated by a Matrigel-based gradient system

BACKGROUND

Advances in three-dimensional culture technologies have led to progression in systems used to model the gonadal microenvironment in vitro. Despite demonstrating basic functionality, tissue organisation is often limited. We have previously detailed a three-dimensional culture model termed the three-layer gradient system to generate rat testicular organoids in vitro. Here we extend the model to human first-trimester embryonic gonadal tissue.

RESULTS

Testicular cell suspensions reorganised into testis-like organoids with distinct seminiferous-like cords situated within an interstitial environment after 7 days. In contrast, tissue reorganisation failed to occur when mesonephros, which promotes testicular development in vivo, was included in the tissue digest. Organoids generated from dissociated female gonad cell suspensions formed loosely organised cords after 7 days. In addition to displaying testis-specific architecture, testis-like organoids demonstrated evidence of somatic cell differentiation. Within the 3-LGS, we observed the onset of AMH expression in the cytoplasm of SOX9-positive Sertoli cells within reorganised testicular cords. Leydig cell differentiation and onset of steroidogenic capacity was also revealed in the 3-LGS through the expression of key steroidogenic enzymes StAR and CYP17A1 within the interstitial compartment. While the 3-LGS generates a somatic cell environment capable of supporting germ cell survival in ovarian organoids germ cell loss was observed in testicular organoids.

CONCLUSION

The 3-LGS can be used to generate organised whole gonadal organoids within 7 days. The 3-LGS brings a new opportunity to explore gonadal organogenesis and contributes to the development of more complex in vitro models in the field of developmental and regenerative medicine.

Exposures during the prepuberty period and future offspring's health: evidence from human cohort studies†

Emerging evidence suggests that exposures in prepuberty, particularly in fathers-to-be, may impact the phenotype of future offspring. Analyses of the RHINESSA cohort find that offspring of father’s exposed to tobacco smoking or overweight that started in prepuberty demonstrate poorer respiratory health in terms of more asthma and lower lung function. A role of prepuberty onset smoking for offspring fat mass is suggested in the RHINESSA and ALSPAC cohorts, and historic studies suggest that ancestral nutrition during prepuberty plays a role for grand-offspring’s health and morbidity. Support for causal relationships between ancestral exposures and (grand-)offspring’s health in humans has been enhanced by advancements in statistical analyses that optimize the gain while accounting for the many complexities and deficiencies in human multigeneration data. The biological mechanisms underlying such observations have been explored in experimental models. A role of sperm small RNA in the transmission of paternal exposures to offspring phenotypes has been established, and chemical exposures and overweight have been shown to influence epigenetic programming in germ cells. For example, exposure of adolescent male mice to smoking led to differences in offspring weight and alterations in small RNAs in the spermatozoa of the exposed fathers. It is plausible that male prepuberty may be a time window of particular susceptibility, given the extensive epigenetic reprogramming taking place in the spermatocyte precursors at this age. In conclusion, epidemiological studies in humans, mechanistic research, and biological plausibility, all support the notion that exposures in the prepuberty of males may influence the phenotype of future offspring.

Gene expression profiles of bovine genital ridges during sex determination and early differentiation of the gonads†

Most current knowledge of sex determination in mammals has emerged from mouse and human studies. To investigate the molecular regulation of the sex determination process in cattle, we used an RNA sequencing strategy to analyze the transcriptome landscape of male and female bovine fetal gonads collected in vivo at key developmental stages: before, during, and after SRY gene activation on fetal days D35 (bipotential gonad formation), D39 (peak SRY expression), and D43 (early gonad differentiation). Differentially expressed genes (DEGs) were identified in male vs. female germinal ridges and among group genes showing similar expression profiles during the three periods. There were 143, 96, and 658 DEG between males and female fetuses at D35, D39, and D43, respectively. On D35, genes upregulated in females were enriched in translation, nuclear export, RNA localization, and mRNA splicing events, whereas those upregulated in males were enriched in cell proliferation regulation and male sex determination terms. In time-course experiments, 767 DEGs in males and 545 DEGs in females were identified between D35 vs. D39, and 3157 DEGs in males and 2008 in females were identified between D39 vs. D43. Results highlight unique aspects of sex determination in cattle, such as the expression of several Y chromosome genes (absent in mice and humans) before SRY expression and an abrupt increase in the nuclear expression of SOX10 (instead of SOX9 expression in the Sertoli cell cytoplasm as observed in mice) during male determination and early differentiation.

Dynamics of the transcriptional landscape during human fetal testis and ovary development

STUDY QUESTION

Which transcriptional program triggers sex differentiation in bipotential gonads and downstream cellular events governing fetal testis and ovary development in humans?

SUMMARY ANSWER

The characterization of a dynamically regulated protein-coding and non-coding transcriptional landscape in developing human gonads of both sexes highlights a large number of potential key regulators that show an early sexually dimorphic expression pattern.

WHAT IS KNOWN ALREADY

Gonadal sex differentiation is orchestrated by a sexually dimorphic gene expression program in XX and XY developing fetal gonads. A comprehensive characterization of its non-coding counterpart offers promising perspectives for deciphering the molecular events underpinning gonad development and for a complete understanding of the etiology of disorders of sex development in humans.

STUDY DESIGN, SIZE, DURATION

To further investigate the protein-coding and non-coding transcriptional landscape during gonad differentiation, we used RNA-sequencing (RNA-seq) and characterized the RNA content of human fetal testis (N = 24) and ovaries (N = 24) from 6 to 17 postconceptional week (PCW), a key period in sex determination and gonad development.

PARTICIPANTS/MATERIALS, SETTING, METHODS

First trimester fetuses (6-12 PCW) and second trimester fetuses (13-14 and 17 PCW) were obtained from legally induced normally progressing terminations of pregnancy. Total RNA was extracted from whole human fetal gonads and sequenced as paired-end 2 × 50 base reads. Resulting sequences were mapped to the human genome, allowing for the assembly and quantification of corresponding transcripts.

MAIN RESULTS AND THE ROLE OF CHANCE

This RNA-seq analysis of human fetal testes and ovaries at seven key developmental stages led to the reconstruction of 22 080 transcripts differentially expressed during testicular and/or ovarian development. In addition to 8935 transcripts displaying sex-independent differential expression during gonad development, the comparison of testes and ovaries enabled the discrimination of 13 145 transcripts that show a sexually dimorphic expression profile. The latter include 1479 transcripts differentially expressed as early as 6 PCW, including 39 transcription factors, 40 long non-coding RNAs and 20 novel genes. Despite the use of stringent filtration criteria (expression cut-off of at least 1 fragment per kilobase of exon model per million reads mapped, fold change of at least 2 and false discovery rate adjusted P values of less than <1%), the possibility of assembly artifacts and of false-positive differentially expressed transcripts cannot be fully ruled out.

LARGE-SCALE DATA

Raw data files (fastq) and a searchable table (.xlss) containing information on genomic features and expression data for all refined transcripts have been submitted to the NCBI GEO under accession number GSE116278.

LIMITATIONS, REASONS FOR CAUTION

The intrinsic nature of this bulk analysis, i.e. the sequencing of transcripts from whole gonads, does not allow direct identification of the cellular origin(s) of the transcripts characterized. Potential cellular dilution effects (e.g. as a result of distinct proliferation rates in XX and XY gonads) may account for a few of the expression profiles identified as being sexually dimorphic. Finally, transcriptome alterations that would result from exposure to pre-abortive drugs cannot be completely excluded. Although we demonstrated the high quality of the sorted cell populations used for experimental validations using quantitative RT-PCR, it cannot be totally excluded that some germline expression may correspond to cell contamination by, for example, macrophages.

WIDER IMPLICATIONS OF THE FINDINGS

For the first time, this study has led to the identification of 1000 protein-coding and non-coding candidate genes showing an early, sexually dimorphic, expression pattern that have not previously been associated with sex differentiation. Collectively, these results increase our understanding of gonad development in humans, and contribute significantly to the identification of new candidate genes involved in fetal gonad differentiation. The results also provide a unique resource that may improve our understanding of the fetal origin of testicular and ovarian dysgenesis syndromes, including cryptorchidism and testicular cancers.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the French National Institute of Health and Medical Research (Inserm), the University of Rennes 1, the French School of Public Health (EHESP), the Swiss National Science Foundation [SNF n° CRS115_171007 to B.J.], the French National Research Agency [ANR n° 16-CE14-0017-02 and n° 18-CE14-0038-02 to F.C.], the Medical Research Council [MR/L010011/1 to P.A.F.] and the European Community's Seventh Framework Programme (FP7/2007-2013) [under grant agreement no 212885 to P.A.F.] and from the European Union's Horizon 2020 Research and Innovation Programme [under grant agreement no 825100 to P.A.F. and S.M.G.]. There are no competing interests related to this study.

Single-cell analysis of the developing human testis reveals somatic niche cell specification and fetal germline stem cell establishment

Human testis development in prenatal life involves complex changes in germline and somatic cell identity. To better understand, we profiled and analyzed ∼32,500 single-cell transcriptomes of testicular cells from embryonic, fetal, and infant stages. Our data show that at 6-7 weeks postfertilization, as the testicular cords are established, the Sertoli and interstitial cells originate from a common heterogeneous progenitor pool, which then resolves into fetal Sertoli cells (expressing tube-forming genes) or interstitial cells (including Leydig-lineage cells expressing steroidogenesis genes). Almost 10 weeks later, beginning at 14-16 weeks postfertilization, the male primordial germ cells exit mitosis, downregulate pluripotent transcription factors, and transition into cells that strongly resemble the state 0 spermatogonia originally defined in the infant and adult testes. Therefore, we called these fetal spermatogonia "state f0." Overall, we reveal multiple insights into the coordinated and temporal development of the embryonic, fetal, and postnatal male germline together with the somatic niche.

Heterozygous deletion of Sox9 in mouse mimics the gonadal sex reversal phenotype associated with campomelic dysplasia in humans

Heterozygous mutations in the human SOX9 gene cause the skeletal malformation syndrome campomelic dysplasia which in 75% of 46, XY individuals is associated with male-to-female sex reversal. Although studies in homozygous Sox9 knockout mouse models confirmed that SOX9 is critical for testis development, mice heterozygous for the Sox9-null allele were reported to develop normal testes. This led to the belief that the SOX9 dosage requirement for testis differentiation is different between humans, which often require both alleles, and mice, in which one allele is sufficient. However, in prior studies, gonadal phenotypes in heterozygous Sox9 XY mice were assessed only by either gross morphology, histological staining or analyzed on a mixed genetic background. In this study, we conditionally inactivated Sox9 in somatic cells of developing gonads using the Nr5a1-Cre mouse line on a pure C57BL/6 genetic background. Section and whole-mount immunofluorescence for testicular and ovarian markers showed that XY Sox9 heterozygous gonads developed as ovotestes. Quantitative droplet digital PCR confirmed a 50% reduction of Sox9 mRNA as well as partial sex reversal shown by an upregulation of ovarian genes. Our data show that haploinsufficiency of Sox9 can perturb testis development in mice, suggesting that mice may provide a more accurate model of human disorders/differences of sex development than previously thought.

Sex determination, gonadal sex differentiation, and plasticity in vertebrate species

A diverse array of sex determination (SD) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed SD mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate SD was the discovery of the SRY gene in 1990. Since that time, many attempts to clone an SRY ortholog from nonmammalian vertebrates remained unsuccessful, until 2002, when DMY/dmrt1by was discovered as the SD gene of a small fish, medaka. Surprisingly, however, DMY/dmrt1by was found in only 2 species among more than 20 species of medaka, suggesting a large diversity of SD genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how SD and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate SD and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, a review of various SD mechanisms among vertebrates suggests that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.

Dysregulation of FGFR signalling by a selective inhibitor reduces germ cell survival in human fetal gonads of both sexes and alters the somatic niche in fetal testes

STUDY QUESTION

Does experimental manipulation of fibroblast growth factor 9 (FGF9)-signalling in human fetal gonads alter sex-specific gonadal differentiation?

SUMMARY ANSWER

Inhibition of FGFR signalling following SU5402 treatment impaired germ cell survival in both sexes and severely altered the developing somatic niche in testes, while stimulation of FGF9 signalling promoted Sertoli cell proliferation in testes and inhibited meiotic entry of germ cells in ovaries.

WHAT IS KNOWN ALREADY

Sex-specific differentiation of bipotential gonads involves a complex signalling cascade that includes a combination of factors promoting either testicular or ovarian differentiation and inhibition of the opposing pathway. In mice, FGF9/FGFR2 signalling has been shown to promote testicular differentiation and antagonize the female developmental pathway through inhibition of WNT4.

STUDY DESIGN, SIZE, DURATION

FGF signalling was manipulated in human fetal gonads in an established ex vivo culture model by treatments with recombinant FGF9 (25 ng/ml) and the tyrosine kinase inhibitor SU5402 (10 μM) that was used to inhibit FGFR signalling. Human fetal testis and ovary tissues were cultured for 14 days and effects on gonadal development and expression of cell lineage markers were determined.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Gonadal tissues from 44 male and 33 female embryos/fetuses from first trimester were used for ex vivo culture experiments. Tissues were analyzed by evaluation of histology and immunohistochemical analysis of markers for germ cells, somatic cells, proliferation and apoptosis. Culture media were collected throughout the experimental period and production of steroid hormone metabolites was analyzed in media from fetal testis cultures by liquid chromatography-tandem mass spectrometry (LC-MS/MS).

MAIN RESULTS AND THE ROLE OF CHANCE

Treatment with SU5402 resulted in near complete loss of gonocytes (224 vs. 14 OCT4+ cells per mm2, P < 0.05) and oogonia (1456 vs. 28 OCT4+ cells per mm2, P < 0.001) in human fetal testes and ovaries, respectively. This was a result of both increased apoptosis and reduced proliferation in the germ cells. Addition of exogenous FGF9 to the culture media resulted in a reduced number of germ cells entering meiosis in fetal ovaries (102 vs. 60 γH2AX+ germ cells per mm2, P < 0.05), while in fetal testes FGF9 stimulation resulted in an increased number of Sertoli cells (2503 vs. 3872 SOX9+ cells per mm2, P < 0.05). In fetal testes, inhibition of FGFR signalling by SU5402 treatment altered seminiferous cord morphology and reduced the AMH expression as well as the number of SOX9-positive Sertoli cells (2503 vs. 1561 SOX9+ cells per mm2, P < 0.05). In interstitial cells, reduced expression of COUP-TFII and increased expression of CYP11A1 and CYP17A1 in fetal Leydig cells was observed, although there were no subsequent changes in steroidogenesis.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Ex vivo culture may not replicate all aspects of fetal gonadal development and function in vivo. Although the effects of FGF9 were studied in ex vivo culture experiments, there is no direct evidence that FGF9 acts in vivo during human fetal gonadogenesis. The FGFR inhibitor (SU5402) used in this study is not specific to FGFR2 but inhibits all FGF receptors and off-target effects on unrelated tyrosine kinases should be considered.

WIDER IMPLICATIONS OF THE FINDINGS

The findings of this study suggest that dysregulation of FGFR-mediated signalling may affect both testicular and ovarian development, in particular impacting the fetal germ cell populations in both sexes.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported in part by an ESPE Research Fellowship, sponsored by Novo Nordisk A/S to A.JØ. Additional funding was obtained from the Erichsen Family Fund (A.JØ.), the Aase and Ejnar Danielsens Fund (A.JØ.), the Danish Government's support for the EDMaRC programme (A.JU.) and a Wellcome Trust Intermediate Clinical Fellowship (R.T.M., Grant no. 098522). The Medical Research Council (MRC) Centre for Reproductive Health (R.T.M.) is supported by an MRC Centre Grant (MR/N022556/1). The authors have no conflict of interest to disclose.

Challenging human somatic testicular cell reassembly by protein kinase inhibition -setting up a functional in vitro test system

Signalling pathways and cellular interactions defining initial processes of testis morphogenesis, i.e. cord formation, are poorly understood. In vitro cell-based systems modelling cord formation can be utilised as platforms to interrogate processes of tubulogenesis. We aimed at testing our established cord formation in vitro model using adult human testicular cells as a quantitative assay that can facilitate future studies on cord morphogenesis. We challenged the responsiveness of our system with a broad-spectrum protein kinase inhibitor, K252a. Cultured testicular cells were treated with various K252a concentrations under constant exposure and compound withdrawal. To quantify cell reaggregation changes, we performed computer-assisted phase-contrast image analysis of aggregate size and number. Cell reaggregation was analysed in detail by categorisation of aggregates into size groups and accounting for changes in aggregate number per size category. We found a dose-related disturbance of testicular cell reaggregation. K252a decreased aggregate size (IC50 of 203.3 nM) and reduced the large aggregate numbers. Video recordings revealed that treatment with K252a at a concentration above IC50 interfered with aggregate coalescence into cords. Short-term exposure and compound wash-out induced irreversible decrease in large aggregates. We propose our in vitro model as a functional platform to quantitatively investigate seminiferous tubulogenesis under pharmacological impact.

Disorders of Sex Development-Novel Regulators, Impacts on Fertility, and Options for Fertility Preservation

Disorders (or differences) of sex development (DSD) are a heterogeneous group of congenital conditions with variations in chromosomal, gonadal, or anatomical sex. Impaired gonadal development is central to the pathogenesis of the majority of DSDs and therefore a clear understanding of gonadal development is essential to comprehend the impacts of these disorders on the individual, including impacts on future fertility. Gonadal development was traditionally considered to involve a primary 'male' pathway leading to testicular development as a result of expression of a small number of key testis-determining genes. However, it is increasingly recognized that there are several gene networks involved in the development of the bipotential gonad towards either a testicular or ovarian fate. This includes genes that act antagonistically to regulate gonadal development. This review will highlight some of the novel regulators of gonadal development and how the identification of these has enhanced understanding of gonadal development and the pathogenesis of DSD. We will also describe the impact of DSDs on fertility and options for fertility preservation in this context.

Testis Development

Production of sperm and androgens is the main function of the testis. This depends on normal development of both testicular somatic cells and germ cells. A genetic program initiated from the Y chromosome gene sex-determining region Y (SRY) directs somatic cell specification to Sertoli cells that orchestrate further development. They first guide fetal germ cell differentiation toward spermatogenic destiny and then take care of the full service to spermatogenic cells during spermatogenesis. The number of Sertoli cells sets the limits of sperm production. Leydig cells secrete androgens that determine masculine development. Testis development does not depend on germ cells; that is, testicular somatic cells also develop in the absence of germ cells, and the testis can produce testosterone normally to induce full masculinization in these men. In contrast, spermatogenic cell development is totally dependent on somatic cells. We herein review germ cell differentiation from primordial germ cells to spermatogonia and development of the supporting somatic cells. Testicular descent to scrota is necessary for normal spermatogenesis, and cryptorchidism is the most common male birth defect. This is a mild form of a disorder of sex differentiation. Multiple genetic reasons for more severe forms of disorders of sex differentiation have been revealed during the last decades, and these are described along with the description of molecular regulation of testis development.

Update on the genetics of differences of sex development (DSD)

Human gonadal development is regulated by the temporospatial expression of many different genes with critical dosage effects. Subsequent sex steroid hormone production requires several consecutive enzymatic steps and functional hormone receptors. Disruption of this complex process can result in atypical sex development and lead to conditions referred to as differences (disorders) of sex development (DSD). With the advent of massively parallel sequencing technologies, in silico protein modeling and innovative tools for the generation of animal models, new genes and pathways have been implicated in the pathogenesis of these conditions. Here, we provide an overview of the currently known DSD genes and mechanisms involved in the process of gonadal and phenotypical sex development and highlight phenotypic findings that may trigger further diagnostic investigations.

Deciphering Cell Lineage Specification during Male Sex Determination with Single-Cell RNA Sequencing

The gonad is a unique biological system for studying cell-fate decisions. However, major questions remain regarding the identity of somatic progenitor cells and the transcriptional events driving cell differentiation. Using time-series single-cell RNA sequencing on XY mouse gonads during sex determination, we identified a single population of somatic progenitor cells prior to sex determination. A subset of these progenitors differentiates into Sertoli cells, a process characterized by a highly dynamic genetic program consisting of sequential waves of gene expression. Another subset of multipotent cells maintains their progenitor state but undergoes significant transcriptional changes restricting their competence toward a steroidogenic fate required for the differentiation of fetal Leydig cells. Our findings confirm the presence of a unique multipotent progenitor population in the gonadal primordium that gives rise to both supporting and interstitial lineages. These also provide the most granular analysis of the transcriptional events occurring during testicular cell-fate commitment.

A Study on Chromosomal Analysis of Patients with Primary Amenorrhea

Background:

Primary amenorrhea is one of the most common disorders seen as gynecological problems in adolescent girls. It refers to the participants who did not attain menarche by the age of 11–15 years. Chromosome abnormalities contribute as one of the etiological factors in patients with primary amenorrhea.

Aims:

The aim of this study was to evaluate the frequency of chromosomal abnormalities and to investigate the abnormal karyotypes in patients referred with the symptom of primary amenorrhea for better management and counseling.

Setting and Design:

One hundred and seventy-four cases of primary amenorrhea were referred from the obstetrics and gynecology department to our cytogenetic laboratory for chromosomal analysis. G-banded chromosomes were karyotyped, and chromosomal analysis of all patients was done.

Results:

Out of 174 patients, we observed 23 (13.22%) participants with abnormal karyotype. In 23 cases of chromosomal abnormalities, 10 cases were sex reversal female (46,XY) and Turner karyotype (45,X) in 6 females. Other numerical and structural abnormalities were also seen such as 47,XXX; 45,X/47,XXX; 45,X/46, X,dic(X); 46,XX, inv (9); 45,X/46,X,i(Xq); 46,X,mar(X); and 45,X/46,XY in the primary amenorrhea cases.

Conclusion:

This study definitely attests the importance of chromosomal analysis in the etiologic diagnosis of primary amenorrhea patients. Karyotyping will help to counsel and manage the cases of primary amenorrhea in a better way. This study reveals the frequencies and different types of chromosomal abnormalities found in primary amenorrhea individuals and that might help to make the national database on primary amenorrhea in relation to chromosomal aberrations.

Human sex reversal is caused by duplication or deletion of core enhancers upstream of SOX9

Disorders of sex development (DSDs) are conditions affecting development of the gonads or genitalia. Variants in two key genes, SRY and its target SOX9, are an established cause of 46,XY DSD, but the genetic basis of many DSDs remains unknown. SRY-mediated SOX9 upregulation in the early gonad is crucial for testis development, yet the regulatory elements underlying this have not been identified in humans. Here, we identified four DSD patients with overlapping duplications or deletions upstream of SOX9. Bioinformatic analysis identified three putative enhancers for SOX9 that responded to different combinations of testis-specific regulators. All three enhancers showed synergistic activity and together drive SOX9 in the testis. This is the first study to identify SOX9 enhancers that, when duplicated or deleted, result in 46,XX or 46,XY sex reversal, respectively. These enhancers provide a hitherto missing link by which SRY activates SOX9 in humans, and establish SOX9 enhancer mutations as a significant cause of DSD.

SRY and its target SOX9 are known key determinants in testis development. Here the authors by studying duplications and deletions upstream of SOX9 from patient samples with disorders of sex development (DSD) reveal enhancers for SOX9 critical for human sex development and DSD.

Nodal Signaling Regulates Germ Cell Development and Establishment of Seminiferous Cords in the Human Fetal Testis

Disruption of human fetal testis development is widely accepted to underlie testicular germ cell cancer (TGCC) origin and additional disorders within testicular dysgenesis syndrome (TDS). However, the mechanisms for the development of testicular dysgenesis in humans are unclear. We used ex vivo culture and xenograft approaches to investigate the importance of Nodal and Activin signaling in human fetal testis development. Inhibition of Nodal, and to some extent Activin, signaling disrupted seminiferous cord formation, abolished AMH expression, reduced androgen secretion, and decreased gonocyte numbers. Subsequent xenografting of testicular tissue rescued the disruptive effects on seminiferous cords and somatic cells but not germ cell effects. Stimulation of Nodal signaling increased the number of germ cells expressing pluripotency factors, and these persisted after xenografting. Our findings suggest a key role for Nodal signaling in the regulation of gonocyte differentiation and early human testis development with implications for the understanding of TGCC and TDS origin.

A Human Gonadal Cell Model From Induced Pluripotent Stem Cells

Sertoli cells are main players in the male gonads development and their study may shed light on 46,XY disorders of sex development (DSD). Mature primary Sertoli cells are incapable of proliferating in prolonged in vitro cultures and the available Sertoli cell models have several limitations since they derive from mouse or human cancer tissues. We differentiated human fibroblasts (HFs)-derived induced pluripotent stem cells into Sertoli-like cells (SLC) and, in order to characterize this new Sertoli cell model, we performed gene expression analyses by NextGeneration Sequencing techniques. This approach revealed that our putative SLC have reduced expression of pluripotency markers and expressed Sertoli cell markers such as SRY-Related HMG-Box 9 (SOX9), vimentin (VIM), and claudin-11 (CLDN-11). More in detail, the transcriptional profile analysis suggested that these cells are in an early stage of Sertoli cells maturation. Harnessing the power of induced pluripotent stem cells, we were able to generate SLC that show genetic and functional similarities to human Sertoli cells (HSerCs). SLC could become an excellent source of patient-specific Sertoli cells that could be of paramount benefit for both basic research and personalized medicine in sex development and reproductive medicine.

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.

The molecular pathways underlying early gonadal development

The body of knowledge surrounding reproductive development spans the fields of genetics, anatomy, physiology and biomedicine, to build a comprehensive understanding of the later stages of reproductive development in humans and animal models. Despite this, there remains much to learn about the bi-potential progenitor structure that the ovary and testis arise from, known as the genital ridge (GR). This tissue forms relatively late in embryonic development and has the potential to form either the ovary or testis, which in turn produce hormones required for development of the rest of the reproductive tract. It is imperative that we understand the genetic networks underpinning GR development if we are to begin to understand abnormalities in the adult. This is particularly relevant in the contexts of disorders of sex development (DSDs) and infertility, two conditions that many individuals struggle with worldwide, with often no answers as to their aetiology. Here, we review what is known about the genetics of GR development. Investigating the genetic networks required for GR formation will not only contribute to our understanding of the genetic regulation of reproductive development, it may in turn open new avenues of investigation into reproductive abnormalities and later fertility issues in the adult.

Experimental models of testicular development and function using human tissue and cells

The mammalian testis has two main roles, production of gametes for reproduction and synthesis of steroid- and peptide hormones for masculinization. These processes are tightly regulated and involve complex interactions between a number of germ and somatic cell-types that comprise a unique microenvironment known as the germ stem cell niche. In humans, failure of normal testicular development or function is associated with susceptibility to a variety of male reproductive disorders including disorders of sex development, infertility and testicular cancer. Whilst studies in rodent models have provided detailed insight into the signaling pathways and molecular mechanisms that regulate the testis, there are important species differences in testicular development, function and reproductive disorders that highlight the need for suitable experimental models utilising human testicular tissues or cells. In this review, we outline experimental approaches used to sustain cells and tissue from human testis at different developmental time-points and discuss relevant end-points. These include survival, proliferation and differentiation of cell lineages within the testis as well as autocrine, paracrine and endocrine function. We also highlight the utility of these experimental approaches for modelling the effects of environmental exposures on testicular development and function.

A role for SOX9 in post-transcriptional processes: insights from the amphibian oocyte

Sox9 is a member of the gene family of SOX transcription factors, which is highly conserved among vertebrates. It is involved in different developmental processes including gonadogenesis. In all amniote species examined thus far, Sox9 is expressed in the Sertoli cells of the male gonad, suggesting an evolutionarily conserved role in testis development. However, in the anamniotes, fishes and amphibians, it is also expressed in the oocyte but the significance of such an expression remains to be elucidated. Here, we have investigated the nuclear localization of the SOX9 protein in the oocyte of three amphibian species, the urodelan Pleurodeles waltl, and two anurans, Xenopus laevis and Xenopus tropicalis. We demonstrate that SOX9 is associated with ribonucleoprotein (RNP) transcripts of lampbrush chromosomes in an RNA-dependent manner. This association can be visualized by Super-resolution Structured Illumination Microscopy (SIM). Our results suggest that SOX9, known to bind DNA, also carries an additional function in the posttranscriptional processes. We also discuss the significance of the acquisition or loss of Sox9 expression in the oocyte during evolution at the transition between anamniotes and amniotes.

A Case of Primary Amenorrhea with Swyer Syndrome

Swyer syndrome with complete gonadal dysgenesis is associated with an absence of testicular differentiation in a phenotypic female with a 46, XY karyotype. A 14-year-old unmarried girl was referred with complaints of primary amenorrhea and nondevelopment of breast. Her built was normal. Examination of her secondary sexual characteristics revealed no breast development, absent axillary hairs, and sparse pubic hairs. External genitalia was of female type. Karyotype showed genotype of 46, XY. Magnetic resonance imaging revealed hypoplastic uterus with absent fallopian tubes and ovaries. A diagnosis of Swyer syndrome was made. Laparoscopy showed infantile uterus, normal fallopian tubes, and streak gonads. Laparoscopic removal of streak gonads was done as there is a risk of gonadoblastoma in such cases. The patient was started on hormonal replacement therapy. Swyer syndrome results mainly due to mutation in certain genes such as SRY gene, which leads to failure of development of testis.

Segregation of mitochondrial DNA heteroplasmy through a developmental genetic bottleneck in human embryos

Mitochondrial DNA (mtDNA) mutations cause inherited diseases and are implicated in the pathogenesis of common late-onset disorders, but how they arise is not clear1,2. Here we show that mtDNA mutations are present in primordial germ cells (PGCs) within healthy female human embryos. Isolated PGCs have a profound reduction in mtDNA content, with discrete mitochondria containing ~5 mtDNA molecules. Single-cell deep mtDNA sequencing of in vivo human female PGCs showed rare variants reaching higher heteroplasmy levels in late PGCs, consistent with the observed genetic bottleneck. We also saw the signature of selection against non-synonymous protein-coding, tRNA gene and D-loop variants, concomitant with a progressive upregulation of genes involving mtDNA replication and transcription, and linked to a transition from glycolytic to oxidative metabolism. The associated metabolic shift would expose deleterious mutations to selection during early germ cell development, preventing the relentless accumulation of mtDNA mutations in the human population predicted by Muller's ratchet. Mutations escaping this mechanism will show shifts in heteroplasmy levels within one human generation, explaining the extreme phenotypic variation seen in human pedigrees with inherited mtDNA disorders.

The evolutionary process of mammalian sex determination genes focusing on marsupial SRYs

BACKGROUND

Maleness in mammals is genetically determined by the Y chromosome. On the Y chromosome SRY is known as the mammalian male-determining gene. Both placental mammals (Eutheria) and marsupial mammals (Metatheria) have SRY genes. However, only eutherian SRY genes have been empirically examined by functional analyses, and the involvement of marsupial SRY in male gonad development remains speculative.

RESULTS

In order to demonstrate that the marsupial SRY gene is similar to the eutherian SRY gene in function, we first examined the sequence differences between marsupial and eutherian SRY genes. Then, using a parsimony method, we identify 7 marsupial-specific ancestral substitutions, 13 eutherian-specific ancestral substitutions, and 4 substitutions that occurred at the stem lineage of therian SRY genes. A literature search and molecular dynamics computational simulations support that the lineage-specific ancestral substitutions might be involved with the functional differentiation between marsupial and eutherian SRY genes. To address the function of the marsupial SRY gene in male determination, we performed luciferase assays on the testis enhancer of Sox9 core (TESCO) using the marsupial SRY. The functional assay shows that marsupial SRY gene can weakly up-regulate the luciferase expression via TESCO.

CONCLUSIONS

Despite the sequence differences between the marsupial and eutherian SRY genes, our functional assay indicates that the marsupial SRY gene regulates SOX9 as a transcription factor in a similar way to the eutherian SRY gene. Our results suggest that SRY genes obtained the function of male determination in the common ancestor of Theria (placental mammals and marsupials). This suggests that the marsupial SRY gene has a function in male determination, but additional experiments are needed to be conclusive.

The bovine sex-determining region Y (Sry) gene and its mRNA transcript are present in Y sperm but not X sperm of bulls

The aims of this study were to establish whether the sex-determining region Y gene and its mRNA transcript are present in the Y sperm and X sperm of bulls and, if present, determine their cellular localization. Semen was collected from three bulls and sorted by flow cytometry into X- and Y-chromosome populations. Reverse transcription-polymerase chain reaction (RT-PCR) was used to determine Sry mRNA expression in X sperm and Y sperm. The presence and localization of Sry DNA and RNA were investigated by fluorescence in situ hybridization (FISH). RT-PCR detected a single Sry transcript of 142 bp in Y sperm but not in X sperm. In Y sperm, the FISH-positive rates for Sry DNA and Sry RNA did not differ significantly from the re-analyzed Y sperm purity. In further experiments, there were no significant differences between the FISH-positive rate for Sry RNA and the re-analyzed Y sperm purity for X-sorted, Y-sorted, or unsorted sperm. In conclusion, FISH analysis revealed that Sry transcripts are present at the edges of the sperm heads of Y sperm but are absent from X sperm.

Temporal expression pattern of genes during the period of sex differentiation in human embryonic gonads

The precise timing and sequence of changes in expression of key genes and proteins during human sex-differentiation and onset of steroidogenesis was evaluated by whole-genome expression in 67 first trimester human embryonic and fetal ovaries and testis and confirmed by qPCR and immunohistochemistry (IHC). SRY/SOX9 expression initiated in testis around day 40 pc, followed by initiation of AMH and steroidogenic genes required for androgen production at day 53 pc. In ovaries, gene expression of RSPO1, LIN28, FOXL2, WNT2B, and ETV5, were significantly higher than in testis, whereas GLI1 was significantly higher in testis than ovaries. Gene expression was confirmed by IHC for GAGE, SOX9, AMH, CYP17A1, LIN28, WNT2B, ETV5 and GLI1. Gene expression was not associated with the maternal smoking habits. Collectively, a precise temporal determination of changes in expression of key genes involved in human sex-differentiation is defined, with identification of new genes of potential importance.

Progress and future prospect of in vitro spermatogenesis

Infertility has become a major health issue in the world. It affects the social life of couples and of all infertility cases; approximately 40–50% is due to “male factor” infertility. Male infertility could be due to genetic factors, environment or due to gonadotoxic treatment. Developments in reproductive biotechnology have made it possible to rescue fertility and uphold biological fatherhood. In vitro production of haploid male germ cell is a powerful tool, not only for the treatment of infertility including oligozoospermic or azoospermic patient, but also for the fertility preservation in pre-pubertal boys whose gonadal function is threatened by gonadotoxic therapies. Genomic editing of in-vitro cultured germ cells could also potentially cure flaws in spermatogenesis due to genomic mutation. Furthermore, this ex-vivo maturation technique with genomic editing may be used to prevent paternal transmission of genomic diseases. Here, we summarize the historical progress of in vitro spermatogenesis research by using organ and cell culture techniques and the future clinical application of in vitro spermatogenesis.

Gametogenesis from Pluripotent Stem Cells

The germ cell lineage originates early in development and undergoes a series of complex developmental processes that culminate in the generation of fully matured gametes, the spermatozoa and the oocytes. Remarkably, researchers have been recapitulating these developmental pathways using mouse and human pluripotent stem cells (PSCs). With further studies, including those involving non-human primate models, human gametogenesis may be fully reconstituted from PSCs, which would profoundly facilitate our understanding of human germ cell development and infertility. Here we discuss groundbreaking studies that lay the foundation for this achievement, the current state of the field, and challenges for deriving gametes from hPSCs.