A mesodermal factor, T, specifies mouse germ cell fate by directly activating germline determinants

Stem cell-based embryo models: a tool to study early human development

How a mammalian fertilized egg acquires totipotency and develops into a full-term offspring is a fundamental scientific question. Human embryonic development is difficult to study due to limited resources, technical challenges and ethics. Moreover, the precise regulatory mechanism underlying early human embryonic development remains unknown. In recent years, the emergence of stem cell-based embryo models (SCBEM) provides the opportunity to reconstitute pre- to post-implantation development in vitro. These models to some extent mimic the embryo morphologically and transcriptionally, and thus may be used to study key events in mammalian pre- and post-implantation development. Many groups have successfully generated SCBEM of the mouse and human. Here, we provide a comparative review of the mouse and human SCBEM, discuss the capability of these models to mimic natural embryos and give a perspective on their potential future applications.

Single-cell multi-omics delineates the dynamics of distinct epigenetic codes coordinating mouse gastrulation

Gastrulation represents a crucial stage in embryonic development and is tightly controlled by a complex network involving epigenetic reprogramming. However, the molecular coordination among distinct epigenetic layers entailing the progressive restriction of lineage potency remains unclear. Here, we present a multi-omics map of H3K27ac and H3K4me1 single-cell ChIP-seq profiles of mouse embryos collected at six sequential time points. Significant epigenetic priming, as reflected by H3K27ac signals, is evident, yet asynchronous cell fate commitment of each germ layer at distinct histone modification levels are observed. Integrated scRNA-seq and single-cell ChIP-seq analysis unveil a "time lag" transition pattern between enhancer activation and gene expression during germ-layer specification. Notably, by utilizing the H3K27ac and H3K4me1 co-marked active enhancers, we construct a gene regulatory network centered on pivotal transcription factors, highlighting the potential critical role of Cdkn1c in mesoderm lineage specification. Together, our study broadens the current understanding of intricate epigenetic regulatory networks governing mouse gastrulation and sheds light on their relevance to congenital diseases.

Evolving Lessons on Metazoan Primordial Germ Cells in Diversity and Development

Germ cells are pivotal for the continuation of biological species. The metazoan germline develops from primordial germ cells (PGCs) that undergo multiple rounds of mitotic divisions. The PGCs are specified by either maternal inheritance of asymmetrically polarized cytoplasmic mRNAs/proteins (found in roundworms, flies, fishes, frogs, and fowl) or via direct induction of epiblast cells from adjacent extraembryonic ectoderm in mammals. In all vertebrates, PGCs remain uncommitted to meiosis and migrate to colonize the developing gonadal ridge before sex determination. Multiple RNA-binding proteins (e.g., Vasa, Dnd, Dazl, etc.) play crucial roles in PGC identity, expansion, survival, and migration. Postsex determination in mouse embryos, Gata4, expressing nascent gonads, induces Dazl expression in newly arriving germ cells that supports retinoic acid-mediated induction of meiotic onset. This article briefly discusses the developmental events regulating the PGC specification and commitment in metazoans. We also highlight the recent progress towards the in vitro generation of functional PGC-like cells in rodents and humans.

Single-cell transcriptomic atlas of the human testis across the reproductive lifespan

Testicular aging is associated with declining reproductive health, but the molecular mechanisms are unclear. Here we generate a dataset of 214,369 single-cell transcriptomes from testicular cells of 35 individuals aged 21-69, offering a resource for studying testicular aging and physiology. Machine learning analysis reveals a stronger aging response in somatic cells compared to germ cells. Two waves of aging-related changes are identified: the first in peritubular cells of donors in their 30s, marked by increased basement membrane thickness, indicating a priming state for aging. In their 50s, testicular cells exhibit functional changes, including altered steroid metabolism in Leydig cells and immune responses in macrophages. Further analyses reveal the impact of body mass index on spermatogenic capacity as age progresses, particularly after age 45. Altogether, our findings illuminate molecular alterations during testis aging and their relationship with body mass index, providing a foundation for future research and offering potential diagnostic markers and therapeutic targets.

The establishment and regulation of human germ cell lineage

The specification of primordial germ cells (PGCs) during early embryogenesis initiates the development of the germ cell lineage that ensures the perpetuation of genetic and epigenetic information from parents to offspring. Defects in germ cell development may lead to infertility or birth defects. Historically, our understanding of human PGCs (hPGCs) regulation has primarily been derived from studies in mice, given the ethical restrictions and practical limitations of human embryos at the stage of PGC specification. However, recent studies have increasingly highlighted significant mechanistic differences for PGC development in humans and mice. The past decade has witnessed the establishment of human pluripotent stem cell (hPSC)-derived hPGC-like cells (hPGCLCs) as new models for studying hPGC fate specification and differentiation. In this review, we systematically summarize the current hPSC-derived models for hPGCLC induction, and how these studies uncover the regulatory machinery for human germ cell fate specification and differentiation, forming the basis for reconstituting gametogenesis in vitro from hPSCs for clinical applications and disease modeling.

Proteomic analysis of wanxi white goose testicles in different reproductive stages by data-independent acquisition (DIA) strategy

Wanxi white goose is an important male parent in crossbreeding of Chinese geese, but its short reproductive cycle restricts its application in Northeast China. Therefore, understanding the potential mechanism of breeding period regulation in Wanxi white goose will help to provide more options for crossbreeding. In this study, the reproductive period was divided into prophase (T1), metaphase (T2) and anaphase (T3) according to the laying rhythm of geese. Based on this, testicular tissues were stained and further analyzed by DIA proteomics. A total of 69577 Unique spectra, 70325 Unique peptides and 6275 proteins were identified. Among them, various differentially expressed proteins were enriched to metabolism-related pathways. Furthermore, trend analysis was performed based on the differentially expressed proteins in which four significant differences of protein expression trends were obtained. Additionally, the PPI protein network analysis helped us to further locate the core genes, such as CDC42, RAC2, IFT81, CSTF2, MRPS35 and BIRC5. Finally, we detected the protein expression of SOX9, RAC1, RhoA and Lyn, which was significantly correlated with the sequencing results. Generally, in this study, several genes and proteins involved in developmental biology and metabolic pathways were involved in the reproductive regulation of Wanxi white goose, which will lay a foundation for us to further study the reproductive regulation of Wanxi white goose.

Pluripotency factor Tex10 finetunes Wnt signaling for spermatogenesis and primordial germ cell development

Testis-specific transcript 10 (Tex10) is highly expressed in the testis, embryonic stem cells (ESCs), and primordial germ cells (PGCs). We previously generated a Tex10 knockout mouse model demonstrating its critical roles in ESC pluripotency and preimplantation development. Here, using conditional knockout mice and dTAG-degron ESCs, we show Tex10 is required for spermatogenesis and ESC-to-PGCLC differentiation. Specifically, Tex10-null spermatocytes arrest at metaphase I, compromising round spermatid formation. Tex10 depletion and overexpression compromise and enhance ESC-to-PGCLC differentiation, respectively. Mechanistically, bulk and single-cell RNA sequencing reveals that Tex10 depletion downregulates genes involved in pluripotency, PGC development, and spermatogenesis while upregulating genes promoting somatic programs. Chromatin occupancy study reveals that Tex10 binds to H3K4me3-marked promoters of Psmd3 and Psmd7, negative regulators of Wnt signaling, and activates their expression, thereby restraining Wnt signaling. Our study identifies Tex10 as a previously unappreciated factor in spermatogenesis and PGC development, offering potential therapeutic insights for treating male infertility.

The role of KLF4 in human primordial germ cell development

Primordial germ cells (PGCs) are the founder cells that develop into mature gametes. PGCs emerge during weeks 2-3 of human embryo development. Pluripotency genes are reactivated during PGC specification, including Krüppel-like factor KLF4, but its precise role in PGC development is unclear. Here, we investigated the role of KLF4 in PGC development using our in vitro model for human PGC-like cells (hPGCLCs). We demonstrate that the depletion of KLF4 reduces the efficiency of hPGCLC specification, resulting in hPGCLCs with an aberrant transcriptome. Cut-and-run and transcriptomic analyses reveal that KLF4 represses somatic markers involved in neuronal and endodermal differentiation while promoting the expression of genes associated with PGC specification, such as PAX5, and epigenetic regulators, including DNMT3L and REST. KLF4 targets in hPGCLCs showed significant co-enrichment of motifs for SP and STAT factors, which are known to regulate cell cycle and migration genes. KLF4 contributes to human PGC development by activating genes involved in PGC specification and cell cycle regulation, while repressing somatic genes to maintain PGC identity.

In vitro gametogenesis: Towards competent oocytes: Limitations and future improvements for generating oocytes from pluripotent stem cells in culture

Production of oocytes from pluripotent cell cultures in a dish represents a new paradigm in stem cell and developmental biology and has implications for how we think about life. The spark of life for the next generation occurs at fertilization when sperm and oocyte fuse. In animals, gametes are the only cells that transmit their genomes to the next generation. Oocytes contain in addition a large cytoplasm with factors that direct embryonic development. Reconstitution of mouse oocyte and embryonic development in culture provides experimental opportunities and facilitates an unprecedented understanding of molecular mechanisms. However, the application of in vitro gametogenesis to reproductive medicine or infertility treatment remains challenging. One significant concern is the quality of in vitro-derived oocytes. Here, we review the current understanding and identify limitations in generating oocytes in vitro. From this basis, we explore opportunities for future improvements of the in vitro approach to generating high-quality oocytes.

piRNA Defense Against Endogenous Retroviruses

Infection by retroviruses and the mobilization of transposable elements cause DNA damage that can be catastrophic for a cell. If the cell survives, the mutations generated by retrotransposition may confer a selective advantage, although, more commonly, the effect of new integrants is neutral or detrimental. If retrotransposition occurs in gametes or in the early embryo, it introduces genetic modifications that can be transmitted to the progeny and may become fixed in the germline of that species. PIWI-interacting RNAs (piRNAs) are single-stranded, 21-35 nucleotide RNAs generated by the PIWI clade of Argonaute proteins that maintain the integrity of the animal germline by silencing transposons. The sequence specific manner by which piRNAs and germline-encoded PIWI proteins repress transposons is reminiscent of CRISPR, which retains memory for invading pathogen sequences. piRNAs are processed preferentially from the unspliced transcripts of piRNA clusters. Via complementary base pairing, mature antisense piRNAs guide the PIWI clade of Argonaute proteins to transposon RNAs for degradation. Moreover, these piRNA-loaded PIWI proteins are imported into the nucleus to modulate the co-transcriptional repression of transposons by initiating histone and DNA methylation. How retroviruses that invade germ cells are first recognized as foreign by the piRNA machinery, as well as how endogenous piRNA clusters targeting the sequences of invasive genetic elements are acquired, is not known. Currently, koalas (Phascolarctos cinereus) are going through an epidemic due to the horizontal and vertical transmission of the KoRV-A gammaretrovirus. This provides an unprecedented opportunity to study how an exogenous retrovirus becomes fixed in the genome of its host, and how piRNAs targeting this retrovirus are generated in germ cells of the infected animal. Initial experiments have shown that the unspliced transcript from KoRV-A proviruses in koala testes, but not the spliced KoRV-A transcript, is directly processed into sense-strand piRNAs. The cleavage of unspliced sense-strand transcripts is thought to serve as an initial innate defense until antisense piRNAs are generated and an adaptive KoRV-A-specific genome immune response is established. Further research is expected to determine how the piRNA machinery recognizes a new foreign genetic invader, how it distinguishes between spliced and unspliced transcripts, and how a mature genome immune response is established, with both sense and antisense piRNAs and the methylation of histones and DNA at the provirus promoter.

Autophagy accompanying the developmental process of male germline stem cells

Germline stem cells are a crucial type of stem cell that can stably pass on genetic information to the next generation, providing the necessary foundation for the reproduction and survival of organisms. Male mammalian germline stem cells are unique cell types that include primordial germ cells and spermatogonial stem cells. They can differentiate into germ cells, such as sperm and eggs, thereby facilitating offspring reproduction. In addition, they continuously generate stem cells through self-renewal mechanisms to support the normal function of the reproductive system. Autophagy involves the use of lysosomes to degrade proteins and organelles that are regulated by relevant genes. This process plays an important role in maintaining the homeostasis of germline stem cells and the synthesis, degradation, and recycling of germline stem cell products. Recently, the developmental regulatory mechanism of germline stem cells has been further elucidated, and autophagy has been shown to be involved in the regulation of self-renewal and differentiation of germline stem cells. In this review, we introduce autophagy accompanying the development of germline stem cells, focusing on the autophagy process accompanying the development of male spermatogonial stem cells and the roles of related genes and proteins. We also briefly outline the effects of autophagy dysfunction on germline stem cells and reproduction.

Temporal BMP4 effects on mouse embryonic and extraembryonic development

The developing placenta, which in mice originates through the extraembryonic ectoderm (ExE), is essential for mammalian embryonic development. Yet unbiased characterization of the differentiation dynamics of the ExE and its interactions with the embryo proper remains incomplete. Here we develop a temporal single-cell model of mouse gastrulation that maps continuous and parallel differentiation in embryonic and extraembryonic lineages. This is matched with a three-way perturbation approach to target signalling from the embryo proper, the ExE alone, or both. We show that ExE specification involves early spatial and transcriptional bifurcation of uncommitted ectoplacental cone cells and chorion progenitors. Early BMP4 signalling from chorion progenitors is required for proper differentiation of uncommitted ectoplacental cone cells and later for their specification towards trophoblast giant cells. We also find biphasic regulation by BMP4 in the embryo. The early ExE-originating BMP4 signal is necessary for proper mesoendoderm bifurcation and for allantois and primordial germ cell specification. However, commencing at embryonic day 7.5, embryo-derived BMP4 restricts the primordial germ cell pool size by favouring differentiation of their extraembryonic mesoderm precursors towards an allantois fate. ExE and embryonic tissues are therefore entangled in time, space and signalling axes, highlighting the importance of their integrated understanding and modelling in vivo and in vitro.

Integrin signaling in pluripotent cells acts as a gatekeeper of mouse germline entry

Primordial germ cells (PGCs) are the precursors of gametes and the sole mechanism by which animals transmit genetic information across generations. In the mouse embryo, the transcriptional and epigenetic regulation of PGC specification has been extensively characterized. However, the initial event that triggers the soma-germline segregation remains poorly understood. Here, we uncover a critical role for the basement membrane in regulating germline entry. We show that PGCs arise in a region of the mouse embryo that lacks contact with the basement membrane, and the addition of exogenous extracellular matrix (ECM) inhibits both PGC and PGC-like cell (PGCLC) specification in mouse embryos and stem cell models, respectively. Mechanistically, we demonstrate that the engagement of β1 integrin with laminin blocks PGCLC specification by preventing the Wnt signaling-dependent down-regulation of the PGC transcriptional repressor, Otx2. In this way, the physical segregation of cells away from the basement membrane acts as a morphogenetic fate switch that controls the soma-germline bifurcation.

Elucidating the Transcriptional States of Spermatogenesis-Joint Analysis of Germline and Supporting Cell, Mice and Human, Normal and Perturbed, Bulk and Single-Cell RNA-Seq

In studying the molecular underpinning of spermatogenesis, we expect to understand the fundamental biological processes better and potentially identify genes that may lead to novel diagnostic and therapeutic strategies toward precision medicine in male infertility. In this review, we emphasized our perspective that the path forward necessitates integrative studies that rely on complementary approaches and types of data. To comprehensively analyze spermatogenesis, this review proposes four axes of integration. First, spanning the analysis of spermatogenesis in the healthy state alongside pathologies. Second, the experimental analysis of model systems (in which we can deploy treatments and perturbations) alongside human data. Third, the phenotype is measured alongside its underlying molecular profiles using known markers augmented with unbiased profiles. Finally, the testicular cells are studied as ecosystems, analyzing the germ cells alongside the states observed in the supporting somatic cells. Recently, the study of spermatogenesis has been advancing using single-cell RNA sequencing, where scientists have uncovered the unique stages of germ cell development in mice, revealing new regulators of spermatogenesis and previously unknown cell subtypes in the testis. An in-depth analysis of meiotic and postmeiotic stages led to the discovery of marker genes for spermatogonia, Sertoli and Leydig cells and further elucidated all the other germline and somatic cells in the testis microenvironment in normal and pathogenic conditions. The outcome of an integrative analysis of spermatogenesis using advanced molecular profiling technologies such as scRNA-seq has already propelled our biological understanding, with additional studies expected to have clinical implications for the study of male fertility. By uncovering new genes and pathways involved in abnormal spermatogenesis, we may gain insights into subfertility or sterility.

DNMT1 can induce primary germ layer differentiation through de novo DNA methylation

DNA methyltransferases and Ten-Eleven Translocation (TET) proteins regulate the DNA methylation and demethylation cycles during mouse embryonic development. Although DNMT1 mainly plays a role in the maintenance of DNA methylation after DNA replication, it is also reported to possess de novo methyltransferase capacity. However, its physiological significance remains unclear. Here, we demonstrate that full-length DNMT1 (FL) and a mutant lacking the N-terminus necessary for its maintenance activity (602) confer the differentiation potential of mouse Dnmt1, Dnmt3a, and Dnmt3b (Dnmts-TKO) embryonic stem cells (ESCs). Both FL and 602 inhibit the spontaneous differentiation of Dnmts-TKO ESCs in the undifferentiated state. Dnmts-TKO ESCs showed loss of DNA methylation and de-repression of primitive endoderm-related genes, but these defects were partially restored in Dnmts-TKO + FL and Dnmts-TKO + 602 ESCs. Upon differentiation, Dnmts-TKO + FL ESCs show increased 5mC and 5hmC levels across chromosomes, including pericentromeric regions. In contrast, Dnmts-TKO + 602 ESCs didn't accumulate 5mC, and sister chromatids showed 5hmC asynchronously. Furthermore, in comparison with DNMT1_602, DNMT1_FL effectively promoted commitment to the epiblast-like cells and beyond, driving cell-autonomous mesendodermal and germline differentiation through embryoid body-based methods. With precise target selectivity achieved by its N-terminal region, DNMT1 may play a role in gene regulation leading to germline development.

HELQ deficiency impairs the induction of primordial germ cell-like cells

Helicase POLQ-like (HELQ) is a DNA helicase essential for the maintenance of genome stability. A recent study identified two HELQ missense mutations in some cases of infertile men. However, the functions of HELQ in the process of germline specification are not well known and whether its function is conserved between mouse and human remains unclear. Here, we revealed that Helq knockout (Helq-/-) could significantly reduce the efficiency of mouse primordial germ cell-like cell (PGCLC) induction. In addition, Helq-/- embryonic bodies exhibited a severe apoptotic phenotype on day 6 of mouse PGCLC induction. p53 inhibitor treatment could partially rescue the generation of mouse PGCLCs from Helq mutant mouse embryonic stem cells. Finally, the genetic ablation of HELQ could also significantly impede the induction of human PGCLCs. Collectively, our study sheds light on the involvement of HELQ in the induction of both mouse and human PGCLCs, providing new insights into the mechanisms underlying germline differentiation and the genetic studies of human fertility.

Generation of marmoset primordial germ cell-like cells under chemically defined conditions

Primordial germ cells (PGCs) are the embryonic precursors of sperm and oocytes, which transmit genetic/epigenetic information across generations. Mouse PGC and subsequent gamete development can be fully reconstituted in vitro, opening up new avenues for germ cell studies in biomedical research. However, PGCs show molecular differences between rodents and humans. Therefore, to establish an in vitro system that is closely related to humans, we studied PGC development in vivo and in vitro in the common marmoset monkey Callithrix jacchus (cj). Gonadal cjPGCs at embryonic day 74 express SOX17, AP2Ɣ, BLIMP1, NANOG, and OCT4A, which is reminiscent of human PGCs. We established transgene-free induced pluripotent stem cell (cjiPSC) lines from foetal and postnatal fibroblasts. These cjiPSCs, cultured in defined and feeder-free conditions, can be differentiated into precursors of mesendoderm and subsequently into cjPGC-like cells (cjPGCLCs) with a transcriptome similar to human PGCs/PGCLCs. Our results not only pave the way for studying PGC development in a non-human primate in vitro under experimentally controlled conditions, but also provide the opportunity to derive functional marmoset gametes in future studies.

Emerging Contributions of Pluripotent Stem Cells to Reproductive Technologies in Veterinary Medicine

The generation of mature gametes and competent embryos in vitro from pluripotent stem cells has been successfully achieved in a few species, mainly in mice, with recent advances in humans and scarce preliminary reports in other domestic species. These biotechnologies are very attractive as they facilitate the understanding of developmental mechanisms and stages that are generally inaccessible during early embryogenesis, thus enabling advanced reproductive technologies and contributing to the generation of animals of high genetic merit in a short period. Studies on the production of in vitro embryos in pigs and cattle are currently used as study models for humans since they present more similar characteristics when compared to rodents in both the initial embryo development and adult life. This review discusses the most relevant biotechnologies used in veterinary medicine, focusing on the generation of germ-cell-like cells in vitro through the acquisition of totipotent status and the production of embryos in vitro from pluripotent stem cells, thus highlighting the main uses of pluripotent stem cells in livestock species and reproductive medicine.

A high throughput CRISPR perturbation screen identifies epigenetic regulators impacting primordial germ cell development

Certain environmental factors can impact fertility and reproductive parameters such as the number and quality of sperm and eggs. One possible mechanism is the perturbation of epigenetic landscapes in the germline. To explore this possibility, we conducted a CRISPRi screen of epigenetic-related genes to identify those that specifically perturb the differentiation of embryonic stem cells (ESCs) into primordial germ cell-like cells (PGCLCs), exploiting a highly scalable cytokine-free platform. Of the 701 genes screened, inhibition of 53 decreased the efficiency of PGCLC formation. NCOR2, a transcriptional repressor that acts via recruitment of Class I and Class IIa histone deacetylases (HDACs) to gene targets, was particularly potent in suppressing PGCLC differentiation. Consistent with evidence that histone deacetylation is crucial for germline differentiation, we found that the HDAC inhibitors (HDACi) valproic acid (VPA; an anti-convulsant) and sodium butyrate (SB; a widely-used dietary supplement) also suppressed ESC>PGCLC differentiation. Furthermore, exposure of developing mouse embryos to SB or VPA caused hypospermatogenesis. Transcriptome analyses of HDACi-treated, differentiating ESC>PGCLC cultures revealed suppression of germline-associated pathways and enhancement of somatic pathways. This work demonstrates the feasibility of conducting large-scale functional screens of genes, chemicals, or other agents that may impact germline development.

Scalable and Efficient Generation of Mouse Primordial Germ Cell-like Cells

Primordial germ cells (PGCs) are the founder cells of the germline. The ability to generate PGC-like cells (PGCLCs) from pluripotent stem cells has advanced our knowledge of gametogenesis and holds promise for developing infertility treatments. However, generating an ample supply of PGCLCs for demanding applications such as high-throughput genetic screens has been a limitation. Here, we demonstrated that simultaneous overexpressing 4 transcriptional factors - Nanog and three PGC master regulators Prdm1, Prdm14 and Tfap2c - in suspended mouse epiblast like cells (EpiLCs) and formative embryonic stem cells (ESCs) results in efficient and cost-effective production of PGCLCs. The overexpression of Nanog enhances the PGC regulatory network and suppresses differentiation of somatic lineages, enabling a significant improvement in the efficiency of PGCLC production. Transcriptomic analysis reveals that differentiated PGCLCs exhibit similarities to in vivo PGCs and are more advanced compared to cytokine-induced PGCLCs. These differentiated PGCLCs could be sustained over prolonged periods of culture and could differentiate into spermatogonia-like cells in vitro. Importantly, the ability to produce PGCLCs at scale, without using costly cytokines, enables biochemical and functional genomic screens to dissect mechanisms of germ cell development and infertility.

The many dimensions of germline competence

Primordial germ cell (PGC) specification is the first step in the development of the germline. Recent work has elucidated human-mouse differences in PGC differentiation and identified cell states with enhanced competency for PGC-like cell (PGCLC) differentiation in vitro in both species. However, it remains a subject of debate how different PGC competent states in vitro relate to each other, to embryonic development, and to the origin of PGCs in vivo. Here we review recent literature on human PGCLC differentiation in the context of mouse and non-human primate models. In contrast to what was previously thought, recent work suggests human pluripotent stem cells (hPSCs) are highly germline competent. We argue that paradoxical observations regarding the origin and signaling requirements of hPGCLCs may be due to local cell interactions. These confound assays of competence by generating endogenous signaling gradients and spatially modulating the ability to receive exogenous inductive signals. Furthermore, combinatorial signaling suggests that there is no unique germline competent state: rather than a one-dimensional spectrum of developmental progression, competence should be considered in a higher dimensional landscape of cell states.

Genetic clues to reprogramming power and formation of mouse oocyte

Oocyte features the unique capacity to reprogram not only sperm but also somatic nuclei to totipotency, yet the scarcity of oocytes has hindered the exploration and application of their reprogramming ability. In the meanwhile, the formation of oocytes, which involves extensive intracellular alterations and interactions, has also attracted tremendous interest. This review discusses developmental principles and regulatory mechanisms associated with ooplasm reprogramming and oocyte formation from a genetic perspective, with knowledge derived from mouse models. We also discuss future directions, especially to address the lack of insight into the regulatory networks that shape the identity of female germ cells or drive transitions in their developmental programs.

Nutritional and metabolic control of germ cell fate through O-GlcNAc regulation

Fate determination of primordial germ cells (PGCs) is regulated in a multi-layered manner, involving signaling pathways, epigenetic mechanisms, and transcriptional control. Chemical modification of macromolecules, including epigenetics, is expected to be closely related with metabolic mechanisms but the detailed molecular machinery linking these two layers remains poorly understood. Here, we show that the hexosamine biosynthetic pathway controls PGC fate determination via O-linked β-N-acetylglucosamine (O-GlcNAc) modification. Consistent with this model, reduction of carbohydrate metabolism via a maternal ketogenic diet that decreases O-GlcNAcylation levels causes repression of PGC formation in vivo. Moreover, maternal ketogenic diet intake until mid-gestation affects the number of ovarian germ cells in newborn pups. Taken together, we show that nutritional and metabolic mechanisms play a previously unappreciated role in PGC fate determination.

Laying out the mammalian body plan to a T

In this issue of Developmental Cell, Schülle et al. address the mechanism that lays down the mammalian body plan and show that the first step in this process is associated with the activity of two T-box transcription factors whose interactions outline territories that will be further elaborated during gastrulation.

Monolayer platform to generate and purify primordial germ-like cells in vitro provides insights into human germline specification

Generating primordial germ cell-like cells (PGCLCs) from human pluripotent stem cells (hPSCs) advances studies of human reproduction and development of infertility treatments, but often entails complex 3D aggregates. Here we develop a simplified, monolayer method to differentiate hPSCs into PGCs within 3.5 days. We use our simplified differentiation platform and single-cell RNA-sequencing to achieve further insights into PGCLC specification. Transient WNT activation for 12 h followed by WNT inhibition specified PGCLCs; by contrast, sustained WNT induced primitive streak. Thus, somatic cells (primitive streak) and PGCLCs are related-yet distinct-lineages segregated by temporally-dynamic signaling. Pluripotency factors including NANOG are continuously expressed during the transition from pluripotency to posterior epiblast to PGCs, thus bridging pluripotent and germline states. Finally, hPSC-derived PGCLCs can be easily purified by virtue of their CXCR4+PDGFRA-GARP- surface-marker profile and single-cell RNA-sequencing reveals that they harbor transcriptional similarities with fetal PGCs.

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.

Gastruloid-derived primordial germ cell-like cells develop dynamically within integrated tissues

Primordial germ cells (PGCs) are the early embryonic precursors of gametes - sperm and egg cells. PGC-like cells (PGCLCs) can currently be derived in vitro from pluripotent cells exposed to signalling cocktails and aggregated into large embryonic bodies, but these do not recapitulate the native embryonic environment during PGC formation. Here, we show that mouse gastruloids, a three-dimensional in vitro model of gastrulation, contain a population of gastruloid-derived PGCLCs (Gld-PGCLCs) that resemble early PGCs in vivo. Importantly, the conserved organisation of mouse gastruloids leads to coordinated spatial and temporal localisation of Gld-PGCLCs relative to surrounding somatic cells, even in the absence of specific exogenous PGC-specific signalling or extra-embryonic tissues. In gastruloids, self-organised interactions between cells and tissues, including the endodermal epithelium, enables the specification and subsequent maturation of a pool of Gld-PGCLCs. As such, mouse gastruloids represent a new source of PGCLCs in vitro and, owing to their inherent co-development, serve as a novel model to study the dynamics of PGC development within integrated tissue environments.

Clinical Challenges in the Management of Malignant Ovarian Germ Cell Tumours

Nonepithelial ovarian cancers (NEOC) are a group of rare malignancies, including germ cell tumours (GCT) and sex cord-stromal tumours (SCST), along with small-cell carcinomas and sarcomas. GCTs represent 2-5% of ovarian cancers, with a yearly incidence of 4:100,000, and they usually affect young women and adolescents. Precursory germ cells of the ovary form the basis of GCT. They are histologically classified into primitive GCT, teratomas, and monodermal and somatic-type tumours associated with dermoid cysts. A primitive GCT can be either a yolk sac tumour (YST), dysgerminoma, or mixed germ cell neoplasm. Teratomas are either mature (benign) or immature (malignant). Given that malignant GCTs occur rarely compared to epithelial ovarian tumours (EOC), greater focus is required in their diagnosis and treatment. In this article, we review the epidemiology, clinical manifestations, diagnosis, and molecular biology, along with the management and therapeutic challenges.

Review: A barnyard in the lab: prospect of generating animal germ cells for breeding and conservation

In vitro gametogenesis (IVG) offers broad opportunities for gaining detailed new mechanistic knowledge of germ cell biology that will enable progress in the understanding of human infertility, as well as for applications in the conservation of endangered species and for accelerating genetic selection of livestock. The realisation of this potential depends on overcoming key technical challenges and of gaining more detailed knowledge of the ontogeny and developmental programme in different species. Important differences in the molecular mechanisms of germ cell determination and epigenetic reprogramming between mice and other animals have been elucidated in recent years. These must be carefully considered when developing IVG protocols, as cellular kinetics in mice may not accurately reflect mechanisms in other mammals. Similarly, diverse stem cell models with potential for germ cell differentiation may reflect alternative routes to successful IVG. In conclusion, the fidelity of the developmental programme recapitulated during IVG must be assessed against reference information from each species to ensure the production of healthy animals using these methods, as well as for developing genuine models of gametogenesis.

The pluripotency factor Tex10 finetunes Wnt signaling for PGC and male germline development

Testis-specific transcript 10 (Tex10) is a critical factor for pluripotent stem cell maintenance and preimplantation development. Here, we dissect its late developmental roles in primordial germ cell (PGC) specification and spermatogenesis using cellular and animal models. We discover that Tex10 binds the Wnt negative regulator genes, marked by H3K4me3, at the PGC-like cell (PGCLC) stage in restraining Wnt signaling. Depletion and overexpression of Tex10 hyperactivate and attenuate the Wnt signaling, resulting in compromised and enhanced PGCLC specification efficiency, respectively. Using the Tex10 conditional knockout mouse models combined with single-cell RNA sequencing, we further uncover critical roles of Tex10 in spermatogenesis with Tex10 loss causing reduced sperm number and motility associated with compromised round spermatid formation. Notably, defective spermatogenesis in Tex10 knockout mice correlates with aberrant Wnt signaling upregulation. Therefore, our study establishes Tex10 as a previously unappreciated player in PGC specification and male germline development by fine-tuning Wnt signaling.

Induction of primordial germ cell-like cells from common marmoset embryonic stem cells by inhibition of WNT and retinoic acid signaling

Reconstitution of the germ cell lineage using pluripotent stem cells provides a unique platform to deepen our understanding of the mechanisms underlying germ cell development and to produce functional gametes for reproduction. This study aimed to establish a culture system that induces a robust number of primordial germ cell-like cells (PGCLCs) from common marmoset (Callithrix jacchus) embryonic stem cells. The robust induction was achieved by not only activation of the conserved PGC-inducing signals, WNT and BMP4, but also temporal inhibitions of WNT and retinoic acid signals, which prevent mesodermal and neural differentiation, respectively, during PGCLC differentiation. Many of the gene expression and differentiation properties of common marmoset PGCLCs were similar to those of human PGCLCs, making this culture system a reliable and useful primate model. Finally, we identified PDPN and KIT as surface marker proteins by which PGCLCs can be isolated from embryonic stem cells without genetic manipulation. This study will expand the opportunities for research on germ cell development and production of functional gametes to the common marmoset.

Oocyte Arrested at Metaphase II Stage were Derived from Human Pluripotent Stem Cells in vitro

Initiation of meiosis is the most difficult aspect of inducing competent oocytes differentiation from human stem cells in vitro. Human induced pluripotent stem cells (hiPSCs) and embryonic stem cells (hESCs) were cultured with follicle fluid, cytokines and small molecule to induced oocyte-like cells (OLCs) formation through a three-step induction procedure. Expression of surface markers and differentiation potential of germ cells were analyzed in vitro by flow cytometry, gene expression, immunocytochemistry, western blotting and RNA Sequencing. To induce the differentiation of hiPSCs into OLCs, cells were firstly cultured with a primordial germ cell medium for 10 days. The cells exhibited similar morphological features to primordial germ cells (PGCs), high expressing of germ cell markers and primordial follicle development associated genes. The induced PGCs were then cultured with the primordial follicle-like cell medium for 5 days to form the induced follicle-like structures (iFLs), which retained both primordial oocytes-like cells and granulosa-like cells. In the third step, the detached iFLs were harvested and transferred to the OLC-medium for additional 10 days. The cultured cells developed cumulus-oocyte-complexes (COCs) structures and OLCs with different sizes (50-150 μm diameter) and a zona pellucida. The in vitro matured OLCs had polar bodies and were arrested at metaphase II (MII) stage. Some OLCs were self-activated and spontaneously developed into multiple-cell structures similar to preimplantation embryos, indicating that OLCs were parthenogenetically activated though in vitro fertilization potential of OLCs are yet to be proved. in vitro maturation of OLCs derived from hiPSCs provides a new means to study human germ cell formation and oogenesis.

Short telomeres impede germ cell specification by upregulating MAPK and TGFβ signaling

Functional telomeres protect chromosome ends and play important roles in stem cell maintenance and differentiation. Short telomeres negatively impact germ cell development and can contribute to age-associated infertility. Moreover, telomere syndrome resulting from mutations of telomerase or telomere-associated genes exhibits short telomeres and reduced fertility. It remains elusive whether and how telomere lengths affect germ cell specification. We report that functional telomere is required for the coordinated germ cell and somatic cell fate decisions. Using telomerase gene Terc deficient mice as a model, we show that short telomeres restrain germ cell specification of epiblast cells but promote differentiation towards somatic lineage. Short telomeres increase chromatin accessibility to elevate TGFβ and MAPK/ERK signaling for somatic cell differentiation. Notably, elevated Fst expression in TGFβ pathway represses the BMP4-pSmad signaling pathway, thus reducing germ cell formation. Re-elongation of telomeres by targeted knock-in of Terc restores normal chromatin accessibility to suppress TGFβ and MAPK signaling, thereby facilitating germ cell formation. Taken together, our data reveal that functional telomeres are required for germ cell specification by repressing TGFβ and MAPK signaling.

Efficient generation of marmoset primordial germ cell-like cells using induced pluripotent stem cells

Reconstitution of germ cell fate from pluripotent stem cells provides an opportunity to understand the molecular underpinnings of germ cell development. Here, we established robust methods for induced pluripotent stem cell (iPSC) culture in the common marmoset (Callithrix jacchus [cj]), allowing stable propagation in an undifferentiated state. Notably, iPSCs cultured on a feeder layer in the presence of a WNT signaling inhibitor upregulated genes related to ubiquitin-dependent protein catabolic processes and enter a permissive state that enables differentiation into primordial germ cell-like cells (PGCLCs) bearing immunophenotypic and transcriptomic similarities to pre-migratory cjPGCs in vivo. Induction of cjPGCLCs is accompanied by transient upregulation of mesodermal genes, culminating in the establishment of a primate-specific germline transcriptional network. Moreover, cjPGCLCs can be expanded in monolayer while retaining the germline state. Upon co-culture with mouse testicular somatic cells, these cells acquire an early prospermatogonia-like phenotype. Our findings provide a framework for understanding and reconstituting marmoset germ cell development in vitro, thus providing a comparative tool and foundation for a preclinical modeling of human in vitro gametogenesis.

Specification of human germ cell fate with enhanced progression capability supported by hindgut organoids

Human primordial germ cells (hPGCs), the precursors of sperm and eggs, are specified during weeks 2-3 after fertilization. Few studies on ex vivo and in vitro cultured human embryos reported plausible hPGCs on embryonic day (E) 12-13 and in an E16-17 gastrulating embryo. In vitro, hPGC-like cells (hPGCLCs) can be specified from the intermediary pluripotent stage or peri-gastrulation precursors. Here, we explore the broad spectrum of hPGCLC precursors and how different precursors impact hPGCLC development. We show that resetting precursors can give rise to hPGCLCs (rhPGCLCs) in response to BMP. Strikingly, rhPGCLCs co-cultured with human hindgut organoids progress at a pace reminiscent of in vivo hPGC development, unlike those derived from peri-gastrulation precursors. Moreover, rhPGCLC specification depends on both EOMES and TBXT, not just on EOMES as for peri-gastrulation hPGCLCs. Importantly, our study provides the foundation for developing efficient in vitro models of human gametogenesis.

Efficient generation of ETX embryoids that recapitulate the entire window of murine egg cylinder development

The murine embryonic-trophoblast-extra-embryonic endoderm (ETX) model is an integrated stem cell-based model to study early postimplantation development. It is based on the self-assembly potential of embryonic, trophoblast, and hypoblast/primitive/visceral endoderm-type stem cell lines (ESC, TSC, and XEN, respectively) to arrange into postimplantation egg cylinder-like embryoids. Here, we provide an optimized method for reliable and efficient generation of ETX embryoids that develop into late gastrulation in static culture conditions. It is based on transgenic Gata6-overproducing ESCs and modified assembly and culture conditions. Using this method, up to 43% of assembled ETX embryoids exhibited a correct spatial distribution of the three stem cell derivatives at day 4 of culture. Of those, 40% progressed into ETX embryoids that both transcriptionally and morphologically faithfully mimicked in vivo postimplantation mouse development between E5.5 and E7.5. The ETX model system offers the opportunity to study the murine postimplantation egg cylinder stages and could serve as a source of various cell lineage precursors.

Robust induction of primordial germ cells of white rhinoceros on the brink of extinction

In vitro gametogenesis, the process of generating gametes from pluripotent cells in culture, is a powerful tool for improving our understanding of germ cell development and an alternative source of gametes. Here, we induced primordial germ cell-like cells (PGCLCs) from pluripotent stem cells of the northern white rhinoceros (NWR), a species for which only two females remain, and southern white rhinoceros (SWR), the closest species to the NWR. PGCLC differentiation from SWR embryonic stem cells is highly reliant on bone morphogenetic protein and WNT signals. Genetic analysis revealed that SRY-box transcription factor 17 (SOX17) is essential for SWR-PGCLC induction. Under the defined condition, NWR induced pluripotent stem cells differentiated into PGCLCs. We also identified cell surface markers, CD9 and Integrin subunit alpha 6 (ITGA6), that enabled us to isolate PGCLCs without genetic alteration in pluripotent stem cells. This study provides a first step toward the production of NWR gametes in culture and understanding of the basic mechanism of primordial germ cell specification in a large animal.

Reconstitution of reproductive organ system that produces functional oocytes

Reproductive organs have unique developmental and functional properties that enable them to manage both germ cell development and the endocrine system in a sex-dependent manner. Proper reconstitution of the reproductive organs, therefore, will contribute to a deeper understanding of the mechanisms underlying germ cell development and sex-determination. However, reproductive organs have not yet been systematically reconstituted from pluripotent stem cells. This is largely due to technical problems in the reconstitution of the germ cell and somatic cell lineages, which have very different developmental trajectories. Accordingly, faithful construction of reproductive organoids requires that the reconstitution and evaluation of these two different cell lineages be performed separately. Here, we update the state-of-the-art in the reconstitution of reproductive organoids that produce functional oocytes.

Decoding the Spermatogenesis Program: New Insights from Transcriptomic Analyses

Spermatogenesis is a complex differentiation process coordinated spatiotemporally across and along seminiferous tubules. Cellular heterogeneity has made it challenging to obtain stage-specific molecular profiles of germ and somatic cells using bulk transcriptomic analyses. This has limited our ability to understand regulation of spermatogenesis and to integrate knowledge from model organisms to humans. The recent advancement of single-cell RNA-sequencing (scRNA-seq) technologies provides insights into the cell type diversity and molecular signatures in the testis. Fine-grained cell atlases of the testis contain both known and novel cell types and define the functional states along the germ cell developmental trajectory in many species. These atlases provide a reference system for integrated interspecies comparisons to discover mechanistic parallels and to enable future studies. Despite recent advances, we currently lack high-resolution data to probe germ cell-somatic cell interactions in the tissue environment, but the use of highly multiplexed spatial analysis technologies has begun to resolve this problem. Taken together, recent single-cell studies provide an improvedunderstanding of gametogenesis to examine underlying causes of infertility and enable the development of new therapeutic interventions.

Naïve pluripotent-like characteristics of non-tumorigenic Muse cells isolated from human amniotic membrane

Multilineage-differentiating stress-enduring (Muse) cells are non-tumorigenic pluripotent-like stem cells that exhibit triploblastic differentiation and self-renewability at the single-cell level, and are collectable as pluripotent surface marker SSEA-3(+) from the bone marrow (BM), peripheral blood, and organ connective tissues. SSEA-3(+) cells from human amniotic membrane mesenchymal stem cells (hAMSCs) were compared with hBM-Muse cells. Similar to hBM-Muse cells, hAMSC-SSEA-3(+) cells expressed pluripotency genes (OCT3/4, NANOG, and SOX2), differentiated into triploblastic cells from a single cell, self-renewed, and exhibited non-tumorigenicity. Notably, however, they exhibited unique characteristics not seen in hBM-Muse cells, including higher expression of genes related to germline- and extraembryonic cell-lineages compared with those in hBM-Muse cells in single-cell RNA-sequencing; and enhanced expression of markers relevant to germline- (PRDM14, TFAP2C, and NANOS3) and extraembryonic cell- (CDX2, GCM1, and ID2) lineages when induced by cytokine subsets, suggesting a broader differentiation potential similar to naïve pluripotent stem cells. t-SNE dimensionality reduction and Gene ontology analysis visualized hAMSC-SSEA-3(+) cells comprised a large undifferentiated subpopulation between epithelial- and mesenchymal-cell states and a small mesenchymal subpopulation expressing genes relevant to the placental formation. The AM is easily accessible by noninvasive approaches. These unique cells are a potentially interesting target naïve pluripotent stem cell-like resource without tumorigenicity.

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.

Regulation of both transcription and RNA turnover contribute to germline specification

The nuanced mechanisms driving primordial germ cells (PGC) specification remain incompletely understood since genome-wide transcriptional regulation in developing PGCs has previously only been defined indirectly. Here, using SLAMseq analysis, we determined genome-wide transcription rates during the differentiation of embryonic stem cells (ESCs) to form epiblast-like (EpiLC) cells and ultimately PGC-like cells (PGCLCs). This revealed thousands of genes undergoing bursts of transcriptional induction and rapid shut-off not detectable by RNAseq analysis. Our SLAMseq datasets also allowed us to infer RNA turnover rates, which revealed thousands of mRNAs stabilized and destabilized during PGCLC specification. mRNAs tend to be unstable in ESCs and then are progressively stabilized as they differentiate. For some classes of genes, mRNA turnover regulation collaborates with transcriptional regulation, but these processes oppose each other in a surprisingly high frequency of genes. To test whether regulated mRNA turnover has a physiological role in PGC development, we examined three genes that we found were regulated by RNA turnover: Sox2, Klf2 and Ccne1. Circumvention of their regulated RNA turnover severely impaired the ESC-to-EpiLC and EpiLC-to-PGCLC transitions. Our study demonstrates the functional importance of regulated RNA stability in germline development and provides a roadmap of transcriptional and post-transcriptional regulation during germline specification.

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.

A New Horizon in Reproductive Research with Pluripotent Stem Cells: Successful In Vitro Gametogenesis in Rodents, Its Application to Large Animals, and Future In Vitro Reconstitution of Reproductive Organs Such as “Uteroid” and “Oviductoid”

Simple Summary

Functional gametes, such as oocytes and spermatozoa, have been derived from rodent pluripotent stem cells, which can be applied to large animals and ultimately, to humans. In addition to summarizing these topics, we also review additional approaches for in vitro reconstitution of reproductive organs. This review illustrates intensive past efforts and future challenges on stem cell research for in vitro biogenesis in various mammalian models.

Abstract

Recent success in derivation of functional gametes (oocytes and spermatozoa) from pluripotent stem cells (PSCs) of rodents has made it feasible for future application to large animals including endangered species and to ultimately humans. Here, we summarize backgrounds and recent studies on in vitro gametogenesis from rodent PSCs, and similar approaches using PSCs from large animals, including livestock, nonhuman primates (NHPs), and humans. We also describe additional developing approaches for in vitro reconstitution of reproductive organs, such as the ovary (ovarioid), testis (testisoid), and future challenges in the uterus (uteroid) and oviduct (oviductoid), all of which may be derived from PSCs. Once established, these in vitro systems may serve as a robust platform for elucidating the pathology of infertility-related disorders and ectopic pregnancy, principle of reproduction, and artificial biogenesis. Therefore, these possibilities, especially when using human cells, require consideration of ethical issues, and international agreements and guidelines need to be raised before opening “Pandora’s Box”.

BMP4 drives primed to naïve transition through PGC-like state

Multiple pluripotent states have been described in mouse and human stem cells. Here, we apply single-cell RNA-seq to a newly established BMP4 induced mouse primed to naïve transition (BiPNT) system and show that the reset is not a direct reversal of cell fate but goes through a primordial germ cell-like cells (PGCLCs) state. We first show that epiblast stem cells bifurcate into c-Kit⁺ naïve and c-Kit^- trophoblast-like cells, among which, the naïve branch undergoes further transition through a PGCLCs intermediate capable of spermatogenesis in vivo. Mechanistically, we show that DOT1L inhibition permits the transition from primed pluripotency to PGCLCs in part by facilitating the loss of H3K79me2 from Gata3/6. In addition, Prdm1/Blimp1 is required for PGCLCs and naïve cells, while Gata2 inhibits PGC-like state by promoting trophoblast-like fate. Our work not only reveals an alternative route for primed to naïve transition, but also gains insight into germ cell development.

Transcriptional control of human gametogenesis

The pathways of gametogenesis encompass elaborate cellular specialization accompanied by precise partitioning of the genome content in order to produce fully matured spermatozoa and oocytes. Transcription factors are an important class of molecules that function in gametogenesis to regulate intrinsic gene expression programs, play essential roles in specifying (or determining) germ cell fate and assist in guiding full maturation of germ cells and maintenance of their populations. Moreover, in order to reinforce or redirect cell fate in vitro, it is transcription factors that are most frequently induced, over-expressed or activated. Many reviews have focused on the molecular development and genetics of gametogenesis, in vivo and in vitro, in model organisms and in humans, including several recent comprehensive reviews: here, we focus specifically on the role of transcription factors. Recent advances in stem cell biology and multi-omic studies have enabled deeper investigation into the unique transcriptional mechanisms of human reproductive development. Moreover, as methods continually improve, in vitro differentiation of germ cells can provide the platform for robust gain- and loss-of-function genetic analyses. These analyses are delineating unique and shared human germ cell transcriptional network components that, together with somatic lineage specifiers and pluripotency transcription factors, function in transitions from pluripotent stem cells to gametes. This grand theme review offers additional insight into human infertility and reproductive disorders that are linked predominantly to defects in the transcription factor networks and thus may potentially contribute to the development of novel treatments for infertility.

Canonical Wnt Signaling Promotes Formation of Somatic Permeability Barrier for Proper Germ Cell Differentiation

Morphogen-mediated signaling is critical for proper organ development and stem cell function, and well-characterized mechanisms spatiotemporally limit the expression of ligands, receptors, and ligand-binding cell-surface glypicans. Here, we show that in the developing Drosophila ovary, canonical Wnt signaling promotes the formation of somatic escort cells (ECs) and their protrusions, which establish a physical permeability barrier to define morphogen territories for proper germ cell differentiation. The protrusions shield germ cells from Dpp and Wingless morphogens produced by the germline stem cell (GSC) niche and normally only received by GSCs. Genetic disruption of EC protrusions allows GSC progeny to also receive Dpp and Wingless, which subsequently disrupt germ cell differentiation. Our results reveal a role for canonical Wnt signaling in specifying the ovarian somatic cells necessary for germ cell differentiation. Additionally, we demonstrate the morphogen-limiting function of this physical permeability barrier, which may be a common mechanism in other organs across species.

Germline specification from pluripotent stem cells

Reproduction is a key event in life guaranteeing the propagation and evolution of a species. Infertility caused by abnormal germ cell development is a topic of extensive concern. Herein, in vitro germline specification studies provide a modeling platform to investigate gametogenesis. The differentiation of pluripotent stem cells (PSCs) into germ cells has been studied for more than 30 years, and there have been many astonishing breakthroughs in the last decade. Fertile sperm and oocytes can be obtained from mouse embryonic stem cells (ESCs) through a primordial germ cell (PGC)-based method. Moreover, human PGC-like cells (PGCLCs) can be derived with a similar strategy as that used for mouse PGCLC derivation. In this review, we describe the reconstitution of PGCs and the subsequent meiosis, as well as the signaling pathways and factors involved in these processes.

Dual role of Ovol2 on the germ cell lineage segregation during gastrulation in mouse embryogenesis

In mammals, primordial germ cells (PGCs), the origin of the germ line, are specified from the epiblast at the posterior region where gastrulation simultaneously occurs, yet the functional relationship between PGC specification and gastrulation remains unclear. Here, we show that OVOL2, a transcription factor conserved across the animal kingdom, balances these major developmental processes by repressing the epithelial-to-mesenchymal transition (EMT) that drives gastrulation and the upregulation of genes associated with PGC specification. Ovol2a, a splice variant encoding a repressor domain, directly regulates EMT-related genes and, consequently, induces re-acquisition of potential pluripotency during PGC specification, whereas Ovol2b, another splice variant missing the repressor domain, directly upregulates genes associated with PGC specification. Taken together, these results elucidate the molecular mechanism underlying allocation of the germ line among epiblast cells differentiating into somatic cells through gastrulation.

This article has an associated ‘The people behind the papers’ interview.

Generation of Miniaturized Ovaries by In Vitro Culture from Mouse Gonads

The incidence of infertility among individuals of reproductive age has been growing due to genetic and environmental factors, and considerable research efforts are focused on solving this issue. Ovarian development is an overly complex process in the body, involving the interaction between primordial germ cells and gonad somatic cells. However, follicles located in the center of the in vitro ovary are poorly formed owing to ovarian complexity, nutrient deficiency, and signaling deficiency. In the present study, we optimized methods for dissociating gonads and culture conditions for the in vitro generation of miniaturized ovaries. The gonads from embryos were dissociated into cell masses and cultured on a Transwell-COL membrane for 3–5 weeks. Approximately 12 follicles were present per in vitro ovary. We observed that miniaturized ovaries successfully matured to MII oocytes in vitro from 150 to 100 µm gonad masses. This method will be useful for investigating follicle development and oocyte production.