Principles of early human development and germ cell program from conserved model systems

Factors within the Developing Embryo and Ovarian Microenvironment That Influence Primordial Germ Cell Fate

BACKGROUND

Primordial germ cell (PGC) fate is dictated by the designation, taxis, and influence of the surrounding embryonic somatic cells. Whereas gonadal sex determination results from a balance of factors within the tissue microenvironment.

SUMMARY

Our understanding of mammalian ovary development is formed in large part from developmental time courses established using murine models. Genomic tools where genes implicated in the PGC designation or gonadal sex determination have been modulated through complete or conditional knockouts in vivo, and studies in in situ models with inhibitors or cultures that alter the native gonadal environment have pieced together the interplay of pioneering transcription factors, co-regulators and chromosomes critical for the progression of PGCs to oocytes. Tools such as pluripotent stem cell derivation, genomic modifications, and aggregate differentiation cultures have yielded some insight into the human condition. Additional understanding of sex determination, both gonadal and anatomical, may be inferred from phenotypes that arise from de novo or inherited gene variants in humans who have differences in sex development.

KEY MESSAGES

This review highlights major factors critical for PGC specification and migration, and in ovarian gonad specification by reviewing seminal murine models. These pathways are compared to what is known about the human condition from expression profiles of fetal gonadal tissue, use of human pluripotent stem cells, or disorders resulting from disease variants. Many of these pathways are challenging to decipher in human tissues. However, the impact of new single-cell technologies and whole-genome sequencing to reveal disease variants of idiopathic reproductive tract phenotypes will help elucidate the mechanisms involved in human ovary development.

John JC. Does supplementation of oocytes with additional mtDNA influence developmental outcome?

Introducing extra mitochondrial DNA (mtDNA) into oocytes at fertilization can rescue poor quality oocytes. However, supplementation alters DNA methylation and gene expression profiles of preimplantation embryos. To determine if these alterations impacted offspring, we introduced mtDNA from failed-to-mature sister (autologous) or third party (heterologous) oocytes into mature oocytes and transferred zygotes into surrogates. Founders exhibited significantly greater daily weight gain (heterologous) and growth rates (heterologous and autologous) to controls. In weaners, cholesterol, bilirubin (heterologous and autologous), anion gap, and lymphocyte count (autologous) were elevated. In mature pigs, potassium (heterologous) and bicarbonate (autologous) were altered. mtDNA and imprinted gene analyses did not reveal aberrant profiles. Neither group exhibited gross anatomical, morphological, or histopathological differences that would lead to clinically significant lesions. Female founders were fertile and their offspring exhibited modified weight and height gain, biochemical, and hematological profiles. mtDNA supplementation induced minor differences that did not affect health and well-being.

Loss-of-function mutations of SOX17 lead to YAP/TEAD activation-dependent malignant transformation in endometrial cancer

Aberrant hyperactivation of the Hippo pathway effector YAP/TEAD complex causes tissue overgrowth and tumorigenesis in various cancers, including endometrial cancer (EC). The transcription factor SOX17 (SRY [sex-determining region Y]-box 17) is frequently mutated in EC; however, SOX17 mutations are rare in other cancer types. The molecular mechanisms underlying SOX17 mutation-induced EC tumorigenesis remain poorly understood. Here, we demonstrate that SOX17 serves as a tumor suppressor to restrict the proliferation, migration, invasion, and anchorage-independent growth of EC cells, partly by suppressing the transcriptional outputs of the Hippo-YAP/TEAD pathway. SOX17 binds to TEAD transcription factors through its HMG domain and attenuates the DNA-binding ability of TEAD. SOX17 loss by inactivating mutations leads to the malignant transformation of EC cells, which can be reversed by small-molecule inhibitors of YAP/TEAD or cabozantinib, an FDA-approved drug targeting the YAP/TEAD transcriptional target AXL. Our findings reveal novel molecular mechanisms underlying Hippo-YAP/TEAD pathway-driven EC tumorigenesis, and suggest potential therapeutic strategies targeting the Hippo-YAP/TEAD pathway in SOX17-mutated EC.

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.

Stay on the road: from germ cell specification to gonadal colonization in mammals

The founder cells of the gametes are primordial germ cells (PGCs). In mammals, PGCs are specified early during embryonic development, at the boundary between embryonic and extraembryonic tissue, long before their later residences, the gonads, have developed. Despite the differences in form and behaviour when differentiated into oocytes or sperm cells, in the period between specification and gonadal colonization, male and female PGCs are morphologically indistinct and largely regulated by similar mechanisms. Here, we compare different modes and mechanisms that lead to the formation of PGCs, putting in context protocols that are in place to differentiate both human and mouse pluripotent stem cells into PGC-like cells. In addition, we review important aspects of the migration of PGCs to the gonadal ridges, where they undergo further sex-specific differentiation. Defects in migration need to be effectively corrected, as misplaced PGCs can become tumorigenic. Concluding, a combination of in vivo studies and the development of adequate innovative in vitro models, ensuring both robustness and standardization, are providing us with the tools for a greater understanding of the first steps of gametogenesis and to develop disease models to study the origin of germ cell tumours.

This article is part of the theme issue ‘Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom’.

Generation of germ cell-deficient pigs by NANOS3 knockout

NANOS3 is an evolutionarily conserved gene expressed in primordial germ cells that is important for germ cell development. Germ cell deletion by NANOS3 knockout has been reported in several mammalian species, but its function in pigs is unclear. In the present study, we investigated the germline effects of NANOS3 knockout in pigs using CRISPR/Cas9. Embryo transfer of CRISPR/Cas9-modified embryos produced ten offspring, of which one showed wild-type NANOS3 alleles, eight had two mutant NANOS3 alleles, and the other exhibited mosaicism (four mutant alleles). Histological analysis revealed no germ cells in the testes or ovaries of any of the nine mutant pigs. These results demonstrated that NANOS3 is crucial for porcine germ cell production.

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.

Laminin111-based defined culture promoting self-renewing human pluripotent stem cells with properties of the early post-implantation epiblast

In the mammalian embryo, a formative pluripotent phase is proposed to exist at the early post-implantation period, during the transition from the pre-implantation naive-to the post-implantation primed-epiblast. By recapitulating a laminin component of the extracellular matrix niche during embryonic formative transition, and defined culture conditions, we generated cultures highly enriched for self-renewing human pluripotent stem cells (hPSCs), exhibiting properties of early post-implantation epiblast cells. These hPSCs display post-implantation-epiblast gene expression profiles. FGF and TGF-β signaling maintain their self-renewal for multiple passages. They have inactive canonical Wnt signaling, do not express primitive streak markers, and are competent to initiate differentiation toward germline and somatic fates. hPSCs exhibiting early post-implantation epiblast properties may shed light on human embryonic PSCs development and may serve for initiating somatic and germ cell specification.

The balance between NANOG and SOX17 mediated by TET proteins regulates specification of human primordial germ cell fate

Background

Human primordial germ cells (hPGCs) initiate from the early post-implantation embryo at week 2–3 and undergo epigenetic reprogramming during development. However, the regulatory mechanism of DNA methylation during hPGC specification is still largely unknown due to the difficulties in analyzing early human embryos. Using an in vitro model of hPGC induction, we found a novel function of TET proteins and NANOG in the hPGC specification which was different from that discovered in mice.

Methods

Using the CRISPR–Cas9 system, we generated a set of TET1, TET2 and TET3 knockout H1 human embryonic stem cell (hESC) lines bearing a BLIMP1-2A-mKate2 reporter. We determined the global mRNA transcription and DNA methylation profiles of pluripotent cells and induced hPGC-like cells (hPGCLCs) by RNA-seq and whole-genome bisulfite sequencing (WGBS) to reveal the involved signaling pathways after TET proteins knockout. ChIP-qPCR was performed to verify the binding of TET and NANOG proteins in the SOX17 promoter. Real-time quantitative PCR, western blot and immunofluorescence were performed to measure gene expression at mRNA and protein levels. The efficiency of hPGC induction was evaluated by FACS.

Results

In humans, TET1, TET2 and TET3 triple-knockout (TKO) human embryonic stem cells (hESCs) impaired the NODAL signaling pathway and impeded hPGC specification in vitro, while the hyperactivated NODAL signaling pathway led to gastrulation failure when Tet proteins were inactivated in mouse. Specifically, TET proteins stimulated SOX17 through the NODAL signaling pathway and directly regulates NANOG expression at the onset of hPGCLCs induction. Notably, NANOG could bind to SOX17 promoter to regulate its expression in hPGCLCs specification. Furthermore, in TKO hESCs, DNMT3B-mediated hypermethylation of the NODAL signaling-related genes and NANOG/SOX17 promoters repressed their activation and inhibited hPGCLC induction. Knockout of DNMT3B in TKO hESCs partially restored NODAL signaling and NANOG/SOX17 expression, and rescued hPGCLC induction.

Conclusion

Our results show that TETs-mediated oxidation of 5-methylcytosine modulates the NODAL signaling pathway and its downstream genes, NANOG and SOX17, by promoting demethylation in opposition to DNMT3B-mediated methylation, suggesting that the epigenetic balance of DNA methylation and demethylation in key genes plays a fundamental role in early hPGC specification.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00917-0.

Melatonin promotes the proliferation of primordial germ cell-like cells derived from porcine skin-derived stem cells: A mechanistic analysis

In vitro differentiation of stem cells into functional gametes remains of great interest in the biomedical field. Skin-derived stem cells (SDSCs) are an adult stem cells that provides a wide range of clinical applications without inherent ethical restrictions. In this paper, porcine SDSCs were successfully differentiated into primordial germ cell-like cells (PGCLCs) in conditioned media. The PGCLCs were characterized in terms of cell morphology, marker gene expression, and epigenetic properties. Furthermore, we also found that 25 μM melatonin (MLT) significantly increased the proliferation of the SDSC-derived PGCLCs while acting through the MLT receptor type 1 (MT1). RNA-seq results found the mitogen-activated protein kinase (MAPK) signaling pathway was more active when PGCLCs were cultured with MLT. Moreover, the effect of MLT was attenuated by the use of S26131 (MT1 antagonist), crenolanib (platelet-derived growth factor receptor inhibitor), U0126 (mitogen-activated protein kinase kinase inhibitor), or CCG-1423 (serum response factor transcription inhibitor), suggesting that MLT promotes the proliferation processes through the MAPK pathway. Taken together, this study highlights the role of MLT in promoting PGCLCs proliferation. Importantly, this study provides a suitable in vitro model for use in translational studies and could help to answer numerous remaining questions related to germ cell physiology.

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.

In vitro germ cell induction from fertile and infertile monozygotic twin research participants

Human induced pluripotent stem cells (hiPSCs) enable reproductive diseases to be studied when the reproductive health of the participant is known. In this study, monozygotic (MZ) monoamniotic (MA) twins discordant for primary ovarian insufficiency (POI) consent to research to address the hypothesis that discordant POI is due to a shared primordial germ cell (PGC) progenitor pool. If this is the case, reprogramming the twin's skin cells to hiPSCs is expected to restore equivalent germ cell competency to the twins hiPSCs. Following reprogramming, the infertile MA twin's cells are capable of generating human PGC-like cells (hPGCLCs) and amniotic sac-like structures equivalent to her fertile twin sister. Using these hiPSCs together with genome sequencing, our data suggest that POI in the infertile twin is not due to a genetic barrier to amnion or germ cell formation and support the hypothesis that during gestation, amniotic PGCs are likely disproportionately allocated to the fertile twin with embryo splitting.

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.

Fluroxypyr-1-methylheptyl ester induced ROS production and mitochondrial apoptosis through the MAPK signaling cascade in porcine trophectoderm and uterine luminal epithelial cells

FPMH (Fluroxypyr-1-methylheptyl ester) is a synthetic auxin herbicide used in agriculture. The mechanism by which FPMH induces adverse effects in porcine trophectoderm (pTr) and porcine uterine luminal epithelial (pLE) cells, which are involved in porcine implantation, have not been studied yet. Therefore, the present study investigates the toxicological effects of FPMH on pTr and pLE cells. We confirmed that FPMH induced cytotoxic effects on the cells, including apoptosis induction, mitochondrial membrane potential (MMP) depolarization, and ROS production. The phosphorylation of the MAPK pathway (ERK1/2, JNK, and p38) was dysregulated by FPMH administration. In addition, FPMH could suppress cell-cell adhesion and migration abilities of pTr and pLE, which are crucial for implantation. Therefore, exposure to FPMH induced adverse effects in pTr and pLE cells and could result in implantation failure.

Single-cell analysis of embryoids reveals lineage diversification roadmaps of early human development

Despite its clinical and fundamental importance, our understanding of early human development remains limited. Stem cell-derived, embryo-like structures (or embryoids) allowing studies of early development without using natural embryos can potentially help fill the knowledge gap of human development. Herein, transcriptome at the single-cell level of a human embryoid model was profiled at different time points. Molecular maps of lineage diversifications from the pluripotent human epiblast toward the amniotic ectoderm, primitive streak/mesoderm, and primordial germ cells were constructed and compared with in vivo primate data. The comparative transcriptome analyses reveal a critical role of NODAL signaling in human mesoderm and primordial germ cell specification, which is further functionally validated. Through comparative transcriptome analyses and validations with human blastocysts and in vitro cultured cynomolgus embryos, we further proposed stringent criteria for distinguishing between human blastocyst trophectoderm and early amniotic ectoderm cells.

Using Machine Learning to Identify Biomarkers Affecting Fat Deposition in Pigs by Integrating Multisource Transcriptome Information

Fat deposition in pigs is not only closely related to pig production efficiency and pork quality but also an ideal model for human obesity. Transcriptome sequencing is widely used to study fat deposition. However, due to small sample sizes, high false positive rates, and poor consistency of results from different studies, new strategies are urgently needed. Machine learning, a new analysis method, can effectively fit complex data and accurately identify samples and genes. In this study, 36 samples of adipose tissue, muscle tissue, and liver tissue were collected from Songliao black pigs and Landrace pigs, and the mRNA of all the samples was sequenced. In addition, we collected transcriptome data for 64 samples in the GEO database from four different sources. After standardization and imputation of missing values in the data set comprising 100 samples, traditional differential expression analysis was carried out, and different numbers of expressed genes were selected as features for the training model of eight machine learning methods. In the 1000 replications of fourfold cross validation with 100 samples, AdaBoost performed best, with an average prediction accuracy greater than 93% and the highest mean area under the curve in predicting the high- and low-fat content groups among the eight ML methods. According to their performance-based ranks inferred by AdaBoost, 12 genes related to fat deposition were identified; among them, FASN and APOD were specifically expressed in adipose tissue, and APOA1 was specifically expressed in the liver, which could be important candidate biomarkers affecting fat deposition.

Ethalfluralin impairs implantation by aggravation of mitochondrial viability and function during early pregnancy

Ethalfluralin, a dinitroaniline-type herbicide, has been used for decades. As a result, its residues are detected on some farmlands. To determine the molecular mechanisms underlying the detrimental effects of ethalfluralin on early pregnancy, porcine luminal epithelium and trophectoderm cell lines were used. Ethalfluralin was found to inhibit the viability, proliferation, and migration of porcine luminal epithelial (pLE) and porcine trophectoderm (pTr) cells. Additionally, ethalfluralin induced apoptotic cell death by means of an imbalance in calcium homeostasis in both pLE and pTr cells. Ethalfluralin decreased mitochondrial membrane potential (ΔΨ_m) and impaired mitochondrial respiration by downregulating the mitochondrial respiratory complex-related genes. Ethalfluralin also activated endoplasmic reticulum stress signals and autophagy pathways, increased the phosphorylation of P38 MAPK and NF-κB, and suppressed the PI3K/AKT signaling pathway. Taken together, this study elucidated the molecular mechanisms by which ethalfluralin impedes the viability and mitochondrial function in fetal trophectoderm and maternal endometrial cells during early pregnancy.

Cryopreservation of porcine skin-derived stem cells using melatonin or trehalose maintains their ability to self-renew and differentiate

Porcine skin-derived stem cells (pSDSCs) are a type of adult stem cells (ASCs) that retain the ability to self-renew and differentiate. Currently, pSDSCs research has entered an intense period of development; however there has been no research regarding methods of cryopreservation. In this paper, we explored an efficient cryopreservation method for pSDSCs. Our results demonstrated that cryopreserving 50 μm diameter pSDSCs aggregates resulted in a lower apoptosis rate and a greater ability to proliferate to form larger spherical cell aggregates than during single-cell cryopreservation. To further optimize the cryopreservation method, we added different concentrations of melatonin (N-acetyl-5-methoxytryptamine, MLT) and trehalose (d-trehalose anhydrous, TRE) to act as cryoprotectants (CPAs) for the pSDSCs. After comparative experiments, we found that the cryopreservation efficiency of 50 mM TRE was superior. Further experiments demonstrated that the reason why 50 mM TRE improved cryopreservation efficiency was that it reduced the intracellular oxidative stress and mitochondrial damage caused by cryopreservation. Taken together, our results suggest that cryopreserving 50 μm diameter pSDSCs aggregates in F12 medium with 10% dimethyl sulfoxide (DMSO) and 50 mM TRE promotes the long-term storage of pSDSCs.

Distinct Roles of NANOS1 and NANOS3 in the Cell Cycle and NANOS3-PUM1-FOXM1 Axis to Control G2/M Phase in a Human Primordial Germ Cell Model

Nanos RNA-binding proteins are critical factors of germline development throughout the animal kingdom and their dysfunction causes infertility. During evolution, mammalian Nanos paralogues adopted divergent roles in germ cell biology. However, the molecular basis behind this divergence, such as their target mRNAs, remains poorly understood. Our RNA-sequencing analysis in a human primordial germ cell model-TCam-2 cell line revealed distinct pools of genes involved in the cell cycle process downregulated upon NANOS1 and NANOS3 overexpression. We show that NANOS1 and NANOS3 proteins influence different stages of the cell cycle. Namely, NANOS1 is involved in the G1/S and NANOS3 in the G2/M phase transition. Many of their cell cycle targets are known infertility and cancer-germ cell genes. Moreover, NANOS3 in complex with RNA-binding protein PUM1 causes 3′UTR-mediated repression of FOXM1 mRNA encoding a transcription factor crucial for G2/M phase transition. Interestingly, while NANOS3 and PUM1 act as post-transcriptional repressors of FOXM1, FOXM1 potentially acts as a transcriptional activator of NANOS3, PUM1, and itself. Finally, by utilizing publicly available RNA-sequencing datasets, we show that the balance between FOXM1-NANOS3 and FOXM1-PUM1 expression levels is disrupted in testis cancer, suggesting a potential role in this disease.

The functional diversity of the POUV-class proteins across vertebrates

POUV is a relatively newly emerged class of POU transcription factors present in jawed vertebrates (Gnathostomata). The function of POUV-class proteins is inextricably linked to zygotic genome activation (ZGA). A large body of evidence now extends the role of these proteins to subsequent developmental stages. While some functions resemble those of other POU-class proteins and are related to neuroectoderm development, others have emerged de novo. The most notable of the latter functions is pluripotency control by Oct4 in mammals. In this review, we focus on these de novo functions in the best-studied species harbouring POUV proteins-zebrafish, Xenopus (anamniotes) and mammals (amniotes). Despite the broad diversity of their biological functions in vertebrates, POUV proteins exert a common feature related to their role in safeguarding the undifferentiated state of cells. Here we summarize numerous pieces of evidence for these specific functions of the POUV-class proteins and recap available loss-of-function data.

Deconstructing human peri-implantation embryogenesis based on embryos and embryoids

The peri-implantation period from blastula to gastrula is one of the crucial stages of human embryo and stem cell development. During development, human embryos undergo many crucial events, such as embryonic lineage differentiation and development, structural self-assembly, pluripotency state transition, cell communication between lineages, and crosstalk between the embryo and uterus. Abnormalities in these developmental events will result in implantation failure or pregnancy loss. However, because of ethical and technical limits, the developmental dynamics of human peri-implantation embryos and the underlying mechanisms of abnormal development remain in a "black box". In this review, we summarize recent progress made towards our understanding of human peri-implantation embryogenesis based on extended in vitro cultured embryos and stem cell-based embryoids. These findings lay an important foundation for understanding early life, promoting research into human stem cells and their application, and preventing and treating infertility. We also propose key scientific issues regarding peri-implantation embryogenesis and provide an outlook on future study directions. Finally, we sum up China's contribution to the field and future opportunities.

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.

Functional Characterization of Endothelial Cells Differentiated from Porcine Epiblast Stem Cells

Endothelial cells (ECs), lining blood vessels' lumen, play an essential role in regulating vascular functions. As multifunctional components of vascular structures, pluripotent stem cells (PSCs) are the promising source for potential therapeutic applications in various vascular diseases. Our laboratory has previously established an approach for differentiating porcine epiblast stem cells (pEpiSCs) into ECs, representing an alternative and potentially superior cell source. However, the condition of pEpiSCs-derived ECs growth has yet to be determined, and whether pEpiSCs differentiate into functional ECs remained unclear. Changes in morphology, proliferation and functional endothelial marker were assessed in pEpiSCs-derived ECs in vitro. pEpiSCs-derived ECs were subjected to magnetic-activated cell sorting (MACS) to collect CD-31+ of ECs. We found that sorted ECs showed the highest proliferation rate in differentiation media in primary culture and M199 media in the subculture. Next, sorted ECs were examined for their ability to act as typical vascular ECs through capillary-like structure formation assay, Dil-acetylated low-density lipoprotein (Dil-Ac-LDL) uptake, and three-dimensional spheroid sprouting. Consequently, pEpiSCs-derived ECs function as typical vascular ECs, indicating that pEpiSC-derived ECs might be used to develop cell therapeutics for vascular disease.

Mitochondrial supplementation of Sus scrofa metaphase II oocytes alters DNA methylation and gene expression profiles of blastocysts

Background

Mitochondrial DNA (mtDNA) copy number in oocytes correlates with oocyte quality and fertilisation outcome. The introduction of additional copies of mtDNA through mitochondrial supplementation of mtDNA-deficient Sus scrofa oocytes resulted in: (1) improved rates of fertilisation; (2) increased mtDNA copy number in the 2-cell stage embryo; and (3) improved development of the embryo to the blastocyst stage. Furthermore, a subset of genes showed changes in gene expression. However, it is still unknown if mitochondrial supplementation alters global and local DNA methylation patterns during early development.

Results

We generated a series of embryos in a model animal, Sus scrofa, by intracytoplasmic sperm injection (ICSI) and mitochondrial supplementation in combination with ICSI (mICSI). The DNA methylation status of ICSI- and mICSI-derived blastocysts was analysed by whole genome bisulfite sequencing. At a global level, the additional copies of mtDNA did not affect nuclear DNA methylation profiles of blastocysts, though over 2000 local genomic regions exhibited differential levels of DNA methylation. In terms of the imprinted genes, DNA methylation patterns were conserved in putative imprint control regions; and the gene expression profile of these genes and genes involved in embryonic genome activation were not affected by mitochondrial supplementation. However, 52 genes showed significant differences in expression as demonstrated by RNAseq analysis. The affected gene networks involved haematological system development and function, tissue morphology and cell cycle. Furthermore, seven mtDNA-encoded t-RNAs were downregulated in mICSI-derived blastocysts suggesting that extra copies of mtDNA affected tRNA processing and/or turnover, hence protein synthesis in blastocysts. We also showed a potential association between differentially methylated regions and changes in expression for 55 genes due to mitochondrial supplementation.

Conclusions

The addition of just an extra ~ 800 copies of mtDNA into oocytes can have a significant impact on both gene expression and DNA methylation profiles in Sus scrofa blastocysts by altering the epigenetic programming established during oogenesis. Some of these changes may affect specific tissue-types later in life. Consequently, it is important to determine the longitudinal effect of these molecular changes on growth and development before considering human clinical practice.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13072-022-00442-x.

Efficient differentiation of human primordial germ cells through geometric control reveals a key role for Nodal signaling

Human primordial germ cells (hPGCs) form around the time of implantation and are the precursors of eggs and sperm. Many aspects of hPGC specification remain poorly understood because of the inaccessibility of the early postimplantation human embryo for study. Here, we show that micropatterned human pluripotent stem cells (hPSCs) treated with BMP4 give rise to hPGC-like cells (hPGCLC) and use these as a quantitatively reproducible and simple in vitro model to interrogate this important developmental event. We characterize micropatterned hPSCs up to 96 hr and show that hPGCLC populations are stable and continue to mature. By perturbing signaling during hPGCLC differentiation, we identify a previously unappreciated role for Nodal signaling and find that the relative timing and duration of BMP and Nodal signaling are critical parameters controlling the number of hPGCLCs. We formulate a mathematical model for a network of cross-repressive fates driven by Nodal and BMP signaling, which predicts the measured fate patterns after signaling perturbations. Finally, we show that hPSC colony size dictates the efficiency of hPGCLC specification, which led us to dramatically improve the efficiency of hPGCLC differentiation.

Functional primordial germ cell-like cells from pluripotent stem cells in rats

The in vitro generation of germ cells from pluripotent stem cells (PSCs) can have a substantial effect on future reproductive medicine and animal breeding. A decade ago, in vitro gametogenesis was established in the mouse. However, induction of primordial germ cell-like cells (PGCLCs) to produce gametes has not been achieved in any other species. Here, we demonstrate the induction of functional PGCLCs from rat PSCs. We show that epiblast-like cells in floating aggregates form rat PGCLCs. The gonadal somatic cells support maturation and epigenetic reprogramming of the PGCLCs. When rat PGCLCs are transplanted into the seminiferous tubules of germline-less rats, functional spermatids-that is, those capable of siring viable offspring-are generated. Insights from our rat model will elucidate conserved and divergent mechanisms essential for the broad applicability of in vitro gametogenesis.

Generation and characterization of stable pig pregastrulation epiblast stem cell lines

Pig epiblast-derived pluripotent stem cells are considered to have great potential and broad prospects for human therapeutic model development and livestock breeding. Despite ongoing attempts since the 1990s, no stably defined pig epiblast-derived stem cell line has been established. Here, guided by insights from a large-scale single-cell transcriptome analysis of pig embryos from embryonic day (E) 0 to E14, specifically, the tracing of pluripotency changes during epiblast development, we developed an in vitro culture medium for establishing and maintaining stable pluripotent stem cell lines from pig E10 pregastrulation epiblasts (pgEpiSCs). Enabled by chemical inhibition of WNT-related signaling in combination with growth factors in the FGF/ERK, JAK/STAT3, and Activin/Nodal pathways, pgEpiSCs maintain their pluripotency transcriptome features, similar to those of E10 epiblast cells, and normal karyotypes after more than 240 passages and have the potential to differentiate into three germ layers. Strikingly, ultradeep in situ Hi-C analysis revealed functional impacts of chromatin 3D-spatial associations on the transcriptional regulation of pluripotency marker genes in pgEpiSCs. In practice, we confirmed that pgEpiSCs readily tolerate at least three rounds of successive gene editing and generated cloned gene-edited live piglets. Our findings deliver on the long-anticipated promise of pig pluripotent stem cells and open new avenues for biological research, animal husbandry, and regenerative biomedicine.

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.

Research Advances in Gametogenesis and Embryogenesis Using Pluripotent Stem Cells

The previous studies of human gametogenesis and embryogenesis have left many unanswered questions, which hinders the understanding of the physiology of these two vital processes and the development of diagnosis and treatment strategies for related diseases. Although many results have been obtained from animal studies, particularly mouse research, the results cannot be fully applied to humans due to species differences in physiology and pathology. However, due to ethical and material limitations, the direct study of human gametes and embryos is very difficult. The emergence and rapid development of organoids allow the construction of organoid systems that simulate gametogenesis and embryogenesis in vitro, and many studies have successfully established organoid systems for some parts of or even the entire processes of gametogenesis and embryogenesis. These studies typically start with the establishment of mouse models and then modify these models to obtain human organoid models. These organoid models can be used to obtain a better understanding of the signaling pathways, molecular mechanisms, genetics, and epigenetic changes involved in gametogenesis and embryogenesis and could also be applied to clinical applications, such as drug screening. Here, we discuss the formation of primordial stem cell-like cells (PGCLCs), and in vitro-induced gametes and embryoids using pluripotent stem cells (PSCs). We also analyze their applications and limitations.

Capturing Pluripotency and Beyond

During the development of a multicellular organism, the specification of different cell lineages originates in a small group of pluripotent cells, the epiblasts, formed in the preimplantation embryo. The pluripotent epiblast is protected from premature differentiation until exposure to inductive cues in strictly controlled spatially and temporally organized patterns guiding fetus formation. Epiblasts cultured in vitro are embryonic stem cells (ESCs), which recapitulate the self-renewal and lineage specification properties of their endogenous counterparts. The characteristics of totipotency, although less understood than pluripotency, are becoming clearer. Recent studies have shown that a minor ESC subpopulation exhibits expanded developmental potential beyond pluripotency, displaying a characteristic reminiscent of two-cell embryo blastomeres (2CLCs). In addition, reprogramming both mouse and human ESCs in defined media can produce expanded/extended pluripotent stem cells (EPSCs) similar to but different from 2CLCs. Further, the molecular roadmaps driving the transition of various potency states have been clarified. These recent key findings will allow us to understand eutherian mammalian development by comparing the underlying differences between potency network components during development. Using the mouse as a paradigm and recent progress in human PSCs, we review the epiblast's identity acquisition during embryogenesis and their ESC counterparts regarding their pluripotent fates and beyond.

Pluripotent stem cells related to embryonic disc exhibit common self-renewal requirements in diverse livestock species

Despite four decades of effort, robust propagation of pluripotent stem cells from livestock animals remains challenging. The requirements for self-renewal are unclear and the relationship of cultured stem cells to pluripotent cells resident in the embryo uncertain. Here, we avoided using feeder cells or serum factors to provide a defined culture microenvironment. We show that the combination of activin A, fibroblast growth factor and the Wnt inhibitor XAV939 (AFX) supports establishment and continuous expansion of pluripotent stem cell lines from porcine, ovine and bovine embryos. Germ layer differentiation was evident in teratomas and readily induced in vitro. Global transcriptome analyses highlighted commonality in transcription factor expression across the three species, while global comparison with porcine embryo stages showed proximity to bilaminar disc epiblast. Clonal genetic manipulation and gene targeting were exemplified in porcine stem cells. We further demonstrated that genetically modified AFX stem cells gave rise to cloned porcine foetuses by nuclear transfer. In summary, for major livestock mammals, pluripotent stem cells related to the formative embryonic disc are reliably established using a common and defined signalling environment.

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

Summary: We report the derivation of similar, stable and continuously expandable pluripotent stem cells related to embryonic disc epiblast from embryos of pig, sheep and cow.

Epigenetic Regulation during Primordial Germ Cell Development and Differentiation

Germline development varies significantly across metazoans. However, mammalian primordial germ cell (PGC) development has key conserved landmarks, including a critical period of epigenetic reprogramming that precedes sex-specific differentiation and gametogenesis. Epigenetic alterations in the germline are of unique importance due to their potential to impact the next generation. Therefore, regulation of, and by, the non-coding genome is of utmost importance during these epigenomic events. Here, we detail the key chromatin changes that occur during mammalian PGC development and how these interact with the expression of non-coding RNAs alongside broader epitranscriptomic changes. We identify gaps in our current knowledge, in particular regarding epigenetic regulation in the human germline, and we highlight important areas of future research.

The Dynamic Regulation of mRNA Translation and Ribosome Biogenesis During Germ Cell Development and Reproductive Aging

The regulation of mRNA translation, both globally and at the level of individual transcripts, plays a central role in the development and function of germ cells across species. Genetic studies using flies, worms, zebrafish and mice have highlighted the importance of specific RNA binding proteins in driving various aspects of germ cell formation and function. Many of these mRNA binding proteins, including Pumilio, Nanos, Vasa and Dazl have been conserved through evolution, specifically mark germ cells, and carry out similar functions across species. These proteins typically influence mRNA translation by binding to specific elements within the 3′ untranslated region (UTR) of target messages. Emerging evidence indicates that the global regulation of mRNA translation also plays an important role in germ cell development. For example, ribosome biogenesis is often regulated in a stage specific manner during gametogenesis. Moreover, oocytes need to produce and store a sufficient number of ribosomes to support the development of the early embryo until the initiation of zygotic transcription. Accumulating evidence indicates that disruption of mRNA translation regulatory mechanisms likely contributes to infertility and reproductive aging in humans. These findings highlight the importance of gaining further insights into the mechanisms that control mRNA translation within germ cells. Future work in this area will likely have important impacts beyond germ cell biology.

Andrew Johnson (1958-2021)

Andrew Johnson, a pioneer in the development of the amphibian axolotl as a model to study the early stages of metazoan development, died 15th September 2021. Known as ‘AJ’ by his family, and by his friends and colleagues, his older sister Pam referred to him as an unstoppable ‘force of nature’ who at the age of 9 or 10 said to her, ‘I'm going to become a professor’. Here, we reflect on AJ's life and work, paying particular attention to his studies on the establishment of primordial germ cells in vertebrates.

The role of Eif2s3y in mouse spermatogenesis

Eukaryotic translation initiation factor 2 subunit 3 and structural gene Y-linked (Eif2s3y) gene, the gene encoding eIF2γ protein, is located on the mouse Y chromosome short arm. The Eif2s3y gene is globally expressed in all tissues and plays an important role in regulating global and gene-specific mRNA translation initiation. During the process of protein translation initiation, Eif2s3x(its homolog) and Eif2s3y encoded eIF2γ perform similar functions. However, it has been noticed that Eif2s3y plays a crucial role in spermatogenesis, including spermatogonia mitosis, meiosis, and spermiogenesis of spermatids, which may account for infertility. In the period of spermatogenesis, the role of Eif2s3x and Eif2s3y are not equivalent. Importance of Eif2s3y has been observed in ESC and implicated in several aspects, including the pluripotency state and the proliferation rate. Here, we discuss the functional significance of Eif2s3y in mouse spermatogenesis and self-renewal of ESCs.

Corpuscles of Stannius development requires FGF signaling

The corpuscles of Stannius (CS) represent a unique endocrine organ of teleostean fish that secrets stanniocalcin-1 (Stc1) to maintain calcium homeostasis. Appearing at 20-25 somite stage in the distal zebrafish pronephros, stc1-expressing cells undergo apical constriction, and are subsequently extruded to form a distinct gland on top of the distal pronephric tubules at 50 ​h post fertilization (hpf). Several transcription factors (e.g. Hnf1b, Irx3b, Tbx2a/b) and signaling pathways (e.g. Notch) control CS development. We report now that Fgf signaling is required to commit tubular epithelial cells to differentiate into stc1-expressing CS cells. Inhibition of Fgf signaling by SU5402, dominant-negative Fgfr1, or depletion of fgf8a prevented CS formation and stc1 expression. Ablation experiments revealed that CS have the ability to partially regenerate via active cell migration involving extensive filopodia and lamellipodia formation. Activation of Wnt signaling curtailed stc1 expression, but had no effect on CS formation. Thus, our observations identify Fgf signaling as a crucial component of CS cell fate commitment.

Germline competent mesoderm: the substrate for vertebrate germline and somatic stem cells?

In vitro production of tissue-specific stem cells [e.g. haematopoietic stem cells (HSCs)] is a key goal of regenerative medicine. However, recent efforts to produce fully functional tissue-specific stem cells have fallen short. One possible cause of shortcomings may be that model organisms used to characterize basic vertebrate embryology (Xenopus, zebrafish, chick) may employ molecular mechanisms for stem cell specification that are not conserved in humans, a prominent example being the specification of primordial germ cells (PGCs). Germ plasm irreversibly specifies PGCs in many models; however, it is not conserved in humans, which produce PGCs from tissue termed germline-competent mesoderm (GLCM). GLCM is not conserved in organisms containing germ plasm, or even in mice, but understanding its developmental potential could unlock successful production of other stem cell types. GLCM was first discovered in embryos from the axolotl and its conservation has since been demonstrated in pigs, which develop from a flat-disc embryo like humans. Together these findings suggest that GLCM is a conserved basal trait of vertebrate embryos. Moreover, the immortal nature of germ cells suggests that immortality is retained during GLCM specification; here we suggest that the demonstrated pluripotency of GLCM accounts for retention of immortality in somatic stem cell types as well.

This article has an associated Future Leaders to Watch interview with the author of the paper.

Summary: Recent findings that germline and stem cell specification may differ between species may have important implications for regenerative medicine and the future of stem cell biology.

Tracing the emergence of primordial germ cells from bilaminar disc rabbit embryos and pluripotent stem cells

Rabbit embryos develop as bilaminar discs at gastrulation as in humans and most other mammals, whereas rodents develop as egg cylinders. Primordial germ cells (PGCs) appear to originate during gastrulation according to many systematic studies on mammalian embryos. Here, we show that rabbit PGC (rbPGC) specification occurs at the posterior epiblast at the onset of gastrulation. Using newly derived rabbit pluripotent stem cells, we show robust and rapid induction of rbPGC-like cells in vitro with WNT and BMP morphogens, which reveals SOX17 as the critical regulator of rbPGC fate as in several non-rodent mammals. We posit that development as a bilaminar disc is a crucial determinant of the PGC regulators, regardless of the highly diverse development of extraembryonic tissues, including the amnion. We propose that investigations on rabbits with short gestation, large litters, and where gastrulation precedes implantation can contribute significantly to advances in early mammalian development.

Mammalian in vitro gametogenesis

[Figure: see text].

YAP regulates porcine skin-derived stem cells self-renewal partly by repressing Wnt/β-catenin signaling pathway

Skin-derived stem cells (SDSCs) are a class of adult stem cells (ASCs) that have the ability to self-renew and differentiate. The regulation mechanisms involved in the differentiation of SDSCs are a hot topic. In this paper, we explore the link between the transcriptional regulator yes-associated protein (YAP) and the fate of porcine SDSCs (pSDSCs). We found that lysophosphatidylcholine (LPC) activates YAP, promotes pSDSCs pluripotency, and counteracts transdifferentiation of pSDSCs into porcine primordial germ cell-like cells (pPGCLCs). YAP promotes the pluripotent state of pSDSCs by maintaining the high expression of the pluripotency genes Oct4 and Sox2. The overexpression of YAP prevented the differentiation of pSDSCs, and the depletion of YAP by small interfering RNA (siRNAs) suppressed the self-renewal of pSDSCs. In addition, we found that YAP regulates the fate of pSDSCs through a mechanism related to the Wnt/β-catenin signaling pathway. When an activator of the Wnt/β-catenin signaling pathway, CHIR99021, was added to pSDSCs overexpressing YAP, the ability of pSDSCs to differentiate was partially restored. Conversely, when XAV939, an inhibitor of the Wnt/β-catenin signaling pathway, was added to YAP knockdown pSDSCs a higher self-renewal ability resulted. Taken together, our results suggested that YAP and the Wnt/β-catenin signaling pathway interact to regulate the fate of pSDSCs.

Tissue of Origin, but Not XCI State, Influences Germ Cell Differentiation from Human Pluripotent Stem Cells

Human pluripotent stem cells (hPSCs) are not only a promising tool to investigate differentiation to many cell types, including the germline, but are also a potential source of cells to use for regenerative medicine purposes in the future. However, current in vitro models to generate human primordial germ cell-like cells (hPGCLCs) have revealed high variability regarding differentiation efficiency depending on the hPSC lines used. Here, we investigated whether differences in X chromosome inactivation (XCI) in female hPSCs could contribute to the variability of hPGCLC differentiation efficiency during embryoid body (EB) formation. For this, we first characterized the XCI state in different hPSC lines by investigating the expression of XIST and H3K27me3, followed by differentiation and quantification of hPGCLCs. We observed that the XCI state did not influence the efficiency to differentiate to hPGCLCs; rather, hPSCs derived from cells isolated from urine showed an increased trend towards hPGCLCs differentiation compared to skin-derived hPSCs. In addition, we also characterized the XCI state in the generated hPGCLCs. Interestingly, we observed that independent of the XCI state of the hPSCs used, both hPGCLCs and soma cells in the EBs acquired XIST expression, indicative of an inactive X chromosome. In fact, culture conditions for EB formation seemed to promote XIST expression. Together, our results contribute to understanding how epigenetic properties of hPSCs influence differentiation and to optimize differentiation methods to obtain higher numbers of hPGCLCs, the first step to achieve human in vitro gametogenesis.

Improving In Vitro Culture of Human Male Fetal Germ Cells

Male human fetal germ cells (hFGCs) give rise to spermatogonial stem cells (SSCs), which are the adult precursors of the male gametes. Human SSCs are a promising (autologous) source of cells for male fertility preservation; however, in contrast to mouse SSCs, we are still unable to culture them in the long term. Here, we investigated the effect of two different culture media and four substrates (laminin, gelatin, vitronectin and matrigel) in the culture of dissociated second trimester testes, enriched for hFGCs. After 6 days in culture, we quantified the presence of POU5F1 and DDX4 expressing hFGCs. We observed a pronounced difference in hFGC number in different substrates. The combination of gelatin-coated substrate and medium containing GDNF, LIF, FGF2 and EGF resulted in the highest percentage of hFGCs (10% of the total gonadal cells) after 6 days of culture. However, the vitronectin-coated substrate resulted in a comparable percentage of hFGCs regardless of the media used (3.3% of total cells in Zhou-medium and 4.8% of total cells in Shinohara-medium). We provide evidence that not only the choices of culture medium but also choices of the adequate substrate are crucial for optimizing culture protocols for male hFGCs. Optimizing culture conditions in order to improve the expansion of hFGCs will benefit the development of gametogenesis assays in vitro.

Ligand-Receptor Interactions Elucidate Sex-Specific Pathways in the Trajectory From Primordial Germ Cells to Gonia During Human Development

The human germ cell lineage originates from primordial germ cells (PGCs), which are specified at approximately the third week of development. Our understanding of the signaling pathways that control this event has significantly increased in recent years and that has enabled the generation of PGC-like cells (PGCLCs) from pluripotent stem cells in vitro. However, the signaling pathways that drive the transition of PGCs into gonia (prospermatogonia in males or premeiotic oogonia in females) remain unclear, and we are presently unable to mimic this step in vitro in the absence of gonadal tissue. Therefore, we have analyzed single-cell transcriptomics data of human fetal gonads to map the molecular interactions during the sex-specific transition from PGCs to gonia. The CellPhoneDB algorithm was used to identify significant ligand–receptor interactions between germ cells and their sex-specific neighboring gonadal somatic cells, focusing on four major signaling pathways WNT, NOTCH, TGFβ/BMP, and receptor tyrosine kinases (RTK). Subsequently, the expression and intracellular localization of key effectors for these pathways were validated in human fetal gonads by immunostaining. This approach provided a systematic analysis of the signaling environment in developing human gonads and revealed sex-specific signaling pathways during human premeiotic germ cell development. This work serves as a foundation to understand the transition from PGCs to premeiotic oogonia or prospermatogonia and identifies sex-specific signaling pathways that are of interest in the step-by-step reconstitution of human gametogenesis in vitro.

Lineage Differentiation Markers as a Proxy for Embryo Viability in Farm Ungulates

Embryonic losses constitute a major burden for reproductive efficiency of farm animals. Pregnancy losses in ungulate species, which include cattle, pigs, sheep and goats, majorly occur during the second week of gestation, when the embryo experiences a series of cell differentiation, proliferation, and migration processes encompassed under the term conceptus elongation. Conceptus elongation takes place following blastocyst hatching and involves a massive proliferation of the extraembryonic membranes trophoblast and hypoblast, and the formation of flat embryonic disc derived from the epiblast, which ultimately gastrulates generating the three germ layers. This process occurs prior to implantation and it is exclusive from ungulates, as embryos from other mammalian species such as rodents or humans implant right after hatching. The critical differences in embryo development between ungulates and mice, the most studied mammalian model, have precluded the identification of the genes governing lineage differentiation in livestock species. Furthermore, conceptus elongation has not been recapitulated in vitro, hindering the study of these cellular events. Luckily, recent advances on transcriptomics, genome modification and post-hatching in vitro culture are shedding light into this largely unknown developmental window, uncovering possible molecular markers to determine embryo quality. In this review, we summarize the events occurring during ungulate pre-implantation development, highlighting recent findings which reveal that several dogmas in Developmental Biology established by knock-out murine models do not hold true for other mammals, including humans and farm animals. The developmental failures associated to in vitro produced embryos in farm animals are also discussed together with Developmental Biology tools to assess embryo quality, including molecular markers to assess proper lineage commitment and a post-hatching in vitro culture system able to directly determine developmental potential circumventing the need of experimental animals.

25th ANNIVERSARY OF CLONING BY SOMATIC-CELL NUCLEAR TRANSFER: Nuclear transfer and the development of genetically modified/gene edited livestock

The birth and adult development of 'Dolly' the sheep, the first mammal produced by the transfer of a terminally differentiated cell nucleus into an egg, provided unequivocal evidence of nuclear equivalence among somatic cells. This ground-breaking experiment challenged a long-standing dogma of irreversible cellular differentiation that prevailed for over a century and enabled the development of methodologies for reversal of differentiation of somatic cells, also known as nuclear reprogramming. Thanks to this new paradigm, novel alternatives for regenerative medicine in humans, improved animal breeding in domestic animals and approaches to species conservation through reproductive methodologies have emerged. Combined with the incorporation of new tools for genetic modification, these novel techniques promise to (i) transform and accelerate our understanding of genetic diseases and the development of targeted therapies through creation of tailored animal models, (ii) provide safe animal cells, tissues and organs for xenotransplantation, (iii) contribute to the preservation of endangered species, and (iv) improve global food security whilst reducing the environmental impact of animal production. This review discusses recent advances that build on the conceptual legacy of nuclear transfer and – when combined with gene editing – will have transformative potential for medicine, biodiversity and sustainable agriculture. We conclude that the potential of these technologies depends on further fundamental and translational research directed at improving the efficiency and safety of these methods.

Porcine Primordial Germ Cell-Like Cells Generated from Induced Pluripotent Stem Cells Under Different Culture Conditions

Culture conditions regulate the process of pluripotency acquisition and self-renewal. This study aimed to analyse the influence of the in vitro environment on the induction of porcine induced pluripotent stem cell (piPSCs) differentiation into primordial germ cell-like cells (pPGCLCs). piPSC culture with different supplementation strategies (LIF, bFGF, or LIF plus bFGF) promoted heterogeneous phenotypic profiles. Continuous bFGF supplementation during piPSCs culture was beneficial to support a pluripotent state and the differentiation of piPSCs into pPGCLCs. The pPGCLCs were positive for the gene and protein expression of pluripotent and germinative markers. This study can provide a suitable in vitro model for use in translational studies and to help answer numerous remaining questions about germ cells.

Modelling human embryogenesis: embryo-like structures spark ethical and policy debate

BACKGROUND

Studying the human peri-implantation period remains hindered by the limited accessibility of the in vivo environment and scarcity of research material. As such, continuing efforts have been directed towards developing embryo-like structures (ELS) from pluripotent stem cells (PSCs) that recapitulate aspects of embryogenesis in vitro. While the creation of such models offers immense potential for studying fundamental processes in both pre- and early post-implantation development, it also proves ethically contentious due to wide-ranging views on the moral and legal reverence due to human embryos. Lack of clarity on how to qualify and regulate research with ELS thus presents a challenge in that it may either limit this new field of research without valid grounds or allow it to develop without policies that reflect justified ethical concerns.

OBJECTIVE AND RATIONALE

The aim of this article is to provide a comprehensive overview of the existing scientific approaches to generate ELS from mouse and human PSCs, as well as discuss future strategies towards innovation in the context of human development. Concurrently, we aim to set the agenda for the ethical and policy issues surrounding research on human ELS.

SEARCH METHODS

The PubMed database was used to search peer-reviewed articles and reviews using the following terms: 'stem cells', 'pluripotency', 'implantation', 'preimplantation', 'post-implantation', 'blastocyst', 'embryoid bodies', 'synthetic embryos', 'embryo models', 'self-assembly', 'human embryo-like structures', 'artificial embryos' in combination with other keywords related to the subject area. The PubMed and Web of Science databases were also used to systematically search publications on the ethics of ELS and human embryo research by using the aforementioned keywords in combination with 'ethics', 'law', 'regulation' and equivalent terms. All relevant publications until December 2019 were critically evaluated and discussed.

OUTCOMES

In vitro systems provide a promising way forward for uncovering early human development. Current platforms utilize PSCs in both two- and three-dimensional settings to mimic various early developmental stages, including epiblast, trophoblast and amniotic cavity formation, in addition to axis development and gastrulation. Nevertheless, much hinges on the term 'embryo-like'. Extension of traditional embryo frameworks to research with ELS reveals that (i) current embryo definitions require reconsideration, (ii) cellular convertibility challenges the attribution of moral standing on the basis of 'active potentiality' and (iii) meaningful application of embryo protective directives will require rethinking of the 14-day culture limit and moral weight attributed to (non-)viability. Many conceptual and normative (dis)similarities between ELS and embryos thus remain to be thoroughly elucidated.

WIDER IMPLICATIONS

Modelling embryogenesis holds vast potential for both human developmental biology and understanding various etiologies associated with infertility. To date, ELS have been shown to recapitulate several aspects of peri-implantation development, but critically, cannot develop into a fetus. Yet, concurrent to scientific innovation, considering the extent to which the use of ELS may raise moral concerns typical of human embryo research remains paramount. This will be crucial for harnessing the potential of ELS as a valuable research tool, whilst remaining within a robust moral and legal framework of professionally acceptable practices.