Derivation of embryonic germ cells and male gametes from embryonic stem cells

Integrative analysis of single-nucleus RNA-seq and bulk RNA-seq reveals germline cells development dynamics and niches in the Pacific oyster gonad

Gametogenesis drives the maturation of germ cell precursors into functional gametes, facilitated by interactions with the niche environment. However, the molecular mechanisms, especially in invertebrates, remain incompletely understood. In this study, the gonadal microenvironment and gametogenic processes in the Pacific oyster, a model for diffuse gonadal organization and periodic gametogenesis, are investigated. We combine single-nucleus RNA-seq and bulk RNA-seq to analyze gonadal microenvironments in oysters. Twenty-three male and nineteen female gonadal cell clusters are identified, revealing four male and three female germ cell types, alongside follicular cells in females and Sertoli/Leydig cells in males. The NOTCH and BMP (bone morphogenetic protein) signaling pathways play a significant role in the male germline niche, suggesting similarities with mammalian germ cell microenvironment. This study offers valuable insights into germ cell developmental transitions and microenvironmental characteristics.

NRF1 promotes primordial germ cell development, proliferation and survival

Primordial germ cells (PGCs) are the germline precursors that give rise to oocytes and sperm, ensuring the continuation of life. While the PGC specification is extensively studied, it remains elusive how the PGC population is sustained and expanded after they migrate to embryonic gonads before birth. This study demonstrates that NRF1, a known regulator for mitochondrial metabolism, plays critical roles in post-migrating PGC development. We show that NRF1 protein level gradually increases in post-migrating PGCs during embryonic development. Conditional Nrf1 knockout from embryonic germ cells leads to impaired PGC proliferation and survival. In addition, NRF1 may also actively drive PGC derivation from pluripotent stem cells. Using whole genome transcriptome profiling and ChIP-seq analyses, we further reveal that NRF1 directly regulates key signalling molecules in PGC formation, transcription factors in proliferation and cell cycle and enzymes in mitochondrial metabolism. Overall, our findings highlight an essential requirement of NRF1 in regulating a broad transcriptional network to support post-migrating PGC development both in vitro and in vivo.

Bridging the Gap: Animal Models in Next-Generation Reproductive Technologies for Male Fertility Preservation

This review aims to explore advanced reproductive technologies for male fertility preservation, underscoring the essential role that animal models have played in shaping these techniques through historical contexts and into modern applications. Rising infertility concerns have become more prevalent in human populations recently. The surge in male fertility issues has prompted advanced reproductive technologies, with animal models playing a pivotal role in their evolution. Historically, animal models have aided our understanding in the field, from early reproductive basic research to developing techniques like artificial insemination, multiple ovulation, and in vitro fertilization. The contemporary landscape of male fertility preservation encompasses techniques such as sperm cryopreservation, testicular sperm extraction, and intracytoplasmic sperm injection, among others. The relevance of animal models will undoubtedly bridge the gap between traditional methods and revolutionary next-generation reproductive techniques, fortifying our collective efforts in enhancing male fertility preservation strategies. While we possess extensive knowledge about spermatogenesis and its regulation, largely thanks to insights from animal models that paved the way for human infertility treatments, a pressing need remains to further understand specific infertility issues unique to humans. The primary aim of this review is to provide a comprehensive analysis of how animal models have influenced the development and refinement of advanced reproductive technologies for male fertility preservation, and to assess their future potential in bridging the gap between current practices and cutting-edge fertility techniques, particularly in addressing unique human male factor infertility.

Modelling in vitro gametogenesis using induced pluripotent stem cells: a review

In vitro gametogenesis (IVG) has been a topic of great interest in recent years not only because it allows for further exploration of mechanisms of germ cell development, but also because of its prospect for innovative medical applications especially for the treatment of infertility. Elucidation of the mechanisms underlying gamete development in vivo has inspired scientists to attempt to recapitulate the entire process of gametogenesis in vitro. While earlier studies have established IVG methods largely using pluripotent stem cells of embryonic origin, the scarcity of sources for these cells and the ethical issues involved in their use are serious limitations to the progress of IVG research especially in humans. However, with the emergence of induced pluripotent stem cells (iPSCs) due to the revolutionary discovery of dedifferentiation and reprogramming factors, IVG research has progressed remarkably in the last decade. This paper extensively reviews developments in IVG using iPSCs. First, the paper presents key concepts from groundwork studies on IVG including earlier researches demonstrating that IVG methods using embryonic stem cells (ESCs) also apply when using iPSCs. Techniques for the derivation of iPSCs are briefly discussed, highlighting the importance of generating transgene-free iPSCs with a high capacity for germline transmission to improve efficacy when used for IVG. The main part of the paper discusses recent advances in IVG research using iPSCs in various stages of gametogenesis. In addition, current clinical applications of IVG are presented, and potential future applications are discussed. Although IVG is still faced with many challenges in terms of technical issues, as well as efficacy and safety, novel IVG methodologies are emerging, and IVG using iPSCs may usher in the next era of reproductive medicine sooner than expected. This raises both ethical and social concerns and calls for the scientific community to cautiously develop IVG technology to ensure it is not only efficacious but also safe and adheres to social and ethical norms.

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.

Fertility restoration in mice with chemotherapy induced ovarian failure using differentiated iPSCs

BACKGROUND

Treatment options for premature ovarian insufficiency (POI) are limited to hormone replacement and donor oocytes. A novel induced pluripotent stem cell (iPSC) transplant paradigm in a mouse model has potential translational applications for management of POI.

METHODS

Mouse ovarian granulosa cell derived-iPSCS were labelled with green fluorescent protein (GFP) reporter and differentiated in vitro into oocytes. Differentiated cells were assayed for estradiol and progesterone secretion by enzyme-linked immunosorbent assays. After Fluorescence-Activated Cell Sorting (FACS) for the cell surface marker anti-Mullerian hormone receptor (AMHR2), enriched populations of differentiated cells were surgically transplanted into ovaries of mice that had POI secondary to gonadotoxic pre-treatment with alkylating agents. A total of 100 mice were used in these studies in five separate experiments with 56 animals receiving orthotopic ovarian injections of either FACS sorted or unsorted differentiated iPSCSs and the remaining animals receiving sham injections of PBS diluent. Following transplantation surgery, mice were stimulated with gonadotropins inducing oocyte development and underwent oocyte retrieval. Nine transplanted mice were cross bred with wild-type mice to assess fertility. Lineage tracing of resultant oocytes, F1 (30 pups), and F2 (42 pups) litters was interrogated by GFP expression and validation by short tandem repeat (STR) lineage tracing.

FINDINGS

[1] iPSCs differentiate into functional oocytes and steroidogenic ovarian cells which [2] express an ovarian (GJA1) and germ cell (ZP1) markers. [3] Endocrine function and fertility were restored in mice pretreated with gonadotoxic alkylating agents via orthotopic transplantation of differentiated iPSCS, thus generating viable, fertile mouse pups.

INTERPRETATION

iPSC-derived ovarian tissue can reverse endocrine and reproductive sequelae of POI.

FUNDING

Center for Infertility and Reproductive Surgery Research Award, Siezen Foundation award (RMA). Reproductive Scientist Development Program, Marriott Foundation, Saltonstall Foundation, Brigham Ovarian Cancer Research Fund (K.E).

In vitro differentiation of primed human induced pluripotent stem cells into primordial germ cell-like cells

BACKGROUND

Previous studies have shown significant results in the differentiation of mouse-induced pluripotent stem cells (miPSCs) into primordial germ cell-like cells (PGCLCs) and that human iPSCs (hiPSCs) can also differentiate into PGCLCs; however, the efficiency of PGCLC induction from hiPSCs is < 5%. In this study, we examined a new protocol to differentiate hiPSCs into PGCLCs.

METHODS AND RESULTS

hiPSCs-derived embryoid bodies (EBs) were exposed to differentiate inducing factors, bone morphogenetic protein 4 (BMP4), and retinoic acid (RA) for 6 days. Cell differentiation was assessed by reverse transcriptase-polymerase chain reaction (RT-PCR) and immunofluorescence (IF) studies. Our results showed increased expression of the PRDM1 gene on the first day of differentiation. On other days, DAZL, VASA, and STRA8 genes increased, and the expression of PRDM1, NANOG, and OCT4 genes decreased. The expression of VASA, C-KIT, and STRA8 proteins was confirmed by IF. A flow cytometry analysis revealed that ~ 60% of differentiated cells were VASA- and STRA8-positive.

CONCLUSION

EB formation and constant exposure of EBs to BMP4 and RA lead to the differentiation of hiPSCs into PGCLCs.

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.

The role of embryonic stem cells, transcription and growth factors in mammals: A review

Mammals represent a relevant species in worldwide cultures with significant commercial value. These animals are considered an attractive large animal model for biomedical and biotechnology research. The development of large animal experimental models may open alternative strategies for investigating stem cells (SCs) physiology and potential application in the veterinary field. The embryonic stem cells (ESCs) are known to possess natural pluripotency that confers the ability to differentiate into various tissues in vivo and in vitro. These notable characteristics can be useful for research and innovative applications, including biomedicine, agriculture and industry. Transcription factors play a crucial role in preserving stem cell self-renewal, whereas growth factors are involved in both growth and differentiation. However, to date, many questions concerning pluripotency, cellular differentiation regulator genes, and other molecules such as growth factors and their interactions in many mammalian species remain unresolved. The purpose of this review is to provide an overall review regarding the study of ESCs in mammals and briefly discuss the role of transcription and growth factors.

Integrated bioinformatics analysis of the transcription factor-mediated gene regulatory networks in the formation of spermatogonial stem cells

Background: In vitro induction of spermatogonial stem cells (SSCs) from embryonic stem cells (ESCs) provides a promising tool for the treatment of male infertility. A variety of molecules are involved in this complex process, which needs to be further clarified. Undoubtedly, the increased knowledge of SSC formation will be beneficial to facilitate the currently complex induction process. Methods: Based on ATAC-seq, DNase-seq, RNA-seq, and microarray data from GEO datasets, chromatin property data (ATAC-seq, DNase-seq) and gene expression data (RNA-seq, microarray data) were combined to search for SSC-specific transcription factors (TFs) and hub SSC-specific genes by using the WGCNA method. Then, we applied RNA-seq and microarray data screening for key SSC-specific TFs and constructed key SSC-specific TF-mediated gene regulatory networks (GRNs) using ChIP-seq data. Results: First, after analysis of the ATAC-seq and DNase-seq data of mouse ESCs, primordial germ cells (PGCs), and SSCs, 33 SSC-specific TFs and 958 targeting genes were obtained. RNA-seq and WGCNA revealed that the key modules (turquoise and red) were the most significantly related to 958 SSC-specific genes, and a total of 10 hub SSC-specific genes were identified. Next, when compared with the cell-specific TFs in human ESCs, PGCs, and SSCs, we obtained five overlapping SSC-specific TF motifs, including the NF1 family TF motifs (NFIA, NFIB, NFIC, and NFIX), GRE, Fox:Ebox, PGR, and ARE. Among these, Nfib and Nfix exhibited abnormally high expression levels relative to mouse ESCs and PGCs. Moreover, Nfib and Nfix were upregulated in the testis sample with impaired spermatogenesis when compared with the normal group. Finally, the ChIP-seq data results showed that NFIB most likely targeted the hub SSC-specific genes of the turquoise module (Rpl36al, Rps27, Rps21, Nedd8, and Sec61b) and the red module (Vcam1 and Ccl2). Conclusion: Our findings preliminarily revealed cell-specific TFs and cell-specific TF-mediated GRNs in the process of SSC formation. The hub SSC-specific genes and the key SSC-specific TFs were identified and suggested complex network regulation, which may play key roles in optimizing the induction efficiency of the differentiation of ESCs into SSCs in vitro.

Cell-Based Therapy Approaches in Treatment of Non-obstructive Azoospermia

The rate of infertility has globally increased in recent years for a variety of reasons. One of the main causes of infertility in men is azoospermia that is defined by the absence of sperm in the ejaculate and classified into two categories: obstructive azoospermia and non-obstructive azoospermia. In non-obstructive azoospermia, genital ducts are not obstructed, but the testicles do not produce sperm at all, due to various reasons. Non-obstructive azoospermia in most cases has no therapeutic options other than assisted reproductive techniques, which in most cases require sperm donors. Here we discuss cell-based therapy approaches to restore fertility in men with non-obstructive azoospermia including cell-based therapies of non-obstructive azoospermia using regenerative medicine and cell-based therapies of non-obstructive azoospermia by paracrine and anti-inflammatory pathway, technical and ethical challenges for using different cell sources and alternative options will be described, and then the more effectual approaches will be mentioned as future trends.

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.

Meiosis resumption in human primordial germ cells from induced pluripotent stem cells by in vitro activation and reconstruction of ovarian nests

Background

The differentiation of human induced pluripotent stem cells (iPSCs) into oocytes, which involves the transformation from mitosis to meiosis, has been a hotspot of biological research for many years and represents a desirable experimental model and potential strategy for treating infertility. At present, studies have shown that most cells stagnate in the oogonium stage after differentiation into primordial germ cells (PGCs) from human iPSCs.

Methods

iPSCs carrying a SYCP3-mkate2 knock-in reporter were generated by the CRISPR/Cas9 strategy to monitor meiosis status during induced differentiation from iPSCs into oocytes. These induced PGCs/oogonia were activated by small molecules from the Wnt signaling pathway and then cocultured with reconstructed human ovarian nests in vivo for further development.

Results

First, human PGCs and oogonia were efficiently induced from iPSCs. Second, induced dormant PGCs resumed meiosis and then differentiated into primary oocytes through the in vitro activation of the Wnt signaling pathway. Finally, a new coculture system involving the reconstruction of ovarian nests in vitro could facilitate the differentiation of oocytes.

Conclusions

Human PGCs/oogonia induced from iPSCs can be activated and used to resume meiosis by molecules of the Wnt signaling pathway. The coculture of activated PGCs and reconstruction of ovarian nests facilitated differentiation into primary oocytes and the generation of haploid human oocytes in vivo. These findings established a new strategy for germline competence in primary oocytes and provided a keystone for human gametogenesis in vitro and in vivo.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03019-3.

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.

In vitro spermatogenesis: Why meiotic checkpoints matter

Successful in vitro spermatogenesis would generate functional haploid spermatids, and thus, form the basis for novel approaches to treat patients with impaired spermatogenesis or develop alternative strategies for male fertility preservation. Several culture strategies, including cell cultures using various stem cells and ex vivo cultures of testicular tissue, have been investigated to recapitulate spermatogenesis in vitro. Although some studies have described complete meiosis and subsequent generation of functional spermatids, key meiotic events, such as chromosome synapsis and homologous recombination required for successful meiosis and faithful in vitro-derived gametes, are often not reported. To guarantee the generation of in vitro-formed spermatids without persistent DNA double-strand breaks (DSBs) and chromosomal aberrations, criteria to evaluate whether all meiotic events are completely executed in vitro need to be established. In vivo, these meiotic events are strictly monitored by meiotic checkpoints that eliminate aberrant spermatocytes. To establish criteria to evaluate in vitro meiosis, we review the meiotic events and checkpoints that have been investigated by previous in vitro spermatogenesis studies. We found that, although major meiotic events such as initiation of DSBs and recombination, complete chromosome synapsis, and XY-body formation can be achieved in vitro, crossover formation, chiasmata frequency, and checkpoint mechanisms have been mostly ignored. In addition, complete spermiogenesis, during which round spermatids differentiate into elongated spermatids, has not been achieved in vitro by various cell culture strategies. Finally, we discuss the implications of meiotic checkpoints for in vitro spermatogenesis protocols and future clinical use.

Artificial Oocyte: Development and Potential Application

Millions of people around the world suffer from infertility, with the number of infertile couples and individuals increasing every year. Assisted reproductive technologies (ART) have been widely developed in recent years; however, some patients are unable to benefit from these technologies due to their lack of functional germ cells. Therefore, the development of alternative methods seems necessary. One of these methods is to create artificial oocytes. Oocytes can be generated in vitro from the ovary, fetal gonad, germline stem cells (GSCs), ovarian stem cells, or pluripotent stem cells (PSCs). This approach has raised new hopes in both basic research and medical applications. In this article, we looked at the principle of oocyte development, the landmark studies that enhanced our understanding of the cellular and molecular mechanisms that govern oogenesis in vivo, as well as the mechanisms underlying in vitro generation of functional oocytes from different sources of mouse and human stem cells. In addition, we introduced next-generation ART using somatic cells with artificial oocytes. Finally, we provided an overview of the reproductive application of in vitro oogenesis and its use in human fertility.

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.

Induction of meiosis by embryonic gonadal somatic cells differentiated from pluripotent stem cells

Background

Depletion of oocytes leads to ovarian aging-associated infertility, endocrine disruption and related diseases. Excitingly, unlimited oocytes can be generated by differentiation of primordial germ cell like cells (PGCLCs) from pluripotent stem cells. Nevertheless, development of oocytes and follicles from PGCLCs relies on developmentally matched gonadal somatic cells, only available from E12.5 embryos in mice. It is therefore imperative to achieve an in vitro source of E12.5 gonadal somatic cells.

Methods

We explored to identify small molecules, which can induce female embryonic stem cells (ESCs) into gonadal somatic cell like cells.

Results

Using RNA-sequencing, we identified signaling pathways highly upregulated in E12.5_gonadal somatic cells (E12.5_GSCs). Through searching for the activators of these pathways, we identified small-molecule compounds Vitamin C (Vc) and AM580 in combination (V580) for inducing differentiation of female embryonic stem cells (ESCs) into E12.5_GSC-like cells (E12.5_GSCLCs). After V580 treatment for 6 days and sorted by a surface marker CD63, the cell population yielded a transcriptome profile similar to that of E12.5_GSCs, which promoted meiosis progression and folliculogenesis of primordial germ cells. This approach will contribute to the study of germ cell and follicle development and oocyte production and have implications in potentially treating female infertility.

Conclusion

ESCs can be induced into embryonic gonadal somatic cell like cells by small molecules.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02672-4.

Generation of developmentally competent oocytes and fertile mice from parthenogenetic embryonic stem cells

Parthenogenetic embryos, created by activation and diploidization of oocytes, arrest at mid-gestation for defective paternal imprints, which impair placental development. Also, viable offspring has not been obtained without genetic manipulation from parthenogenetic embryonic stem cells (pESCs) derived from parthenogenetic embryos, presumably attributable to their aberrant imprinting. We show that an unlimited number of oocytes can be derived from pESCs and produce healthy offspring. Moreover, normal expression of imprinted genes is found in the germ cells and the mice. pESCs exhibited imprinting consistent with exclusively maternal lineage, and higher X-chromosome activation compared to female ESCs derived from the same mouse genetic background. pESCs differentiated into primordial germ cell-like cells (PGCLCs) and formed oocytes following in vivo transplantation into kidney capsule that produced fertile pups and reconstituted ovarian endocrine function. The transcriptome and methylation of imprinted and X-linked genes in pESC-PGCLCs closely resembled those of in vivo produced PGCs, consistent with efficient reprogramming of methylation and genomic imprinting. These results demonstrate that amplification of germ cells through parthenogenesis faithfully maintains maternal imprinting, offering a promising route for deriving functional oocytes and having potential in rebuilding ovarian endocrine function.

In Vitro Meiosis of Male Germline Stem Cells

In vitro spermatogenesis has been achieved by culturing mouse embryonic stem cells (ESCs) together with a cell suspension of male juvenile gonad. However, for human fertility treatment or preservation, patient-specific ESCs or juvenile gonad is not available. We therefore aim to achieve in vitro spermatogenesis using male germline stem cells (GSCs) without the use of juvenile gonad. GSCs, when cultured on immortalized Sertoli cells, were able to enter meiosis, reach the meiotic metaphase stages, and sporadically form spermatid-like cells. However, the in vitro-formed pachytene-like spermatocytes did not display full chromosome synapsis and did not form meiotic crossovers. Despite this, the meiotic checkpoints that usually eliminate such cells to prevent genomic instabilities from being transmitted to the offspring were not activated, allowing the cells to proceed to the meiotic metaphase stages. In vitro-generated spermatid-like cells should thus be thoroughly investigated before being considered for clinical use.

Germ Cell Derivation from Pluripotent Stem Cells for Understanding In Vitro Gametogenesis

Assisted reproductive technologies (ARTs) have developed considerably in recent years; however, they cannot rectify germ cell aplasia, such as non-obstructive azoospermia (NOA) and oocyte maturation failure syndrome. In vitro gametogenesis is a promising technology to overcome infertility, particularly germ cell aplasia. Early germ cells, such as primordial germ cells, can be relatively easily derived from pluripotent stem cells (PSCs); however, further progression to post-meiotic germ cells usually requires a gonadal niche and signals from gonadal somatic cells. Here, we review the recent advances in in vitro male and female germ cell derivation from PSCs and discuss how this technique is used to understand the biological mechanism of gamete development and gain insight into its application in infertility.

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.

Recent advances: fertility preservation and fertility restoration options for males and females

Fertility preservation is the process of saving gametes, embryos, gonadal tissues and/or gonadal cells for individuals who are at risk of infertility due to disease, medical treatments, age, genetics, or other circumstances. Adult patients have the options to preserve eggs, sperm, or embryos that can be used in the future to produce biologically related offspring with assisted reproductive technologies. These options are not available to all adults or to children who are not yet producing mature eggs or sperm. Gonadal cells/tissues have been frozen for several thousands of those patients worldwide with anticipation that new reproductive technologies will be available in the future. Therefore, the fertility preservation medical and research communities are obligated to responsibly develop next-generation reproductive technologies and translate them into clinical practice. We briefly describe standard options to preserve and restore fertility, but the emphasis of this review is on experimental options, including an assessment of readiness for translation to the human fertility clinic.

H3K4me2 Promotes the Activation of lncCPSET1 by Jun in the Chicken PGC Formation

Primordial germ cells are the ancestors of female and male cells. Current research has shown that long non-coding RNA (lncRNA) and Histone methylation are the pivotal epigenetic factors in the PGC formation. However, there are few studies on the regulatory mechanism of lncRNA in the formation of PGC. Here, we define the lncRNA highly expressed in chicken PGC, lncCPSET1 (chicken-PGC-specifically-expressed transcript 1) This study found that compared with the interference of lncCPSET1/histone methylase Mll2 alone, the PGC formation was severely inhibited with the interference of lncCPSET1 and histone methylase Mll2 jointly in vivo and in vitro. Studies on the transcription level of lncCPSET1 found that H3K4me2 and transcription factor Jun have a positive effect on the activation of lncCPSET1; while DNA hypomethylation inhibits the expression of lncCPSET1. In terms of mechanism, compared with DNA methylation, H3K4me2 dominates lncCPSET1 activation. H3K4me2 can be enriched in the lncCPSET1 promoter, change its chromosome conformation, recruit the transcription factor Jun, and activate the expression of lncCPSET1. Taken together, we confirmed the model that H3K4me2 rather than DNA hypomethylation mediates Jun to regulate lncCPSET1 transcription, which broadens the study of lncCPSET1 pre-transcriptional mechanism.

Therapeutic Potential of HUMSCs in Female Reproductive Aging

With the development of regenerative medicine, stem cells are being considered more frequently for the treatment of reproductive aging. Human umbilical cord mesenchymal stem cells have been reported to improve the reserve function of aging ovaries through their homing and paracrine effects. In this process, paracrine factors secreted by stem cells play an important role in ovarian recovery. Although the transplantation of human umbilical cord mesenchymal stem cells to improve ovarian function has been studied with great success in animal models of reproductive aging, their application in clinical research and therapy is still relatively rare. Therefore, this paper reviews the role of human umbilical cord mesenchymal stem cells in the treatment of reproductive aging and their related mechanisms, and it does so in order to provide a theoretical basis for further research and clinical treatment.

Regenerative Medicine Approaches in Bioengineering Female Reproductive Tissues

Diseases, disorders, and dysfunctions of the female reproductive tract tissues can result in either infertility and/or hormonal imbalance. Current treatment options are limited and often do not result in tissue function restoration, requiring alternative therapeutic approaches. Regenerative medicine offers potential new therapies through the bioengineering of female reproductive tissues. This review focuses on some of the current technologies that could address the restoration of functional female reproductive tissues, including the use of stem cells, biomaterial scaffolds, bio-printing, and bio-fabrication of tissues or organoids. The use of these approaches could also be used to address issues in infertility. Strategies such as cell-based hormone replacement therapy could provide a more natural means of restoring normal ovarian physiology. Engineering of reproductive tissues and organs could serve as a powerful tool for correcting developmental anomalies. Organ-on-a-chip technologies could be used to perform drug screening for personalized medicine approaches and scientific investigations of the complex physiological interactions between the female reproductive tissues and other organ systems. While some of these technologies have already been developed, others have not been translated for clinical application. The continuous evolution of biomaterials and techniques, advances in bioprinting, along with emerging ideas for new approaches, shows a promising future for treating female reproductive tract-related disorders and dysfunctions.

In Vitro Induction of Teleost PGCs

Primordial germ cells (PGCs) are unique cells in an embryo. These cells contain all genetic information and therefore represent the best source to store maternal and paternal genomes until embryo cryopreservation is achieved. However, the number of these cells in an embryo is very low limiting their potential application in cryopreservation and surrogate production. However, it was assumed that the induction of fish PGCs in vitro is not possible because in vivo they inherit germ plasm. In this chapter, we describe a successful differentiation protocol explaining the crucial factors and steps for in vitro PGC generation.

Artificially produced gametes in mice, humans and other species

The production of gametes from pluripotent stem cells in culture, also known as invitro gametogenesis, will make an important contribution to reproductive biology and regenerative medicine, both as a unique tool for understanding germ cell development and as an alternative source of gametes for reproduction. Invitro gametogenesis was developed using mouse pluripotent stem cells but is increasingly being applied in other mammalian species, including humans. In principle, the entire process of germ cell development is nearly reconstitutable in culture using mouse pluripotent stem cells, although the fidelity of differentiation processes and the quality of resultant gametes remain to be refined. The methodology in the mouse system is only partially applicable to other species, and thus it must be optimised for each species. In this review, we update the current status of invitro gametogenesis in mice, humans and other animals, and discuss challenges for further development of this technology.

Meiotic Chromosome Synapsis and XY-Body Formation In Vitro

To achieve spermatogenesis in vitro, one of the most challenging processes to mimic is meiosis. Meiotic problems, like incomplete synapsis of the homologous chromosomes, or impaired homologous recombination, can cause failure of crossover formation and subsequent chromosome nondisjunction, eventually leading to aneuploid sperm. These meiotic events are therefore strictly monitored by meiotic checkpoints that initiate apoptosis of aberrant spermatocytes and lead to spermatogenic arrest. However, we recently found that, in vitro derived meiotic cells proceeded to the first meiotic division (MI) stage, despite displaying incomplete chromosome synapsis, no discernible XY-body and lack of crossover formation. We therefore optimized our in vitro culture system of meiosis from male germline stem cells (mGSCs) in order to achieve full chromosome synapsis, XY-body formation and meiotic crossovers. In comparison to previous culture system, the in vitro-generated spermatocytes were transferred after meiotic initiation to a second culture dish. This dish already contained a freshly plated monolayer of proliferatively inactivated immortalized Sertoli cells supporting undifferentiated mGSCs. In this way we aimed to simulate the multiple layers of germ cell types that support spermatogenesis in vivo in the testis. We found that in this optimized culture system, although independent of the undifferentiated mGSCs, meiotic chromosome synapsis was complete and XY body appeared normal. However, meiotic recombination still occurred insufficiently and only few meiotic crossovers were formed, leading to MI-spermatocytes displaying univalent chromosomes (paired sister chromatids). Therefore, considering that meiotic checkpoints are not necessarily fully functional in vitro, meiotic crossover formation should be closely monitored when mimicking gametogenesis in vitro to prevent generation of aneuploid gametes.

Studying human reproductive biology through single-cell analysis and in vitro differentiation of stem cells into germ cell-like cells

BACKGROUND

Understanding the molecular and cellular mechanisms of human reproductive development has been limited by the scarcity of human samples and ethical constraints. Recently, in vitro differentiation of human pluripotent stem cells into germ cells and single-cell analyses have opened new avenues to directly study human germ cells and identify unique mechanisms in human reproductive development.

OBJECTIVE AND RATIONALE

The goal of this review is to collate novel findings and insightful discoveries with these new methodologies, aiming at introducing researchers and clinicians to the use of these tools to study human reproductive biology and develop treatments for infertility.

SEARCH METHODS

PubMed was used to search articles and reviews with the following main keywords: in vitro differentiation, human stem cells, single-cell analysis, spermatogenesis, oogenesis, germ cells and other key terms related to these subjects. The search period included all publications from 2000 until now.

OUTCOMES

Single-cell analyses of human gonads have identified many important gene markers at different developmental stages and in subpopulations of cells. To validate the functional roles of these gene markers, researchers have used the in vitro differentiation of human pluripotent cells into germ cells and confirmed that some genetic requirements are unique in human germ cells and are not conserved in mouse models. Moreover, transcriptional regulatory networks and the interaction of germ and somatic cells in gonads were elucidated in these studies.

WIDER IMPLICATIONS

Single-cell analyses allow researchers to identify gene markers and potential regulatory networks using limited clinical samples. On the other hand, in vitro differentiation methods provide clinical researchers with tools to examine these newly identify gene markers and study the causative effects of mutations previously associated with infertility. Combining these two methodologies, researchers can identify gene markers and networks which are essential and unique in human reproductive development, thereby producing more accurate diagnostic tools for assessing reproductive disorders and developing treatments for infertility.

In-vitro gametogenesis on YouTube - Epistemological performances from Strasbourg and Los Angeles

YouTube hosts two records of interest for those interested in how human-stem-cell-derived gametes are made: one from the USA and one from France. Human-stem-cell-derived gametes, sometimes called 'artificial gametes' or 'synthetic gametes', are the result of in-vitro gametogenesis (IVG). IVG is a technology in the making that attempts to create oocytes and spermatozoa from embryonic cells or skin cells. This article presents some elements of these videos in written form, and asks what information is publicly available to 'think with', and what is not, when it comes to imagining the future of human reproduction. Focusing on the staging of science, this article argues that these videos represent ways of understanding and interrogating science, and display epistemological performances. The comparison is helpful to analyse how a shared global bioscientific authority is valued in these two locations, pointing at areas 'back stage' that the social sciences can illuminate.

Modeling Klinefelter Syndrome Using Induced Pluripotent Stem Cells Reveals Impaired Germ Cell Differentiation

Klinefelter syndrome (KS), with an incidence between 1/600 and 1/1,000, is the main genetic cause of male infertility. Due to the lack of an accurate study model, the detailed pathogenic mechanisms by which this X chromosome aneuploidy leads to KS features remain unknown. Here, we report the generation and characterization of induced pluripotent stem cells (iPSCs) derived from a patient with KS: 47XXY-iPSCs. In order to compare the potentials of both 47XXY-iPSCs and 46XY-iPSCs to differentiate into the germ cell lineage, we developed a directed differentiation protocol by testing different combinations of factors including bone morphogenetic protein 4 (BMP4), glial-derived neurotrophic factor (GDNF), retinoic acid (RA) and stem cell factor (SCF) for 42 days. Importantly, we found a reduced ability of 47XXY-iPSCs to differentiate into germ cells when compared to 46XY-iPSCs. In particular, upon germ cell differentiation of 47XXY-iPSCs, we found a reduced proportion of cells positive for BOLL, a protein required for germ cell development and spermatogenesis, as well as a reduced proportion of cells positive for MAGEA4, a spermatogonia marker. This reduced ability to generate germ cells was not associated with a decrease of proliferation of 47XXY-iPSC-derived cells but rather with an increase of cell death upon germ cell differentiation as revealed by an increase of LDH release and of capase-3 expression in 47XXY-iPSC-derived cells. Our study supports the idea that 47XXY-iPSCs provides an excellent in vitro model to unravel the pathophysiology and to design potential treatments for KS patients.

In vitro reconstitution of germ cell development†

Germ cell development is a series of highly specialized processes through which diploid pluripotent cells differentiate into haploid gametes. The processes include biologically important events such as epigenetic reprogramming, sex determination, and meiosis. The mechanisms underlying these events are key issues in reproductive and developmental biology, yet they still remain elusive. As a tool to elucidate these mechanisms, in vitro gametogenesis, which reproduces germ cell development in culture, has long been sought for decades. Recently, methods of in vitro gametogenesis have undergone rapid development in association with stem cell biology, opening many possibilities in this field. This new technology is considered an alternative source of gametes for the reproduction of animals and perhaps humans. This review summarizes current advances and problems in in vitro gametogenesis.

Necessity is the mother of invention and the evolutionary force driving the success of in vitro fertilization

During the last few decades, millions of healthy children have been born with the aid of in vitro fertilization (IVF). This success belies the fact that IVF treatment is comprised of a complex series of interventions starting with a customized control ovarian stimulation protocol. This is followed by the induction of oocyte maturation, the retrieval of mature oocytes and in vitro fertilization, which often involves the microinjection of a single sperm into the oocyte. After fertilization, the resulting embryos are cultured for up to 7 days. The best embryos are transferred into the uterus where the embryo implants and hopefully develops into a healthy child. However, frequently the best embryos are biopsied and frozen. The biopsied cells are analyzed to identify those embryos without chromosomal abnormalities. These embryos are eventually thawed and transferred with pregnancy rates as good if not better than embryos that are not biopsied and transferred in a fresh cycle. Thus, IVF treatment requires the coordinated efforts of physicians, nurses, molecular biologists and embryologists to conduct each of these multifaceted phases in a seamless and flawless manner. Even though complex, IVF treatment may seem routine today, but it was not always the case. In this review the evolution of human IVF is presented as a series of innovations that resolved a technical hurdle in one component of IVF while creating challenges that eventually lead to the next major advancement. This step-by-step evolution in the treatment of human infertility is recounted in this review.

Narrow H3K4me2 is required for chicken PGC formation

The development of primordial germ cells (PGCs) undergoes epigenetic modifications. The study of histone methylation in regulating PGCs is beneficial to understand the development and differentiation mechanism of germ stem cells. Notably, it provides a theoretical basis for directed induction and mass acquisition in vitro. However, little is known about the regulation of PGC formation by histone methylation. Here, we found the high enrichment of H3K4me2 in the blastoderm, genital ridges, and testis. Chromatin immunoprecipitation sequencing was performed and the results revealed that genomic H3K4me2 is dynamic in embryonic stem cells, PGCs, and spermatogonial stem cells. This trend was consistent with the H3K4me2 enrichment in the gene promoter region. Additionally, narrow region triggered PGC-related genes (Bmp4, Wnt5a, and Tcf7l2) and signaling pathways (Wnt and transforming growth factor-β). After knocking down histone methylase Mll2 in vitro and vivo, the level of H3K4me2 decreased, inhibiting Cvh and Blimp1 expression, then repressing the formation of PGCs. Taken together, our study revealed the whole genome map of H3K4me2 in the formation of PGCs, contributing to improve the epigenetic study in PGC formation and providing materials for bird gene editing and rescue of endangered birds.

A typology of scientific breakthroughs

Scientific breakthroughs are commonly understood as discoveries that transform the knowledge frontier and have a major impact on science, technology, and society. Prior literature studying breakthroughs generally treats them as a homogeneous group in attempts to identify supportive conditions for their occurrence. In this paper, we argue that there are different types of scientific breakthroughs, which differ in their disciplinary occurrence and are associated with different considerations of use and citation impact patterns. We develop a typology of scientific breakthroughs based on three binary dimensions of scientific discoveries and use this typology to analyze qualitatively the content of 335 scientific articles that report on breakthroughs. For each dimension, we test associations with scientific disciplines, reported use considerations, and scientific impact. We find that most scientific breakthroughs are driven by a question and in line with literature, and that paradigm shifting discoveries are rare. Regarding the scientific impact of breakthrough as measured by citations, we find that an article that answers an unanswered question receives more citations compared to articles that were not motivated by an unanswered question. We conclude that earlier research in which breakthroughs were operationalized as highly cited scientific articles may thus be biased against the latter.

Generation of Artificial Gamete and Embryo From Stem Cells in Reproductive Medicine

In addition to the great growing need for assisted reproduction technologies (ART), additional solutions for patients without functional gametes are strongly needed. Due to ethical restrictions, limited studies can be performed on human gametes and embryos; however, artificial gametes and embryos represent a new hope for clinical application and basic research in the field of reproductive medicine. Here, we provide a review of the research progress and possible application of artificial gametes and embryos from different species, including mice, monkeys and humans. Gametes specification from adult stem cells and embryonic stem cells (ESCs) as well as propagation of stem cells from the reproductive system and from organized embryos, which are similar to blastocysts, have been realized in some nonhuman mammals, but not all achievements can be replicated in humans. This area of research remains noteworthy and requires further study and effort to achieve the reconstitution of the entire cycle of gametogenesis and embryo development in vitro.

Derivation of oocyte-like cells from putative embryonic stem cells and parthenogenetically activated into blastocysts in goat

Germ cells are responsible for the propagation of live animals from generation to generation, but to surprise, a steep increase in infertile problems among livestock poses great threat for economic development of human race. An alternative and robust approach is essential to combat these ailments. Here, we demonstrate that goat putative embryonic stem cells (ESCs) were successfully in vitro differentiated into primordial germ cells and oocyte-like cells using bone morphogenetic protein-4 (BMP-4) and trans-retinoic acid (RA). Oocyte-like cells having distinct zonapellucida recruited adjacent somatic cells in differentiating culture to form cumulus-oocyte complexes (COCs). The putative COCs were found to express the zonapellucida specific (ZP1 and ZP2) and oocyte-specific markers. Primordial germ cell-specific markers VASA, DAZL, STELLA, and PUM1 were detected at protein and mRNA level. In addition to that, the surface architecture of these putative COCs was thoroughly visualized by the scanning electron microscope. The putative COCs were further parthenogenetically activated to develop into healthy morula, blastocysts and hatched blastocyst stage like embryos. Our findings may contribute to the fundamental understanding of mammalian germ cell biology and may provide clinical insights regarding infertility ailments.

In vitro breeding: application of embryonic stem cells to animal production†

Embryonic stem cells (ESCs) are derived from the inner cell mass of preimplantation blastocysts. For decades, attempts to efficiently derive ESCs in animal livestock species have been unsuccessful, but this goal has recently been achieved in cattle. Together with the recent reconstitution of the germ cell differentiation processes from ESCs in mice, these achievements open new avenues for the development of promising technologies oriented toward improving health, animal production, and the environment. In this article, we present a strategy that will notably accelerate genetic improvement in livestock populations by reducing the generational interval, namely in vitro breeding (IVB). IVB combines genomic selection, a widely used strategy for genetically improving livestock, with ESC derivation and in vitro differentiation of germ cells from pluripotent stem cells. We also review the most recent findings in the fields on which IVB is based. Evidence suggests this strategy will be soon within reach.

Haprin-deficient spermatozoa are incapable of in vitro fertilization

Haprin (TRIM36) is a ubiquitin-protein ligase that mediates ubiquitination and subsequent proteasomal degradation of target proteins. It is expressed in the testes in both mice and humans and is thought to be involved in spermiogenesis, the acrosome reaction, and fertilization. However, the functional role of Haprin is poorly understood. The aim of this study was to investigate the physiological role of Haprin in fertility. Homozygous haprin-deficient mice were generated and these mice, and their spermatozoa, were analyzed to detect morphological and fertility-related abnormalities. In these models, normal spermatogenesis was observed but sperm quality was reduced with haprin-deficient mice having poorer sperm morphology and motility than wild-type mice. Interestingly, haprin-deficient mice showed normal in vivo fertility but could not fertilize oocytes under standard in vitro fertilization conditions. In conclusion, this study demonstrated that Haprin deficiency causes morphological abnormalities in spermatozoa, indicating that Haprin is involved in spermiogenesis.

Visualizing structure and transitions in high-dimensional biological data

The high-dimensional data created by high-throughput technologies require visualization tools that reveal data structure and patterns in an intuitive form. We present PHATE, a visualization method that captures both local and global nonlinear structure using an information-geometric distance between data points. We compare PHATE to other tools on a variety of artificial and biological datasets, and find that it consistently preserves a range of patterns in data, including continual progressions, branches and clusters, better than other tools. We define a manifold preservation metric, which we call denoised embedding manifold preservation (DEMaP), and show that PHATE produces lower-dimensional embeddings that are quantitatively better denoised as compared to existing visualization methods. An analysis of a newly generated single-cell RNA sequencing dataset on human germ-layer differentiation demonstrates how PHATE reveals unique biological insight into the main developmental branches, including identification of three previously undescribed subpopulations. We also show that PHATE is applicable to a wide variety of data types, including mass cytometry, single-cell RNA sequencing, Hi-C and gut microbiome data.

Roadmap of germline development and in vitro gametogenesis from pluripotent stem cells

BACKGROUND

The germ cell lineage is a fundamental component of the metazoan life cycle, ensuring the perpetuation and substantial diversification of genetic information across generations. Recent advances in the understanding of mouse germ cell development have culminated in the ability to reconstitute gametogenesis in vitro, thereby enabling the biochemical and molecular analyses of germ cell specification and subsequent development in mice. Similar advances in reconstituting human germ cells in vitro would provide critical insight into the etiology of various reproductive conditions and disorders, including infertility.

OBJECTIVES

This review presents the mechanisms leading to germ cell development in mammals, particularly in mice and non-human primates, as well as the applicability of these animal models to human germ cell development. The induction methods performed to recapitulate germ cell development in vitro are also discussed in this review, specifically focusing on in vitro gametogenesis from pluripotent stem cells.

MATERIALS AND METHODS

This review compiles the key methods and findings of various references relevant to the above-mentioned topic.

RESULTS

Murine models have provided essential mechanistic insight into the process of germ cell lineage development. However, there are several structural differences between mice and humans during early embryogenesis that hinder the extrapolation of findings made in murine models to what may occur in humans. Recent studies using human or non-human primate embryos and human-induced pluripotent stem cell (hiPSC)-derived germ cells shed light on key cellular and genetic mechanisms governing germ cell development in humans.

DISCUSSION

Utilizing the knowledge obtained from studying germ cell development in different animal models, induction methods established by various laboratories now permit partial reconstitution of human gametogenesis in vitro.

CONCLUSION

In vitro gametogenesis will constitute an emergent new field in human reproductive medicine in the near future, although legal and ethical considerations must be taken into account.

Retinoic acid and 17β-estradiol improve male germ cell differentiation from mouse-induced pluripotent stem cells

This research aimed to explore the impacts of retinoic acid (RA)/17β-estradiol (E) induction and embryoid body formation to enhance differentiation of mouse-induced pluripotent stem cells (miPSCs) into male germ cells in vitro. Flow cytometry and qPCR were conducted to describe miPSCs differentiation process. Various temporal expression profiles of germ cell-related genes were traced. Stra8 gene expression increased in the RA group on the 4th day compared to other groups. The RA group experienced a more significant increase than E group. The expression of Sycp3 increased in RA + E group on 4th day compared with other groups. Expression of AKAP3 enhanced in the RA + E group than other groups on day 4. Moreover, miPSCs showed that this gene expression in the RA + E group was increased in comparison to RA and E groups on day 7. AKAP3 gene expression on day 7 of miPSCs decreased in RA and E groups. Flow cytometry data indicated that 3%-8% of the cells in sub-G1 stage were haploid after RA and E induction compared to other groups on day 4. This study showed that miPSCs possess the power for differentiating into male germ cells in vitro via formation of embryoid body by RA with/or E induction.

Evolving Role of RING1 and YY1 Binding Protein in the Regulation of Germ-Cell-Specific Transcription

Separation of germline cells from somatic lineages is one of the earliest decisions of embryogenesis. Genes expressed in germline cells include apoptotic and meiotic factors, which are not transcribed in the soma normally, but a number of testis-specific genes are active in numerous cancer types. During germ cell development, germ-cell-specific genes can be regulated by specific transcription factors, retinoic acid signaling and multimeric protein complexes. Non-canonical polycomb repressive complexes, like ncPRC1.6, play a critical role in the regulation of the activity of germ-cell-specific genes. RING1 and YY1 binding protein (RYBP) is one of the core members of the ncPRC1.6. Surprisingly, the role of Rybp in germ cell differentiation has not been defined yet. This review is focusing on the possible role of Rybp in this process. By analyzing whole-genome transcriptome alterations of the Rybp^-/- embryonic stem (ES) cells and correlating this data with experimentally identified binding sites of ncPRC1.6 subunits and retinoic acid receptors in ES cells, we propose a model how germ-cell-specific transcription can be governed by an RYBP centered regulatory network, underlining the possible role of RYBP in germ cell differentiation and tumorigenesis.

Contribution of Single-Cell Transcriptomics to the Characterization of Human Spermatogonial Stem Cells: Toward an Application in Male Fertility Regenerative Medicine?

Ongoing progress in genomic technologies offers exciting tools that can help to resolve transcriptome and genome-wide DNA modifications at single-cell resolution. These methods can be used to characterize individual cells within complex tissue organizations and to highlight various molecular interactions. Here, we will discuss recent advances in the definition of spermatogonial stem cells (SSC) and their progenitors in humans using the single-cell transcriptome sequencing (scRNAseq) approach. Exploration of gene expression patterns allows one to investigate stem cell heterogeneity. It leads to tracing the spermatogenic developmental process and its underlying biology, which is highly influenced by the microenvironment. scRNAseq already represents a new diagnostic tool for the personalized investigation of male infertility. One may hope that a better understanding of SSC biology could facilitate the use of these cells in the context of fertility preservation of prepubertal children, as a key component of regenerative medicine.

Pluripotent Cell Models for Gonadal Research

Sex development is a complex process involving many genes and hormones. Defects in this process lead to Differences of Sex Development (DSD), a group of heterogeneous conditions not as rare as previously thought. Part of the obstacles in proper management of these patients is due to an incomplete understanding of the genetics programs and molecular pathways involved in sex development and DSD. Several challenges delay progress and the lack of a proper model system for the single patient severely hinders advances in understanding these diseases. The revolutionary techniques of cellular reprogramming and guided in vitro differentiation allow us now to exploit the versatility of induced pluripotent stem cells to create alternatives models for DSD, ideally on a patient-specific personalized basis.

Germ cell differentiation of mouse embryonic stem cells can be influenced by the culture medium

Although embryonic stem (ES) cells can differentiate into germ cells, little is known about the influence of culture media on this process. We investigated the effect of two culture media on the capacity of ES cells to differentiate into germ cells using embryoid body (EB) and monolayer culture protocols. Germ cell differentiation was induced in mouse ES cells under four experimental conditions: EB/Dulbecco's modified Eagle's medium (EB/DMEM), EB/knockout Dulbecco's modified Eagle's medium (EB/KO-DMEM), monolayer/Dulbecco's modified Eagle's medium (monolayer/DMEM), and monolayer/knockout Dulbecco's modified Eagle's medium (monolayer/KO-DMEM). After incubation for 6 days, quantitative real-time polymerase chain reaction (qRT-PCR) was used to assess expression of the germ cell markers, Mvh, Oct4, Rec8, Scp1, Scp3 and Stra8. Also, Oct4 and Mvh expressions at the protein level were assessed using immunocytochemistry; we evaluated alkaline phosphatase activity in addition to cell number and viability. Germ cell-specific marker expression was increased significantly in cells differentiated in KO-DMEM for both EB and monolayer protocols; the highest level was in cultures using the EB protocol. The highest cell proliferation rate was observed using the monolayer/KO-DMEM protocol and the lowest using the EB/DMEM protocol. Generally, KO-DMEM exhibited the greatest impact on germ cell differentiation and cell proliferation. Optimization of germ cell differentiation of ES cells requires careful selection of culture medium.

Distinct roles of retinoic acid and BMP4 pathways in the formation of chicken primordial germ cells and spermatogonial stem cells

This study demonstrated different effects of bone morphogenetic protein 4 (BMP4) and retinoic acid (RA) signaling on the induction of germ cell formation in chickens. In vitro, BMP4 significantly promoted primordial germ cell (PGC) formation, while RA promoted spermatogonial stem cell (SSC) formation. Hematoxylin-Eosin (HE) staining of reproductive ridge and testicular slices showed that BMP4 signaling was activated during PGC formation but was inhibited during PGC differentiation into SSC. In contrast, RA signaling was significantly activated during PGC differentiation to SSC. Mechanistically, elevated expression of phosphorylated mothers against decapentaplegic homolog 5 (p-Smad5) activated BMP4 signaling, while inhibition of p-Smad5 significantly reduced the PGC formation. Additionally, BMP4 regulated the PGC formation through histone acetylation and DNA methylation in deleted in azoospermia-like (DAZL) gene. Luciferase report showed RA binding to RARα regulated stimulated by RA 8 (Stra8) promoter activity during SSC formation, while mutations in RAR binding sites inhibited the Stra8 expression and SSC formation. Further, both HAT and HDAC regulated the RARα isoform, and HAT binding to RARα activated the Stra8 transcription. RNA-seq of embryonic stem cells (ESC), PGC, and SSC showed inverse expression of genes related to the BMP4 and RA pathways during PGC and SSC formation. Additionally, Smad5 and Smurf were critical for the interactions between the two pathways. Specifically, through Smurf promotion of Smad5 ubiquitination, RA could inhibit the BMP4 signal transduction. In conclusion, the BMP4 and RA signaling pathways play opposing roles in germ cell formation, driven by epigenetic processes such as phosphorylation, ubiquitination, and histone acetylation.