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

The establishment and regulation of human germ cell lineage

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

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

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

Rapid human oogonia-like cell specification via transcription factor-directed differentiation

The generation of germline cells from human induced pluripotent stem cells (hiPSCs) represents a milestone toward in vitro gametogenesis. Methods to recapitulate germline development beyond primordial germ cells in vitro have relied on long-term cell culture, such as 3-dimensional organoid co-culture for ~four months. Using a pipeline with highly parallelized screening, this study identifies combinations of TFs that directly and rapidly convert hiPSCs to induced oogonia-like cells (iOLCs). We demonstrate that co-expression of five TFs - namely, ZNF281, LHX8, SOHLH1, ZGLP1, and ANHX, induces high efficiency DDX4-positive iOLCs in only four days in a feeder-free monolayer culture condition. We also show improved production of human primordial germ cell-like cells (hPGCLCs) from hiPSCs by expression of DLX5, HHEX, and FIGLA. We characterize these TF-based iOLCs and hPGCLCs via gene and protein expression analyses and demonstrate their similarity to in vivo and in vitro-derived oogonia and primordial germ cells. Together, these results identify new regulatory factors that enhance human germ cell specification in vitro, and further establish unique computational and experimental tools for human in vitro oogenesis research.

Epigenetic reprogramming in mouse and human primordial germ cells

Primordial germ cells (PGCs) are the precursors of sperm and eggs. They undergo genome-wide epigenetic reprogramming to erase epigenetic memory and reset the genomic potential for totipotency. Global DNA methylation erasure is a crucial part of epigenetic resetting when DNA methylation levels decrease across the genome to <5%. However, certain localized regions exhibit slower demethylation or resistance to reprogramming. Since DNA methylation plays a crucial role in transcriptional regulation, this depletion in PGCs requires mechanisms independent of DNA methylation to regulate transcriptional control during PGC reprogramming. Histone modifications are predicted to compensate for the loss of DNA methylation in gene regulation. Different histone modifications exhibit distinct patterns in PGCs undergoing epigenetic programming at the genomic level during PGC development in conjunction with changes in DNA methylation. Together, they contribute to PGC-specific genomic regulation. Recent findings related to these processes provide a comprehensive overview of germline epigenetic reprogramming and its importance in mouse and human PGC development. Additionally, we evaluated the extent to which in vitro culture techniques have replicated the development processes of human PGCs.

Early human development and stem cell-based human embryo models

The use of stem cells to model the early human embryo promises to transform our understanding of developmental biology and human reproduction. In this review, we present our current knowledge of the first 2 weeks of human embryo development. We first focus on the distinct cell lineages of the embryo and the derivation of stem cell lines. We then discuss the intercellular crosstalk that guides early embryo development and how this crosstalk is recapitulated in vitro to generate stem cell-based embryo models. We highlight advances in this fast-developing field, discuss current limitations, and provide a vision for the future.

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

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

In vitro reconstitution of epigenetic reprogramming in the human germ line

Epigenetic reprogramming resets parental epigenetic memories and differentiates primordial germ cells (PGCs) into mitotic pro-spermatogonia or oogonia. This process ensures sexually dimorphic germ cell development for totipotency1. In vitro reconstitution of epigenetic reprogramming in humans remains a fundamental challenge. Here we establish a strategy for inducing epigenetic reprogramming and differentiation of pluripotent stem-cell-derived human PGC-like cells (hPGCLCs) into mitotic pro-spermatogonia or oogonia, coupled with their extensive amplification (about >1010-fold). Bone morphogenetic protein (BMP) signalling is a key driver of these processes. BMP-driven hPGCLC differentiation involves attenuation of the MAPK (ERK) pathway and both de novo and maintenance DNA methyltransferase activities, which probably promote replication-coupled, passive DNA demethylation. hPGCLCs deficient in TET1, an active DNA demethylase abundant in human germ cells2,3, differentiate into extraembryonic cells, including amnion, with de-repression of key genes that bear bivalent promoters. These cells fail to fully activate genes vital for spermatogenesis and oogenesis, and their promoters remain methylated. Our study provides a framework for epigenetic reprogramming in humans and an important advance in human biology. Through the generation of abundant mitotic pro-spermatogonia and oogonia-like cells, our results also represent a milestone for human in vitro gametogenesis research and its potential translation into reproductive medicine.

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

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

Induction of Meiosis from Human Pluripotent Stem Cells

An in vitro model of human meiosis would accelerate research into this important reproductive process and development of therapies for infertility. We have developed a method to induce meiosis starting from male or female human pluripotent stem cells. We demonstrate that DNMT1 inhibition, retinoid signaling activation, and overexpression of regulatory factors (anti-apoptotic BCL2, and pro-meiotic HOXB5, BOLL, or MEIOC) rapidly activates meiosis, with leptonema beginning at 6 days, zygonema at 9 days, and pachynema at 12 days. Immunofluorescence microscopy shows key aspects of meiosis, including chromosome synapsis and sex body formation. The meiotic cells express genes similar to meiotic oogonia in vivo, including all synaptonemal complex components and machinery for meiotic recombination. These findings establish an accessible system for inducing human meiosis in vitro.

The journey of a generation: advances and promises in the study of primordial germ cell migration

The germline provides the genetic and non-genetic information that passes from one generation to the next. Given this important role in species propagation, egg and sperm precursors, called primordial germ cells (PGCs), are one of the first cell types specified during embryogenesis. In fact, PGCs form well before the bipotential somatic gonad is specified. This common feature of germline development necessitates that PGCs migrate through many tissues to reach the somatic gonad. During their journey, PGCs must respond to select environmental cues while ignoring others in a dynamically developing embryo. The complex multi-tissue, combinatorial nature of PGC migration is an excellent model for understanding how cells navigate complex environments in vivo. Here, we discuss recent findings on the migratory path, the somatic cells that shepherd PGCs, the guidance cues somatic cells provide, and the PGC response to these cues to reach the gonad and establish the germline pool for future generations. We end by discussing the fate of wayward PGCs that fail to reach the gonad in diverse species. Collectively, this field is poised to yield important insights into emerging reproductive technologies.

Human primordial germ cell-like cells specified from resetting precursors develop in human hindgut organoids

Human primordial germ cells (hPGCs), the precursors of eggs and sperm, start their complex development shortly after specification and during their migration to the primitive gonads. Here, we describe protocols for specifying hPGC-like cells (hPGCLCs) from resetting precursors and progressing them with the support of human hindgut organoids. Resetting hPGCLCs (rhPGCLCs) are specified from human embryonic stem cells (hESCs) transitioning from the primed into the naive state of pluripotency. Hindgut organoids are also derived from hESCs after a sequential differentiation into a posterior endoderm/hindgut fate. Both rhPGCLCs and hindgut organoids are combined and co-cultured for 25 d. The entire procedure takes ~1.5 months and can be successfully implemented by a doctoral or graduate student with basic skills and experience in hESC cultures. The co-culture system supports the progression of rhPGCLCs at a developmental timing analogous to that observed in vivo. Compared with previously developed hPGCLC progression protocols, which depend on co-cultures with mouse embryonic gonadal tissue, our co-culture system represents a developmentally relevant model closer to the environment that hPGCs first encounter after specification. Together with the potential for investigations of events during hPGC specification and early development, these protocols provide a practical approach to designing efficient models for in vitro gametogenesis. Notably, the rhPGCLC-hindgut co-culture system can also be adapted to study failings in hPGC migration, which are associated with the etiology of some forms of infertility and germ cell tumors.

Using Embryo Models to Understand the Development and Progression of Embryonic Lineages: A Focus on Primordial Germ Cell Development

BACKGROUND

Recapitulating mammalian cell type differentiation in vitro promises to improve our understanding of how these processes happen in vivo, while bringing additional prospects for biomedical applications. The establishment of stem cell-derived embryo models and embryonic organoids, which have experienced explosive growth over the last few years, opens new avenues for research due to their scale, reproducibility, and accessibility. Embryo models mimic various developmental stages, exhibit different degrees of complexity, and can be established across species. Since embryo models exhibit multiple lineages organized spatially and temporally, they are likely to provide cellular niches that, to some degree, recapitulate the embryonic setting and enable "co-development" between cell types and neighbouring populations. One example where this is already apparent is in the case of primordial germ cell-like cells (PGCLCs).

SUMMARY

While directed differentiation protocols enable the efficient generation of high PGCLC numbers, embryo models provide an attractive alternative as they enable the study of interactions of PGCLCs with neighbouring cells, alongside the regulatory molecular and biophysical mechanisms of PGC competency. Additionally, some embryo models can recapitulate post-specification stages of PGC development (including migration or gametogenesis), mimicking the inductive signals pushing PGCLCs to mature and differentiate and enabling the study of PGCLC development across stages. Therefore, in vitro models may allow us to address questions of cell type differentiation, and PGC development specifically, that have hitherto been out of reach with existing systems.

KEY MESSAGE

This review evaluates the current advances in stem cell-based embryo models, with a focus on their potential to model cell type-specific differentiation in general and in particular to address open questions in PGC development and gametogenesis.

Making human eggs in a dish: are we close?

In the space of 50 years, we have seen incredible achievements in human reproductive medicine. With these leaps forward, it is no wonder that there is a major interest in women's reproductive health research, including extension of reproductive lifespan. Substantial effort is currently being made to address this challenge, including from the commercial sector. In vitro gametogenesis (IVG) in mice is a spectacular breakthrough and has the potential to offer hope to women with intractable infertility. However, with such lofty goals, some reflection may be called for: mastering all of the techniques required for complete and safe IVG in women is likely to be extraordinarily difficult.

Highly cooperative chimeric super-SOX induces naive pluripotency across species

Our understanding of pluripotency remains limited: iPSC generation has only been established for a few model species, pluripotent stem cell lines exhibit inconsistent developmental potential, and germline transmission has only been demonstrated for mice and rats. By swapping structural elements between Sox2 and Sox17, we built a chimeric super-SOX factor, Sox2-17, that enhanced iPSC generation in five tested species: mouse, human, cynomolgus monkey, cow, and pig. A swap of alanine to valine at the interface between Sox2 and Oct4 delivered a gain of function by stabilizing Sox2/Oct4 dimerization on DNA, enabling generation of high-quality OSKM iPSCs capable of supporting the development of healthy all-iPSC mice. Sox2/Oct4 dimerization emerged as the core driver of naive pluripotency with its levels diminished upon priming. Transient overexpression of the SK cocktail (Sox+Klf4) restored the dimerization and boosted the developmental potential of pluripotent stem cells across species, providing a universal method for naive reset in mammals.

The many dimensions of germline competence

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

DMRT1 regulates human germline commitment

Germline commitment following primordial germ cell (PGC) specification during early human development establishes an epigenetic programme and competence for gametogenesis. Here we follow the progression of nascent PGC-like cells derived from human embryonic stem cells in vitro. We show that switching from BMP signalling for PGC specification to Activin A and retinoic acid resulted in DMRT1 and CDH5 expression, the indicators of migratory PGCs in vivo. Moreover, the induction of DMRT1 and SOX17 in PGC-like cells promoted epigenetic resetting with striking global enrichment of 5-hydroxymethylcytosine and locus-specific loss of 5-methylcytosine at DMRT1 binding sites and the expression of DAZL representing DNA methylation-sensitive genes, a hallmark of the germline commitment programme. We provide insight into the unique role of DMRT1 in germline development for advances in human germ cell biology and in vitro gametogenesis.

The developmental dynamics of the human male germline

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

From in vivo to in vitro: exploring the key molecular and cellular aspects of human female gametogenesis

Human oogenesis is a highly complex and not yet fully understood process due to ethical and technological barriers that limit studies in the field. In this context, replicating female gametogenesis in vitro would not only provide a solution for some infertility problems, but also be an excellent study model to better understand the biological mechanisms that determine the formation of the female germline. In this review, we explore the main cellular and molecular aspects involved in human oogenesis and folliculogenesis in vivo, from the specification of primordial germ cells (PGCs) to the formation of the mature oocyte. We also sought to describe the important bidirectional relationship between the germ cell and the follicular somatic cells. Finally, we address the main advances and different methodologies used in the search for obtaining cells of the female germline in vitro.