Single-cell transcriptomic characterization of a gastrulating human embryo

Unravelling early hematoendothelial development through the chick model: Insights and future perspectives

The chicken embryo has been an important model in advancing our understanding of early hematoendothelial development, particularly in the formation of hematopoietic stem cells (HSCs) and the endothelial-to-hematopoietic transition (EHT). The accessibility and ease of manipulation of chicken embryos have made them an invaluable tool for researching development of blood and endothelial cells. Early research using this model provided pivotal insights, demonstrating that intra-embryonic regions, such as the dorsal aorta (DA), are primary sources of HSCs, rather than the yolk sac (YS), as previously believed. The identification of intra-aortic hematopoietic clusters (IAHCs) and the process of EHT in the chicken embryo laid the foundation for similar discoveries in other vertebrate species, including mice and zebrafish. Recent advances in genetic tools, such as transgenic chickens expressing fluorescent proteins, have further enhanced the precision of cell lineage tracing and real-time imaging of dynamic cellular processes. This review highlights both historical contributions and contemporary advancements facilitated by the chicken model, underscoring its continued relevance in developmental biology. By examining key findings and methodological innovations, we aim to demonstrate the importance of the chicken embryo as a model system for understanding hematoendothelial development and its potential for informing therapeutic applications in regenerative medicine and blood disorders. Finally, we will underscore potential applications of the chicken model for comparative and omics-level studies in conjunction with other model systems and what future directions lie ahead.

Deciphering signaling mechanisms and developmental dynamics in extraembryonic mesoderm specification from hESCs

Extraembryonic mesoderm (ExM) is crucial for human development, yet its specification is poorly understood. Human embryonic stem cell (hESC)-based models, including embryoids and differentiated derivatives, are emerging as promising tools for studying ExM development. Despite this, the signaling mechanisms and developmental dynamics that underlie ExM specification from hESCs remain challenging to study. Here, we report that the modulation of BMP, WNT, and Nodal signaling pathways can rapidly (4-5 days) and efficiently ( ~90%) induce the differentiation of both naive and primed hESCs into ExM-like cells (ExMs). We reveal that ExM specification from hESCs predominantly proceeds through intermediates exhibiting a primitive streak (PS)-like gene expression pattern and delineate the regulatory roles of WNT and Nodal signaling in this process. Furthermore, we find that the initial pluripotent state governs hESC-based ExM specification by influencing signal response, cellular composition, developmental progression, and transcriptional characteristics of the resulting ExMs. Our study provides promising models for dissecting human ExM development and sheds light on the signaling principles, developmental dynamics, and influences of pluripotency states underlying ExM specification from hESCs.

Meeting Review of the '1st Azerbaijan Neuroscience School: Introduction'

The '1st Azerbaijan Neuroscience School: Introduction' program marked a historic milestone as the first neuroscience education initiative in Azerbaijan. This 12-week online program aimed to bridge the gap between Azerbaijani neuroscience enthusiasts and international experts by delivering foundational knowledge, cutting-edge research insights, and critical thinking skills. The program was open to a diverse audience, ranging from high school students to university-level medical and biology students, as well as early-career researchers and faculty members. The virtual format of the program allowed participants to enroll and join in, regardless of geographical location, which fostered scientific exchange, mentorship, and professional networking. The success of this initiative paved the way for the follow-up events: the 'Azerbaijan Neuroscience School: Advanced', 'International Brain Bee Competition', and the '1st Azerbaijan Neuroscience Conference' in 2024.

Post-gastrulation amnioids as a stem cell-derived model of human extra-embryonic development

The amnion, an extra-embryonic tissue in mammalian embryos, is thought to provide crucial signaling, structural, and nutritional support during pregnancy. Despite its pivotal importance, studying human amnion formation and function has been hampered by the lack of accurate in vitro models. Here, we present an embryonic stem cell-derived 3D model of the post-gastrulation amnion, post-gastrulation amnioids (PGAs), that faithfully recapitulates extra-embryonic development up to 4 weeks post-fertilization, closely mimicking the functional traits of the human amniotic sac. PGAs self-organize, forming the amnion and the yolk sac, and are surrounded by the extra-embryonic mesoderm. Using PGAs, we show that GATA3 is required and sufficient for amniogenesis and that an autoregulatory feedback loop governs amnion formation, whereby extra-embryonic signals promote amnion specification. The reproducibility and scalability of the PGA system, with its precise cellular, structural, and functional integrity, opens avenues for investigating embryo-amnion interactions beyond gastrulation and offers an ideal platform for large-scale pharmacological and clinical studies.

MIXL1 activation in endoderm differentiation of human induced pluripotent stem cells

Human induced pluripotent stem cells (hiPSCs) possess the ability to differentiate into a multitude of cell and tissue types but display heterogeneous propensity of differentiation into specific lineage. Characterization of the transcriptome of 11 hiPSC lines showed that activation of MIXL1 at the early stage of stem cell differentiation correlated with higher efficacy in generating definitive endoderm and advancing differentiation and maturation of endoderm derivatives. Enforced expression of MIXL1 in the endoderm-inefficient hiPSCs enhanced the propensity of endoderm differentiation, suggesting that modulation of key drivers of lineage differentiation can re-wire hiPSC to the desired lineage propensity to generate the requisite stem cell products.

Extended culture of 2D gastruloids to model human mesoderm development

Micropatterned human pluripotent stem cells treated with BMP4 (two-dimensional (2D) gastruloids) are among the most widely used stem cell models for human gastrulation. Due to its simplicity and reproducibility, this system is ideal for high-throughput quantitative studies of tissue patterning and has led to many insights into the mechanisms of mammalian gastrulation. However, 2D gastruloids have been studied only up to about 2 days owing to a loss of organization beyond this time with earlier protocols. Here we report an extended 2D gastruloid model to up to 10 days. We discovered a phase of highly reproducible morphogenesis between 2 and 4 days during which directed migration from the primitive streak-like region gives rise to a mesodermal layer beneath an epiblast-like layer. Multiple types of mesoderm arise with striking spatial organization including lateral plate mesoderm-like cells on the colony border and paraxial mesoderm-like cells further inside the colony. Single-cell transcriptomics showed strong similarity of these cells to mesoderm in human and nonhuman primate embryos. Our results illustrate that extended culture of 2D gastruloids provides a powerful model for human mesoderm differentiation and morphogenesis.

3D reconstruction of a human Carnegie stage 9 embryo provides a snapshot of early body plan formation

The Carnegie stage 9 (CS9) embryo is a pivotal phase signifying the conclusion of gastrulation and the onset of early organogenesis, crucial for initiating major organ system development. Utilizing spatial transcriptomics, we analyzed an intact CS9 human embryo in a spatially detailed manner. Through the examination of 75 transverse cryosections, we digitally reconstructed a 3D model, allowing us to identify diverse cell types, including those from brain and spine regions, the primitive gut tube, distinct somite formation stages, somatic mesoderm, splanchnic mesoderm, etc. Notably, we observed two distinct trajectories of hindbrain development, pinpointed the isthmic organizer at the midbrain-hindbrain boundary, delineated the bi-layered structure of neuromesodermal progenitor (NMP) cells, and described the early aorta formation and primordial germ cells (PGCs) presence in the aorta-gonad-mesonephros (AGM) region. This study provides key insights into the transcriptomic and spatial intricacies shaping the human body plan.

Human and non-human primate female in vitro gametogenesis toward meiotic entry: a systematic review

In vitro gametogenesis offers a powerful platform to explore the complexities of female germline development while bypassing ethical and technical barriers in human and non-human primate research. This systematic review examined 23 articles that reported meiotic entry from differentiated pluripotent stem cells or ex vivo-cultured fetal germ cells from humans, cynomolgus monkeys or marmosets and were published between 2009 and 2025. By comparing methodologies and outcomes, the review highlighted current progress and ongoing challenges in inducing meiotic progression in primates. Although complete oogenesis using in vitro gametogenesis has been successfully achieved in mice, extending this success to primates remains a major hurdle, with meiotic entry representing a key milestone toward realizing in vitro gametogenesis in humans.

α-Ketoglutarate promotes trophectoderm induction and maturation from naive human embryonic stem cells

Development and lineage choice are driven by interconnected transcriptional, epigenetic and metabolic changes. Specific metabolites, such as α-ketoglutarate (αKG), function as signalling molecules affecting the activity of chromatin-modifying enzymes. However, how metabolism coordinates cell-state changes, especially in human pre-implantation development, remains unclear. Here we uncover that inducing naive human embryonic stem cells towards the trophectoderm lineage results in considerable metabolic rewiring, characterized by αKG accumulation. Elevated αKG levels potentiate the capacity of naive embryonic stem cells to specify towards the trophectoderm lineage. Moreover, increased αKG levels promote blastoid polarization and trophectoderm maturation. αKG supplementation does not affect global histone methylation levels; rather, it decreases acetyl-CoA availability, reduces histone acetyltransferase activity and weakens the pluripotency network. We propose that metabolism functions as a positive feedback loop aiding in trophectoderm fate induction and maturation, highlighting that global metabolic rewiring can promote specificity in cell fate decisions through intricate regulation of signalling and chromatin.

Advances and applications in single-cell and spatial genomics

The applications of single-cell and spatial technologies in recent times have revolutionized the present understanding of cellular states and the cellular heterogeneity inherent in complex biological systems. These advancements offer unprecedented resolution in the examination of the functional genomics of individual cells and their spatial context within tissues. In this review, we have comprehensively discussed the historical development and recent progress in the field of single-cell and spatial genomics. We have reviewed the breakthroughs in single-cell multi-omics technologies, spatial genomics methods, and the computational strategies employed toward the analyses of single-cell atlas data. Furthermore, we have highlighted the advances made in constructing cellular atlases and their clinical applications, particularly in the context of disease. Finally, we have discussed the emerging trends, challenges, and opportunities in this rapidly evolving field.

HAND1 level controls the specification of multipotent cardiac and extraembryonic progenitors from human pluripotent stem cells

Diverse sets of progenitors contribute to the development of the embryonic heart, but the mechanisms of their specification have remained elusive. Here, using a human pluripotent stem cell (hPSC) model, we deciphered cardiac and non-cardiac lineage trajectories in differentiation and identified transcription factors underpinning cell specification, identity and function. We discovered a concentration-dependent, fate determining function for the basic helix-loop-helix transcription factor HAND1 in mesodermal progenitors and uncovered its gene regulatory network. At low expression level, HAND1 directs differentiation towards multipotent juxta-cardiac field progenitors able to make cardiomyocytes and epicardial cells, whereas at high level it promotes the development of extraembryonic mesoderm. Importantly, HAND1-low progenitors can be propagated in their multipotent state. This detailed mechanistic insight into human development has the potential to accelerate the delivery of effective disease modelling, including for congenital heart disease, and cell therapy-based regenerative medicine.

HAND1, partially mediated through ape-specific LTR binding, is essential for human extra-embryonic mesenchyme derivation from iPSCs

The specification of extra-embryonic mesenchyme (ExMC) is a prime example of developmental divergence between mouse and human. Derived from definitive mesoderm during mouse gastrulation, the human ExMC first appears at peri-implantation prior to gastrulation and therefore its human cellular origin, still unknown, must differ. In a human pluripotent stem cell model, we report that ExMC shares progenitor cells with trophoblast, suggesting a trophectoderm origin. This ability to form ExMC appears to extend to human trophoblast stem cell lines. We define HAND1 as an essential regulator of ExMC specification, with null cells remaining in the trophoblast lineage. Bound by HAND1, ape-specific, endogenous retrovirus-derived LTR2B contributes to unique features of ExMC. Additionally, ExMC supports the maintenance of pluripotent stem cells, possibly reflecting a role in maintaining epiblast pluripotency through peri-implantation development. Our data emphasize the nascent evolutionary innovation in human early development and provide a cellular system to study this.

METTL3-dependent m6A RNA methylation regulates transposable elements and represses human naïve pluripotency through transposable element-derived enhancers

N 6-methyladenosine (m6A) is the most prevalent messenger RNA modification with diverse regulatory roles in mammalian cells. While its functions are well-documented in mouse embryonic stem cells (mESCs), its role in human pluripotent stem cells (hPSCs) remains to be fully explored. METTL3 is the main enzyme responsible for m6A deposition. Here, using a METTL3 inducible knockout (iKO) system, we uncovered that, unlike in mESCs, METTL3 was indispensable for hPSC maintenance. Importantly, loss of METTL3 caused significant upregulation of pluripotency factors including naïve pluripotency genes and failure to exit pluripotency, thus impairing stem cell differentiation towards both embryonic and extraembryonic cell lineages. Mechanistically, METTL3 iKO in hPSCs promoted expression and enhancer activities of two primate-specific transposable elements (TEs), SVA_D and HERVK/LTR5_Hs. At SVA_D elements, loss of METTL3 leads to reduced H3K9me3 deposition. On the other hand, the activation of LTR5_Hs in the METTL3 iKO cells is accompanied by increased chromatin accessibility and binding pluripotency factors. The activated SVA_D and LTR5_Hs elements can act as enhancers and promote nearby naïve gene expression by directly interacting with their promoters. Together these findings reveal that METTL3-dependent m6A RNA methylation plays critical roles in suppressing TE expression and in regulating the human pluripotency network.

Exploring early extraembryonic cells of epiblast origin: Questions on human amniotic ectoderm and extraembryonic mesoderm

Extraembryonic tissues are essential for proper fetal development and exhibit great diversity across species. Despite its importance, human extraembryonic development has been relatively overlooked. Previously, we established an in vitro model to study human amniogenesis and extraembryonic mesoderm formation. In this article, I develop discussions on four topics inspired by this study: (1) Features of amniotic cell populations described to date. A recently reported early amniotic cell type is examined based on its signature genes to consider how this population should be incorporated into models of primate amniogenesis. (2) Molecular mechanisms underlying the effect of cell density in regulating non-neural ectoderm specification. Fate specification by positional cues in mouse is revisited and possible mechanisms are suggested by drawing insights from human epiblast models. (3) Potential applications of the three-dimensional culture we established. Primate amniotic ectoderm is postulated as a gastrulation-inducing signaling center, and our technique could be used to effectively model its interactions with epiblast. (4) Extraembryonic mesoderm development in human embryos. The obscure origin of primate extraembryonic mesoderm and implications from recent in vitro differentiation models using human pluripotent stem cells are explained. The key concepts explored here will stimulate further studies into both amnion and extraembryonic mesoderm during human and non-human primate development.

Bloodhounds chasing the origin of blood cells

The generation of blood cells during embryonic development involves a process resembling lineage reprogramming, where specialized cells within the vasculature become blood forming, or hemogenic. These hemogenic cells undergo rapid transcriptional and morphological changes as they appear to switch from an endothelial to blood identity. What controls this process and the exact nature of the hemogenic cells remains debated, with evidence supporting several hypotheses. In this opinion, we synthesize current knowledge and propose a model reconciling conflicting observations, integrating evolutionary and mechanistic insights into blood cell emergence.

Decoding the human prenatal immune system with single-cell multi-omics

The human immune system is made up of a huge variety of cell types each with unique functions. Local networks of resident immune cells are poised to sense and protect against pathogen entry, whereas more widespread innate and adaptive immune networks provide first rapid, then long-lasting and targeted responses. However, how we develop such a diverse and complex system remains unknown. Studying human development directly has been challenging in the past, but recent advances in single-cell and spatial genomics, together with the co-ordinated efforts of the Human Cell Atlas and other initiatives, have led to new studies that map the development of the human immune system in unprecedented detail. In this Review, we consider the timings, transitions, cell types and tissue microenvironments that are crucial for building the human immune system. We also compare and contrast the human system with model species and in vitro systems, and discuss how an understanding of prenatal immune system development will improve our knowledge of human disease.

Pluripotent cell states and fates in human embryo models

Pluripotency, the capacity to generate all cells of the body, is a defining property of a transient population of epiblast cells found in pre-, peri- and post-implantation mammalian embryos. As development progresses, the epiblast cells undergo dynamic transitions in pluripotency states, concurrent with the specification of extra-embryonic and embryonic lineages. Recently, stem cell-based models of pre- and post-implantation human embryonic development have been developed using stem cells that capture key properties of the epiblast at different developmental stages. Here, we review early primate development, comparing pluripotency states of the epiblast in vivo with cultured pluripotent cells representative of these states. We consider how the pluripotency status of the starting cells influences the development of human embryo models and, in turn, what we can learn about the human pluripotent epiblast. Finally, we discuss the limitations of these models and questions arising from the pioneering studies in this emerging field.

Origin, fate and function of extraembryonic tissues during mammalian development

Extraembryonic tissues have pivotal roles in morphogenesis and patterning of the early mammalian embryo. Developmental programmes mediated through signalling pathways and gene regulatory networks determine the sequence in which fate determination and lineage commitment of extraembryonic tissues take place, and epigenetic processes allow the memory of cell identity and state to be sustained throughout and beyond embryo development, even extending across generations. In this Review, we discuss the molecular and cellular mechanisms necessary for the different extraembryonic tissues to develop and function, from their initial specification up until the end of gastrulation, when the body plan of the embryo and the anatomical organization of its supporting extraembryonic structures are established. We examine the interaction between extraembryonic and embryonic tissues during early patterning and morphogenesis, and outline how epigenetic memory supports extraembryonic tissue development.

Transient chemical-mediated epigenetic modulation confers unrestricted lineage potential on human primed pluripotent stem cells

Human primed pluripotent stem cells are capable of generating all the embryonic lineages. However, their extraembryonic trophectoderm potentials are limited. It remains unclear how to expand their developmental potential to trophectoderm lineages. Here we show that transient treatment with a cocktail of small molecule epigenetic modulators imparts trophectoderm lineage potentials to human primed pluripotent stem cells while preserving their embryonic potential. These chemically treated cells can generate trophectoderm-like cells and downstream trophoblast stem cells, diverging into syncytiotrophoblast and extravillous trophoblast lineages. Transcriptomic and CUT&Tag analyses reveal that these induced cells share transcriptional profiles with in vivo trophectoderm and cytotrophoblast, and exhibit reduced H3K27me3 modification at gene loci specific to trophoblast lineages compared with primed pluripotent cells. Mechanistic exploration highlighted the critical roles of epigenetic modulators HDAC2, EZH1/2, and KDM5s in the activation of trophoblast lineage potential. Our findings demonstrate that transient epigenetic resetting activates unrestricted lineage potential in human primed pluripotent stem cells, and offer new mechanistic insights into human trophoblast lineage specification and in vitro models for studying placental development and related disorders.

The emergence of human primordial germ cell-like cells in stem cell-derived gastruloids

Most advances in early human postimplantation development depend on animal studies and stem cell-based embryo models. Here, we present self-organized three-dimensional human gastruloids (hGs) derived from embryonic stem cells. The transcriptome profile of day 3 hGs aligned with Carnegie stage 7 human gastrula, with cell types and differentiation trajectories consistent with human gastrulation. Notably, we observed the emergence of nascent primordial germ cell-like cells (PGCLCs), but without exogenous bone morphogenetic protein (BMP) signaling, which is essential for the PGCLC fate. A mutation in the ISL1 gene affects amnion-like cells and leads to a loss of PGCLCs; the addition of exogenous BMP2 rescues the PGCLC fate, indicating that the amnion may provide endogenous BMP signaling. Our model of early human embryogenesis will enable further exploration of the germ line and other early human lineages.

Programming of pluripotency and the germ line co-evolved from a Nanog ancestor

Francois Jacob proposed that evolutionary novelty arises through incremental tinkering with pre-existing genetic mechanisms. Vertebrate evolution was predicated on pluripotency, the ability of embryonic cells to form somatic germ layers and primordial germ cells (PGCs). The origins of pluripotency remain unclear, as key regulators, such as Nanog, are not conserved outside of vertebrates. Given NANOG's role in mammalian development, we hypothesized that NANOG activity might exist in ancestral invertebrate genes. Here, we find that Vent from the hemichordate Saccoglossus kowalevskii exhibits NANOG activity, programming pluripotency in Nanog-/- mouse pre-induced pluripotent stem cells (iPSCs) and NANOG-depleted axolotl embryos. Vent from the cnidarian Nematostella vectensis showed partial activity, whereas Vent from sponges and vertebrates had no activity. VENTX knockdown in axolotls revealed a role in germline-competent mesoderm, which Saccoglossus Vent could rescue but Nematostella Vent could not. This suggests that the last deuterostome ancestor had a Vent gene capable of programming pluripotency and germline competence.

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.

Cardiac differentiation roadmap for analysis of plasticity and balanced lineage commitment

Stem cell-based models of human heart tissue and cardiac differentiation employ monolayer and 3D organoid cultures with different properties, cell type composition, and maturity. Here we show how cardiac monolayer, embryoid body, and engineered heart tissue trajectories compare in a single-cell roadmap of atrial and ventricular differentiation conditions. Using a multiomic approach and gene-regulatory network inference, we identified regulators of the epicardial, atrial, and ventricular cardiomyocyte lineages. We identified ZNF711 as a regulatory switch and safeguard for cardiomyocyte commitment. We show that ZNF711 ablation prevents cardiomyocyte differentiation in the absence of retinoic acid, causing progenitors to be diverted more prominently to epicardial and other lineages. Retinoic acid rescues this shift in lineage commitment and promotes atrial cardiomyocyte differentiation by regulation of shared and complementary target genes, showing interplay between ZNF711 and retinoic acid in cardiac lineage commitment.

Modeling early gastrulation in human blastoids with DNA methylation patterns of natural blastocysts

Blastoids are a promising model for studying early human embryogenesis, but current models have limitations in post-implantation development and lack comprehensive epigenetic assessments, especially regarding genomic imprinting. These issues can lead to failures in accurately modeling early embryonic development. In this study, we developed a high-fidelity blastoid model using 4 chemicals + leukemia inhibitory factor (LIF) (4CL) naive human pluripotent stem cells (hPSCs) (4CL blastoids). 4CL blastoids closely resemble human blastocysts in morphology and transcriptional profiles, exhibiting similar DNA methylation and gene imprinting patterns. By extending the 3D culture to 14 days, these blastoids mimic early gastrulation, demonstrating the specification and migration of cells. They also show the transcriptional signature of hemogenic angioblast (HAB) cells at Carnegie stage 6 (CS6). This model bridges pre- and post-implantation stages, offering valuable insights into early tissue formation and human development.

Decoding the blueprints of embryo development with single-cell and spatial omics

Embryonic development is a complex and intricately regulated process that encompasses precise control over cell differentiation, morphogenesis, and the underlying gene expression changes. Recent years have witnessed a remarkable acceleration in the development of single-cell and spatial omic technologies, enabling high-throughput profiling of transcriptomic and other multi-omic information at the individual cell level. These innovations offer fresh and multifaceted perspectives for investigating the intricate cellular and molecular mechanisms that govern embryonic development. In this review, we provide an in-depth exploration of the latest technical advancements in single-cell and spatial multi-omic methodologies and compile a systematic catalog of their applications in the field of embryonic development. We deconstruct the research strategies employed by recent studies that leverage single-cell sequencing techniques and underscore the unique advantages of spatial transcriptomics. Furthermore, we delve into both the current applications, data analysis algorithms and the untapped potential of these technologies in advancing our understanding of embryonic development. With the continuous evolution of multi-omic technologies, we anticipate their widespread adoption and profound contributions to unraveling the intricate molecular foundations underpinning embryo development in the foreseeable future.

Extra-embryonic mesoderm during development and in in vitro models

Extra-embryonic tissues provide protection and nutrition in vertebrates, as well as a connection to the maternal tissues in mammals. The extra-embryonic mesoderm is an essential and understudied germ layer present in amniotes. It is involved in hematopoiesis, as well as in the formation of extra-embryonic structures such as the amnion, umbilical cord and placenta. The origin and specification of extra-embryonic mesoderm are not entirely conserved across species, and the molecular mechanisms governing its formation and function are not fully understood. This Review begins with an overview of the embryonic origin and function of extra-embryonic mesoderm in vertebrates from in vivo studies. We then compare in vitro models that generate extra-embryonic mesoderm-like cells. Finally, we discuss how insights from studying both embryos and in vitro systems can aid in designing even more advanced stem cell-based embryo models.

Ectopic expression of DNMT3L in human trophoblast stem cells restores features of the placental methylome

The placental DNA methylation landscape is unique, with widespread partially methylated domains (PMDs). The placental "methylome" is conserved across mammals, a shared feature of many cancers, and extensively studied for links with pregnancy complications. Human trophoblast stem cells (hTSCs) offer exciting potential for functional studies to better understand this epigenetic feature; however, whether the hTSC epigenome recapitulates primary trophoblast remains unclear. We find that hTSCs exhibit an atypical methylome compared with trophectoderm and 1st trimester cytotrophoblast. Regardless of cell origin, oxygen levels, or culture conditions, hTSCs show localized DNA methylation within transcribed gene bodies and a complete loss of PMDs. Unlike early human trophoblasts, hTSCs display a notable absence of DNMT3L expression, which is necessary for PMD establishment in mouse trophoblasts. Remarkably, we demonstrate that ectopic expression of DNMT3L in hTSCs restores placental PMDs, supporting a conserved role for DNMT3L in de novo methylation in trophoblast development in human embryogenesis.

Atlas of multilineage stem cell differentiation reveals TMEM88 as a developmental regulator of blood pressure

Pluripotent stem cells provide a scalable approach to analyse molecular regulation of cell differentiation across developmental lineages. Here, we engineer barcoded induced pluripotent stem cells to generate an atlas of multilineage differentiation from pluripotency, encompassing an eight-day time course with modulation of WNT, BMP, and VEGF signalling pathways. Annotation of in vitro cell types with reference to in vivo development reveals diverse mesendoderm lineage cell types including lateral plate and paraxial mesoderm, neural crest, and primitive gut. Interrogation of temporal and signalling-specific gene expression in this atlas, evaluated against cell type-specific gene expression in human complex trait data highlights the WNT-inhibitor gene TMEM88 as a regulator of mesendodermal lineages influencing cardiovascular and anthropometric traits. Genetic TMEM88 loss of function models show impaired differentiation of endodermal and mesodermal derivatives in vitro and dysregulated arterial blood pressure in vivo. Together, this study provides an atlas of multilineage stem cell differentiation and analysis pipelines to dissect genetic determinants of mammalian developmental physiology.

Spatial transcriptomic characterization of a Carnegie stage 7 human embryo

Gastrulation marks a pivotal stage in mammalian embryonic development, establishing the three germ layers and body axis through lineage diversification and morphogenetic movements. However, studying human gastrulating embryos is challenging due to limited access to early tissues. Here we show the use of spatial transcriptomics to analyse a fully intact Carnegie stage 7 human embryo at single-cell resolution, along with immunofluorescence validations in a second embryo. Employing 82 serial cryosections and Stereo-seq technology, we reconstructed a three-dimensional model of the embryo. Our findings reveal early specification of distinct mesoderm subtypes and the presence of the anterior visceral endoderm. Notably, primordial germ cells were located in the connecting stalk, and haematopoietic stem cell-independent haematopoiesis was observed in the yolk sac. This study advances our understanding of human gastrulation and provides a valuable dataset for future research in early human development.

Extraembryonic mesoderm cells derived from human embryonic stem cells rely on Wnt pathway activation

Extraembryonic mesoderm cells (EXMCs) are involved in the development of multiple embryonic lineages and umbilical cord formation, where they subsequently develop into mesenchymal stem cells (MSCs). Although EXMCs can be generated from human naïve embryonic stem cells (ESCs), it is unclear whether human primed ESCs (hpESCs) can differentiate into EXMCs that subsequently produce MSCs. The present report described a three-dimensional differentiation protocol to induce hpESCs into EXMCs by activating the Wnt pathway using CHIR99021. Single-cell transcriptome and immunostaining analyses revealed that the EXMC characteristics were similar to those of post-implantation embryonic EXMCs. Cell sorting was used to purify and expand the EXMCs. Importantly, these EXMCs secreted extracellular matrix proteins, including COL3A1 and differentiated into MSCs. Inconsistent with other MSC types, these MSCs exhibited a strong differentiation potential for chondrogenic and osteogenic cells and lacked adipocyte differentiation. Together, these findings provided a protocol to generate EXMCs and subsequent MSCs from hpESCs.

c-JUN: a chromatin repressor that limits mesoderm differentiation in human pluripotent stem cells

Cell fate determination at the chromatin level is not fully comprehended. Here, we report that c-JUN acts on chromatin loci to limit mesoderm cell fate specification as cells exit pluripotency. Although c-JUN is widely expressed across various cell types in early embryogenesis, it is not essential for maintaining pluripotency. Instead, it functions as a repressor to constrain mesoderm development while having a negligible impact on ectoderm differentiation. c-JUN interacts with MBD3-NuRD complex, which helps maintain chromatin in a low accessibility state at mesoderm-related genes during the differentiation of human pluripotent stem cells into mesoderm. Furthermore, c-JUN specifically inhibits the activation of key mesoderm factors, such as EOMES and GATA4. Knocking out c-JUN or inhibiting it with a JNK inhibitor can alleviate this suppression, promoting mesoderm cell differentiation. Consistently, knockdown of MBD3 enhances mesoderm generation, whereas MBD3 overexpression impedes it. Overexpressing c-JUN redirects differentiation toward a fibroblast-like lineage. Collectively, our findings suggest that c-JUN acts as a chromatin regulator to restrict the mesoderm cell fate.

A comprehensive human embryo reference tool using single-cell RNA-sequencing data

Stem cell-based embryo models offer unprecedented experimental tools for studying early human development. The usefulness of embryo models hinges on their molecular, cellular and structural fidelities to their in vivo counterparts. To authenticate human embryo models, single-cell RNA sequencing has been utilized for unbiased transcriptional profiling. However, an organized and integrated human single-cell RNA-sequencing dataset, serving as a universal reference for benchmarking human embryo models, remains unavailable. Here we developed such a reference through the integration of six published human datasets covering development from the zygote to the gastrula. Lineage annotations are contrasted and validated with available human and nonhuman primate datasets. Using stabilized Uniform Manifold Approximation and Projection, we constructed an early embryogenesis prediction tool, where query datasets can be projected on the reference and annotated with predicted cell identities. Using this reference tool, we examined published human embryo models, highlighting the risk of misannotation when relevant references are not utilized for benchmarking and authentication.

In vitro culture of cynomolgus monkey embryos from blastocyst to early organogenesis

Human early embryonic development is the cornerstone of a healthy baby. Abnormal early embryonic development may lead to developmental and pregnancy-related disorders. Accordingly, understanding the developmental events and mechanisms of human early embryonic development is very important. However, attempts to reveal these events and mechanisms are greatly hindered by the extreme inaccessibility of in vivo early human embryos. Fortunately, the emergence of in vitro culture (IVC) systems for mammalian embryos provides an alternative strategy. In recent years, different two-dimensional and three-dimensional IVC systems have been developed for human embryos. Ethical limitations restrict the IVC of human embryos beyond 14 days, which makes non-human primate embryos an ideal model for studying primate developmental events. Different culture systems have supported the development of monkey embryos to days postfertilization 14 and 25, respectively. The successful recapitulation of in vivo developmental events by these IVC embryos has greatly enriched our understanding of human early embryonic development, which undoubtedly helps us to develop possible strategies to predict or treat various gestation-related diseases and birth defects. In this protocol, we establish different two-dimensional and three-dimensional IVC systems for primate embryos, provide step-by-step culture procedures and notes, and summarize the advantages and limitations of different culture systems. Replicating this protocol requires a moderate level of experience in mammalian embryo IVC, and the embryo culture requires strict adherence to the procedures we have described.

Transcriptomic profiling of day 3 human embryos of poor quality reveals molecular links to divergent developmental trajectories

Poor-quality (PQ) embryos are commonly formed in assisted reproduction and often discarded, but emerging evidence suggests that some can develop into healthy children. Here, we performed single-embryo transcriptome profiling of day 3 human embryos of both good quality (GQ) and PQ. Our analyses revealed that day 3 PQ embryos can be categorized into two distinct subgroups: genuine PQ (gPQ) and morphological PQ (mPQ) embryos. This was further supported by additional transcriptomic data of GQ and PQ embryos donated by the same individuals. gPQ embryos showed greater impairments in both RNA decay and zygotic genome activation (ZGA) compared to mPQ embryos. Blastocysts that emerged from PQ embryos, more likely from mPQ embryos, demonstrated transcriptomic similarity to normal blastocysts. We also identified genes highly upregulated in day 3 gPQ embryos. These molecular insights may improve embryo selection and offer valuable clues for enhancing embryo quality in the future.

mTOR activity paces human blastocyst stage developmental progression

Many mammals can temporally uncouple conception from parturition by pacing down their development around the blastocyst stage. In mice, this dormant state is achieved by decreasing the activity of the growth-regulating mTOR signaling pathway. It is unknown whether this ability is conserved in mammals in general and in humans in particular. Here, we show that decreasing the activity of the mTOR signaling pathway induces human pluripotent stem cells (hPSCs) and blastoids to enter a dormant state with limited proliferation, developmental progression, and capacity to attach to endometrial cells. These in vitro assays show that, similar to other species, the ability to enter dormancy is active in human cells around the blastocyst stage and is reversible at both functional and molecular levels. The pacing of human blastocyst development has potential implications for reproductive therapies.

Single-cell multi-modal integrative analyses highlight functional dynamic gene regulatory networks directing human cardiac development

Illuminating the precise stepwise genetic programs directing cardiac development provides insights into the mechanisms of congenital heart disease and strategies for cardiac regenerative therapies. Here, we integrate in vitro and in vivo human single-cell multi-omic studies with high-throughput functional genomic screening to reveal dynamic, cardiac-specific gene regulatory networks (GRNs) and transcriptional regulators during human cardiomyocyte development. Interrogating developmental trajectories reconstructed from single-cell data unexpectedly reveal divergent cardiomyocyte lineages with distinct gene programs based on developmental signaling pathways. High-throughput functional genomic screens identify key transcription factors from inferred GRNs that are functionally relevant for cardiomyocyte lineages derived from each pathway. Notably, we discover a critical heat shock transcription factor 1 (HSF1)-mediated cardiometabolic GRN controlling cardiac mitochondrial/metabolic function and cell survival, also observed in fetal human cardiomyocytes. Overall, these multi-modal genomic studies enable the systematic discovery and validation of coordinated GRNs and transcriptional regulators controlling the development of distinct human cardiomyocyte populations.

Morphogen-driven differentiation is precluded by physical confinement in human iPSCs spheroids

Introduction

Cell lineage specification is tightly associated with profound morphological changes in the developing human embryo, particularly during gastrulation. The interplay between mechanical forces and biochemical signals is poorly understood.

Methods

Here, we dissect the effects of biochemical cues and physical confinement on a 3D in vitro model based on spheroids formed from human induced pluripotent stem cells (hiPSCs).

Results

First, we compare self-renewing versus differentiating media conditions in free-floating cultures and observe the emergence of tri-germ layers. In these unconfined conditions, BMP4 exposure induces polarised expression of SOX17 in conjunction with spheroid elongation. We then physically confine spheroids using PEG-peptide hydrogels and observe dramatically reduced SOX17 expression, albeit rescued if gels that soften over time are used instead.

Discussion

Our study combines high-content imaging, synthetic hydrogels, and hiPSCs-derived models of early development to define the drivers that cause changes in the shape and the emergence of germ layers.

Population size interacts with reproductive longevity to shape the germline mutation rate

Mutation rates vary across the tree of life by many orders of magnitude, with lower mutation rates in species that reproduce quickly and maintain large effective population sizes. A compelling explanation for this trend is that large effective population sizes facilitate selection against weakly deleterious "mutator alleles" such as variants that interfere with the molecular efficacy of DNA repair. However, in multicellular organisms, the relationship of the mutation rate to DNA repair efficacy is complicated by variation in reproductive age. Long generation times leave more time for mutations to accrue each generation, and late reproduction likely amplifies the fitness consequences of any DNA repair defect that creates extra mutations in the sperm or eggs. Here, we present theoretical and empirical evidence that a long generation time amplifies the strength of selection for low mutation rates in the spermatocytes and oocytes. This leads to the counterintuitive prediction that the species with the highest germline mutation rates per generation are also the species with most effective mechanisms for DNA proofreading and repair in their germ cells. In contrast, species with different generation times accumulate similar mutation loads during embryonic development. Our results parallel recent findings that the longest-lived species have the lowest mutation rates in adult somatic tissues, potentially due to selection to keep the lifetime mutation load below a harmful threshold.

Reconsidering the 14-day rule in human embryo research: Advice from the Dutch Health Council

The Dutch Health Council has advised to extend the 14-day rule to 28 days and to subsume integrated stem cell-based embryo models under the same legislative regime as natural embryos. Public discussion is necessary due to the ethical issues that may emerge from studying integrated and non-integrated embryo models.

Navigating human embryogenesis through tailored model selection

Rapid advances in stem cell and bioengineering technologies have sparked a revolution in developmental biology, with stem cell-based embryo models emerging as crucial tools to uncover the intricacies of human embryogenesis. However, making progress relies on precisely posing our questions and selecting our models.

Human yolk sac-derived innate lymphoid-biased multipotent progenitors emerge prior to hematopoietic stem cell formation

Hematopoietic stem cell (HSC)-independent lymphopoiesis has been elucidated in murine embryos. However, our understanding regarding human embryonic counterparts remains limited. Here, we demonstrated the presence of human yolk sac-derived lymphoid-biased progenitors (YSLPs) expressing CD34, IL7R, LTB, and IRF8 at Carnegie stage 10, much earlier than the first HSC emergence. The number and lymphopoietic potential of these progenitors were both significantly higher in the yolk sac than the embryo proper at this early stage. Importantly, single-cell/bulk culture and CITE-seq have elucidated the tendency of YSLP to differentiate into innate lymphoid cells and dendritic cells. Notably, lymphoid progenitors in fetal liver before and after HSC seeding displayed distinct transcriptional features, with the former closely resembling those of YSLPs. Overall, our data identified the origin, potential, and migratory dynamics of innate lymphoid-biased multipotent progenitors in human yolk sac before HSC emergence, providing insights for understanding the stepwise establishment of innate immune system in humans.

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.

Marmoset and human trophoblast stem cells differ in signaling requirements and recapitulate divergent modes of trophoblast invasion

Early human trophoblast development has remained elusive due to the inaccessibility of the early conceptus. Non-human primate models recapitulate many features of human development and allow access to early postimplantation stages. Here, we tracked the pre- to postimplantation transition of the trophoblast lineage in superficially implanting marmoset embryos in vivo. We differentiated marmoset naive pluripotent stem cells into trophoblast stem cells (TSCs), which exhibited trophoblast-specific transcriptome, methylome, differentiation potential, and long-term self-renewal. Notably, human TSC culture conditions failed to support marmoset TSC derivation, instead inducing an extraembryonic mesoderm-like fate in marmoset cells. We show that combined MEK, TGF-β/NODAL, and histone deacetylase inhibition stabilizes a periimplantation trophoblast-like identity in marmoset TSCs. By contrast, these conditions differentiated human TSCs toward extravillous trophoblasts. Our work presents a paradigm to harness the evolutionary divergence in implantation strategies to elucidate human trophoblast development and invasion.

ZIC2 and ZIC3 promote SWI/SNF recruitment to safeguard progression towards human primed pluripotency

The primed epiblast acts as a transitional stage between the relatively homogeneous naïve epiblast and the gastrulating embryo. Its formation entails coordinated changes in regulatory circuits driven by transcription factors and epigenetic modifications. Using a multi-omic approach in human embryonic stem cell models across the spectrum of peri-implantation development, we demonstrate that the transcription factors ZIC2 and ZIC3 have overlapping but essential roles in opening primed-specific enhancers. Together, they are essential to facilitate progression to and maintain primed pluripotency. ZIC2/3 accomplish this by recruiting SWI/SNF to chromatin and loss of ZIC2/3 or degradation of SWI/SNF both prevent enhancer activation. Loss of ZIC2/3 also results in transcriptome changes consistent with perturbed Polycomb activity and a shift towards the expression of genes linked to differentiation towards the mesendoderm. Additionally, we find an intriguing dependency on the transcriptional machinery for sustained recruitment of ZIC2/3 over a subset of primed-hESC specific enhancers. Taken together, ZIC2 and ZIC3 regulate highly dynamic lineage-specific enhancers and collectively act as key regulators of human primed pluripotency.

Retinoic acid induces human gastruloids with posterior embryo-like structures

Gastruloids are a powerful in vitro model of early human development. However, although elongated and composed of all three germ layers, human gastruloids do not morphologically resemble post-implantation human embryos. Here we show that an early pulse of retinoic acid (RA), together with later Matrigel, robustly induces human gastruloids with posterior embryo-like morphological structures, including a neural tube flanked by segmented somites and diverse cell types, including neural crest, neural progenitors, renal progenitors and myocytes. Through in silico staging based on single-cell RNA sequencing, we find that human RA-gastruloids progress further than other human or mouse embryo models, aligning to E9.5 mouse and CS11 cynomolgus monkey embryos. We leverage chemical and genetic perturbations of RA-gastruloids to confirm that WNT and BMP signalling regulate somite formation and neural tube length in the human context, while transcription factors TBX6 and PAX3 underpin presomitic mesoderm and neural crest, respectively. Looking forward, RA-gastruloids are a robust, scalable model for decoding early human embryogenesis.

The indispensable role of time in autonomous development

Advances in single-cell analysis have led to a picture of development largely in agreement with Waddington's eponymous epigenetic landscape, in which a cell's fate is determined by its basin of attraction in a high-dimensional gene-expression space. Yet conceptual gaps remain as to how a single progenitor can simultaneously generate multiple endpoints, and why time should be required of the process at all. We propose a theoretical model based on the Hamiltonian mechanics of n-dimensional rotational motion, which resolves these paradoxes. We derive the result that systems which become different from themselves over time must initially move in a direction not towards their ultimate endpoints, and propose that this process of resolving ambiguity can be quantified (in an information-theoretic sense) by the volume subtended in gene-expression space by the trajectories taken by the system towards its endpoints. We discuss the implications of this theory for the analysis of single-cell gene-expression data in studies of development.

Single‑cell RNA sequencing analysis of human embryos from the late Carnegie to fetal development

BACKGROUND

The cell development atlas of transition stage from late Carnegie to fetal development (7-9 weeks) remain unclear. It can be seen that the early period of human embryos (7-9 weeks) is a critical research gap. Therefore, we employed single‑cell RNA sequencing to identify cell types and elucidate differentiation relationships.

RESULTS

The single‑cell RNA sequencing analysis determines eighteen cell clusters in human embryos during the 7-9 weeks period. We uncover two distinct pathways of cellular development and differentiation. Initially, mesenchymal progenitor cells differentiated into osteoblast progenitor cells and neural stem cells, respectively. Neural stem cells further differentiated into neurons. Alternatively, multipotential stem cells differentiated into adipocyte, hematopoietic stem cells and neutrophil, respectively. Additionally, COL1A2-(ITGA1 + ITGB1) mediated the cell communication between mesenchymal progenitor cells and osteoblast progenitor cells. NCAM1-FGFR1 facilitated the cell communication between mesenchymal progenitor cells and neural stem cells. Notably, NCAM1-NCAM1 as a major contributor mediated the cell communication between neural stem cells and neurons. Moreover, CGA-FSHR simultaneously mediated the communication between multipotential stem cells, adipocyte, hematopoietic stem cells and neutrophil. Distinct cell clusters activated specific transcription factors such as HIC1, LMX1B, TWIST1, and et al., which were responsible for their specific functions. These coregulators, such as HOXB13, VSX2, PAX5, and et al., may mediate cell development and differentiation in human embryos.

CONCLUSIONS

We provide the cell development atlas for human embryos (7-9 weeks). Two distinct cell development and differentiation pathways are revealed.

An integrated approach identifies the molecular underpinnings of murine anterior visceral endoderm migration

The anterior visceral endoderm (AVE) differs from the surrounding visceral endoderm (VE) in its migratory behavior and ability to restrict primitive streak formation to the opposite side of the mouse embryo. To characterize the molecular bases for the unique properties of the AVE, we combined single-cell RNA sequencing of the VE prior to and during AVE migration with phosphoproteomics, high-resolution live-imaging, and short-term lineage labeling and intervention. This identified the transient nature of the AVE with attenuation of "anteriorizing" gene expression as cells migrate and the emergence of heterogeneities in transcriptional states relative to the AVE's position. Using cell communication analysis, we identified the requirement of semaphorin signaling for normal AVE migration. Lattice light-sheet microscopy showed that Sema6D mutants have abnormalities in basal projections and migration speed. These findings point to a tight coupling between transcriptional state and position of the AVE and identify molecular controllers of AVE migration.

Mechano-osmotic signals control chromatin state and fate transitions in pluripotent stem cells

Acquisition of specific cell shapes and morphologies is a central component of cell fate transitions. Although signaling circuits and gene regulatory networks that regulate pluripotent stem cell differentiation have been intensely studied, how these networks are integrated in space and time with morphological transitions and mechanical deformations to control state transitions remains a fundamental open question. Here, we focus on two distinct models of pluripotency, primed pluripotent stem cells and pre-implantation inner cell mass cells of human embryos to discover that cell fate transitions associate with rapid changes in nuclear shape and volume which collectively alter the nuclear mechanophenotype. Mechanistic studies in human induced pluripotent stem cells further reveal that these phenotypical changes and the associated active fluctuations of the nuclear envelope arise from growth factor signaling-controlled changes in chromatin mechanics and cytoskeletal confinement. These collective mechano-osmotic changes trigger global transcriptional repression and a condensation-prone environment that primes chromatin for a cell fate transition by attenuating repression of differentiation genes. However, while this mechano-osmotic chromatin priming has the potential to accelerate fate transitions and differentiation, sustained biochemical signals are required for robust induction of specific lineages. Our findings uncover a critical mechanochemical feedback mechanism that integrates nuclear mechanics, shape and volume with biochemical signaling and chromatin state to control cell fate transition dynamics.