Pluripotent stem cell-derived model of the post-implantation human embryo

Cellular mechanisms of monozygotic twinning: clues from assisted reproduction

BACKGROUND

Monozygotic (MZ) twins are believed to arise from the fission of a single fertilized embryo at different stages. Monochorionic MZ twins, who share one chorion, originate from the splitting of the inner cell mass (ICM) within a single blastocyst. In the classic model for dichorionic MZ twins, the embryo splits before compaction, developing into two blastocysts. However, there are a growing number of ART cases where a single blastocyst transfer results in dichorionic MZ twins, indicating that embryo splitting may occur even after blastocyst formation.

OBJECTIVE AND RATIONALE

For monochorionic MZ twins, we conducted a comprehensive analysis of the cellular mechanisms involved in ICM splitting, drawing from both ART cases and animal experiments. In addition, we critically re-examine the classic early splitting model for dichorionic MZ twins. We explore cellular mechanisms leading to two separated blastocysts in ART, potentially causing dichorionic MZ twins.

SEARCH METHODS

Relevant studies including research articles, reviews, and conference papers were searched in the PubMed database. Cases of MZ twins from IVF clinics were found by using combinations of terms including 'monozygotic twins' with 'IVF case report', 'ART', 'single embryo transfer', or 'dichorionic'. The papers retrieved were categorized based on the implicated mechanisms or as those with unexplained mechanisms. Animal experiments relating to MZ twins were found using 'mouse embryo monozygotic twins', 'mouse 8-shaped hatching', 'zebrafish janus mutant', and 'nine-banded armadillo embryo', along with literature collected through day-to-day reading. The search was limited to articles in English, with no restrictions on publication date or species.

OUTCOMES

For monochorionic MZ twins, ART cases and mouse experiments demonstrate evidence that a looser ICM in blastocysts has an increased chance of ICM separation. Physical forces facilitated by blastocoel formation or 8-shaped hatching are exerted on the ICM, resulting in monochorionic MZ twins. For dichorionic MZ twins, the classic model resembles artificial cloning of mouse embryos in vitro, requiring strictly controlled splitting forces, re-joining prevention, and proper aggregation, which allows the formation of two separate human blastocysts under physiological circumstances. In contrast, ART procedures involving the transfer of a single blastocysts after atypical hatching or vitrified-warmed cycles might lead to blastocyst separation. Differences in morphology, molecular mechanisms, and timing across various animal model systems for MZ twinning can impede this research field. As discussed in future directions, recent developments of innovative in vitro models of human embryos may offer promising avenues for providing fundamental novel insights into the cellular mechanisms of MZ twinning during human embryogenesis.

WIDER IMPLICATIONS

Twin pregnancies pose high risks to both the fetuses and the mother. While single embryo transfer is commonly employed to prevent dizygotic twin pregnancies in ART, it cannot prevent the occurrence of MZ twins. Drawing from our understanding of the cellular mechanisms underlying monochorionic and dichorionic MZ twinning, along with insights into the genetic mechanisms, could enable improved prediction, prevention, and even intervention strategies during ART procedures.

REGISTRAITON NUMBER

N/A.

Blastoid: The future of human development in the laboratory

Research on early human development is crucial for understanding the origins of life and mechanisms underlying disease onset. However, these studies have significant challenges owing to ethical restrictions and technical limitations. Stem cell technology advancement has led to the development of blastoids to overcome these challenges." Blastoids are three-dimensional structures produced by pluripotent stem cells (PSCs) that resemble the blastocyst stage of human embryos. Research on blastoids can enhance our understanding of early human development and drive innovations in regenerative medicine and disease modeling. This review outlines the background of blastoid development and highlights the limitations of existing organoid research. It presents developments in blastoid research, from previous studies using animal models to the latest developments using human stem cell-derived blastoids in early human development studies. Additionally, this review provides a comparative analysis of the methods used to develop blastoids across various studies, evaluating their potential as ethical alternatives for regenerative medicine, human developmental biology, and embryonic research. It further assesses the ethical and social considerations surrounding blastoid research, the current strategies to address these concerns, and the potential long-term impact on science and medicine. We aimed to provide a comprehensive understanding of the current trends in blastoid research, offer new insights into early human development, and suggest novel directions and approaches for researchers.

Pseudo-trajectory inference for identifying essential regulations and molecules in cell fate decisions

Cell fate decision is crucial in biological development and plays fundamental roles in normal development and functional maintenance of organisms. By identifying key regulatory interactions and molecules involved in these fate decisions, we can shed light on the intricate mechanisms underlying the cell fates. This understanding ultimately reveals the fundamental principles driving biological development and the origins of various diseases. In this study, we present an overarching framework which integrates pseudo-trajectory inference and differential analysis to determine critical regulatory interactions and molecules during cell fate transitions. To demonstrate feasibility and reliability of the approach, we employ the differentiation networks of hepatobiliary system and embryonic stem cells as representative model systems. By applying pseudo-trajectory inference to biological data, we aim to identify critical regulatory interactions and molecules during the cell fate transition processes. Consistent with experimental observations, the approach can allow us to infer dynamical cell fate decision processes and gain insights into the underlying mechanisms which govern cell state decisions.

Deep learning-based models for preimplantation mouse and human embryos based on single-cell RNA sequencing

The rapid growth of single-cell transcriptomic technology has produced an increasing number of datasets for both embryonic development and in vitro pluripotent stem cell-derived models. This avalanche of data surrounding pluripotency and the process of lineage specification has meant it has become increasingly difficult to define specific cell types or states in vivo, and compare these with in vitro differentiation. Here we utilize a set of deep learning tools to integrate and classify multiple datasets. This allows the definition of both mouse and human embryo cell types, lineages and states, thereby maximizing the information one can garner from these precious experimental resources. Our approaches are built on recent initiatives for large-scale human organ atlases, but here we focus on material that is difficult to obtain and process, spanning early mouse and human development. Using publicly available data for these stages, we test different deep learning approaches and develop a model to classify cell types in an unbiased fashion at the same time as defining the set of genes used by the model to identify lineages, cell types and states. We used our models trained on in vivo development to classify pluripotent stem cell models for both mouse and human development, showcasing the importance of this resource as a dynamic reference for early embryogenesis.

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.

Off to a good start: The importance of the placental exchange surface - Lessons from the mouse

The role of the chorio-allantoic placenta as the critical nutrient- and oxygen-supplying organ to nourish the demands of the fetus has been well recognized. This function relies on the successful establishment of the placental feto-maternal exchange unit, or interhaemal barrier, across which all nutrients as well as waste products must pass to cross from the maternal to the fetal blood circulation, or vice versa, respectively. As a consequence, defects in the establishment of this elaborate interface lead to fetal growth retardation or even embryonic lethality, depending on the severity of the defect. Beyond this essential role, however, it has also emerged that the functionality of the feto-maternal interface dictates the proper development of specific embryonic organs, with tightest links observed to the formation of the heart. In this article, we build on the foundational strength of the mouse as experimental model in which the placental causality of embryonic defects can be genetically proven. We discuss in detail the formation of the interhaemal barrier that makes up the labyrinth layer of the murine placenta, including insights into drivers of its formation and the interdependence of the cell types that make up this essential interface, from in vivo and in vitro data using mouse trophoblast stem cells. We highlight mouse genetic tools that enable the elucidation of cause-effect relationships between defects driven by either the trophoblast cells of the placenta or by embryonic cell types. We specifically emphasize gene knockouts for which a placental causality of embryonic heart defects has been demonstrated. This in-depth perspective provides much-needed insights while highlighting remaining gaps in knowledge that are essential for gaining a better understanding of the multi-facetted roles of the placenta in setting us up for a healthy start in life well beyond nutritional support alone.

Don't be so naïve

New evidence sheds light on actin regulation of pluripotency in human embryonic stem cells.

ALKBH5 governs human endoderm fate by regulating the DKK1/4-mediated Wnt/β-catenin activation

N6-methyladenonsine (m6A) is ubiquitously distributed in mammalian mRNA. However, the precise involvement of m6A in early development has yet to be fully elucidated. Here, we report that deletion of the m6A demethylase ALKBH5 in human embryonic stem cells (hESCs) severely impairs definitive endoderm (DE) differentiation. ALKBH5-/- hESCs fail to undergo the primitive streak (PS) intermediate transition that precedes endoderm specification. Mechanistically, we show that ALKBH5 deficiency induces m6A hypermethylation around the 3' untranslated region (3'UTR) of GATA6 transcripts and destabilizes GATA6 mRNA in a YTHDF2-dependent manner. Moreover, GATA6 binds to the promoters of critical regulatory genes involved in Wnt/β-catenin signaling transduction, including the canonical Wnt antagonist DKK1 and DKK4, which are unexpectedly repressed upon the dysregulation of GATA6 mRNA metabolism. Remarkably, DKK1 and DKK4 both exhibit a pleiotropic effect in modulating the Wnt/β-catenin cascade and guard the endogenous signaling activation underlying DE formation as potential downstream targets of the ALKBH5-GATA6 regulation. Here, we unravel a role of ALKBH5 in human endoderm formation in vitro by modulating the canonical Wnt signaling logic through the previously unrecognized functions of DKK1/4, thus capturing a more comprehensive role of m6A in early human embryogenesis.

The logic of monsters: development and morphological diversity in stem-cell-based embryo models

Organoids and stem-cell-based embryo models (SEMs) are imperfect organ or embryo representations that explore a much larger space of possible forms, or morphospace, compared to their in vivo counterparts. Here, we discuss SEM biology in light of seminal work by Pere Alberch, a leading figure in early evo-devo, interpreting SEMs as developmental 'monstrosities' in the Alberchian sense. Alberch suggested that ordered patterns in aberrant development-i.e. 'the logic of monsters'-reveal developmental constraints on possible morphologies. In the same vein, we detail how SEMs have begun to shed light on structural features of normal development, such as developmental variability, the relative importance of internal versus external constraints, boundary conditions and design principles governing robustness and canalization. We argue that SEMs represent a powerful experimental tool to explore and expand developmental morphospace and propose that the 'monstrosity' of SEMs can be leveraged to uncover the 'hidden' rules and developmental constraints that robustly shape and pattern the embryo.

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.

An explainable map of human gastruloid morphospace reveals gastrulation failure modes and predicts teratogens

Human gastrulation is a critical stage of development where many pregnancies fail due to poorly understood mechanisms. Using the 2D gastruloid, a stem cell model of human gastrulation, we combined high-throughput drug perturbations and mathematical modelling to create an explainable map of gastruloid morphospace. This map outlines patterning outcomes in response to diverse perturbations and identifies variations in canonical patterning and failure modes. We modeled morphogen dynamics to embed simulated gastruloids into experimentally-determined morphospace to explain how developmental parameters drive patterning. Our model predicted and validated the two greatest sources of patterning variance: cell density-based modulations in Wnt signaling and SOX2 stability. Assigning these parameters as axes of morphospace imparted interpretability. To demonstrate its utility, we predicted novel teratogens that we validated in zebrafish. Overall, we show how stem cell models of development can be used to build a comprehensive and interpretable understanding of the set of developmental outcomes.

Advances in human pluripotent stem cell reporter systems

The capability of human pluripotent stem cells (hPSCs) to self-renew and differentiate into any cell type has greatly contributed to the advancement of biomedicine. Reporter lines derived from hPSCs have played a crucial role in elucidating the mechanisms underlying human development and diseases by acting as an alternative reporter system that cannot be used in living humans. To bring hPSCs closer to clinical application in transplantation, scientists have generated reporter lines for isolating the desired cell populations, as well as improving graft quality and treatment outcomes. This review presents an overview of the applications of hPSC reporter lines and the important variables in designing a reporter system, including options for gene delivery and editing tools, design of reporter constructs, and selection of reporter genes. It also provides insights into the prospects of hPSC reporter lines and the challenges that must be overcome to maximize the potential of hPSC reporter lines.

Assembly of a stem cell-derived human postimplantation embryo model

The embryonic and extraembryonic tissue interactions underlying human embryogenesis at implantation stages are not currently understood. We have generated a pluripotent stem cell-derived model that mimics aspects of peri-implantation development, allowing tractable experimentation otherwise impossible in the human embryo. Activation of the extraembryonic lineage-specific transcription factors GATA6 and SOX17 (hypoblast factors) or GATA3 and TFAP2C (encoding AP2γ; trophoblast factors) in human embryonic stem (ES) cells drive conversion to extraembryonic-like cells. When combined with wild-type ES cells, self-organized embryo-like structures form in the absence of exogenous factors, termed human inducible embryoids (hiEmbryoids). The epiblast-like domain of hiEmbryoids polarizes and differentiates in response to extraembryonic-secreted extracellular matrix and morphogen cues. Extraembryonic mesenchyme, amnion and primordial germ cells are specified in hiEmbryoids in a stepwise fashion. After establishing stable inducible ES lines and converting ES cells to RSeT culture media, the protocol takes 7-10 d to generate hiEmbryoids. Generation of hiEmbryoids can be performed by researchers with basic expertise in stem cell culture.

A primate-specific endogenous retroviral envelope protein sequesters SFRP2 to regulate human cardiomyocyte development

Endogenous retroviruses (ERVs) occupy a significant part of the human genome, with some encoding proteins that influence the immune system or regulate cell-cell fusion in early extra-embryonic development. However, whether ERV-derived proteins regulate somatic development is unknown. Here, we report a somatic developmental function for the primate-specific ERVH48-1 (SUPYN/Suppressyn). ERVH48-1 encodes a fragment of a viral envelope that is expressed during early embryonic development. Loss of ERVH48-1 led to impaired mesoderm and cardiomyocyte commitment and diverted cells to an ectoderm-like fate. Mechanistically, ERVH48-1 is localized to sub-cellular membrane compartments through a functional N-terminal signal peptide and binds to the WNT antagonist SFRP2 to promote its polyubiquitination and degradation, thus limiting SFRP2 secretion and blocking repression of WNT/β-catenin signaling. Knockdown of SFRP2 or expression of a chimeric SFRP2 with the ERVH48-1 signal peptide rescued cardiomyocyte differentiation. This study demonstrates how ERVH48-1 modulates WNT/β-catenin signaling and cell type commitment in somatic development.

The pluripotency state of human embryonic stem cells derived from single blastomeres of eight-cell embryos

Human embryonic stem cells (hESCs) derived from blastocyst stage embryos present a primed state of pluripotency, whereas mouse ESCs (mESCs) display naïve pluripotency. Their unique characteristics make naïve hESCs more suitable for particular applications in biomedical research. This work aimed to derive hESCs from single blastomeres and determine their pluripotency state, which is currently unclear. We derived hESC lines from single blastomeres of 8-cell embryos and from whole blastocysts, and analysed several naïve pluripotency indicators, their transcriptomic profile and their trilineage differentiation potential. No significant differences were observed between blastomere-derived hESCs (bm-hESCs) and blastocyst-derived hESCs (bc-hESCs) for most naïve pluripotency indicators, including TFE3 localization, mitochondrial activity, and global DNA methylation and hydroxymethylation, nor for their trilineage differentiation potential. Nevertheless, bm-hESCs showed an increased single-cell clonogenicity and a higher expression of naïve pluripotency markers at early passages than bc-hESCs. Furthermore, RNA-seq revealed that bc-hESCs overexpressed a set of genes related to the post-implantational epiblast. Altogether, these results suggest that bm-hESCs, although displaying primed pluripotency, would be slightly closer to the naïve end of the pluripotency continuum than bc-hESCs.

Revisiting trophectoderm-inner cell mass lineage segregation in the mammalian preimplantation embryo

In the first days of life, cells of the mammalian embryo segregate into two distinct lineages, trophectoderm and inner cell mass. Unlike nonmammalian species, mammalian development does not proceed from predetermined factors in the oocyte. Rather, asymmetries arise de novo in the early embryo incorporating cues from cell position, contractility, polarity, and cell-cell contacts. Molecular heterogeneities, including transcripts and non-coding RNAs, have now been characterized as early as the 2-cell stage. However, it's debated whether these early heterogeneities bias cells toward one fate or the other or whether lineage identity arises stochastically at the 16-cell stage. This review summarizes what is known about early blastomere asymmetries and our understanding of lineage allocation in the context of historical models. Preimplantation development is reviewed coupled with what is known about changes in morphology, contractility, and transcription factor networks. The addition of single-cell atlases of human embryos has begun to reveal key differences between human and mouse, including the timing of events and core transcription factors. Furthermore, the recent generation of blastoid models will provide valuable tools to test and understand fate determinants. Lastly, new techniques are reviewed, which may better synthesize existing knowledge with emerging data sets and reconcile models with the regulative capacity unique to the mammalian embryo.

Appreciating animal induced pluripotent stem cells to shape plant cell reprogramming strategies

Animals and plants have developed resilience mechanisms to effectively endure and overcome physical damage and environmental challenges throughout their life span. To sustain their vitality, both animals and plants employ mechanisms to replenish damaged cells, either directly, involving the activity of adult stem cells, or indirectly, via dedifferentiation of somatic cells that are induced to revert to a stem cell state and subsequently redifferentiate. Stem cell research has been a rapidly advancing field in animal studies for many years, driven by its promising potential in human therapeutics, including tissue regeneration and drug development. A major breakthrough was the discovery of induced pluripotent stem cells (iPSCs), which are reprogrammed from somatic cells by expressing a limited set of transcription factors. This discovery enabled the generation of an unlimited supply of cells that can be differentiated into specific cell types and tissues. Equally, a keen interest in the connection between plant stem cells and regeneration has been developed in the last decade, driven by the demand to enhance plant traits such as yield, resistance to pathogens, and the opportunities provided by CRISPR/Cas-mediated gene editing. Here we discuss how knowledge of stem cell biology benefits regeneration technology, and we speculate on the creation of a universal genotype-independent iPSC system for plants to overcome regenerative recalcitrance.

Pig blastocyst-like structure models from embryonic stem cells

Pluripotent stem cells have the potential to generate embryo models that can recapitulate developmental processes in vitro. Large animals such as pigs may also benefit from stem-cell-based embryo models for improving breeding. Here, we report the generation of blastoids from porcine embryonic stem cells (pESCs). We first develop a culture medium 4FIXY to derive pESCs. We develop a 3D two-step differentiation strategy to generate porcine blastoids from the pESCs. The resulting blastoids exhibit similar morphology, size, cell lineage composition, and single-cell transcriptome characteristics to blastocysts. These porcine blastoids survive and expand for more than two weeks in vitro under two different culture conditions. Large animal blastoids such as those derived from pESCs may enable in vitro modeling of early embryogenesis and improve livestock species' breeding practices.

Technology to the rescue: how to uncover the role of transposable elements in preimplantation development

Transposable elements (TEs) are highly expressed in preimplantation development. Preimplantation development is the phase when the cells of the early embryo undergo the first cell fate choice and change from being totipotent to pluripotent. A range of studies have advanced our understanding of TEs in preimplantation, as well as their epigenetic regulation and functional roles. However, many questions remain about the implications of TE expression during early development. Challenges originate first due to the abundance of TEs in the genome, and second because of the limited cell numbers in preimplantation. Here we review the most recent technological advancements promising to shed light onto the role of TEs in preimplantation development. We explore novel avenues to identify genomic TE insertions and improve our understanding of the regulatory mechanisms and roles of TEs and their RNA and protein products during early development.

The progress of induced pluripotent stem cells derived from pigs: a mini review of recent advances

Pigs (Sus scrofa) are widely acknowledged as an important large mammalian animal model due to their similarity to human physiology, genetics, and immunology. Leveraging the full potential of this model presents significant opportunities for major advancements in the fields of comparative biology, disease modeling, and regenerative medicine. Thus, the derivation of pluripotent stem cells from this species can offer new tools for disease modeling and serve as a stepping stone to test future autologous or allogeneic cell-based therapies. Over the past few decades, great progress has been made in establishing porcine pluripotent stem cells (pPSCs), including embryonic stem cells (pESCs) derived from pre- and peri-implantation embryos, and porcine induced pluripotent stem cells (piPSCs) using a variety of cellular reprogramming strategies. However, the stabilization of pPSCs was not as straightforward as directly applying the culture conditions developed and optimized for murine or primate PSCs. Therefore, it has historically been challenging to establish stable pPSC lines that could pass stringent pluripotency tests. Here, we review recent advances in the establishment of stable porcine PSCs. We focus on the evolving derivation methods that eventually led to the establishment of pESCs and transgene-free piPSCs, as well as current challenges and opportunities in this rapidly advancing field.

The fusion of physics and biology in early mammalian embryogenesis

Biomechanics in embryogenesis is a dynamic field intertwining the physical forces and biological processes that shape the first days of a mammalian embryo. From the first cell fate bifurcation during blastulation to the complex symmetry breaking and tissue remodeling in gastrulation, mechanical cues appear critical in cell fate decisions and tissue patterning. Recent strides in mouse and human embryo culture, stem cell modeling of mammalian embryos, and biomaterial design have shed light on the role of cellular forces, cell polarization, and the extracellular matrix in influencing cell differentiation and morphogenesis. This chapter highlights the essential functions of biophysical mechanisms in blastocyst formation, embryo implantation, and early gastrulation where the interplay between the cytoskeleton and extracellular matrix stiffness orchestrates the intricacies of embryogenesis and placenta specification. The advancement of in vitro models like blastoids, gastruloids, and other types of embryoids, has begun to faithfully recapitulate human development stages, offering new avenues for exploring the biophysical underpinnings of early development. The integration of synthetic biology and advanced biomaterials is enhancing the precision with which we can mimic and study these processes. Looking ahead, we emphasize the potential of CRISPR-mediated genomic perturbations coupled with live imaging to uncover new mechanosensitive pathways and the application of engineered biomaterials to fine-tune the mechanical conditions conducive to embryonic development. This synthesis not only bridges the gap between experimental models and in vivo conditions to advancing fundamental developmental biology of mammalian embryogenesis, but also sets the stage for leveraging biomechanical insights to inform regenerative medicine.

Toward developing human organs via embryo models and chimeras

Developing functional organs from stem cells remains a challenging goal in regenerative medicine. Existing methodologies, such as tissue engineering, bioprinting, and organoids, only offer partial solutions. This perspective focuses on two promising approaches emerging for engineering human organs from stem cells: stem cell-based embryo models and interspecies organogenesis. Both approaches exploit the premise of guiding stem cells to mimic natural development. We begin by summarizing what is known about early human development as a blueprint for recapitulating organogenesis in both embryo models and interspecies chimeras. The latest advances in both fields are discussed before highlighting the technological and knowledge gaps to be addressed before the goal of developing human organs could be achieved using the two approaches. We conclude by discussing challenges facing embryo modeling and interspecies organogenesis and outlining future prospects for advancing both fields toward the generation of human tissues and organs for basic research and translational applications.

Dynamic models of human development and concepts of the individual

Stem cells can be coaxed to self-organize into dynamic models of human development and early embryo formation. Despite their scientific promise, might widespread use of these technologies alter people's beliefs about what it means to be a human individual? Attention to some important philosophical distinctions may help navigate our thinking.

Reconstructing axial progenitor field dynamics in mouse stem cell-derived embryoids

Embryogenesis requires substantial coordination to translate genetic programs to the collective behavior of differentiating cells, but understanding how cellular decisions control tissue morphology remains conceptually and technically challenging. Here, we combine continuous Cas9-based molecular recording with a mouse embryonic stem cell-based model of the embryonic trunk to build single-cell phylogenies that describe the behavior of transient, multipotent neuro-mesodermal progenitors (NMPs) as they commit into neural and somitic cell types. We find that NMPs show subtle transcriptional signatures related to their recent differentiation and contribute to downstream lineages through a surprisingly broad distribution of individual fate outcomes. Although decision-making can be heavily influenced by environmental cues to induce morphological phenotypes, axial progenitors intrinsically mature over developmental time to favor the neural lineage. Using these data, we present an experimental and analytical framework for exploring the non-homeostatic dynamics of transient progenitor populations as they shape complex tissues during critical developmental windows.

Axis formation in annual killifish: Nodal and β-catenin regulate morphogenesis without Huluwa prepatterning

Axis formation in fish and amphibians typically begins with a prepattern of maternal gene products. Annual killifish embryogenesis, however, challenges prepatterning models as blastomeres disperse and then aggregate to form the germ layers and body axes. We show that huluwa, a prepatterning factor thought to break symmetry by stabilizing β-catenin, is truncated and inactive in Nothobranchius furzeri. Nuclear β-catenin is not selectively stabilized on one side of the blastula but accumulates in cells forming the aggregate. Blocking β-catenin activity or Nodal signaling disrupts aggregate formation and germ layer specification. Nodal signaling coordinates cell migration, establishing an early role for this signaling pathway. These results reveal a surprising departure from established mechanisms of axis formation: Huluwa-mediated prepatterning is dispensable, and β-catenin and Nodal regulate morphogenesis.

Oct4 controls basement membrane development during human embryogenesis

Basement membranes (BMs) are sheet-like structures of extracellular matrix (ECM) that provide structural support for many tissues and play a central role in signaling. They are key regulators of cell behavior and tissue functions, and defects in their assembly or composition are involved in numerous human diseases. Due to the differences between human and animal embryogenesis, ethical concerns, legal constraints, the scarcity of human tissue material, and the inaccessibility of the in vivo condition, BM regulation during human embryo development has remained elusive. Using the post-implantation amniotic sac embryoid (PASE), we delineate BM assembly upon post-implantation development and BM disassembly during primitive streak (PS) cell dissemination. Further, we show that the transcription factor Oct4 regulates the expression of BM structural components and receptors and controls BM development by regulating Akt signaling and the small GTPase Rac1. These results represent a relevant step toward a more comprehensive understanding of early human development.

Opinions on research involving human embryo models by researchers and the general public

Rules and ethical considerations regarding research on embryo models have been debated across numerous countries. In this paper, we provide insights from our attitude survey conducted among Japanese researchers, including members of the Japanese Society for Regenerative Medicine, and among the general public residing in Japan, the US, the UK, Canada, and Australia. Our survey revealed that many researchers expressed the need for clear guidelines for embryo model research. Furthermore, a minority but significant portion of the general public in each country expressed opposition to research on embryo models but did not oppose research involving real embryos.

Synthetic human gonadal tissues for toxicology

The process of mammalian reproduction involves the development of fertile germ cells in the testis and ovary, supported by the surrounders. Fertilization leads to embryo development and ultimately the birth of offspring inheriting parental genome information. Any disruption in this process can result in disorders such as infertility and cancer. Chemical toxicity affecting the reproductive system and embryogenesis can impact birth rates, overall health, and fertility, highlighting the need for animal toxicity studies during drug development. However, the translation of animal data to human health remains challenging due to interspecies differences. In vitro culture systems offer a promising solution to bridge this gap, allowing the study of mammalian cells in an environment that mimics the physiology of the human body. Current advances on in vitro culture systems, such as organoids, enable the development of biomaterials that recapitulate the physiological state of reproductive organs. Application of these technologies to human gonadal cells would provide effective tools for drug screening and toxicity testing, and these models would be a powerful tool to study reproductive biology and pathology. This review focuses on the 2D/3D culture systems of human primary testicular and ovarian cells, highlighting the novel approaches for in vitro study of human reproductive toxicology, specifically in the context of testis and ovary.

3D reconstruction of a gastrulating human embryo

Progress in understanding early human development has been impeded by the scarcity of reference datasets from natural embryos, particularly those with spatial information during crucial stages like gastrulation. We conducted high-resolution spatial transcriptomics profiling on 38,562 spots from 62 transverse sections of an intact Carnegie stage (CS) 8 human embryo. From this spatial transcriptomic dataset, we constructed a 3D model of the CS8 embryo, in which a range of cell subtypes are identified, based on gene expression patterns and positional register, along the anterior-posterior, medial-lateral, and dorsal-ventral axis in the embryo. We further characterized the lineage trajectories of embryonic and extra-embryonic tissues and associated regulons and the regionalization of signaling centers and signaling activities that underpin lineage progression and tissue patterning during gastrulation. Collectively, the findings of this study provide insights into gastrulation and post-gastrulation development of the human embryo.

The beginning of becoming a human

According to birth certificates, the life of a child begins once their body comes out of the mother's womb. But when does their organismal life begin? Science holds a palette of answers-depending on how one defines a human life. In 1984, a commission on the regulatory framework for human embryo experimentation opted not to answer this question, instead setting a boundary, 14 days post-fertilization, beyond which any experiments were forbidden. Recently, as the reproductive technologies developed and the demand for experimentation grew stronger, this boundary may be set aside leaving the ultimate decision to local oversight committees. While science has not come closer to setting a zero point for human life, there has been significant progress in our understanding of early mammalian embryogenesis. It has become clear that the 14-day stage does in fact possess features, which make it a foundational time point for a developing human. Importantly, this stage defines the separation of soma from the germline and marks the boundary between rejuvenation and aging. We explore how different levels of life organization emerge during human development and suggest a new meaning for the 14-day stage in organismal life that is grounded in recent mechanistic advances and insights from aging studies.

Developmental regulation of primitive erythropoiesis

PURPOSE OF REVIEW

In this review, we present an overview of recent studies of primitive erythropoiesis, focusing on advances in deciphering its embryonic origin, defining species-specific differences in its developmental regulation, and better understanding the molecular and metabolic pathways involved in terminal differentiation.

RECENT FINDINGS

Single-cell transcriptomics combined with state-of-the-art lineage tracing approaches in unperturbed murine embryos have yielded new insights concerning the origin of the first (primitive) erythroid cells that arise from mesoderm-derived progenitors. Moreover, studies examining primitive erythropoiesis in rare early human embryo samples reveal an overall conservation of primitive erythroid ontogeny in mammals, albeit with some interesting differences such as localization of erythropoietin (EPO) production in the early embryo. Mechanistically, the repertoire of transcription factors that critically regulate primitive erythropoiesis has been expanded to include regulators of transcription elongation, as well as epigenetic modifiers such as the histone methyltransferase DOT1L. For the latter, noncanonical roles aside from enzymatic activity are being uncovered. Lastly, detailed surveys of the metabolic and proteomic landscape of primitive erythroid precursors reveal the activation of key metabolic pathways such as pentose phosphate pathway that are paralleled by a striking loss of mRNA translation machinery.

SUMMARY

The ability to interrogate single cells in vivo continues to yield new insights into the birth of the first essential organ system of the developing embryo. A comparison of the regulation of primitive and definitive erythropoiesis, as well as the interplay of the different layers of regulation - transcriptional, epigenetic, and metabolic - will be critical in achieving the goal of faithfully generating erythroid cells in vitro for therapeutic purposes.

Why study human embryo development?

Understanding the processes and mechanisms underlying early human embryo development has become an increasingly active and important area of research. It has potential for insights into important clinical issues such as early pregnancy loss, origins of congenital anomalies and developmental origins of adult disease, as well as fundamental insights into human biology. Improved culture systems for preimplantation embryos, combined with the new tools of single cell genomics and live imaging, are providing new insights into the similarities and differences between human and mouse development. However, access to human embryo material is still restricted and extended culture of early embryos has regulatory and ethical concerns. Stem cell-derived models of different phases of human development can potentially overcome these limitations and provide a scalable source of material to explore the early postimplantation stages of human development. To date, such models are clearly incomplete replicas of normal development but future technological improvements can be envisaged. The ethical and regulatory environment for such studies remains to be fully resolved.

Current strategies using 3D organoids to establish in vitro maternal-embryonic interaction

IMPORTANCE

The creation of robust maternal-embryonic interactions and implantation models is important for comprehending the early stages of embryonic development and reproductive disorders. Traditional two-dimensional (2D) cell culture systems often fail to accurately mimic the highly complex in vivo conditions. The employment of three-dimensional (3D) organoids has emerged as a promising strategy to overcome these limitations in recent years. The advancements in the field of organoid technology have opened new avenues for studying the physiology and diseases affecting female reproductive tract.

OBSERVATIONS

This review summarizes the current strategies and advancements in the field of 3D organoids to establish maternal-embryonic interaction and implantation models for use in research and personalized medicine in assisted reproductive technology. The concepts of endometrial organoids, menstrual blood flow organoids, placental trophoblast organoids, stem cell-derived blastoids, and in vitro-generated embryo models are discussed in detail. We show the incorportaion of organoid systems and microfluidic technology to enhance tissue performance and precise management of the cellular surroundings.

CONCLUSIONS AND RELEVANCE

This review provides insights into the future direction of modeling maternal-embryonic interaction research and its combination with other powerful technologies to interfere with this dialogue either by promoting or hindering it for improving fertility or methods for contraception, respectively. The merging of organoid systems with microfluidics facilitates the creation of sophisticated and functional organoid models, enhancing insights into organ development, disease mechanisms, and personalized medical investigations.

Induced pluripotent stem cells (iPSCs): molecular mechanisms of induction and applications

The induced pluripotent stem cell (iPSC) technology has transformed in vitro research and holds great promise to advance regenerative medicine. iPSCs have the capacity for an almost unlimited expansion, are amenable to genetic engineering, and can be differentiated into most somatic cell types. iPSCs have been widely applied to model human development and diseases, perform drug screening, and develop cell therapies. In this review, we outline key developments in the iPSC field and highlight the immense versatility of the iPSC technology for in vitro modeling and therapeutic applications. We begin by discussing the pivotal discoveries that revealed the potential of a somatic cell nucleus for reprogramming and led to successful generation of iPSCs. We consider the molecular mechanisms and dynamics of somatic cell reprogramming as well as the numerous methods available to induce pluripotency. Subsequently, we discuss various iPSC-based cellular models, from mono-cultures of a single cell type to complex three-dimensional organoids, and how these models can be applied to elucidate the mechanisms of human development and diseases. We use examples of neurological disorders, coronavirus disease 2019 (COVID-19), and cancer to highlight the diversity of disease-specific phenotypes that can be modeled using iPSC-derived cells. We also consider how iPSC-derived cellular models can be used in high-throughput drug screening and drug toxicity studies. Finally, we discuss the process of developing autologous and allogeneic iPSC-based cell therapies and their potential to alleviate human diseases.

The SWI/SNF ATP-dependent chromatin remodeling complex in cell lineage priming and early development

ATP dependent chromatin remodelers have pivotal roles in transcription, DNA replication and repair, and maintaining genome integrity. SWI/SNF remodelers were first discovered in yeast genetic screens for factors involved in mating type switching or for using alternative energy sources therefore termed SWI/SNF complex (short for SWItch/Sucrose NonFermentable). The SWI/SNF complexes utilize energy from ATP hydrolysis to disrupt histone-DNA interactions and shift, eject, or reposition nucleosomes making the underlying DNA more accessible to specific transcription factors and other regulatory proteins. In development, SWI/SNF orchestrates the precise activation and repression of genes at different stages, safe guards the formation of specific cell lineages and tissues. Dysregulation of SWI/SNF have been implicated in diseases such as cancer, where they can drive uncontrolled cell proliferation and tumor metastasis. Additionally, SWI/SNF defects are associated with neurodevelopmental disorders, leading to disruption of neural development and function. This review offers insights into recent developments regarding the roles of the SWI/SNF complex in pluripotency and cell lineage primining and the approaches that have helped delineate its importance. Understanding these molecular mechanisms is crucial for unraveling the intricate processes governing embryonic stem cell biology and developmental transitions and may potentially apply to human diseases linked to mutations in the SWI/SNF complex.

Unlocking trophectoderm mysteries: In vivo and in vitro perspectives on human and mouse trophoblast fate induction

In recent years, the pursuit of inducing the trophoblast stem cell (TSC) state has gained prominence as a compelling research objective, illuminating the establishment of the trophoblast lineage and unlocking insights into early embryogenesis. In this review, we examine how advancements in diverse technologies, including in vivo time course transcriptomics, cellular reprogramming to TSC state, chemical induction of totipotent stem-cell-like state, and stem-cell-based embryo-like structures, have enriched our insights into the intricate molecular mechanisms and signaling pathways that define the mouse and human trophectoderm/TSC states. We delve into disparities between mouse and human trophectoderm/TSC fate establishment, with a special emphasis on the intriguing role of pluripotency in this context. Additionally, we re-evaluate recent findings concerning the potential of totipotent-stem-like cells and embryo-like structures to fully manifest the trophectoderm/trophoblast lineage's capabilities. Lastly, we briefly discuss the potential applications of induced TSCs in pregnancy-related disease modeling.

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, open 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 organised 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.

Self-organization of embryonic stem cells into a reproducible embryo model through epigenome editing

Embryonic stem cells (ESCs) can self-organize in vitro into developmental patterns with spatial organization and molecular similarity to that of early embryonic stages. This self-organization of ESCs requires transmission of signaling cues, via addition of small molecule chemicals or recombinant proteins, to induce distinct embryonic cellular fates and subsequent assembly into structures that can mimic aspects of early embryonic development. During natural embryonic development, different embryonic cell types co-develop together, where each cell type expresses specific fate-inducing transcription factors through activation of non-coding regulatory elements and interactions with neighboring cells. However, previous studies have not fully explored the possibility of engineering endogenous regulatory elements to shape self-organization of ESCs into spatially-ordered embryo models. Here, we hypothesized that cell-intrinsic activation of a minimum number of such endogenous regulatory elements is sufficient to self-organize ESCs into early embryonic models. Our results show that CRISPR-based activation (CRISPRa) of only two endogenous regulatory elements in the genome of pluripotent stem cells is sufficient to generate embryonic patterns that show spatial and molecular resemblance to that of pre-gastrulation mouse embryonic development. Quantitative single-cell live fluorescent imaging showed that the emergence of spatially-ordered embryonic patterns happens through the intrinsic induction of cell fate that leads to an orchestrated collective cellular motion. Based on these results, we propose a straightforward approach to efficiently form 3D embryo models through intrinsic CRISPRa-based epigenome editing and independent of external signaling cues. CRISPRa-Programmed Embryo Models (CPEMs) show highly consistent composition of major embryonic cell types that are spatially-organized, with nearly 80% of the structures forming an embryonic cavity. Single cell transcriptomics confirmed the presence of main embryonic cell types in CPEMs with transcriptional similarity to pre-gastrulation mouse embryos and revealed novel signaling communication links between different embryonic cell types. Our findings offer a programmable embryo model and demonstrate that minimum intrinsic epigenome editing is sufficient to self-organize ESCs into highly consistent pre-gastrulation embryo models.

Hallmarks of totipotent and pluripotent stem cell states

Though totipotency and pluripotency are transient during early embryogenesis, they establish the foundation for the development of all mammals. Studying these in vivo has been challenging due to limited access and ethical constraints, particularly in humans. Recent progress has led to diverse culture adaptations of epiblast cells in vitro in the form of totipotent and pluripotent stem cells, which not only deepen our understanding of embryonic development but also serve as invaluable resources for animal reproduction and regenerative medicine. This review delves into the hallmarks of totipotent and pluripotent stem cells, shedding light on their key molecular and functional features.

Modelling post-implantation human development to yolk sac blood emergence

Implantation of the human embryo begins a critical developmental stage that comprises profound events including axis formation, gastrulation and the emergence of haematopoietic system1,2. Our mechanistic knowledge of this window of human life remains limited due to restricted access to in vivo samples for both technical and ethical reasons3-5. Stem cell models of human embryo have emerged to help unlock the mysteries of this stage6-16. Here we present a genetically inducible stem cell-derived embryoid model of early post-implantation human embryogenesis that captures the reciprocal codevelopment of embryonic tissue and the extra-embryonic endoderm and mesoderm niche with early haematopoiesis. This model is produced from induced pluripotent stem cells and shows unanticipated self-organizing cellular programmes similar to those that occur in embryogenesis, including the formation of amniotic cavity and bilaminar disc morphologies as well as the generation of an anterior hypoblast pole and posterior domain. The extra-embryonic layer in these embryoids lacks trophoblast and shows advanced multilineage yolk sac tissue-like morphogenesis that harbours a process similar to distinct waves of haematopoiesis, including the emergence of erythroid-, megakaryocyte-, myeloid- and lymphoid-like cells. This model presents an easy-to-use, high-throughput, reproducible and scalable platform to probe multifaceted aspects of human development and blood formation at the early post-implantation stage. It will provide a tractable human-based model for drug testing and disease modelling.

Hypoblast from human pluripotent stem cells regulates epiblast development

Recently, several studies using cultures of human embryos together with single-cell RNA-seq analyses have revealed differences between humans and mice, necessitating the study of human embryos1-8. Despite the importance of human embryology, ethical and legal restrictions have limited post-implantation-stage studies. Thus, recent efforts have focused on developing in vitro self-organizing models using human stem cells9-17. Here, we report genetic and non-genetic approaches to generate authentic hypoblast cells (naive hPSC-derived hypoblast-like cells (nHyCs))-known to give rise to one of the two extraembryonic tissues essential for embryonic development-from naive human pluripotent stem cells (hPSCs). Our nHyCs spontaneously assemble with naive hPSCs to form a three-dimensional bilaminar structure (bilaminoids) with a pro-amniotic-like cavity. In the presence of additional naive hPSC-derived analogues of the second extraembryonic tissue, the trophectoderm, the efficiency of bilaminoid formation increases from 20% to 40%, and the epiblast within the bilaminoids continues to develop in response to trophectoderm-secreted IL-6. Furthermore, we show that bilaminoids robustly recapitulate the patterning of the anterior-posterior axis and the formation of cells reflecting the pregastrula stage, the emergence of which can be shaped by genetically manipulating the DKK1/OTX2 hypoblast-like domain. We have therefore successfully modelled and identified the mechanisms by which the two extraembryonic tissues efficiently guide the stage-specific growth and progression of the epiblast as it establishes the post-implantation landmarks of human embryogenesis.

Topical section: embryonic models (2023) for Current Opinion in Genetics & Development

Stem cell-based mammalian embryo models facilitate the discovery of developmental mechanisms because they are more amenable to genetic and epigenetic perturbations than natural embryos. Here, we highlight exciting recent advances that have yielded a plethora of models of embryonic development. Imperfections in these models highlight gaps in our current understanding and outline future research directions, ushering in an exciting new era for embryology.

Transcriptional consequences of trisomy 21 on neural induction

Introduction

Down syndrome, caused by trisomy 21, is a complex developmental disorder associated with intellectual disability and reduced growth of multiple organs. Structural pathologies are present at birth, reflecting embryonic origins. A fundamental unanswered question is how an extra copy of human chromosome 21 contributes to organ-specific pathologies that characterize individuals with Down syndrome, and, relevant to the hallmark intellectual disability in Down syndrome, how trisomy 21 affects neural development. We tested the hypothesis that trisomy 21 exerts effects on human neural development as early as neural induction.

Methods

Bulk RNA sequencing was performed on isogenic trisomy 21 and euploid human induced pluripotent stem cells (iPSCs) at successive stages of neural induction: embryoid bodies at Day 6, early neuroectoderm at Day 10, and differentiated neuroectoderm at Day 17.

Results

Gene expression analysis revealed over 1,300 differentially expressed genes in trisomy 21 cells along the differentiation pathway compared to euploid controls. Less than 5% of the gene expression changes included upregulated chromosome 21 encoded genes at every timepoint. Genes involved in specific growth factor signaling pathways (WNT and Notch), metabolism (including oxidative stress), and extracellular matrix were altered in trisomy 21 cells. Further analysis uncovered heterochronic expression of genes.

Conclusion

Trisomy 21 impacts discrete developmental pathways at the earliest stages of neural development. The results suggest that metabolic dysfunction arises early in embryogenesis in trisomy 21 and may affect development and function more broadly.

Self-renewing human naïve pluripotent stem cells dedifferentiate in 3D culture and form blastoids spontaneously

Human naïve pluripotent stem cells (hnPSCs) can generate integrated models of blastocysts termed blastoids upon switch to inductive medium. However, the underlying mechanisms remain obscure. Here we report that self-renewing hnPSCs spontaneously and efficiently give rise to blastoids upon three dimensional (3D) suspension culture. The spontaneous blastoids mimic early stage human blastocysts in terms of structure, size, and transcriptome characteristics and are capable of progressing to post-implantation stages. This property is conferred by the glycogen synthase kinase-3 (GSK3) signalling inhibitor IM-12 present in 5iLAF self-renewing medium. IM-12 upregulates oxidative phosphorylation-associated genes that underly the capacity of hnPSCs to generate blastoids spontaneously. Starting from day one of self-organization, hnPSCs at the boundary of all 3D aggregates dedifferentiate into E5 embryo-like intermediates. Intermediates co-express SOX2/OCT4 and GATA6 and by day 3 specify trophoblast fate, which coincides with cavity and blastoid formation. In summary, spontaneous blastoid formation results from 3D culture triggering dedifferentiation of hnPSCs into earlier embryo-like intermediates which are then competent to segregate blastocyst fates.

VGLL1 cooperates with TEAD4 to control human trophectoderm lineage specification

In contrast to rodents, the mechanisms underlying human trophectoderm and early placenta specification are understudied due to ethical barriers and the scarcity of embryos. Recent reports have shown that human pluripotent stem cells (PSCs) can differentiate into trophectoderm (TE)-like cells (TELCs) and trophoblast stem cells (TSCs), offering a valuable in vitro model to study early placenta specification. Here, we demonstrate that the VGLL1 (vestigial-like family member 1), which is highly expressed during human and non-human primate TE specification in vivo but is negligibly expressed in mouse, is a critical regulator of cell fate determination and self-renewal in human TELCs and TSCs derived from naïve PSCs. Mechanistically, VGLL1 partners with the transcription factor TEAD4 (TEA domain transcription factor 4) to regulate chromatin accessibility at target gene loci through histone acetylation and acts in cooperation with GATA3 and TFAP2C. Our work is relevant to understand primate early embryogenesis and how it differs from other mammalian species.

The emergence of human gastrulation upon in vitro attachment

While studied extensively in model systems, human gastrulation remains obscure. The scarcity of fetal biological material as well as ethical considerations limit our understanding of this process. In vitro attachment of natural blastocysts shed light on aspects of the second week of human development in the absence of the morphological manifestation of gastrulation. Stem cell-derived blastocyst models, blastoids, provide the opportunity to reconstitute pre- to post-implantation development in vitro. Here we show that upon in vitro attachment, human blastoids self-organize a BRA+ population and undergo gastrulation. Single-cell RNA sequencing of these models replicates the transcriptomic signature of the human gastrula. Analysis of developmental timing reveals that in both blastoid models and natural human embryos, the onset of gastrulation as defined by molecular markers, can be traced to timescales equivalent to 12 days post fertilization. In all, natural human embryos and blastoid models self-organize primitive streak and mesoderm derivatives upon in vitro attachment.

Transparency in controversial research: A review of human embryo research publication ethical disclosure statements

In 2021, the International Society for Stem Cell Research (ISSCR) released updated guidelines that included human embryo research guidance. Requiring ethics statements in publications using human embryos is one way to verify adherence to these guidelines. A review of top-tier biomedical journal requirements identified only one publisher that requires a human embryo statement. A review of articles using human embryos from top-tier biomedical journals found that all contain some form of ethics statement, but they differ in content and location. Requiring ethics statements with specific elements could improve transparency and adherence to research guidelines.

Roles of lipid mediators in early pregnancy events

Background

Early pregnancy events, including embryo implantation, are critical for maintaining a healthy pregnancy and facilitating childbirth. Despite numerous signaling pathways implicated in establishing early pregnancy, a comprehensive understanding of implantation remains elusive.

Methods

This paper provides a comprehensive review of the current research on lipids in the context of early pregnancy, with a particular focus on feto-maternal communications.

Main Findings

Embryo implantation entails direct interaction between uterine tissues and embryos. Introducing embryos triggers significant changes in uterine epithelial morphology and stromal differentiation, facilitating embryo implantation through communication with uterine tissue. Studies employing genetic models and chemical compounds targeting enzymes and receptors have elucidated the crucial roles of lipid mediators-prostaglandins, lysophosphatidic acid, sphingosine-1-phosphate, and cannabinoids-in early pregnancy events.

Conclusion

Given the high conservation of lipid synthases and receptors across species, lipid mediators likely play pivotal roles in rodents and humans. Further investigations into lipids hold promise for developing novel diagnostic and therapeutic approaches for infertility in humans.

Human peri-gastruloids: a significant advancement in embryology research

The peri-gastruloids comprise both embryonic (epiblast) and extraembryonic (hypoblast) tissues, faithfully mirroring crucial developmental events spanning from the immediate post-implantation phase to early organogenesis, encompassing the emergence of amniotic and yolk sac cavities, as well as the progression from bilaminar to trilaminar embryonic discs.