Stem-cell-based embryo models for fundamental research and translation

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.

Autophagy in reproduction and pregnancy-associated diseases

As advantageous as sexual reproduction is during progeny generation, it is also an expensive and treacherous reproductive strategy. The viviparous eukaryote has evolved to survive stress before, during, and after pregnancy. An important and conserved intracellular pathway for the control of metabolic stress is autophagy. The autophagy process occurs in multiple stages through the coordinated action of autophagy-related genes. This review summarizes the evidence that autophagy is an integral component of reproduction. Additionally, we discuss emerging in vitro techniques that will enable cellular and molecular studies of autophagy and its associated pathways in reproduction. Finally, we discuss the role of autophagy in the pathogenesis and progression of several pregnancy-related disorders such as preterm birth, preeclampsia, and intra-uterine growth restriction, and its potential as a therapeutic target.

Substrates mimicking the blastocyst geometry revert pluripotent stem cell to naivety

Naive pluripotent stem cells have the highest developmental potential but their in vivo existence in the blastocyst is transient. Here we report a blastocyst motif substrate for the in vitro reversion of mouse and human pluripotent stem cells to a naive state. The substrate features randomly varied microstructures, which we call motifs, mimicking the geometry of the blastocyst. Motifs representing mouse-blastocyst-scaled curvature ranging between 15 and 62 mm-1 were the most efficient in promoting reversion to naivety, as determined by time-resolved correlative analysis. In these substrates, apical constriction enhances E-cadherin/RAC1 signalling and activates the mechanosensitive nuclear transducer YAP, promoting the histone modification of pluripotency genes. This results in enhanced levels of pluripotency transcription factor NANOG, which persist even after cells are removed from the substrate. Pluripotent stem cells cultured in blastocyst motif substrates display a higher development potential in generating embryoid bodies and teratomas. These findings shed light on naivety-promoting substrate design and their large-scale implementation.

Rapid Whole-Plate Cell and Tissue Micropatterning Using a Budget 3D Resin Printer

The ability to precisely pattern cells and proteins is crucial in various scientific disciplines, including cell biology, bioengineering, and materials chemistry. Current techniques, such as microcontact stamping, 3D bioprinting, and direct photopatterning, have limitations in terms of cost, versatility, and throughput. In this Article, we present an accessible approach that combines the throughput of photomask systems with the versatility of programmable light patterning using a low-cost consumer LCD resin printer. The method involves utilizing a bioinert hydrogel, poly(ethylene glycol) diacrylate (PEGDA), and a 405 nm sensitive photoinitiator (LAP) that are selectively cross-linked to form a hydrogel upon light exposure, creating specific regions that are protein and cell-repellent. Our result highlights that a low-cost LCD resin printer can project virtual photomasks onto the hydrogel, allowing for reasonable resolution and large-area printing at a fraction of the cost of traditional systems. The study demonstrates the calibration of exposure times for optimal resolution and accuracy and shape corrections to overcome the inherent challenges of wide-field resin printing. The potential of this approach is validated through widely studied 2D and 3D stem cell applications, showcasing its biocompatibility and ability to replicate complex tissue engineering patterns. We also validate the method with a cell-adhesive polymer (gelatin methacrylate; GelMA). The combination of low cost, high throughput, and accessibility makes this method broadly applicable across fields for enabling rapid and precise fabrication of cells and tissues in standard laboratory culture vessels.

Exploring the black box of human reproduction: endometrial organoids and assembloids - generation, implantation modeling, and future clinical perspectives

One of the critical processes in human reproduction that is still poorly understood is implantation. The implantation of an early human embryo is considered a significant limitation of successful pregnancy. Therefore, researchers are trying to develop an ideal model of endometrium in vitro that can mimic the endometrial micro-environment in vivo as much as possible. The ultimate goal of endometrial modeling is to study the molecular interactions at the embryo-maternal interface and to use this model as an in vitro diagnostic tool for infertility. Significant progress has been made over the years in generating such models. The first experiments of endometrial modeling involved animal models, which are undoubtedly valuable, but at the same time, their dissimilarities with human tissue represent a significant obstacle to further research. This fact led researchers to develop basic monolayer coculture systems using uterine cells obtained from biopsies and, later on, complex and multilayer coculture models. With successful tissue engineering methods and various cultivation systems, it is possible to form endometrial two-dimensional (2D) models to three-dimensional (3D) organoids and novel assembloids that can recapitulate many aspects of endometrial tissue architecture and cell composition. These organoids have already helped to provide new insight into the embryo-endometrium interplay. The main aim of this paper is a comprehensive review of past and current approaches to endometrial model generation, their feasibility, and potential clinical application for infertility treatment.

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.

The role of DNA damage response in human embryonic stem cells exposed to atmospheric oxygen tension: Implications for embryo development and differentiation

Previous retrospective cohort studies have found that, compared with oxygen tension in the uterus and fallopian tubes (2 %-8 %), exposure of pre-implantation embryos to atmospheric oxygen tension (AtmO2, 20 %) during assisted reproductive technology(ART) can affect embryo quality, pregnancy outcomes and offspring health. However, current research on the effects and mechanisms of AtmO2 on the development of embryos and offspring is mainly limited to animal experiments. Human embryonic stem cells (hESCs) play a special and irreplaceable role in the study of early human embryonic development. In this study, we used hESCs as a model to elucidate the possible effects and mechanisms of AtmO2 exposure on human embryonic development. We found that exposure to AtmO2 can reduce cell viability, produce oxidative stress, increase DNA damage, initiate DNA repair, activate autophagy, and increase cell apoptosis. We also noticed that approximately 50 % of hESCs survived, adapted and proliferated through high expression of self-renewal and pluripotency regulatory factors, and affected embryoid body differentiation. These data indicate that hESCs experience oxidative stress, accumulation of DNA damage, and activate DNA damage response under the selective pressure of AtmO2.Some hESCs undergo cell death, whereas other hESCs adapt and proliferate through increased expression of self-renewal genes. The current findings provide in vitro evidence that exposure to AtmO2 during the early preimplantation stage negatively affects hESCs.

3D Multispheroid Assembly Strategies towards Tissue Engineering and Disease Modeling

Cell spheroids (esp. organoids) as 3D culture platforms are popular models for representing cell-cell and cell-extracellular matrix (ECM) interactions, bridging the gap between 2D cell cultures and natural tissues. 3D cell models with spatially organized multiple cell types are preferred for gaining comprehensive insights into tissue pathophysiology and constructing in vitro tissues and disease models because of the complexities of natural tissues. In recent years, an assembly strategy using cell spheroids (or organoids) as living building blocks has been developed to construct complex 3D tissue models with spatial organization. Here, a comprehensive overview of recent advances in multispheroid assembly studies is provided. The different mechanisms of the multispheroid assembly techniques, i.e., automated directed assembly, noncontact remote assembly, and programmed self-assembly, are introduced. The processing steps, advantages, and technical limitations of the existing methodologies are summarized. Applications of the multispheroid assembly strategies in disease modeling, drug screening, tissue engineering, and organogenesis are reviewed. Finally, this review concludes by emphasizing persistent issues and future perspectives, encouraging researchers to adopt multispheroid assembly techniques for generating advanced 3D cell models that better resemble real tissues.

Technological advances and challenges in constructing complex gut organoid systems

Recent advancements in organoid technology have heralded a transformative era in biomedical research, characterized by the emergence of gut organoids that replicate the structural and functional complexity of the human intestines. These stem cell-derived structures provide a dynamic platform for investigating intestinal physiology, disease pathogenesis, and therapeutic interventions. This model outperforms traditional two-dimensional cell cultures in replicating cell interactions and tissue dynamics. Gut organoids represent a significant leap towards personalized medicine. They provide a predictive model for human drug responses, thereby minimizing reliance on animal models and paving the path for more ethical and relevant research approaches. However, the transition from basic organoid models to more sophisticated, biomimetic systems that encapsulate the gut's multifaceted environment-including its interactions with microbial communities, immune cells, and neural networks-presents significant scientific challenges. This review concentrates on recent technological strides in overcoming these barriers, emphasizing innovative engineering approaches for integrating diverse cell types to replicate the gut's immune and neural components. It also explores the application of advanced fabrication techniques, such as 3D bioprinting and microfluidics, to construct organoids that more accurately replicate human tissue architecture. They provide insights into the intricate workings of the human gut, fostering the development of targeted, effective treatments. These advancements hold promise in revolutionizing disease modeling and drug discovery. Future research directions aim at refining these models further, making them more accessible and scalable for wider applications in scientific inquiry and clinical practice, thus heralding a new era of personalized and predictive medicine.

ST6GALNAC1-mediated sialylation in uterine endometrial epithelium facilitates the epithelium-embryo attachment

INTRODUCTION

Embryo implantation requires synergistic interaction between the embryo and the receptive endometrium. Glycoproteins and glycan-binding proteins are involved in endometrium-embryo attachment. Sialyl Tn (sTn), a truncated O-glycan, is catalyzed by ST6 N-Acetylgalactosaminide Alpha-2,6-Sialyltransferase 1 (ST6GALNAC1) and can be detected by specific Sialic-acid-binding immunoglobulin-like lectins (Siglecs). Whether the sTn-Siglecs axis supports embryo implantation remains unknown.

OBJECTIVES

This paper aims to study the role of ST6GALNAC1/sTn-Siglecs axis in embryo implantation.

METHODS

ST6GALNAC1 and sTn in human endometrium were analyzed by immunohistochemistry. An in vitro implantation model was conducted to evaluate the effects of ST6GALNAC1/sTn on the receptivity of human endometrial AN3CA cells to JAR spheroids. Immunoprecipitation combined with mass spectrometry analysis was carried out to identify the key proteins modified by sTn in endometrial cells. Siglec-6 in human embryos was analyzed by published single-cell RNA sequencing (scRNA-seq) datasets. Protein interaction assay was applied to verify the bond between the Siglec-6 with sTn-modified CD44. St6galnac1 siRNAs and anti-sTn antibodies were injected into the uterine horn of the mouse at the pre-implantation stage to evaluate the role of endometrial St6galnac1/sTn in embryo implantation. Siglec-G in murine embryos was analyzed by immunofluorescence staining. The function of Siglec-G is evidenced by uterine horn injection and protein interaction assay.

RESULTS

Both human and murine endometrium at the receptive stage exhibit higher ST6GALNAC1 and sTn levels compared to the non-receptive stage. Overexpression of ST6GALNAC1 significantly enhanced the receptivity of AN3CA cells to JAR spheroids. Inhibition of endometrial ST6GALNAC1/sTn substantially impaired embryo implantation in vivo. CD44 was identified as a carrier for sTn in the endometrial cells of both species. Siglec-6 and Siglec-G, expressed in the embryonic trophectoderm, were found to promote embryo attachment, which may be achieved through binding with sTn-modified CD44.

CONCLUSION

ST6GALNAC1-regulated sTn in the endometrium aids in embryo attachment through interaction with trophoblastic Siglecs.

Reprogramming fibroblast into human iBlastoids

The study of early human embryogenesis has relied on the use of blastocysts donated to research or simple stem cell culture systems such as pluripotent and trophoblast stem cells, which have been seminal in shedding light on many key developmental processes. However, simple culture systems lack the necessary complexity to adequately model the spatiotemporal, cellular and molecular dynamics occurring during the early phases of embryonic development. As such, an in vitro model of the human blastocyst is advantageous in many aspects to decipher human embryogenesis. Here we describe a step-by-step protocol for the generation of induced blastoids (iBlastoids), an in vitro integrated model of the human blastocyst derived via somatic reprogramming. This protocol details the workflow for reprogramming of human dermal fibroblasts and subsequent generation of iBlastoids using the reprogramming intermediates, which together takes ~27 days (21 days for reprogramming and 6 days for iBlastoid generation). We also discuss several characterization/functional assays that can be used on the iBlastoids. We believe that a person trained in cell culture with ~1 year of experience with human somatic cell and reprogramming/cell differentiation assays would be able to perform this protocol. In short, the iBlastoids present an alternative tool as a model to the blastocyst to facilitate the scientific community in the exploration of early human development.

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.

The reprotoxic adverse side effects of neurogenic and neuroprotective drugs: current use of human organoid modeling as a potential alternative to preclinical models

The management of neurological disorders heavily relies on neurotherapeutic drugs, but notable concerns exist regarding their possible negative effects on reproductive health. Traditional preclinical models often fail to accurately predict reprotoxicity, highlighting the need for more physiologically relevant systems. Organoid models represent a promising approach for concurrently studying neurotoxicity and reprotoxicity, providing insights into the complex interplay between neurotherapeutic drugs and reproductive systems. Herein, we have examined the molecular mechanisms underlying neurotherapeutic drug-induced reprotoxicity and discussed experimental findings from case studies. Additionally, we explore the utility of organoid models in elucidating the reproductive complications of neurodrug exposure. Have discussed the principles of organoid models, highlighting their ability to recapitulate neurodevelopmental processes and simulate drug-induced toxicity in a controlled environment. Challenges and future perspectives in the field have been addressed with a focus on advancing organoid technologies to improve reprotoxicity assessment and enhance drug safety screening. This review underscores the importance of organoid models in unraveling the complex relationship between neurotherapeutic drugs and reproductive health.

A PLURIPOTENT STEM CELL PLATFORM FOR IN VITRO SYSTEMS GENETICS STUDIES OF MOUSE DEVELOPMENT

The directed differentiation of pluripotent stem cells (PSCs) from panels of genetically diverse individuals is emerging as a powerful experimental system for characterizing the impact of natural genetic variation on developing cell types and tissues. Here, we establish new PSC lines and experimental approaches for modeling embryonic development in a genetically diverse, outbred mouse stock (Diversity Outbred mice). We show that a range of inbred and outbred PSC lines can be stably maintained in the primed pluripotent state (epiblast stem cells -- EpiSCs) and establish the contribution of genetic variation to phenotypic differences in gene regulation and directed differentiation. Using pooled in vitro fertilization, we generate and characterize a genetic reference panel of Diversity Outbred PSCs (n = 230). Finally, we demonstrate the feasibility of pooled culture of Diversity Outbred EpiSCs as "cell villages", which can facilitate the differentiation of large numbers of EpiSC lines for forward genetic screens. These data can complement and inform similar efforts within the stem cell biology and human genetics communities to model the impact of natural genetic variation on phenotypic variation and disease-risk.

Precise and scalable self-organization in mammalian pseudo-embryos

Gene expression is inherently noisy, posing a challenge to understanding how precise and reproducible patterns of gene expression emerge in mammals. Here we investigate this phenomenon using gastruloids, a three-dimensional in vitro model for early mammalian development. Our study reveals intrinsic reproducibility in the self-organization of gastruloids, encompassing growth dynamics and gene expression patterns. We observe a remarkable degree of control over gene expression along the main body axis, with pattern boundaries positioned with single-cell precision. Furthermore, as gastruloids grow, both their physical proportions and gene expression patterns scale proportionally with system size. Notably, these properties emerge spontaneously in self-organizing cell aggregates, distinct from many in vivo systems constrained by fixed boundary conditions. Our findings shed light on the intricacies of developmental precision, reproducibility and size scaling within a mammalian system, suggesting that these phenomena might constitute fundamental features of multicellularity.

Combinatorial interpretation of BMP and WNT controls the decision between primitive streak and extraembryonic fates

BMP signaling is essential for mammalian gastrulation, as it initiates a cascade of signals that control self-organized patterning. As development is highly dynamic, it is crucial to understand how time-dependent combinatorial signaling affects cellular differentiation. Here, we show that BMP signaling duration is a crucial control parameter that determines cell fates upon the exit from pluripotency through its interplay with the induced secondary signal WNT. BMP signaling directly converts cells from pluripotent to extraembryonic fates while simultaneously upregulating Wnt signaling, which promotes primitive streak and mesodermal specification. Using live-cell imaging of signaling and cell fate reporters together with a simple mathematical model, we show that this circuit produces a temporal morphogen effect where, once BMP signal duration is above a threshold for differentiation, intermediate and long pulses of BMP signaling produce specification of mesoderm and extraembryonic fates, respectively. Our results provide a systems-level picture of how these signaling pathways control the landscape of early human development.

The Imperative for Innovative Enteric Nervous System-Intestinal Organoid Co-Culture Models: Transforming GI Disease Modeling and Treatment

This review addresses the need for innovative co-culture systems integrating the enteric nervous system (ENS) with intestinal organoids. The breakthroughs achieved through these techniques will pave the way for a transformative era in gastrointestinal (GI) disease modeling and treatment strategies. This review serves as an introduction to the companion protocol paper featured in this journal. The protocol outlines the isolation and co-culture of myenteric and submucosal neurons with small intestinal organoids. This review provides an overview of the intestinal organoid culture field to establish a solid foundation for effective protocol application. Remarkably, the ENS surpasses the number of neurons in the spinal cord. Referred to as the "second brain", the ENS orchestrates pivotal roles in GI functions, including motility, blood flow, and secretion. The ENS is organized into myenteric and submucosal plexuses. These plexuses house diverse subtypes of neurons. Due to its proximity to the gut musculature and its cell type complexity, there are methodological intricacies in studying the ENS. Diverse approaches such as primary cell cultures, three-dimensional (3D) neurospheres, and induced ENS cells offer diverse insights into the multifaceted functionality of the ENS. The ENS exhibits dynamic interactions with the intestinal epithelium, the muscle layer, and the immune system, influencing epithelial physiology, motility, immune responses, and the microbiome. Neurotransmitters, including acetylcholine (ACh), serotonin (5-HT), and vasoactive intestinal peptide (VIP), play pivotal roles in these intricate interactions. Understanding these dynamics is imperative, as the ENS is implicated in various diseases, ranging from neuropathies to GI disorders and neurodegenerative diseases. The emergence of organoid technology presents an unprecedented opportunity to study ENS interactions within the complex milieu of the small and large intestines. This manuscript underscores the urgent need for standardized protocols and advanced techniques to unravel the complexities of the ENS and its dynamic relationship with the gut ecosystem. The insights gleaned from such endeavors hold the potential to revolutionize GI disease modeling and treatment paradigms.

Emerging Contributions of Pluripotent Stem Cells to Reproductive Technologies in Veterinary Medicine

The generation of mature gametes and competent embryos in vitro from pluripotent stem cells has been successfully achieved in a few species, mainly in mice, with recent advances in humans and scarce preliminary reports in other domestic species. These biotechnologies are very attractive as they facilitate the understanding of developmental mechanisms and stages that are generally inaccessible during early embryogenesis, thus enabling advanced reproductive technologies and contributing to the generation of animals of high genetic merit in a short period. Studies on the production of in vitro embryos in pigs and cattle are currently used as study models for humans since they present more similar characteristics when compared to rodents in both the initial embryo development and adult life. This review discusses the most relevant biotechnologies used in veterinary medicine, focusing on the generation of germ-cell-like cells in vitro through the acquisition of totipotent status and the production of embryos in vitro from pluripotent stem cells, thus highlighting the main uses of pluripotent stem cells in livestock species and reproductive medicine.

Lumen expansion is initially driven by apical actin polymerization followed by osmotic pressure in a human epiblast model

Post-implantation, the pluripotent epiblast in a human embryo forms a central lumen, paving the way for gastrulation. Osmotic pressure gradients are considered the drivers of lumen expansion across development, but their role in human epiblasts is unknown. Here, we study lumenogenesis in a pluripotent-stem-cell-based epiblast model using engineered hydrogels. We find that leaky junctions prevent osmotic pressure gradients in early epiblasts and, instead, forces from apical actin polymerization drive lumen expansion. Once the lumen reaches a radius of ∼12 μm, tight junctions mature, and osmotic pressure gradients develop to drive further growth. Computational modeling indicates that apical actin polymerization into a stiff network mediates initial lumen expansion and predicts a transition to pressure-driven growth in larger epiblasts to avoid buckling. Human epiblasts show transcriptional signatures consistent with these mechanisms. Thus, actin polymerization drives lumen expansion in the human epiblast and may serve as a general mechanism of early lumenogenesis.

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.

Contribution of the ELRs to the development of advanced in vitro models

Developing in vitro models that accurately mimic the microenvironment of biological structures or processes holds substantial promise for gaining insights into specific biological functions. In the field of tissue engineering and regenerative medicine, in vitro models able to capture the precise structural, topographical, and functional complexity of living tissues, prove to be valuable tools for comprehending disease mechanisms, assessing drug responses, and serving as alternatives or complements to animal testing. The choice of the right biomaterial and fabrication technique for the development of these in vitro models plays an important role in their functionality. In this sense, elastin-like recombinamers (ELRs) have emerged as an important tool for the fabrication of in vitro models overcoming the challenges encountered in natural and synthetic materials due to their intrinsic properties, such as phase transition behavior, tunable biological properties, viscoelasticity, and easy processability. In this review article, we will delve into the use of ELRs for molecular models of intrinsically disordered proteins (IDPs), as well as for the development of in vitro 3D models for regenerative medicine. The easy processability of the ELRs and their rational design has allowed their use for the development of spheroids and organoids, or bioinks for 3D bioprinting. Thus, incorporating ELRs into the toolkit of biomaterials used for the fabrication of in vitro models, represents a transformative step forward in improving the accuracy, efficiency, and functionality of these models, and opening up a wide range of possibilities in combination with advanced biofabrication techniques that remains to be explored.

Time-integrated BMP signaling determines fate in a stem cell model for early human development

How paracrine signals are interpreted to yield multiple cell fate decisions in a dynamic context during human development in vivo and in vitro remains poorly understood. Here we report an automated tracking method to follow signaling histories linked to cell fate in large numbers of human pluripotent stem cells (hPSCs). Using an unbiased statistical approach, we discover that measured BMP signaling history correlates strongly with fate in individual cells. We find that BMP response in hPSCs varies more strongly in the duration of signaling than the level. However, both the level and duration of signaling activity control cell fate choices only by changing the time integral. Therefore, signaling duration and level are interchangeable in this context. In a stem cell model for patterning of the human embryo, we show that signaling histories predict the fate pattern and that the integral model correctly predicts changes in cell fate domains when signaling is perturbed. Our data suggest that mechanistically, BMP signaling is integrated by SOX2.

Human blastoid as an in vitro model of human blastocysts

Human development is a highly coordinated process, with any abnormalities during the early embryonic stages that can often have detrimental consequences. The complexity and nuances of human development underpin its significance in embryo research. However, this research is often hindered by limited availability and ethical considerations associated with the use of donated blastocysts from in vitro fertilization (IVF) surplus. Human blastoids offer promising alternatives as they can be easily generated and manipulated in the laboratory while preserving key characteristics of human blastocysts. In this way, they hold the potential to serve as a scalable and ethically permissible resource in embryology research. By utilizing such human embryo models, we can establish a transformative platform that complements the study with IVF embryos, ultimately enhancing our understanding of human embryogenesis.

Stem Cell-Derived Microfluidic Amniotic Sac Embryoid (μPASE)

The microfluidic amniotic sac embryoid (μPASE) is a human pluripotent stem cell (hPSC)-derived multicellular human embryo-like structure with molecular and morphological features resembling the progressive development of the early post-implantation human embryonic sac. The microfluidic device is specifically designed to control the formation of hPSC clusters and expose the clusters to different morphogen environments, allowing the development of μPASEs in a highly controllable, reproducible, and scalable fashion. The μPASE model displays human embryonic developmental landmarks such as lumenogenesis of the epiblast, amniotic cavity formation, and the specification of primordial germ cells and gastrulating cells (or mesendoderm cells). Here, we provide detailed instructions needed to reproduce μPASEs, including the immunofluorescence staining and cell retrieval protocols for characterizing μPASEs obtained under different experimental conditions.

Generation of Human Blastoids from Naive Pluripotent Stem Cells

Under certain culture conditions, naive human pluripotent stem cells can generate human blastocyst-like structures (called human blastoids). Human blastoids serve as an accessible model for human blastocysts and are amenable for large-scale production. Here, we describe a detailed step-by-step protocol for the robust and high-efficient generation of human blastoids from naive human pluripotent stem cells.

The many dimensions of germline competence

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

Exploring the promising potential of induced pluripotent stem cells in cancer research and therapy

The advent of iPSCs has brought about a significant transformation in stem cell research, opening up promising avenues for advancing cancer treatment. The formation of cancer is a multifaceted process influenced by genetic, epigenetic, and environmental factors. iPSCs offer a distinctive platform for investigating the origin of cancer, paving the way for novel approaches to cancer treatment, drug testing, and tailored medical interventions. This review article will provide an overview of the science behind iPSCs, the current limitations and challenges in iPSC-based cancer therapy, the ethical and social implications, and the comparative analysis with other stem cell types for cancer treatment. The article will also discuss the applications of iPSCs in tumorigenesis, the future of iPSCs in tumorigenesis research, and highlight successful case studies utilizing iPSCs in tumorigenesis research. The conclusion will summarize the advancements made in iPSC-based tumorigenesis research and the importance of continued investment in iPSC research to unlock the full potential of these cells.

Intelligent Vascularized 3D/4D/5D/6D-Printed Tissue Scaffolds

Blood vessels are essential for nutrient and oxygen delivery and waste removal. Scaffold-repairing materials with functional vascular networks are widely used in bone tissue engineering. Additive manufacturing is a manufacturing technology that creates three-dimensional solids by stacking substances layer by layer, mainly including but not limited to 3D printing, but also 4D printing, 5D printing and 6D printing. It can be effectively combined with vascularization to meet the needs of vascularized tissue scaffolds by precisely tuning the mechanical structure and biological properties of smart vascular scaffolds. Herein, the development of neovascularization to vascularization to bone tissue engineering is systematically discussed in terms of the importance of vascularization to the tissue. Additionally, the research progress and future prospects of vascularized 3D printed scaffold materials are highlighted and presented in four categories: functional vascularized 3D printed scaffolds, cell-based vascularized 3D printed scaffolds, vascularized 3D printed scaffolds loaded with specific carriers and bionic vascularized 3D printed scaffolds. Finally, a brief review of vascularized additive manufacturing-tissue scaffolds in related tissues such as the vascular tissue engineering, cardiovascular system, skeletal muscle, soft tissue and a discussion of the challenges and development efforts leading to significant advances in intelligent vascularized tissue regeneration is presented.

Local cellular interactions during the self-organization of stem cells

Stem cell models for early mammalian development offer new experimental opportunities to access spatio-temporal details of the cell-cell interactions that govern cell differentiation and tissue patterning. This review summarizes recent studies that have used stem cell models to investigate the spatial range of developmental cell-cell communication systems. A key message from these works is that important biochemical signals for cell differentiation in these systems, such as Nodal and fibroblast growth factors (FGFs), often act over short distances of only a few cell diameters. The formation of long-range patterns at the tissue scale associated with these signals then results from signal relays and cell rearrangements. The modular view of differentiation and patterning emerging from research on stem cell models can offer a fresh perspective on the corresponding processes in the embryo.

Pathway to Independence: the future of developmental biology

In 2022, Development launched its Pathway to Independence (PI) Programme, aimed at supporting postdocs as they transition to their first independent position. We selected eight talented researchers as the first cohort of PI Fellows. In this article, each of our Fellows provides their perspective on the future of their field. Together, they paint an exciting picture of the current state of and open questions in developmental biology.

Mechanically enhanced biogenesis of gut spheroids with instability-driven morphomechanics

Region-specific gut spheroids are precursors for gastrointestinal and pulmonary organoids that hold great promise for fundamental studies and translations. However, efficient production of gut spheroids remains challenging due to a lack of control and mechanistic understanding of gut spheroid morphogenesis. Here, we report an efficient biomaterial system, termed micropatterned gut spheroid generator (μGSG), to generate gut spheroids from human pluripotent stem cells through mechanically enhanced tissue morphogenesis. We show that μGSG enhances the biogenesis of gut spheroids independent of micropattern shape and size; instead, mechanically enforced cell multilayering and crowding is demonstrated as a general, geometry-insensitive mechanism that is necessary and sufficient for promoting spheroid formation. Combining experimental findings and an active-phase-field morphomechanics theory, our study further reveals an instability-driven mechanism and a mechanosensitive phase diagram governing spheroid pearling and fission in μGSG. This work unveils mechanobiological paradigms based on tissue architecture and surface tension for controlling tissue morphogenesis and advancing organoid technology.

In Vitro Embryogenesis and Gastrulation Using Stem Cells in Mice and Humans

During early mammalian embryonic development, fertilized one-cell embryos develop into pre-implantation blastocysts and subsequently establish three germ layers through gastrulation during post-implantation development. In recent years, stem cells have emerged as a powerful tool to study embryogenesis and gastrulation without the need for eggs, allowing for the generation of embryo-like structures known as synthetic embryos or embryoids. These in vitro models closely resemble early embryos in terms of morphology and gene expression and provide a faithful recapitulation of early pre- and post-implantation embryonic development. Synthetic embryos can be generated through a combinatorial culture of three blastocyst-derived stem cell types, such as embryonic stem cells, trophoblast stem cells, and extraembryonic endoderm cells, or totipotent-like stem cells alone. This review provides an overview of the progress and various approaches in studying in vitro embryogenesis and gastrulation in mice and humans using stem cells. Furthermore, recent findings and breakthroughs in synthetic embryos and gastruloids are outlined. Despite ethical considerations, synthetic embryo models hold promise for understanding mammalian (including humans) embryonic development and have potential implications for regenerative medicine and developmental research.

Population-level antagonism between FGF and BMP signaling steers mesoderm differentiation in embryonic stem cells

The mesodermal precursor populations for different internal organ systems are specified during gastrulation by the combined activity of extracellular signaling systems such as BMP, Wnt, Nodal and FGF. The BMP, Wnt and Nodal signaling requirements for the differentiation of specific mesoderm subtypes in mammals have been mapped in detail, but how FGF shapes mesodermal cell type diversity is not precisely known. It is also not clear how FGF signaling integrates with the activity of other signaling systems involved in mesoderm differentiation. Here, we address these questions by analyzing the effects of targeted signaling manipulations in differentiating stem cell populations at single-cell resolution. We identify opposing functions of BMP and FGF, and map FGF-dependent and -independent mesodermal lineages. Stimulation with exogenous FGF boosts the expression of endogenous Fgf genes while repressing Bmp ligand genes. This positive autoregulation of FGF signaling, coupled with the repression of BMP signaling, may contribute to the specification of reproducible and coherent cohorts of cells with the same identity via a community effect, both in the embryo and in synthetic embryo-like systems.

Shifting early embryology paradigms: Applications of stem cell-based embryo models in bioengineering

Technologies to reproduce specific aspects of early mammalian embryogenesis in vitro using stem cells have skyrocketed over the last several years. With these advances, we have gained new perspectives on how embryonic and extraembryonic cells self-organize to form the embryo. These reductionist approaches hold promise for the future implementation of precise environmental and genetic controls to understand variables affecting embryo development. Our review discusses recent progress in cellular models of early mammalian embryo development and bioengineering advancements that can be leveraged to study the embryo-maternal interface. We summarize current gaps in the field, emphasizing the importance of understanding how intercellular interactions at this interface contribute to reproductive and developmental health.

Technical challenges of studying early human development

Recent years have seen exciting progress across human embryo research, including new methods for culturing embryos, transcriptional profiling of embryogenesis and gastrulation, mapping lineage trajectories, and experimenting on stem cell-based embryo models. These advances are beginning to define the dynamical principles of development across stages, tissues and organs, enabling a better understanding of human development before birth in health and disease, and potentially leading to improved treatments for infertility and developmental disorders. However, there are still significant roadblocks en route to this goal. Here, we highlight technical challenges to studying early human development and propose ways and means to overcome some of these constraints.

Dynamic and self-biodegradable polysaccharide hydrogel stores embryonic stem cell construct under ambient condition

The proper microenvironment is critical for the storage and transportation of embryonic stem cells (ESCs). To mimic a dynamic 3D microenvironment as it exists in vivo and consider "off-the-shelf" availability reaching the destination, we proposed an alternative approach that allows for facile storage and transportation of stem cells in the form of ESCs-dynamic hydrogel construct (CDHC) under ambient conditions. To form CDHC, mouse embryonic stem cells (mESCs) were in-situ encapsulated within a polysaccharide-based dynamic and self-biodegradable hydrogel. After storing CDHC in a sterile and hermetic environment for 3 days and then transferring to a sealed vessel with fresh medium for another 3 days, the large and compact colonies retained a 90% survival rate and pluripotency. Furthermore, after transporting and arriving at the destination, the encapsulated stem cell could be automatically released from the self-biodegradable hydrogel. After continuous cultivation of 15 generations of retrieved cells, automatically released from the CDHC, the mESCs underwent 3D encapsulation, storage, transportation, release, and continuous long-term subculture; resumed colony forming capacity and pluripotency were revealed by stem cell markers both in protein and mRNA levels. We believe that the dynamic and self-biodegradable hydrogel provides a simple, cost-effective, and valuable tool for storing and transporting "ready-to-use" CDHC under ambient conditions, facilitating "off-the-shelf" availability and widespread applications.

Generating human blastoids modeling blastocyst-stage embryos and implantation

Human early development sets the stage for embryonic and adult life but remains difficult to investigate. A solution came from the ability of stem cells to organize into structures resembling preimplantation embryos-blastocysts-that we termed blastoids. This embryo model is available in unlimited numbers and could thus support scientific and medical advances. However, its predictive power depends on how faithfully it recapitulates the blastocyst. Here, we describe how we formed human blastoids that (1) efficiently achieve the morphology of the blastocyst and (2) form lineages according to the pace and sequence of blastocyst development, (3) ultimately forming cells that transcriptionally reflect the blastocyst (preimplantation stage). We employ three different commercially available 96- and 24-well microwell plates with results similar to our custom-made ones, and show that blastoids form in clinical in vitro fertilization medium and can be cryopreserved for shipping. Finally, we explain how blastoids replicate the directional process of implantation into endometrial organoids, specifically when these are hormonally stimulated. It takes 4 d for human blastoids to form and 10 d to prepare the endometrial implantation assay, and we have cultured blastoids up to 6 d (time-equivalent of day 13). On the basis of our experience, we anticipate that a person with ~1 year of human pluripotent stem cell culture experience and of organoid culture should be able to perform the protocol. Altogether, blastoids offer an opportunity to establish scientific and biomedical discovery programs for early pregnancy, and an ethical alternative to the use of embryos.

The time integral of BMP signaling determines fate in a stem cell model for early human development

How paracrine signals are interpreted to yield multiple cell fate decisions in a dynamic context during human development in vivo and in vitro remains poorly understood. Here we report an automated tracking method to follow signaling histories linked to cell fate in large numbers of human pluripotent stem cells (hPSCs). Using an unbiased statistical approach, we discovered that measured BMP signaling history correlates strongly with fate in individual cells. We found that BMP response in hPSCs varies more strongly in the duration of signaling than the level. However, we discovered that both the level and duration of signaling activity control cell fate choices only by changing the time integral of signaling and that duration and level are therefore interchangeable in this context. In a stem cell model for patterning of the human embryo, we showed that signaling histories predict the fate pattern and that the integral model correctly predicts changes in cell fate domains when signaling is perturbed. Using an RNA-seq screen we then found that mechanistically, BMP signaling is integrated by SOX2.

Dissecting embryonic and extra-embryonic lineage crosstalk with stem cell co-culture

Faithful embryogenesis requires precise coordination between embryonic and extraembryonic tissues. Although stem cells from embryonic and extraembryonic origins have been generated for several mammalian species(Bogliotti et al., 2018; Choi et al., 2019; Cui et al., 2019; Evans and Kaufman, 1981; Kunath et al., 2005; Li et al., 2008; Martin, 1981; Okae et al., 2018; Tanaka et al., 1998; Thomson et al., 1998; Vandevoort et al., 2007; Vilarino et al., 2020; Yu et al., 2021b; Zhong et al., 2018), they are grown in different culture conditions with diverse media composition, which makes it difficult to study cross-lineage communication. Here, by using the same culture condition that activates FGF, TGF-β and WNT signaling pathways, we derived stable embryonic stem cells (ESCs), extraembryonic endoderm stem cells (XENs) and trophoblast stem cells (TSCs) from all three founding tissues of mouse and cynomolgus monkey blastocysts. This allowed us to establish embryonic and extraembryonic stem cell co-cultures to dissect lineage crosstalk during early mammalian development. Co-cultures of ESCs and XENs uncovered a conserved and previously unrecognized growth inhibition of pluripotent cells by extraembryonic endoderm cells, which is in part mediated through extracellular matrix signaling. Our study unveils a more universal state of stem cell self-renewal stabilized by activation, as opposed to inhibition, of developmental signaling pathways. The embryonic and extraembryonic stem cell co-culture strategy developed here will open new avenues for creating more faithful embryo models and developing more developmentally relevant differentiation protocols.

Progress and challenges in directing the differentiation of human iPSCs into spinal motor neurons

Motor neuron (MN) diseases, including amyotrophic lateral sclerosis, progressive bulbar palsy, primary lateral sclerosis and spinal muscular atrophy, cause progressive paralysis and, in many cases, death. A better understanding of the molecular mechanisms of pathogenesis is urgently needed to identify more effective therapies. However, studying MNs has been extremely difficult because they are inaccessible in the spinal cord. Induced pluripotent stem cells (iPSCs) can generate a theoretically limitless number of MNs from a specific patient, making them powerful tools for studying MN diseases. However, to reach their potential, iPSCs need to be directed to efficiently differentiate into functional MNs. Here, we review the reported differentiation protocols for spinal MNs, including induction with small molecules, expression of lineage-specific transcription factors, 2-dimensional and 3-dimensional cultures, as well as the implementation of microfluidics devices and co-cultures with other cell types, including skeletal muscle. We will summarize the advantages and disadvantages of each strategy. In addition, we will provide insights into how to address some of the remaining challenges, including reproducibly obtaining mature and aged MNs.

Bead-jet printing enabled sparse mesenchymal stem cell patterning augments skeletal muscle and hair follicle regeneration

Transplantation of mesenchymal stem cells (MSCs) holds promise to repair severe traumatic injuries. However, current transplantation practices limit the potential of this technique, either by losing the viable MSCs or reducing the performance of resident MSCs. Herein, we design a "bead-jet" printer, specialized for high-throughput intra-operative formulation and printing of MSCs-laden Matrigel beads. We show that high-density encapsulation of MSCs in Matrigel beads is able to augment MSC function, increasing MSC proliferation, migration, and extracellular vesicle production, compared with low-density bead or high-density bulk encapsulation of the equivalent number of MSCs. We find that the high-density MSCs-laden beads in sparse patterns demonstrate significantly improved therapeutic performance, by regenerating skeletal muscles approaching native-like cell density with reduced fibrosis, and regenerating skin with hair follicle growth and increased dermis thickness. MSC proliferation within 1-week post-transplantation and differentiation at 3 - 4 weeks post-transplantation are suggested to contribute therapy augmentation. We expect this "bead-jet" printing system to strengthen the potential of MSC transplantation.

Human Maternal-Fetal Interface Cellular Models to Assess Antiviral Drug Toxicity during Pregnancy

Pregnancy is a period of elevated risk for viral disease severity, resulting in serious health consequences for both the mother and the fetus; yet antiviral drugs lack comprehensive safety and efficacy data for use among pregnant women. In fact, pregnant women are systematically excluded from therapeutic clinical trials to prevent potential fetal harm. Current FDA-recommended reproductive toxicity assessments are studied using small animals which often do not accurately predict the human toxicological profiles of drug candidates. Here, we review the potential of human maternal-fetal interface cellular models in reproductive toxicity assessment of antiviral drugs. We specifically focus on the 2- and 3-dimensional maternal placental models of different gestational stages and those of fetal embryogenesis and organ development. Screening of drug candidates in physiologically relevant human maternal-fetal cellular models will be beneficial to prioritize selection of safe antiviral therapeutics for clinical trials in pregnant women.

Regulatory of miRNAs in tri-lineage differentiation of C3H10T1/2

MicroRNAs (miRNAs) are non-coding single-stranded RNA molecules encoded by endogenous genes, which play a vital role in cell generation, metabolism, apoptosis and stem cell differentiation. C3H10T1/2, a mesenchymal cell extracted from mouse embryos, is capable of osteogenic differentiation, adipogenic differentiation and chondrogenic differentiation. Extensive studies have shown that not only miRNAs can directly trigger targeted genes to regulate the tri-lineage differentiation of C3H10T1/2, but it also can indirectly regulate the differentiation by triggering different signaling pathways or various downstream molecules. This paper aims to clarify the regulatory roles of different miRNAs on C3H10T1/2 differentiation, and discussing their balance effect among osteogenic differentiation, adipogenic differentiation and chondrogenic differentiation of C3H10T1/2. We also review the biogenesis of miRNAs, Wnt signaling pathways, MAPK signaling pathways and BMP signaling pathways and provide some specific examples of how these signaling pathways act on C3H10T1/2 tri-lineage differentiation. On this basis, we hope that a deeper understanding of the differentiation and regulation mechanism of miRNAs in C3H10T1/2 can provide a promising therapeutic method for the clinical treatment of bone defects, osteoporosis, osteoarthritis and other diseases.

Serial Gene Expression Profiling of Neural Stem Cells Shows Transcriptome Switch by Long-Term Physioxia from Metabolic Adaption to Cell Signaling Profile

Oxygen is an essential factor in the cellular microenvironment with pivotal effects on neural development with a particular sensitivity of midbrain neural stem cells (NSCs) to high atmospheric oxygen tension. However, most experiments are still performed at atmospheric O₂ levels (21%, normoxia), whereas mammalian brain tissue is physiologically exposed to substantially lower O₂ tensions around 3% (physioxia). We here performed serial Affymetrix gene array analyses to detect expression changes in mouse fetal NSCs from both midbrain and cortical tissues when kept at physioxia compared to normoxia. We identified more than 400 O₂-regulated genes involved in cellular metabolism, cell proliferation/differentiation, and various signaling pathways. NSCs from both regions showed a low number but high conformity of regulated genes (9 genes in midbrain vs. 34 in cortical NSCs; 8 concordant expression changes) after short-term physioxia (2 days) with metabolic processes and cellular processes being the most prominent GO categories pointing to cellular adaption to lower oxygen levels. Gene expression profiles changed dramatically after long-term physioxia (13 days) with a higher number of regulated genes and more diverse expression patterns when comparing the two NSC types (338 genes in midbrain vs. 121 in cortical NSCs; 75 concordant changes). Most prominently, we observed a reduction of hits in metabolic processes but an increase in biological regulation and signaling pointing to a switch towards signaling processes and stem cell maintenance. Our data may serve as a basis for identifying potential signaling pathways that maintain stem cell characteristics in cortical versus midbrain physioxic stem cell niches.

Carnegie in 4D? Stem-cell-based models of human embryo development

How cells build embryos is still a major mystery. Many unresolved questions require the study of the processes that pattern and shape the embryo in live specimens, in toto, across spatial and temporal scales. In mammalian embryogenesis, this remains a major challenge as the embryo develops in utero, precluding easy accessibility. For human embryos, technical, ethical and legal limitations further hamper the in-depth investigation of embryogenesis, especially beyond gastrulation stages. This has resulted in an over-reliance on model organisms, particularly mice, to understand mammalian development. However, recent efforts show critical differences between rodent and primate embryos, including timing, architecture and transcriptional regulation. Thus, a human-centric understanding of embryogenesis is much needed. To empower this, novel in vitro approaches, which coax human pluripotent stem cells to form embryonic organoids that model embryo development, are pivotal. Here, we summarize these emergent technologies that recapitulate aspects of human development "in a dish". We show how these technologies can provide insights into the molecular, cellular and morphogenetic processes that fuel the formation of a fully formed fetus, and discuss the potential of these platforms to revolutionize our understanding of human development in health and disease. Despite their clear promise, we caution against over-interpreting the extent to which these in vitro platforms model the natural embryo. In particular, we discuss how fate, form and function - a tightly coupled trinity in vivo, can be disconnected in vitro. Finally, we propose how careful benchmarking of existing models, in combination with rational protocol design based on an increased understanding of in vivo developmental dynamics and insights from mouse in vitro models of embryo development, will help guide the establishment of better models of human embryo development.

Approaches to benchmark and characterize in vitro human model systems

In vitro human models, such as gastruloids and organoids, are complex three-dimensional (3D) structures often consist of cells from multiple germ layers that possess some attributes of a developing embryo or organ. To use these models to interrogate human development and organogenesis, these 3D models must accurately recapitulate aspects of their in vivo counterparts. Recent advances in single-cell technologies, including sequencing and spatial approaches, have enabled efforts to better understand and directly compare organoids with native tissues. For example, single-cell genomic efforts have created cell and organ atlases that enable benchmarking of in vitro models and can also be leveraged to gain novel biological insights that can be used to further improve in vitro models. This Spotlight discusses the state of current in vitro model systems, the efforts to create large publicly available atlases of the developing human and how these data are being used to improve organoids. Limitations and perspectives on future efforts are also discussed.

Stem cell-based models of early mammalian development

The complex process by which a single-celled zygote develops into a viable embryo is nothing short of a miraculous wonder of the natural world. Elucidating how this process is orchestrated in humans has long eluded the grasp of scientists due to ethical and practical limitations. Thankfully, pluripotent stem cells that resemble early developmental cell types possess the ability to mimic specific embryonic events. As such, murine and human stem cells have been leveraged by scientists to create in vitro models that aim to recapitulate different stages of early mammalian development. Here, we examine the wide variety of stem cell-based embryo models that have been developed to recapitulate and study embryonic events, from pre-implantation development through to early organogenesis. We discuss the applications of these models, key considerations regarding their importance within the field, and how such models are expected to grow and evolve to achieve exciting new milestones in the future.