A combined human gastruloid model of cardiogenesis and neurogenesis

Matrigel inhibits elongation and drives endoderm differentiation in aggregates of mouse embryonic stem cells

Modelling peri-implantation mammalian development using the self-organising properties of stem cells is a rapidly growing field that has advanced our understanding of cell fate decisions occurring in the early embryo. Matrigel, a basement membrane matrix, is a critical substrate used in various protocols for its efficacy in promoting stem cell growth and self-organisation. However, its role in driving stem cell lineage commitment, and whether this effect is driven by biochemical or physical cues, is not currently clear. Here, we grow embryoid bodies in suspension, Matrigel and agarose, an inert polysaccharide, to attempt to decouple the physical and biochemical roles of Matrigel and better understand how it drives stem cell differentiation. We use a combination of light microscopy, quantitative PCR and immunostaining to investigate gene and protein changes in our different culture conditions. We show that stem cell aggregates in Matrigel are hindered in their ability to elongate compared with those grown in agarose or in suspension, indicating that prohibitive role in self-organisation. Aggregates in Matrigel are also driven to differentiate into endoderm, with ectoderm differentiation inhibited. Furthermore, these effects are not due to the physical presence of Matrigel, as the same effects are not witnessed in aggregates grown in agarose. Our results thus indicate that Matrigel has a significant and complex effect on the differentiation and morphology of embryoid bodies.

Identifying cross-lineage dependencies of cell-type-specific regulators in mouse gastruloids

Correct gene expression levels are crucial for normal development. Advances in genomics enable the inference of gene regulatory programs active during development but cannot capture the complex multicellular interactions occurring during mammalian embryogenesis in utero. In vitro models of mammalian development, like gastruloids, can overcome this limitation. Using time-resolved single-cell chromatin accessibility analysis, we delineated the regulatory profile during mouse gastruloid development, identifying critical drivers of developmental transitions. Gastruloids develop from bipotent progenitor cells driven by the transcription factors (TFs) OCT4, SOX2, and TBXT, differentiating into the mesoderm (characterized by the mesogenin 1 [MSGN1]) and spinal cord (characterized by CDX2). ΔCDX gastruloids fail to form spinal cord, while Msgn1 ablation inhibits paraxial mesoderm and spinal cord development. Chimeric gastruloids with ΔMSGN1 and wild-type cells formed both tissues, indicating that inter-tissue communication is necessary for spinal cord formation. Our work has important implications for studying inter-tissue communication and gene regulatory programs in development.

Synthetic embryology of the human heart

The evolution of stem cell-based heart models from cells and tissues to organoids and assembloids and recently synthetic embryology gastruloids, is poised to revolutionize our understanding of cardiac development, congenital to adult diseases, and patient customized therapies. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have already been integrated into transplantable patches and are in preclinical efforts to reverse fibrotic scarring from myocardial infarctions. To inform on the complexity of heart diseases, multi-tissue morphogenic heart models are needed that replicate fundamental components of heart function to heart organogenesis in vitro and which require a deep understanding of heart development. Organoid and assembloid models capture selected multicellular cardiac processes, such as chamber formation and priming events for vascularization. Gastruloid heart models offer deeper insights as synthetic embryology to mimic multi-staged developmental events of in vivo heart organogenesis including established heart fields, crescent formation and heart tube development along with vascular systemic foundation and even further steps. The human Elongating Multi-Lineage Organized Cardiac (EMLOC) gastruloid model captures these stages and additional events including chamber genesis, patterned vascularization, and extrinsic central and intrinsic cardiac nervous system (CNS-ICNS) integration guided by spatiotemporal and morphogenic processes with neural crest cells. Gastruloid synthetic embryology heart models offer new insights into previously hidden processes of development and provide powerful platforms for addressing heart disease that extends beyond cardiomyocytes, such as arrhythmogenic diseases, congenital defects, and systemic injury interactions, as in spinal cord injuries. The holistic view that is emerging will reveal heart development and disease in unprecedented detail to drive transformative state-of-the-art innovative applications for heart health.

Three-dimensional stem cell models of mammalian gastrulation

Gastrulation is a key milestone in the development of an organism. It is a period of cell proliferation and coordinated cellular rearrangement, that creates an outline of the body plan. Our current understanding of mammalian gastrulation has been improved by embryo culture, but there are still many open questions that are difficult to address because of the intrauterine development of the embryos and the low number of specimens. In the case of humans, there are additional difficulties associated with technical and ethical challenges. Over the last few years, pluripotent stem cell models are being developed that have the potential to become useful tools to understand the mammalian gastrulation. Here we review these models with a special emphasis on gastruloids and provide a survey of the methods to produce them robustly, their uses, relationship to embryos, and their prospects as well as their limitations.

In Vitro Models of Cardiovascular Disease: Embryoid Bodies, Organoids and Everything in Between

Cardiovascular disease comprises a group of disorders affecting or originating within tissues and organs of the cardiovascular system; most, if not all, will eventually result in cardiomyocyte dysfunction or death, negatively impacting cardiac function. Effective models of cardiac disease are thus important for understanding crucial aspects of disease progression, while recent advancements in stem cell biology have allowed for the use of stem cell populations to derive such models. These include three-dimensional (3D) models such as stem cell-based models of embryos (SCME) as well as organoids, many of which are frequently derived from embryoid bodies (EB). Not only can they recapitulate 3D form and function, but the developmental programs governing the self-organization of cell populations into more complex tissues as well. Many different organoids and SCME constructs have been generated in recent years to recreate cardiac tissue and the complex developmental programs that give rise to its cellular composition and unique tissue morphology. It is thus the purpose of this narrative literature review to describe and summarize many of the recently derived cardiac organoid models as well as their use for the recapitulation of genetic and acquired disease. Owing to the cellular composition of the models examined, this review will focus on disease and tissue injury associated with embryonic/fetal tissues.

Gastruloids are competent to specify both cardiac and skeletal muscle lineages

Cardiopharyngeal mesoderm contributes to the formation of the heart and head muscles. However, the mechanisms governing cardiopharyngeal mesoderm specification remain unclear. Here, we reproduce cardiopharyngeal mesoderm specification towards cardiac and skeletal muscle lineages with gastruloids from mouse embryonic stem cells. By conducting a comprehensive temporal analysis of cardiopharyngeal mesoderm development and differentiation in gastruloids compared to mouse embryos, we present the evidence for skeletal myogenesis in gastruloids. We identify different subpopulations of cardiomyocytes and skeletal muscles, the latter of which most likely correspond to different states of myogenesis with "head-like" and "trunk-like" skeletal myoblasts. In this work, we unveil the potential of gastruloids to undergo specification into both cardiac and skeletal muscle lineages, allowing the investigation of the mechanisms of cardiopharyngeal mesoderm differentiation in development and how this could be affected in congenital diseases.

Generation of self-renewing neuromesodermal progenitors with neuronal and skeletal muscle bipotential from human embryonic stem cells

Progress has been made in generating spinal cord and trunk derivatives from neuromesodermal progenitors (NMPs). However, maintaining the self-renewal of NMPs in vitro remains a challenge. In this study, we developed a cocktail of small molecules and growth factors that induces human embryonic stem cells to produce self-renewing NMPs (srNMPs) under chemically defined conditions. These srNMPs maintain the state of neuromesodermal progenitors in prolonged culture and have the potential to generate mesodermal cells and neurons, even at the single-cell level. Additionally, suspended srNMP aggregates can spontaneously differentiate into all tissue types of early embryonic trunks. Furthermore, transplanted srNMP-derived muscle satellite cells or progenitors of motor neurons were integrated into skeletal muscle or the spinal cord, respectively, and contributed to regeneration in mouse models. In summary, srNMPs hold great promise for applications in developmental biology and as renewable cell sources for cell therapy for trunk and spinal cord injuries.

Cardiac Development and Factors Influencing the Development of Congenital Heart Defects (CHDs): Part I

The traditional description of cardiac development involves progression from a cardiac crescent to a linear heart tube, which in the phase of transformation into a mature heart forms a cardiac loop and is divided with the septa into individual cavities. Cardiac morphogenesis involves numerous types of cells originating outside the initial cardiac crescent, including neural crest cells, cells of the second heart field origin, and epicardial progenitor cells. The development of the fetal heart and circulatory system is subject to regulatation by both genetic and environmental processes. The etiology for cases with congenital heart defects (CHDs) is largely unknown, but several genetic anomalies, some maternal illnesses, and prenatal exposures to specific therapeutic and non-therapeutic drugs are generally accepted as risk factors. New techniques for studying heart development have revealed many aspects of cardiac morphogenesis that are important in the development of CHDs, in particular transposition of the great arteries.

Three-dimensional cardiac models: a pre-clinical testing platform

Major advancements in human pluripotent stem cell (hPSC) technology over recent years have yielded valuable tools for cardiovascular research. Multi-cell type 3-dimensional (3D) cardiac models in particular, are providing complementary approaches to animal studies that are better representatives than simple 2-dimensional (2D) cultures of differentiated hPSCs. These human 3D cardiac models can be broadly divided into two categories; namely those generated through aggregating pre-differentiated cells and those that form self-organizing structures during their in vitro differentiation from hPSCs. These models can either replicate aspects of cardiac development or enable the examination of interactions among constituent cell types, with some of these models showing increased maturity compared with 2D systems. Both groups have already emerged as physiologically relevant pre-clinical platforms for studying heart disease mechanisms, exhibiting key functional attributes of the human heart. In this review, we describe the different cardiac organoid models derived from hPSCs, their generation methods, applications in cardiovascular disease research and use in drug screening. We also address their current limitations and challenges as pre-clinical testing platforms and propose potential improvements to enhance their efficacy in cardiac drug discovery.

Exploring the prospects, advancements, and challenges of in vitro modeling of the heart-brain axis

Research on bidirectional communication between the heart and brain has often relied on studies involving nonhuman animals. Dependance on animal models offer limited applicability to humans and a lack of high-throughput screening. Recently, the field of 3D cell biology, specifically organoid technology, has rapidly emerged as a valuable tool for studying interactions across organ systems, i.e., gut-brain axis. The initial success of organoid models indicates the usefulness of 3D cultures for elucidating the intricate interactivity of the autonomic nervous system and overall health. This perspective aims to explore the potential of advancing in vitro modeling of the heart-brain axis by discussing the benefits, applications, and adaptability of organoid technologies. We closely examine the current state of brain organoids in conjunction with the advancements of cardiac organoids. Moreover, we explore the use of combined organoid systems to investigate pathophysiology and provide a platform for treatment discovery. Finally, we address the challenges that accompany the use of 3D models for studying the heart-brain axis with an emphasis on generating tailored engineering strategies for further refinement of dynamic organ system modeling in vitro.

Proposal for considerations during human iPSC-derived cardiac organoid generation for cardiotoxicity drug testing

Human iPSC-derived cardiac organoids (hiPSC-COs) for cardiotoxicity drug testing via the variety of cell lines and unestablished protocols may lead to differences in response results due to a lack of criteria for generation period and size. To ensure reliable drug testing, it is important for researchers to set optimal generation period and size of COs according to the cell line and protocol applied in their studies. Hence, we sought to propose a process to establish minimum criteria for the generation duration and size of hiPSC-COs for cardiotoxic drug testing. We generated hiPSC-COs of different sizes based on our protocol and continuously monitored organoids until they indicated a minimal beating rate change as a control that could lead to more accurate beating rate changes on drug testing. Calcium transients and physiological tests to assess the functionality of hiPSC-COs on selected generation period, which showed regular cardiac beating, and immunostaining assays to compare characteristics were performed. We explained the generation period and size that exhibited and maintained regular beating rate changes on hiPSC-COs, and lead to reliable response results to cardiotoxicity drugs. We anticipate that this study will offer valuable insights into considering the appropriate generation period and size of hiPSC-COs ensuring reliable outcomes in cardiotoxicity drug testing.

Engineered platforms for mimicking cardiac development and drug screening

Congenital heart defects are associated with significant health challenges, demanding a deep understanding of the underlying biological mechanisms and, thus, better devices or platforms that can recapitulate human cardiac development. The discovery of human pluripotent stem cells has substantially reduced the dependence on animal models. Recent advances in stem cell biology, genetic editing, omics, microfluidics, and sensor technologies have further enabled remarkable progress in the development of in vitro platforms with increased fidelity and efficiency. In this review, we provide an overview of advancements in in vitro cardiac development platforms, with a particular focus on technological innovation. We categorize these platforms into four areas: two-dimensional solid substrate cultures, engineered substrate architectures that enhance cellular functions, cardiac organoids, and embryos/explants-on-chip models. We conclude by addressing current limitations and presenting future perspectives.

Cardiac construction-Recent advances in morphological and transcriptional modeling of early heart development

During human embryonic development the early establishment of a functional heart is vital to support the growing fetus. However, forming the embryonic heart is an extremely complex process, requiring spatiotemporally controlled cell specification and differentiation, tissue organization, and coordination of cardiac function. These complexities, in concert with the early and rapid development of the embryonic heart, mean that understanding the intricate interplay between these processes that help shape the early heart remains highly challenging. In this review I focus on recent insights from animal models that have shed new light on the earliest stages of heart development. This includes specification and organization of cardiac progenitors, cell and tissue movements that make and shape the early heart tube, and the initiation of the first beat in the developing heart. In addition I highlight relevant in vitro models that could support translation of findings from animal models to human heart development. Finally I discuss challenges that are being addressed in the field, along with future considerations that together may help move us towards a deeper understanding of how our hearts are made.

The new era of cardiovascular research: revolutionizing cardiovascular research with 3D models in a dish

Cardiovascular research has heavily relied on studies using patient samples and animal models. However, patient studies often miss the data from the crucial early stage of cardiovascular diseases, as obtaining primary tissues at this stage is impracticable. Transgenic animal models can offer some insights into disease mechanisms, although they usually do not fully recapitulate the phenotype of cardiovascular diseases and their progression. In recent years, a promising breakthrough has emerged in the form of in vitro three-dimensional (3D) cardiovascular models utilizing human pluripotent stem cells. These innovative models recreate the intricate 3D structure of the human heart and vessels within a controlled environment. This advancement is pivotal as it addresses the existing gaps in cardiovascular research, allowing scientists to study different stages of cardiovascular diseases and specific drug responses using human-origin models. In this review, we first outline various approaches employed to generate these models. We then comprehensively discuss their applications in studying cardiovascular diseases by providing insights into molecular and cellular changes associated with cardiovascular conditions. Moreover, we highlight the potential of these 3D models serving as a platform for drug testing to assess drug efficacy and safety. Despite their immense potential, challenges persist, particularly in maintaining the complex structure of 3D heart and vessel models and ensuring their function is comparable to real organs. However, overcoming these challenges could revolutionize cardiovascular research. It has the potential to offer comprehensive mechanistic insights into human-specific disease processes, ultimately expediting the development of personalized therapies.

Advances in the Generation of Constructed Cardiac Tissue Derived from Induced Pluripotent Stem Cells for Disease Modeling and Therapeutic Discovery

The human heart lacks significant regenerative capacity; thus, the solution to heart failure (HF) remains organ donation, requiring surgery and immunosuppression. The demand for constructed cardiac tissues (CCTs) to model and treat disease continues to grow. Recent advances in induced pluripotent stem cell (iPSC) manipulation, CRISPR gene editing, and 3D tissue culture have enabled a boom in iPSC-derived CCTs (iPSC-CCTs) with diverse cell types and architecture. Compared with 2D-cultured cells, iPSC-CCTs better recapitulate heart biology, demonstrating the potential to advance organ modeling, drug discovery, and regenerative medicine, though iPSC-CCTs could benefit from better methods to faithfully mimic heart physiology and electrophysiology. Here, we summarize advances in iPSC-CCTs and future developments in the vascularization, immunization, and maturation of iPSC-CCTs for study and therapy.

Embryonic Spinal Cord Innervation in Human Trunk Organogenesis Gastruloids: Cardiac Versus Enteric Customization and Beyond

Trunk-biased human gastruloids provide the ability to couple developmentally relevant spinal neurogenesis and organ morphogenesis via spatiotemporal self-organization events from derivatives of the three germ layers. The multi-lineage nature of gastruloids provides the full complexity of regulatory signaling cues that surpasses directed organoids and lays the foundation for an ex vivo self-evolving system. Here we detail two distinct protocols for trunk-biased gastruloids from an elongated, polarized structure with coordinated organ-specific neural patterning. Following an induction phase to caudalize iPSCs to trunk phenotype, divergent features of organogenesis and end-organ innervation yield separate models of enteric and cardiac nervous system formation. Both protocols are permissive to multi-lineage development and allow the study of neural integration events within a native, embryo-like context. We discuss the customizability of human gastruloids and the optimization of initial and extended conditions that maintain a permissive environment for multi-lineage differentiation and integration.

A new era of stem cell and developmental biology: from blastoids to synthetic embryos and beyond

Recent discoveries in stem cell and developmental biology have introduced a new era marked by the generation of in vitro models that recapitulate early mammalian development, providing unprecedented opportunities for extensive research in embryogenesis. Here, we present an overview of current techniques that model early mammalian embryogenesis, specifically noting models created from stem cells derived from two significant species: Homo sapiens, for its high relevance, and Mus musculus, a historically common and technically advanced model organism. We aim to provide a holistic understanding of these in vitro models by tracing the historical background of the progress made in stem cell biology and discussing the fundamental underlying principles. At each developmental stage, we present corresponding in vitro models that recapitulate the in vivo embryo and further discuss how these models may be used to model diseases. Through a discussion of these models as well as their potential applications and future challenges, we hope to demonstrate how these innovative advances in stem cell research may be further developed to actualize a model to be used in clinical practice.

Wnt and BMP signalling direct anterior-posterior differentiation in aggregates of mouse embryonic stem cells

Stem-cell-based embryo models have allowed greater insight into peri-implantation mammalian developmental events that are otherwise difficult to manipulate due to the inaccessibility of the early embryo. The rapid development of this field has resulted in the precise roles of frequently used supplements such as N2, B27 and Chiron in driving stem cell lineage commitment not being clearly defined. Here, we investigate the effects of these supplements on embryoid bodies to better understand their roles in stem cell differentiation. We show that Wnt signalling has a general posteriorising effect on stem cell aggregates and directs differentiation towards the mesoderm, as confirmed through the upregulation of posterior and mesodermal markers. N2 and B27 can mitigate these effects and upregulate the expression of anterior markers. To control the Wnt gradient and the subsequent anterior versus posterior fate, we make use of a BMP4 signalling centre and show that aggregates in these conditions express cephalic markers. These findings indicate that there is an intricate balance between various culture supplements and their ability to guide differentiation in stem cell embryo models.

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.

Advancing organoid design through co-emergence, assembly, and bioengineering

Human adult stem cells and patient-derived induced pluripotent stem cells represent promising tools to understand human biology, development, and disease. Under a permissive environment, stem cell derivatives can self-organize and reconstruct their native milieu, resulting in the creation of organ-like entities known as organoids. Although organoids represent a breakthrough in the stem cell field, there are still considerable shortcomings preventing their widespread use, namely their variability, limited function, and reductionist size. In the past few years, sophisticated methodologies have been proposed to allow the design of organoids with improved biological fidelity and physiological relevance. Here, we summarize these emerging technologies and provide insights into how they can be utilized to fulfill the potential of stem cells.

Systems bioengineering approaches for developmental toxicology

Developmental toxicology is the field of study that examines the effects of chemical and physical agents on developing organisms. By using principles of systems biology and bioengineering, a systems bioengineering approach could be applied to study the complex interactions between developing organisms, the environment, and toxic agents. This approach would result in a holistic understanding of the effects of toxic agents on organisms, by considering the interactions between different biological systems and the impacts of toxicants on those interactions. It would be useful in identifying key biological pathways and mechanisms affected by toxic agents, as well as in the development of predictive models to assess potential risks of exposure to toxicants during development. In this review, we discuss the relevance of systems bioengineering to the field of developmental toxicity and provide up-to-date examples that illustrate the use of engineering principles for this application.

Heart in a dish - choosing the right in vitro model

The heart is the first functional organ established during embryogenesis. Investigating heart development and disease is a fascinating and crucial field of research because cardiovascular diseases remain the leading cause of morbidity and mortality worldwide. Therefore, there is great interest in establishing in vitro models for recapitulating both physiological and pathological aspects of human heart development, tissue function and malfunction. Derived from pluripotent stem cells, a large variety of three-dimensional cardiac in vitro models have been introduced in recent years. In this At a Glance article, we discuss the available methods to generate such models, grouped according to the following classification: cardiac organoids, cardiac microtissues and engineered cardiac tissues. For these models, we provide a systematic overview of their applications for disease modeling and therapeutic development, as well as their advantages and limitations to assist scientists in choosing the most suitable model for their research purpose.

Recent Advances in Generation of In Vitro Cardiac Organoids

Cardiac organoids are in vitro self-organizing and three-dimensional structures composed of multiple cardiac cells (i.e., cardiomyocytes, endothelial cells, cardiac fibroblasts, etc.) with or without biological scaffolds. Since cardiac organoids recapitulate structural and functional characteristics of the native heart to a higher degree compared to the conventional two-dimensional culture systems, their applications, in combination with pluripotent stem cell technologies, are being widely expanded for the investigation of cardiogenesis, cardiac disease modeling, drug screening and development, and regenerative medicine. In this mini-review, recent advances in cardiac organoid technologies are summarized in chronological order, with a focus on the methodological points for each organoid formation. Further, the current limitations and the future perspectives in these promising systems are also discussed.

Developmental Toxicity Studies: The Path towards Humanized 3D Stem Cell-Based Models

Today, it is recognized that medicines will eventually be needed during pregnancy to help prevent to, ameliorate or treat an illness, either due to gestation-related medical conditions or pre-existing diseases. Adding to that, the rate of drug prescription to pregnant women has increased over the past few years, in accordance with the increasing trend to postpone childbirth to a later age. However, in spite of these trends, information regarding teratogenic risk in humans is often missing for most of the purchased drugs. So far, animal models have been the gold standard to obtain teratogenic data, but inter-species differences have limited the suitability of those models to predict human-specific outcomes, contributing to misidentified human teratogenicity. Therefore, the development of physiologically relevant in vitro humanized models can be the key to surpassing this limitation. In this context, this review describes the pathway towards the introduction of human pluripotent stem cell-derived models in developmental toxicity studies. Moreover, as an illustration of their relevance, a particular emphasis will be placed on those models that recapitulate two very important early developmental stages, namely gastrulation and cardiac specification.

Human Heart Morphogenesis: A New Vision Based on In Vivo Labeling and Cell Tracking

Despite the extensive information available on the different genetic, epigenetic, and molecular features of cardiogenesis, the origin of congenital heart defects remains unknown. Most genetic and molecular studies have been conducted outside the context of the progressive anatomical and histological changes in the embryonic heart, which is one of the reasons for the limited knowledge of the origins of congenital heart diseases. We integrated the findings of descriptive studies on human embryos and experimental studies on chick, rat, and mouse embryos. This research is based on the new dynamic concept of heart development and the existence of two heart fields. The first field corresponds to the straight heart tube, into which splanchnic mesodermal cells from the second heart field are gradually recruited. The overall aim was to create a new vision for the analysis, diagnosis, and regionalized classification of congenital defects of the heart and great arteries. In addition to highlighting the importance of genetic factors in the development of congenital heart disease, this study provides new insights into the composition of the straight heart tube, the processes of twisting and folding, and the fate of the conus in the development of the right ventricle and its outflow tract. The new vision, based on in vivo labeling and cell tracking and enhanced by models such as gastruloids and organoids, has contributed to a better understanding of important errors in cardiac morphogenesis, which may lead to several congenital heart diseases.

Gastruloids: A Novel System for Disease Modelling and Drug Testing

By virtue of its inaccessible nature, mammalian implantation stage development has remained one of the most enigmatic and hard to investigate periods of embryogenesis. Derived from pluripotent stem cells, gastruloids recapitulate key aspects of gastrula-stage embryos and have emerged as a powerful in vitro tool to study the architectural features of early post-implantation embryos. While the majority of the work in this emerging field has focused on the use of gastruloids to model embryogenesis, their tractable nature and suitability for high-throughput scaling, has presented an unprecedented opportunity to investigate both developmental and environmental aberrations to the embryo as they occur in vitro. This review summarises the recent developments in the use of gastruloids to model congenital anomalies, their usage in teratogenicity testing, and the current limitations of this emerging field.

Generation of human elongating multi-lineage organized cardiac gastruloids

Human elongating multi-lineage organized (EMLOC) gastruloid technology captures key aspects of trunk neurodevelopment including neural integration with cardiogenesis. We generate multi-chambered, contractile EMLOC gastruloids with integrated central and peripheral neurons using defined culture conditions and signaling factors. hiPSC colonies are primed by activating FGF and Wnt signaling pathways for co-induced lineages. EMLOC gastruloids are then initialized with primed cells in suspension culture using timed exposure to FGF2, HGF, IGF1, and Y-27632. Cardiogenesis is stimulated by FGF2, VEGF, and ascorbic acid. For complete details on the use and execution of this protocol, please refer to Olmsted and Paluh (2022).¹.

Human stem cell-derived neurons and neural circuitry therapeutics: Next frontier in spinal cord injury repair

Spinal cord injury (SCI) remains a life-altering event that devastates those injured and the families that support them. Numerous laboratories are engaged in preclinical and clinical trials to repair the injured spinal cord with stem cell-derived therapeutics. A new developmental paradigm reveals early bifurcation of brain or trunk neurons in mammals via neuromesodermal progenitors (NMPs) relevant to therapies requiring homotypic spinal cord neural populations. Human-induced pluripotent stem cell (hiPSC) NMP-derived spinal motor neurons generated ex vivo following this natural developmental route demonstrate robust survival in vivo when delivered as suspension grafts or as in vitro preformed encapsulated neuronal circuitry when transplanted into a rat C4-C5 hemicontusion injury site. Use of in vitro matured neurons avoids in vivo differentiation challenges of using pluripotent hiPSC or multipotent neural stem cell (NSC) or mesenchymal stem cell therapeutics. In this review, we provide an injury to therapeutics overview focusing on how stem cell and developmental fields are merging to generate exquisitely matched spinal motor neurons for SCI therapeutic studies. The complexity of the SCI microenvironment generated by trauma to neurons and vasculature, along with infiltrating inflammatory cells and scarring, underlies the challenging cytokine microenvironment that therapeutic cells encounter. An overview of evolving but limited stem cell-based SCI therapies that have progressed from preclinical to clinical trials illustrates the challenges and need for additional stem cell-based therapeutic approaches. The focus here on neurons describes how NMP-based neurotechnologies are advancing parallel strategies such as transplantation of preformed neuronal circuitry as well as human in vitro gastruloid multicellular models of trunk central and peripheral nervous system integration with organs. NMP-derived neurons are expected to be powerful drivers of the next generation of SCI therapeutics and integrate well with combination therapies that may utilize alternate biomimetic scaffolds for bridging injuries or flexible biodegradable electronics for electrostimulation.

In vitro models of pre- and post-gastrulation embryonic development

The successful derivation and culture of pluripotent stem cells (PSCs) is tightly connected with the study of embryonic development, and was made largely possible by advances in in vitro fertilization and blastocyst culture during the latter half of the last century [1,2]. Since then, embryonic and induced pluripotent stem cells have been extensively used to derive a plethora of functional cell types in vitro, heavily relying on and utilizing insights into cellular differentiation won from developmental biological studies in model organisms. Excitingly, PSCs are now being increasingly used to reconstitute and analyze complex aspects of mouse and human embryonic development. These bottom-up approaches are starting to provide novel insights into core developmental processes and biological questions and may ultimately help decipher the biological principles that underlie the emergence of form and function during development. This mini review summarizes the latest advances and recent breakthroughs in this rapidly growing field of research on PSC-based in vitro models of early embryonic development.

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.

Bioengineering the human spinal cord

Three dimensional, self-assembled organoids that recapitulate key developmental and organizational events during embryogenesis have proven transformative for the study of human central nervous system (CNS) development, evolution, and disease pathology. Brain organoids have predominated the field, but human pluripotent stem cell (hPSC)-derived models of the spinal cord are on the rise. This has required piecing together the complex interactions between rostrocaudal patterning, which specifies axial diversity, and dorsoventral patterning, which establishes locomotor and somatosensory phenotypes. Here, we review how recent insights into neurodevelopmental biology have driven advancements in spinal organoid research, generating experimental models that have the potential to deepen our understanding of neural circuit development, central pattern generation (CPG), and neurodegenerative disease along the body axis. In addition, we discuss the application of bioengineering strategies to drive spinal tissue morphogenesis in vitro, current limitations, and future perspectives on these emerging model systems.