Implantation initiation of self-assembled embryo-like structures generated using three types of mouse blastocyst-derived stem cells

From pluripotency to totipotency: an experimentalist's guide to cellular potency

Embryonic stem cells (ESCs) are derived from the pre-implantation mammalian blastocyst. At this point in time, the newly formed embryo is concerned with the generation and expansion of both the embryonic lineages required to build the embryo and the extra-embryonic lineages that support development. When used in grafting experiments, embryonic cells from early developmental stages can contribute to both embryonic and extra-embryonic lineages, but it is generally accepted that ESCs can give rise to only embryonic lineages. As a result, they are referred to as pluripotent, rather than totipotent. Here, we consider the experimental potential of various ESC populations and a number of recently identified in vitro culture systems producing states beyond pluripotency and reminiscent of those observed during pre-implantation development. We also consider the nature of totipotency and the extent to which cell populations in these culture systems exhibit this property.

The moral status of human embryo-like structures: potentiality matters?: The moral status of human synthetic embryos

New techniques to generate and culture embryo-like structures from stem cells require a more fine-grained distinction of potential to define the moral status of these structures.

Generation of Artificial Gamete and Embryo From Stem Cells in Reproductive Medicine

In addition to the great growing need for assisted reproduction technologies (ART), additional solutions for patients without functional gametes are strongly needed. Due to ethical restrictions, limited studies can be performed on human gametes and embryos; however, artificial gametes and embryos represent a new hope for clinical application and basic research in the field of reproductive medicine. Here, we provide a review of the research progress and possible application of artificial gametes and embryos from different species, including mice, monkeys and humans. Gametes specification from adult stem cells and embryonic stem cells (ESCs) as well as propagation of stem cells from the reproductive system and from organized embryos, which are similar to blastocysts, have been realized in some nonhuman mammals, but not all achievements can be replicated in humans. This area of research remains noteworthy and requires further study and effort to achieve the reconstitution of the entire cycle of gametogenesis and embryo development in vitro.

Design Approaches for Generating Organ Constructs

Organ constructs are organ-like structures grown in vitro or in vivo that harbor the components, architecture, and function of in vivo organs, in part or in toto. The convergence of stem cell biology, bioengineering, and gene editing tools have substantially broadened our ability to generate various types of organ constructs for regenerative medicine as well as to address pressing biomedical questions. In this Review, we highlight prevailing approaches for generating organ constructs, from organoids to chimeric organ engineering. We also discuss design principles of different approaches, their utility and limitations, and propose strategies to resolve existing hurdles.

Generation of Blastocyst-like Structures from Mouse Embryonic and Adult Cell Cultures

A single mouse blastomere from an embryo until the 8-cell stage can generate an entire blastocyst. Whether laboratory-cultured cells retain a similar generative capacity remains unknown. Starting from a single stem cell type, extended pluripotent stem (EPS) cells, we established a 3D differentiation system that enabled the generation of blastocyst-like structures (EPS-blastoids) through lineage segregation and self-organization. EPS-blastoids resembled blastocysts in morphology and cell-lineage allocation and recapitulated key morphogenetic events during preimplantation and early postimplantation development in vitro. Upon transfer, some EPS-blastoids underwent implantation, induced decidualization, and generated live, albeit disorganized, tissues in utero. Single-cell and bulk RNA-sequencing analysis revealed that EPS-blastoids contained all three blastocyst cell lineages and shared transcriptional similarity with natural blastocysts. We also provide proof of concept that EPS-blastoids can be generated from adult cells via cellular reprogramming. EPS-blastoids provide a unique platform for studying early embryogenesis and pave the way to creating viable synthetic embryos by using cultured cells.

Specification of the First Mammalian Cell Lineages In Vivo and In Vitro

Our understanding of how the first mammalian cell lineages arise has been shaped largely by studies of the preimplantation mouse embryo. Painstaking work over many decades has begun to reveal how a single totipotent cell is transformed into a multilayered structure representing the foundations of the body plan. Here, we review how the first lineage decision is initiated by epigenetic regulation but consolidated by the integration of morphological features and transcription factor activity. The establishment of pluripotent and multipotent stem cell lines has enabled deeper analysis of molecular and epigenetic regulation of cell fate decisions. The capability to assemble these stem cells into artificial embryos is an exciting new avenue of research that offers a long-awaited window into cell fate specification in the human embryo. Together, these approaches are poised to profoundly increase our understanding of how the first lineage decisions are made during mammalian embryonic development.

Synthetic human embryology: towards a quantitative future

Study of early human embryo development is essential for advancing reproductive and regenerative medicine. Traditional human embryological studies rely on embryonic tissue specimens, which are difficult to acquire due to technical challenges and ethical restrictions. The availability of human stem cells with developmental potentials comparable to pre-implantation and peri-implantation human embryonic and extraembryonic cells, together with properly engineered in vitro culture environments, allow for the first time researchers to generate self-organized multicellular structures in vitro that mimic the structural and molecular features of their in vivo counterparts. The development of these stem cell-based, synthetic human embryo models offers a paradigm-shifting experimental system for quantitative measurements and perturbations of multicellular development, critical for advancing human embryology and reproductive and regenerative medicine without using intact human embryos.

In vitro modeling of early mammalian embryogenesis

Synthetic embryology endeavors to use stem cells to recapitulate the first steps of mammalian development that define the body axes and first stages of fate assignment. Well-engineered synthetic systems provide an unparalleled assay to disentangle and quantify the contributions of individual tissues as well as the molecular components driving embryogenesis. Experiments using a mixture of mouse embryonic and extra-embryonic stem cell lines show a surprising degree of self-organization akin to certain milestones in the development of intact mouse embryos. To further advance the field and extend the mouse results to human, it is crucial to develop a better control of the assembly process as well as to establish a deeper understanding of the developmental state and potency of cells used in experiments at each step of the process. We review recent advances in the derivation of embryonic and extraembryonic stem cells, and we highlight recent efforts in reconstructing the structural and signaling aspects of embryogenesis in three-dimensional tissue cultures.

Bioengineered microenvironment to culture early embryos

The abnormalities of early post-implantation embryos can lead to early pregnancy loss and many other syndromes. However, it is hard to study embryos after implantation due to the limited accessibility. The success of embryo culture in vitro can avoid the challenges of embryonic development in vivo and provide a powerful research platform for research in developmental biology. The biophysical and chemical cues of the microenvironments impart significant spatiotemporal effects on embryonic development. Here, we summarize the main strategies which enable researchers to grow embryos outside of the body while overcoming the implantation barrier, highlight the roles of engineered microenvironments in regulating early embryonic development, and finally discuss the future challenges and new insights of early embryo culture.

Defined Stem Cell Culture Conditions to Model Mouse Blastocyst Development

The complex program of mouse development entails specification of the embryonic epiblast (Epi) as well as the extra-embryonic trophectoderm (TE) and primitive endoderm (PrE). These three lineages of mouse blastocyst can be modeled in vitro using stem cells derived from primary tissues. In these cultures, cells self-renew while retaining their developmental potential if put back into a developing embryo. Indeed, embryonic stem cells (ESC), when injected into a blastocyst, readily contribute to all embryonic lineages. Similarly, trophoblast stem cells (TSCs) will give rise to all TE-derived trophoblast lineages, and extraembryonic endoderm cells (XEN) will contribute to the PrE-derived yolk sack. These model systems are a powerful tool to study early development, lineage specification, and placenta formation. Only recently reproducible and chemically defined culture systems of these cells have been described. This overview discusses such novel methods for culturing ESC/TSC/XEN, as well as their molecular signatures and developmental potential. Recent strides in expanding the developmental potential of stem cells as well as achieving models more reminiscent of their in vivo counterparts are discussed. Finally, such in vitro stem cells can self-assemble into structures resembling embryos when used in novel 3D-culture systems. This article discusses the strengths and limitations of such "synthetic embryos" in studying developmental processes. © 2020 by John Wiley & Sons, Inc.

Modification of stem cell states by alcohol and acetaldehyde

Ethanol (EtOH) is a recreationally ingested compound that is both teratogenic and carcinogenic in humans. Because of its abundant consumption worldwide and the vital role of stem cells in the formation of birth defects and cancers, delineating the effects of EtOH on stem cell function is currently an active and urgent pursuit of scientific investigation to explicate some of the mechanisms contributing to EtOH toxicity. Stem cells represent a primordial, undifferentiated phase of development; thus encroachment on normal physiologic processes of differentiation into terminal lineages by EtOH can greatly alter the function of progenitors and terminally differentiated cells, leading to pathological consequences that manifest as fetal alcohol spectrum disorders and cancers. In this review we explore the disruptive role of EtOH in differentiation of stem cells. Our primary objective is to elucidate the mechanisms by which EtOH alters differentiation-related gene expression and lineage specifications, thus modifying stem cells to promote pathological outcomes. We additionally review the effects of a reactive metabolite of EtOH, acetaldehyde (AcH), in causing both differentiation defects in stem cells as well as genomic damage that incites cellular aging and carcinogenesis.

Recent insights into the naïve state of human pluripotency and its applications

The past decade has seen significant interest in the isolation of pluripotent stem cells corresponding to various stages of mammalian embryonic development. Two distinct and well-defined pluripotent states can be derived from mouse embryos: "naïve" pluripotent cells with properties of pre-implantation epiblast, and "primed" pluripotent cells, resembling post-implantation epiblast. Prompted by the successful interconversion between these two stem cell states in the mouse system, several groups have devised strategies for inducing a naïve state of pluripotency in human pluripotent stem cells. Here, we review recent insights into the naïve state of human pluripotency, focusing on two methods that confer defining transcriptomic and epigenomic signatures of the pre-implantation embryo. The isolation of naïve human pluripotent stem cells offers a window into early developmental mechanisms that cannot be adequately modeled in primed cells, such as X chromosome reactivation, metabolic reprogramming, and the regulation of hominid-specific transposable elements. We outline key unresolved questions regarding naïve human pluripotency, including its extrinsic and intrinsic control mechanisms, potential for embryonic and extraembryonic differentiation, and general utility as a model system for human development and disease.

A 3D model of a human epiblast reveals BMP4-driven symmetry breaking

Breaking the anterior-posterior symmetry in mammals occurs at gastrulation. Much of the signalling network underlying this process has been elucidated in the mouse; however, there is no direct molecular evidence of events driving axis formation in humans. Here, we use human embryonic stem cells to generate an in vitro three-dimensional model of a human epiblast whose size, cell polarity and gene expression are similar to a day 10 human epiblast. A defined dose of BMP4 spontaneously breaks axial symmetry, and induces markers of the primitive streak and epithelial-to-mesenchymal transition. We show that WNT signalling and its inhibitor DKK1 play key roles in this process downstream of BMP4. Our work demonstrates that a model human epiblast can break axial symmetry despite the absence of asymmetry in the initial signal and of extra-embryonic tissues or maternal cues. Our three-dimensional model is an assay for the molecular events underlying human axial symmetry breaking.

Self-organization of stem cells into embryos: A window on early mammalian development

Embryonic development is orchestrated by robust and complex regulatory mechanisms acting at different scales of organization. In vivo studies are particularly challenging for mammals after implantation, owing to the small size and inaccessibility of the embryo. The generation of stem cell models of the embryo represents a powerful system with which to dissect this complexity. Control of geometry, modulation of the physical environment, and priming with chemical signals reveal the intrinsic capacity of embryonic stem cells to make patterns. Adding the stem cells for the extraembryonic lineages generates three-dimensional models that are more autonomous from the environment and recapitulate many features of the pre- and postimplantation mouse embryo, including gastrulation. Here, we review the principles of self-organization and how they set cells in motion to create an embryo.