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

Cradle cultures: growing stem cell-derived developmental cell models in vitro

How are three stem cell-derived developmental cell models furthering our understanding of post-implantation human embryo development? And why have recent advancements in these human embryo-like models spurred ethical discussion and the need to refine our definition of 'embryo'? [Formula: see text].

Looking back at five decades of embryo technology in practice

This paper covers developments from International Embryo Technology Society (IETS) meetings over the past 50years. The IETS was officially 'born' at a meeting in Denver, Colorado on Sunday 26 May 1974. There have been 51 IETS meetings (the first was in May 1974), and the first conference at which papers were presented was in 1975. The name of the IETS was changed from International Embryo Transfer Society to International Embryo Technology Society in 2016. The annual IETS conferences are held once a year in January. Embryo technology advanced from the laborious and slow techniques of surgical recovery and transfer of bovine embryos to non-surgical recovery and transfer in many species. Cryopreservation of embryos was initially a slow process that met with only moderate technical success before the development of technology that resulted in high embryo survival rates. The polymerase chain reaction was used to determine the sex of embryos following laminar flow cytology, which came into use to separate X- and Y-bearing spermatozoa. IVF has grown to become widely used, and several factors make it more useful than superovulation and embryo recovery/transfer. Perhaps the most important tool of all was the system that allowed the genetic 'mapping' of the genomic sequence of Bos taurus cattle. Charting key DNA differences by scientists, 'haplotypes' are now used routinely to identify animals with desirable traits of economic importance, discover new genetic disorders, and track carrier status of genotyped animals. Future technology is discussed.

Retinoids: Mechanisms of Action in Neuronal Cell Fate Acquisition

Neuronal differentiation has been shown to be directed by retinoid action during embryo development and has been exploited in various in vitro cell differentiation systems. In this review, we summarize the role of retinoids through the activation of their specific retinoic acid nuclear receptors during embryo development and also in a variety of in vitro strategies for neuronal differentiation, including recent efforts in driving cell specialization towards a range of neuronal subtypes and glial cells. Finally, we highlight the role of retinoic acid in recent protocols recapitulating nervous tissue complexity (cerebral organoids). Overall, we expect that this effort might pave the way for exploring the usage of specific synthetic retinoids for directing complex nervous tissue differentiation.

Molecular mechanisms underlying totipotency

Numerous efforts to understand pluripotency in mammals, using pluripotent stem cells in culture, have enabled the generation of artificially induced pluripotent stem cells, which serve as a valuable source for regenerative medicine and the creation of disease models. In contrast to these tremendous successes in the pluripotency field in the past few decades, our understanding of totipotency, which is highlighted by its broader plasticity than pluripotency, is still limited. This is largely attributable to the scarcity of available materials and the lack of in vitro models. However, recent technological advances have unveiled molecular features that characterize totipotent cells. Single-cell or low-input sequencing technologies allow the dissection of pre- and post-fertilization developmental processes at the molecular level with high resolution. In this review, we describe some of the key findings in understanding totipotency and discuss how totipotency is acquired at the beginning of life.

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.

Complete human day 14 post-implantation embryo models from naive ES cells

The ability to study human post-implantation development remains limited owing to ethical and technical challenges associated with intrauterine development after implantation1. Embryo-like models with spatially organized morphogenesis and structure of all defining embryonic and extra-embryonic tissues of the post-implantation human conceptus (that is, the embryonic disc, the bilaminar disc, the yolk sac, the chorionic sac and the surrounding trophoblast layer) remain lacking1,2. Mouse naive embryonic stem cells have recently been shown to give rise to embryonic and extra-embryonic stem cells capable of self-assembling into post-gastrulation structured stem-cell-based embryo models with spatially organized morphogenesis (called SEMs)3. Here we extend those findings to humans using only genetically unmodified human naive embryonic stem cells (cultured in human enhanced naive stem cell medium conditions)4. Such human fully integrated and complete SEMs recapitulate the organization of nearly all known lineages and compartments of post-implantation human embryos, including the epiblast, the hypoblast, the extra-embryonic mesoderm and the trophoblast layer surrounding the latter compartments. These human complete SEMs demonstrated developmental growth dynamics that resemble key hallmarks of post-implantation stage embryogenesis up to 13-14 days after fertilization (Carnegie stage 6a). These include embryonic disc and bilaminar disc formation, epiblast lumenogenesis, polarized amniogenesis, anterior-posterior symmetry breaking, primordial germ-cell specification, polarized yolk sac with visceral and parietal endoderm formation, extra-embryonic mesoderm expansion that defines a chorionic cavity and a connecting stalk, and a trophoblast-surrounding compartment demonstrating syncytium and lacunae formation. This SEM platform will probably enable the experimental investigation of previously inaccessible windows of human early post implantation up to peri-gastrulation development.

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.

iPSC-based modeling of helicase deficiency reveals impaired cell proliferation and increased apoptosis after NK cell lineage commitment

Cell proliferation is a ubiquitous process required for organismal development and homeostasis. However, individuals with partial loss-of-function variants in DNA replicative helicase components often present with immunodeficiency due to specific loss of natural killer (NK) cells. Such lineage-specific disease phenotypes raise questions on how the proliferation is regulated in cell type-specific manner. We aimed to understand NK cell-specific proliferative dynamics and vulnerability to impaired helicase function using iPSCs from individuals with NK cell deficiency (NKD) due to hereditary compound heterozygous GINS4 variants. We observed and characterized heterogeneous cell populations that arise during the iPSC differentiation along with NK cells. While overall cell proliferation decreased with differentiation, early NK cell precursors showed a short burst of cell proliferation. GINS4 deficiency induced replication stress in these early NK cell precursors, which are poised for apoptosis, and ultimately recapitulate the NKD phenotype.

3D-cultured blastoids model human embryogenesis from pre-implantation to early gastrulation stages

Naive human pluripotent stem cells have the remarkable ability to self-organize into blastocyst-like structures ("blastoids") that model lineage segregation in the pre-implantation embryo. However, the extent to which blastoids can recapitulate the defining features of human post-implantation development remains unexplored. Here, we report that blastoids cultured on thick three-dimensional (3D) extracellular matrices capture hallmarks of early post-implantation development, including epiblast lumenogenesis, rapid expansion and diversification of trophoblast lineages, and robust invasion of extravillous trophoblast cells by day 14. Extended blastoid culture results in the localized activation of primitive streak marker TBXT and the emergence of embryonic germ layers by day 21. We also show that the modulation of WNT signaling alters the balance between epiblast and trophoblast fates in post-implantation blastoids. This work demonstrates that 3D-cultured blastoids offer a continuous and integrated in vitro model system of human embryonic and extraembryonic development from pre-implantation to early gastrulation stages.

Modeling post-implantation stages of human development into early organogenesis with stem-cell-derived peri-gastruloids

In vitro stem cell models that replicate human gastrulation have been generated, but they lack the essential extraembryonic cells needed for embryonic development, morphogenesis, and patterning. Here, we describe a robust and efficient method that prompts human extended pluripotent stem cells to self-organize into embryo-like structures, termed peri-gastruloids, which encompass both embryonic (epiblast) and extraembryonic (hypoblast) tissues. Although peri-gastruloids are not viable due to the exclusion of trophoblasts, they recapitulate critical stages of human peri-gastrulation development, such as forming amniotic and yolk sac cavities, developing bilaminar and trilaminar embryonic discs, specifying primordial germ cells, initiating gastrulation, and undergoing early neurulation and organogenesis. Single-cell RNA-sequencing unveiled transcriptomic similarities between advanced human peri-gastruloids and primary peri-gastrulation cell types found in humans and non-human primates. This peri-gastruloid platform allows for further exploration beyond gastrulation and may potentially aid in the development of human fetal tissues for use in regenerative medicine.

Modeling specific aneuploidies: from karyotype manipulations to biological insights

An abnormal chromosome number, or aneuploidy, underlies developmental disorders and is a common feature of cancer, with different cancer types exhibiting distinct patterns of chromosomal gains and losses. To understand how specific aneuploidies emerge in certain tissues and how they contribute to disease development, various methods have been developed to alter the karyotype of mammalian cells and mice. In this review, we provide an overview of both classic and novel strategies for inducing or selecting specific chromosomal gains and losses in human and murine cell systems. We highlight how these customized aneuploidy models helped expanding our knowledge of the consequences of specific aneuploidies to (cancer) cell physiology.

Advances and challenges toward developing kidney organoids for clinical applications

Kidney organoids have enabled modeling of human development and disease. While methods of generating the nephron lineage are well established, new protocols to induce another lineage, the ureteric bud/collecting duct, have been reported in the past 5 years. Many reports have described modeling of various hereditary kidney diseases, with polycystic kidney disease serving as the archetypal disease, by using patient-derived or genome-edited kidney organoids. The generation of more organotypic kidneys is also becoming feasible. In this review, I also discuss the significant challenges for more sophisticated disease modeling and for realizing the ambitious goal of generating transplantable synthetic kidneys.

Dawn of development: Exploring early human embryogenesis using stem cells

Exploring the early stages of human embryonic development poses significant difficulties owing to ethical and technical limitations. Two recent studies in Nature report the self-organization of human stem cells into 3D embryoids that model features of the early post-implantation stages of human development.1,2.

Tissue tension permits β-catenin phosphorylation to drive mesoderm specification in human embryonic stem cells

The role of morphogenetic forces in cell fate specification is an area of intense interest. Our prior studies suggested that the development of high cell-cell tension in human embryonic stem cells (hESC) colonies permits the Src-mediated phosphorylation of junctional β-catenin that accelerates its release to potentiate Wnt-dependent signaling critical for initiating mesoderm specification. Using an ectopically expressed nonphosphorylatable mutant of β-catenin (Y654F), we now provide direct evidence that impeding tension-dependent Src-mediated β-catenin phosphorylation impedes the expression of Brachyury (T) and the epithelial-to-mesenchymal transition (EMT) necessary for mesoderm specification. Addition of exogenous Wnt3a or inhibiting GSK3β activity rescued mesoderm expression, emphasizing the importance of force dependent Wnt signaling in regulating mechanomorphogenesis. Our work provides a framework for understanding tension-dependent β-catenin/Wnt signaling in the self-organization of tissues during developmental processes including gastrulation.

Human stem cell-derived embryo models: Toward ethically appropriate regulations and policies

Human pluripotent stem cells can be used to derive in vitro models recapitulating post-implantation human embryos. While useful for research purposes, such integrated embryo models raise ethical issues that need to be addressed to facilitate ethically appropriate policies and regulations that permit scientific ingenuity and medical progress.