Modelling post-implantation human development to yolk sac blood emergence

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.

Tissue-intrinsic beta-catenin signals antagonize Nodal-driven anterior visceral endoderm differentiation

The anterior-posterior axis of the mammalian embryo is laid down by the anterior visceral endoderm (AVE), an extraembryonic signaling center that is specified within the visceral endoderm. Current models posit that AVE differentiation is promoted globally by epiblast-derived Nodal signals, and spatially restricted by a BMP gradient established by the extraembryonic ectoderm. Here, we report spatially restricted AVE differentiation in bilayered embryo-like aggregates made from mouse embryonic stem cells that lack an extraembryonic ectoderm. Notably, clusters of AVE cells also form in pure visceral endoderm cultures upon activation of Nodal signaling, indicating that tissue-intrinsic factors can restrict AVE differentiation. We identify β-catenin activity as a tissue-intrinsic factor that antagonizes AVE-inducing Nodal signals. Together, our results show how an AVE-like population can arise through interactions between epiblast and visceral endoderm alone. This mechanism may be a flexible solution for axis patterning in a wide range of embryo geometries, and provide robustness to axis patterning when coupled with signal gradients.

The molecular determinants of microglial developmental dynamics

Microglia constitute the largest population of parenchymal macrophages in the brain and are considered a unique subset of central nervous system glial cells owing to their extra-embryonic origins in the yolk sac. During development, microglial progenitors readily proliferate and eventually colonize the entire brain. In this Review, we highlight the origins of microglial progenitors and their entry routes into the brain and discuss the various molecular and non-molecular determinants of their fate, which may inform their specific functions. Specifically, we explore recently identified mechanisms that regulate microglial colonization of the brain, including the availability of space, and describe how the expansion of highly proliferative microglial progenitors facilitates the occupation of the microglial niche. Finally, we shed light on the factors involved in establishing microglial identity in the brain.

Developmental regulation of primitive erythropoiesis

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Why study human embryo development?

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

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

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

Extra (embryonic) dialogues: Keys to improved stem cell-based embryo models

Despite advances in stem cell cultures, modeling early human development with stem cells in a dish remains challenging. Research by Hislop et al.,1 Okubo et al.,2 and Wei et al.3 paves the way for improved in vitro embryo models and culture conditions, offering valuable insights for regenerative medicine.