In preprints: opportunities to unravel the earliest stages of human development using stem cell-based embryo models

A dynamical perspective: moving towards mechanism in single-cell transcriptomics

As the field of single-cell transcriptomics matures, research is shifting focus from phenomenological descriptions of cellular phenotypes to a mechanistic understanding of the gene regulation underneath. This perspective considers the value of capturing dynamical information at single-cell resolution for gaining mechanistic insight; reviews the available technologies for recording and inferring temporal information in single cells; and explores whether better dynamical resolution is sufficient to adequately capture the causal relationships driving complex biological systems. This article is part of a discussion meeting issue 'Causes and consequences of stochastic processes in development and disease'.

Neuromesodermal specification during head-to-tail body axis formation

The anterior-to-posterior (head-to-tail) body axis is extraordinarily diverse among vertebrates but conserved within species. Body axis development requires a population of axial progenitors that resides at the posterior of the embryo to sustain elongation and is then eliminated once axis extension is complete. These progenitors occupy distinct domains in the posterior (tail-end) of the embryo and contribute to various lineages along the body axis. The subset of axial progenitors with neuromesodermal competency will generate both the neural tube (the precursor of the spinal cord), and the trunk and tail somites (producing the musculoskeleton) during embryo development. These axial progenitors are called Neuromesodermal Competent cells (NMCs) and Neuromesodermal Progenitors (NMPs). NMCs/NMPs have recently attracted interest beyond the field of developmental biology due to their clinical potential. In the mouse, the maintenance of neuromesodermal competency relies on a fine balance between a trio of known signals: Wnt/β-catenin, FGF signalling activity and suppression of retinoic acid signalling. These signals regulate the relative expression levels of the mesodermal transcription factor Brachyury and the neural transcription factor Sox2, permitting the maintenance of progenitor identity when co-expressed, and either mesoderm or neural lineage commitment when the balance is tilted towards either Brachyury or Sox2, respectively. Despite important advances in understanding key genes and cellular behaviours involved in these fate decisions, how the balance between mesodermal and neural fates is achieved remains largely unknown. In this chapter, we provide an overview of signalling and gene regulatory networks in NMCs/NMPs. We discuss mutant phenotypes associated with axial defects, hinting at the potential significant role of lesser studied proteins in the maintenance and differentiation of the progenitors that fuel axial elongation.

Stem cell-derived models of spinal neurulation

Neurulation is a critical step in early embryonic development, giving rise to the neural tube, the primordium of the central nervous system in amniotes. Understanding this complex, multi-scale, multi-tissue morphogenetic process is essential to provide insights into normal development and the etiology of neural tube defects. Innovations in tissue engineering have fostered the generation of pluripotent stem cell-based in vitro models, including organoids, that are emerging as unique tools for delving into neurulation mechanisms, especially in the context of human development. Each model captures specific aspects of neural tube morphogenesis, from epithelialization to neural tissue elongation, folding and cavitation. In particular, the recent models of human and mouse trunk morphogenesis, such as gastruloids, that form a spinal neural plate-like or neural tube-like structure are opening new avenues to study normal and pathological neurulation. Here, we review the morphogenetic events generating the neural tube in the mammalian embryo and questions that remain unanswered. We discuss the advantages and limitations of existing in vitro models of neurulation and possible future technical developments.

Signaling mechanisms that direct cell fate specification and morphogenesis in human embryonic stem cells-based models of human gastrulation

During mammalian gastrulation, a mass of pluripotent cells surrounded by extraembryonic tissues differentiates into germ layers, mesoderm, endoderm, and ectoderm. The three germ layers are then organized into a body plan with organ rudiments via morphogenetic gastrulation movements of emboly, epiboly, convergence, and extension. Emboly is the most conserved gastrulation movement, whereby mesodermal and endodermal progenitors undergo epithelial-to-mesenchymal transition (EMT) and move via a blastopore/primitive streak beneath the ectoderm. Decades of embryologic, genetic, and molecular studies in invertebrates and vertebrates, delineated a BMP > WNT > NODAL signaling cascade underlying mesoderm and endoderm specification. Advances have been made in the research animals in understanding the cellular and molecular mechanisms underlying gastrulation morphogenesis. In contrast, little is known about human gastrulation, which occurs in utero during the third week of gestation and its investigations face ethical and methodological limitations. This is changing with the unprecedented progress in modeling aspects of human development, using human pluripotent stem cells (hPSCs), including embryonic stem cells (hESC)-based embryo-like models (SCEMs). In one approach, hESCs of various pluripotency are aggregated to self-assemble into structures that resemble pre-implantation or post-implantation embryo-like structures that progress to early gastrulation, and some even reach segmentation and neurulation stages. Another approach entails coaxing hESCs with biochemical signals to generate germ layers and model aspects of gastrulation morphogenesis, such as EMT. Here, we review the recent advances in understanding signaling cascades that direct germ layers specification and the early stages of gastrulation morphogenesis in these models. We discuss outstanding questions, challenges, and opportunities for this promising area of developmental biology.