Control of gastruloid patterning and morphogenesis by the Erk and Akt signaling pathways

Towards an integrated view and understanding of embryonic signalling during murine gastrulation

At the onset of mammalian gastrulation, secreted signalling molecules belonging to the Bmp, Wnt, Nodal and Fgf signalling pathways induce and pattern the primitive streak, marking the start for the cellular rearrangements that generate the body plan. Our current understanding of how signalling specifies and organises the germ layers in three dimensions, was mainly derived from genetic experimentation using mouse embryos performed over many decades. However, the exact spatiotemporal sequence of events is still poorly understood, both because of a lack of tractable models that allow for real time visualisation of signalling and differentiation and because of the molecular and cellular complexity of these early developmental events. In recent years, a new wave of in vitro embryo models has begun to shed light on the dynamics of signalling during primitive streak formation. Here we discuss the similarities and differences between a widely adopted mouse embryo model, termed gastruloids, and real embryos from a signalling perspective. We focus on the gene regulatory networks that underlie signalling pathway interactions and outline some of the challenges ahead. Finally, we provide a perspective on how embryo models may be used to advance our understanding of signalling dynamics through computational modelling.

Progress in understanding the vertebrate segmentation clock

The segmentation clock is a molecular oscillator that regulates the periodic formation of somites from the presomitic mesoderm during vertebrate embryogenesis. Synchronous oscillatory expression of a Hairy homologue or Hairy-related basic helix-loop-helix (bHLH) transcriptional repressor in presomitic mesoderm cells regulates periodic expression of downstream factors that control somite segmentation with a periodicity that varies across species. Although many of the key components of the clock have been identified and characterized, less is known about how the clock is synchronized across cells and how species-specific periodicity is achieved. Advances in live imaging, stem cell and organoid technologies, and synthetic approaches have started to uncover the detailed mechanisms underlying these aspects of somitogenesis, providing insight into how morphogenesis is coordinated in space and time during embryonic development.

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.

Recording morphogen signals reveals mechanisms underlying gastruloid symmetry breaking

Aggregates of stem cells can break symmetry and self-organize into embryo-like structures with complex morphologies and gene expression patterns. Mechanisms including reaction-diffusion Turing patterns and cell sorting have been proposed to explain symmetry breaking but distinguishing between these candidate mechanisms of self-organization requires identifying which early asymmetries evolve into subsequent tissue patterns and cell fates. Here we use synthetic 'signal-recording' gene circuits to trace the evolution of signalling patterns in gastruloids, three-dimensional stem cell aggregates that form an anterior-posterior axis and structures resembling the mammalian primitive streak and tailbud. We find that cell sorting rearranges patchy domains of Wnt activity into a single pole that defines the gastruloid anterior-posterior axis. We also trace the emergence of Wnt domains to earlier heterogeneity in Nodal activity even before Wnt activity is detectable. Our study defines a mechanism through which aggregates of stem cells can form a patterning axis even in the absence of external spatial cues.

Endogenous FGFs drive ERK-dependent cell fate patterning in 2D human gastruloids

The role of FGF is the least understood of the morphogens driving mammalian gastrulation. Here we investigated the function of FGF in a stem cell model for human gastrulation known as a 2D gastruloid. We found a ring of FGF-dependent ERK activity that closely follows the emergence of primitive streak (PS)-like cells but expands further inward. We showed that this ERK activity pattern is required for PS-like differentiation and that loss of PS-like cells upon FGF receptor inhibition can be rescued by directly activating ERK. We further demonstrated that the ERK-ring depends on localized activation of basally localized FGF receptors (FGFR) by endogenous FGF gradients. We confirm and extend previous studies in analyzing expression of FGF pathway components, showing the main receptor to be FGFR1 and the key ligands FGF2/4/17, similar to the human and monkey embryo but different from the mouse. In situ hybridization and scRNA-seq revealed that FGF4 and FGF17 expression colocalize with the PS marker TBXT but only FGF17 is maintained in nascent mesoderm and endoderm. FGF4 and FGF17 reduction both reduced ERK activity and differentiation to PS-like cells and their derivatives, indicating overlapping function. Thus, we have identified a previously unknown role for FGF-dependent ERK signaling in 2D gastruloids and possibly the human embryo, driven by a mechanism where FGF4 and FGF17 signal through basally localized FGFR1 to induce PS-like cells.

Precise and scalable self-organization in mammalian pseudo-embryos

Gene expression is inherently noisy, posing a challenge to understanding how precise and reproducible patterns of gene expression emerge in mammals. Here we investigate this phenomenon using gastruloids, a three-dimensional in vitro model for early mammalian development. Our study reveals intrinsic reproducibility in the self-organization of gastruloids, encompassing growth dynamics and gene expression patterns. We observe a remarkable degree of control over gene expression along the main body axis, with pattern boundaries positioned with single-cell precision. Furthermore, as gastruloids grow, both their physical proportions and gene expression patterns scale proportionally with system size. Notably, these properties emerge spontaneously in self-organizing cell aggregates, distinct from many in vivo systems constrained by fixed boundary conditions. Our findings shed light on the intricacies of developmental precision, reproducibility and size scaling within a mammalian system, suggesting that these phenomena might constitute fundamental features of multicellularity.

Robust axis elongation by Nodal-dependent restriction of BMP signaling

Embryogenesis results from the coordinated activities of different signaling pathways controlling cell fate specification and morphogenesis. In vertebrate gastrulation, both Nodal and BMP signaling play key roles in germ layer specification and morphogenesis, yet their interplay to coordinate embryo patterning with morphogenesis is still insufficiently understood. Here, we took a reductionist approach using zebrafish embryonic explants to study the coordination of Nodal and BMP signaling for embryo patterning and morphogenesis. We show that Nodal signaling triggers explant elongation by inducing mesendodermal progenitors but also suppressing BMP signaling activity at the site of mesendoderm induction. Consistent with this, ectopic BMP signaling in the mesendoderm blocks cell alignment and oriented mesendoderm intercalations, key processes during explant elongation. Translating these ex vivo observations to the intact embryo showed that, similar to explants, Nodal signaling suppresses the effect of BMP signaling on cell intercalations in the dorsal domain, thus allowing robust embryonic axis elongation. These findings suggest a dual function of Nodal signaling in embryonic axis elongation by both inducing mesendoderm and suppressing BMP effects in the dorsal portion of the mesendoderm.

Local cellular interactions during the self-organization of stem cells

Stem cell models for early mammalian development offer new experimental opportunities to access spatio-temporal details of the cell-cell interactions that govern cell differentiation and tissue patterning. This review summarizes recent studies that have used stem cell models to investigate the spatial range of developmental cell-cell communication systems. A key message from these works is that important biochemical signals for cell differentiation in these systems, such as Nodal and fibroblast growth factors (FGFs), often act over short distances of only a few cell diameters. The formation of long-range patterns at the tissue scale associated with these signals then results from signal relays and cell rearrangements. The modular view of differentiation and patterning emerging from research on stem cell models can offer a fresh perspective on the corresponding processes in the embryo.