Left-right symmetry of zebrafish embryos requires somite surface tension

Modularity of the segmentation clock and morphogenesis

Vertebrates have evolved great diversity in the number of segments dividing the trunk body, however, the developmental origin of the evolvability of this trait is poorly understood. The number of segments is thought to be determined in embryogenesis as a product of morphogenesis of the pre-somitic mesoderm (PSM) and the periodicity of a molecular oscillator active within the PSM known as the segmentation clock. Here, we explore whether the clock and PSM morphogenesis exhibit developmental modularity, as independent evolution of these two processes may explain the high evolvability of segment number. Using a computational model of the clock and PSM parameterised for zebrafish, we find that the clock is broadly robust to variation in morphogenetic processes such as cell ingression, motility, compaction, and cell division. We show that this robustness is in part determined by the length of the PSM and the strength of phase coupling in the clock. As previous studies report no changes to morphogenesis upon perturbing the clock, we suggest that the clock and morphogenesis of the PSM exhibit developmental modularity.

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.

Application of tissue-scale tension to avian epithelia in vivo to study multiscale mechanics and inter-germ layer coupling

As cross-disciplinary approaches drawing from physics and mechanics have increasingly influenced our understanding of morphogenesis, the tools available to measure and perturb physical aspects of embryonic development have expanded as well. However, it remains a challenge to measure mechanical properties and apply exogenous tissue-scale forces in vivo, particularly for epithelia. Exploiting the size and accessibility of the developing chick embryo, here we describe a simple technique to quantitatively apply exogenous forces on the order of ~1-100 μN to the endodermal epithelium. To demonstrate the utility of this approach, we performed a series of proof-of-concept experiments that reveal fundamental and unexpected mechanical behaviors in the early chick embryo, including mechanotype heterogeneity among cells of the midgut endoderm, complex non-cell autonomous effects of actin disruption, and a high degree of mechanical coupling between the endoderm and adjacent paraxial mesoderm. To illustrate the broader utility of this method, we determined that forces on the order of ~ 10 μN are sufficient to unzip the neural tube during primary neurulation. Together, these findings provide basic insights into the mechanics of embryonic epithelia in vivo in the early avian embryo, and provide a useful tool for future investigations of how morphogenesis is influenced by mechanical factors.

Cellular and molecular control of vertebrate somitogenesis

Segmentation is a fundamental feature of the vertebrate body plan. This metameric organization is first implemented by somitogenesis in the early embryo, when paired epithelial blocks called somites are rhythmically formed to flank the neural tube. Recent advances in in vitro models have offered new opportunities to elucidate the mechanisms that underlie somitogenesis. Notably, models derived from human pluripotent stem cells introduced an efficient proxy for studying this process during human development. In this Review, we summarize the current understanding of somitogenesis gained from both in vivo studies and in vitro studies. We deconstruct the spatiotemporal dynamics of somitogenesis into four distinct modules: dynamic events in the presomitic mesoderm, segmental determination, somite anteroposterior polarity patterning, and epithelial morphogenesis. We first focus on the segmentation clock, as well as signalling and metabolic gradients along the tissue, before discussing the clock and wavefront and other models that account for segmental determination. We then detail the molecular and cellular mechanisms of anteroposterior polarity patterning and somite epithelialization.

An emerging role for tissue plasticity in developmental precision

Reproducible tissue morphology is a fundamental feature of embryonic development. To ensure such robustness during tissue morphogenesis, inherent noise in biological processes must be buffered. While redundant genes, parallel signaling pathways and intricate network topologies are known to reduce noise, over the last few years, mechanical properties of tissues have been shown to play a vital role. Here, taking the example of somite shape changes, I will discuss how tissues are highly plastic in their ability to change shapes leading to increased precision and reproducibility.

Modeling the extracellular matrix in cell migration and morphogenesis: a guide for the curious biologist

The extracellular matrix (ECM) is a highly complex structure through which biochemical and mechanical signals are transmitted. In processes of cell migration, the ECM also acts as a scaffold, providing structural support to cells as well as points of potential attachment. Although the ECM is a well-studied structure, its role in many biological processes remains difficult to investigate comprehensively due to its complexity and structural variation within an organism. In tandem with experiments, mathematical models are helpful in refining and testing hypotheses, generating predictions, and exploring conditions outside the scope of experiments. Such models can be combined and calibrated with in vivo and in vitro data to identify critical cell-ECM interactions that drive developmental and homeostatic processes, or the progression of diseases. In this review, we focus on mathematical and computational models of the ECM in processes such as cell migration including cancer metastasis, and in tissue structure and morphogenesis. By highlighting the predictive power of these models, we aim to help bridge the gap between experimental and computational approaches to studying the ECM and to provide guidance on selecting an appropriate model framework to complement corresponding experimental studies.

Emergence of a left-right symmetric body plan in vertebrate embryos

External bilateral symmetry is a prevalent feature in vertebrates, which emerges during early embryonic development. To begin with, vertebrate embryos are largely radially symmetric before transitioning to bilaterally symmetry, after which, morphogenesis of various bilateral tissues (e.g somites, otic vesicle, limb bud), and structures (e.g palate, jaw) ensue. While a significant amount of work has probed the mechanisms behind symmetry breaking in the left-right axis leading to asymmetric positioning of internal organs, little is known about how bilateral tissues emerge at the same time with the same shape and size and at the same position on the two sides of the embryo. By discussing emergence of symmetry in many bilateral tissues and structures across vertebrate model systems, we highlight that understanding symmetry establishment is largely an open field, which will provide deep insights into fundamental problems in developmental biology for decades to come.

From cells to form: A roadmap to study shape emergence in vivo

Organogenesis arises from the collective arrangement of cells into progressively 3D-shaped tissue. The acquisition of a correctly shaped organ is then the result of a complex interplay between molecular cues, responsible for differentiation and patterning, and the mechanical properties of the system, which generate the necessary forces that drive correct shape emergence. Nowadays, technological advances in the fields of microscopy, molecular biology, and computer science are making it possible to see and record such complex interactions in incredible, unforeseen detail within the global context of the developing embryo. A quantitative and interdisciplinary perspective of developmental biology becomes then necessary for a comprehensive understanding of morphogenesis. Here, we provide a roadmap to quantify the events that lead to morphogenesis from imaging to image analysis, quantification, and modeling, focusing on the discrete cellular and tissue shape changes, as well as their mechanical properties.

Maintaining symmetry during body axis elongation

Bilateral symmetry defines much of the animal kingdom and is crucial for numerous functions of bilaterian organisms. Genetic approaches have discovered highly conserved patterning networks that establish bilateral symmetry in early embryos,1 but how this symmetry is maintained throughout subsequent morphogenetic events remains largely unknown.2 Here we show that the terminal patterning system-which relies on Ras/ERK signaling through activation of the Torso receptor by its ligand Trunk3-is critical for preserving bilateral symmetry during Drosophila body axis elongation, a process driven by cell rearrangements in the two identical lateral regions of the embryo and specified by the dorsal-ventral and anterior-posterior patterning systems.4 We demonstrate that fluctuating asymmetries in this rapid convergent-extension process are attenuated in normal embryos over time, possibly through noise-dissipating forces from the posterior midgut invagination and movement. However, when Torso signaling is attenuated via mutation of Trunk or RNAi directed against downstream Ras/ERK pathway components, body axis elongation results in a characteristic corkscrew phenotype,5 which reflects dramatic reorganization of global tissue flow and is incompatible with viability. Our results reveal a new function downstream of the Drosophila terminal patterning system in potentially active control of bilateral symmetry and should motivate systematic search for similar symmetry-preserving regulatory mechanisms in other bilaterians.

Tissue interplay during morphogenesis

The process by which biological systems such as cells, tissues and organisms acquire shape has been named as morphogenesis and it is central to a plethora of biological contexts including embryo development, wound healing, or even cancer. Morphogenesis relies in both self-organising properties of the system and in environmental inputs (biochemical and biophysical). The classical view of morphogenesis is based on the study of external biochemical molecules, such as morphogens. However, recent studies are establishing that the mechanical environment is also used by cells to communicate within tissues, suggesting that this mechanical crosstalk is essential to synchronise morphogenetic transitions and self-organisation. In this article we discuss how tissue interaction drive robust morphogenesis, starting from a classical biochemical view, to finalise with more recent advances on how the biophysical properties of a tissue feedback with their surroundings to allow form acquisition. We also comment on how in silico models aid to integrate and predict changes in cell and tissue behaviour. Finally, considering recent advances from the developmental biomechanics field showing that mechanical inputs work as cues that promote morphogenesis, we invite to revisit the concept of morphogen.

Alcam-a and Pdgfr-α are essential for the development of sclerotome-derived stromal cells that support hematopoiesis

Mesenchymal stromal cells are essential components of hematopoietic stem and progenitor cell (HSPC) niches, regulating HSPC proliferation and fates. Their developmental origins are largely unknown. In zebrafish, we previously found that the stromal cells of the caudal hematopoietic tissue (CHT), a niche functionally homologous to the mammalian fetal liver, arise from the ventral part of caudal somites. We have now found that this ventral domain is the sclerotome, and that two markers of mammalian mesenchymal stem/stromal cells, Alcam and Pdgfr-α, are distinctively expressed there and instrumental for the emergence and migration of stromal cell progenitors, which in turn conditions the proper assembly of the vascular component of the CHT niche. Furthermore, we find that trunk somites are similarly dependent on Alcam and Pdgfr-α to produce mesenchymal cells that foster HSPC emergence from the aorta. Thus the sclerotome contributes essential stromal cells for each of the key steps of developmental hematopoiesis.

Dand5 is involved in zebrafish tailbud cell movement

During vertebrate development, symmetry breaking occurs in the left-right organizer (LRO). The transfer of asymmetric molecular information to the lateral plate mesoderm is essential for the precise patterning of asymmetric internal organs, such as the heart. However, at the same developmental time, it is crucial to maintain symmetry at the somite level for correct musculature and vertebrae specification. We demonstrate how left-right signals affect the behavior of zebrafish somite cell precursors by using live imaging and fate mapping studies in dand5 homozygous mutants compared to wildtype embryos. We describe a population of cells in the vicinity of the LRO, named Non-KV Sox17:GFP+ Tailbud Cells (NKSTCs), which migrate anteriorly and contribute to future somites. We show that NKSTCs originate in a cluster of cells aligned with the midline, posterior to the LRO, and leave that cluster in a left-right alternating manner, primarily from the left side. Fate mapping revealed that more NKSTCs integrated somites on the left side of the embryo. We then abolished the asymmetric cues from the LRO using dand5-/- mutant embryos and verified that NKSTCs no longer displayed asymmetric patterns. Cell exit from the posterior cluster became bilaterally synchronous in dand5-/- mutants. Our study revealed a new link between somite specification and Dand5 function. The gene dand5 is well known as the first asymmetric gene involved in vertebrate LR development. This study revealed a new link for Dand5 as a player in cell exit from the maturation zone into the presomitic mesoderm, affecting the expression patterns of myogenic factors and tail size.

Deconstructing body axis morphogenesis in zebrafish embryos using robot-assisted tissue micromanipulation

Classic microsurgical techniques, such as those used in the early 1900s by Mangold and Spemann, have been instrumental in advancing our understanding of embryonic development. However, these techniques are highly specialized, leading to issues of inter-operator variability. Here we introduce a user-friendly robotic microsurgery platform that allows precise mechanical manipulation of soft tissues in zebrafish embryos. Using our platform, we reproducibly targeted precise regions of tail explants, and quantified the response in real-time by following notochord and presomitic mesoderm (PSM) morphogenesis and segmentation clock dynamics during vertebrate anteroposterior axis elongation. We find an extension force generated through the posterior notochord that is strong enough to buckle the structure. Our data suggest that this force generates a unidirectional notochord extension towards the tailbud because PSM tissue around the posterior notochord does not let it slide anteriorly. These results complement existing biomechanical models of axis elongation, revealing a critical coupling between the posterior notochord, the tailbud, and the PSM, and show that somite patterning is robust against structural perturbations.

Optokinetic set-point adaptation functions as an internal dynamic calibration mechanism for oculomotor disequilibrium

Experience-dependent brain circuit plasticity underlies various sensorimotor learning and memory processes. Recently, a novel set-point adaptation mechanism was identified that accounts for the pronounced negative optokinetic afternystagmus (OKAN) following a sustained period of unidirectional optokinetic nystagmus (OKN) in larval zebrafish. To investigate the physiological significance of optokinetic set-point adaptation, animals in the current study were exposed to a direction-alternating optokinetic stimulation paradigm that better resembles their visual experience in nature. Our results reveal that not only was asymmetric alternating stimulation sufficient to induce the set-point adaptation and the resulting negative OKAN, but most strikingly, under symmetric alternating stimulation some animals displayed an inherent bias of the OKN gain in one direction, and that was compensated by the similar set-point adaptation. This finding, supported by mathematical modeling, suggests that set-point adaptation allows animals to cope with asymmetric optokinetic behaviors evoked by either external stimuli or innate oculomotor biases.

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Optokinetic set-point adaptation reflects the temporal integration of visual input

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Wild-type zebrafish larvae may display innate optokinetic left-right asymmetries

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The degree of the optokinetic asymmetry among larvae is normally distributed

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The innate optokinetic asymmetry can be compensated by the set-point adaptation

A Dilp8-dependent time window ensures tissue size adjustment in Drosophila

The control of organ size mainly relies on precise autonomous growth programs. However, organ development is subject to random variations, called developmental noise, best revealed by the fluctuating asymmetry observed between bilateral organs. The developmental mechanisms ensuring bilateral symmetry in organ size are mostly unknown. In Drosophila, null mutations for the relaxin-like hormone Dilp8 increase wing fluctuating asymmetry, suggesting that Dilp8 plays a role in buffering developmental noise. Here we show that size adjustment of the wing primordia involves a peak of dilp8 expression that takes place sharply at the end of juvenile growth. Wing size adjustment relies on a cross-organ communication involving the epidermis as the source of Dilp8. We identify ecdysone signaling as both the trigger for epidermal dilp8 expression and its downstream target in the wing primordia, thereby establishing reciprocal hormonal feedback as a systemic mechanism, which controls organ size and bilateral symmetry in a narrow developmental time window.

Mechanisms ensuring developmental precision are poorly understood. Here Blanco-Obregon et al. report reciprocal feedback between Dilp8 and Ecdysone, two hormones required during a precise time window of Drosophila development for organ size adjustment.

Transitions in development - an interview with Rashmi Priya

Rashmi Priya is a Group Leader at The Francis Crick Institute in London, UK. Her research combines genetic, cell biological and biophysical approaches to understand the complex morphogenetic events of organogenesis, using the zebrafish heart as a model system. We met Rashmi at the Crick to learn how she got started as a researcher, and to discuss the challenges of starting a lab in the middle of a global pandemic.

Cell–Fibronectin Interactions and Actomyosin Contractility Regulate the Segmentation Clock and Spatio-Temporal Somite Cleft Formation during Chick Embryo Somitogenesis

Fibronectin is essential for somite formation in the vertebrate embryo. Fibronectin matrix assembly starts as cells emerge from the primitive streak and ingress in the unsegmented presomitic mesoderm (PSM). PSM cells undergo cyclic waves of segmentation clock gene expression, followed by Notch-dependent upregulation of meso1 in the rostral PSM which induces somite cleft formation. However, the relevance of the fibronectin matrix for these molecular processes remains unknown. Here, we assessed the role of the PSM fibronectin matrix in the spatio-temporal regulation of chick embryo somitogenesis by perturbing (1) extracellular fibronectin matrix assembly, (2) integrin–fibronectin binding, (3) Rho-associated protein kinase (ROCK) activity and (4) non-muscle myosin II (NM II) function. We found that integrin–fibronectin engagement and NM II activity are required for cell polarization in the nascent somite. All treatments resulted in defective somitic clefts and significantly perturbed meso1 and segmentation clock gene expression in the PSM. Importantly, inhibition of actomyosin-mediated contractility increased the period of hairy1/hes4 oscillations from 90 to 120 min. Together, our work strongly suggests that the fibronectin–integrin–ROCK–NM II axis regulates segmentation clock dynamics and dictates the spatio-temporal localization of somitic clefts.