Changes in the shape of the developing vertebrate nervous system analyzed experimentally, mathematically and by computer simulation

PCP-dependent polarized mechanics in the cortex of individual cells during convergent extension

Convergent extension (CE) is a key process for tissue elongation during vertebrate development and is driven by polarized cell behaviors. Here, we used a novel image-based technique to investigate the mechanical properties of individual cells undergoing CE. Our results suggest a PCP- and Septin-dependent mechanical gradient, where cortical tension is higher at the anterior face of the cells compared with their posterior face. Disruption of PCP protein Vangl2 or its downstream effector Septin7 eliminates this mechanical polarity. These findings demonstrate a link between actin organization, PCP signaling, and mechanical polarization, providing new avenues into the mechanochemical regulation of cellular behaviors during CE.

Stretch-induced endogenous electric fields drive directed collective cell migration in vivo

Directed collective cell migration is essential for morphogenesis, and chemical, electrical, mechanical and topological features have been shown to guide cell migration in vitro. Here we provide in vivo evidence showing that endogenous electric fields drive the directed collective cell migration of an embryonic stem cell population-the cephalic neural crest of Xenopus laevis. We demonstrate that the voltage-sensitive phosphatase 1 is a key component of the molecular mechanism, enabling neural crest cells to specifically transduce electric fields into a directional cue in vivo. Finally, we propose that endogenous electric fields are mechanically established by the convergent extension movements of the ectoderm, which generate a membrane tension gradient that opens stretch-activated ion channels. Overall, these findings establish a role for electrotaxis in tissue morphogenesis, highlighting the functions of endogenous bioelectrical stimuli in non-neural contexts.

In vivo imaging in transgenic songbirds reveals superdiffusive neuron migration in the adult brain

Neuron migration is a key phase of neurogenesis, critical for the assembly and function of neuronal circuits. In songbirds, this process continues throughout life, but how these newborn neurons disperse through the adult brain is unclear. We address this question using in vivo two-photon imaging in transgenic zebra finches that express GFP in young neurons and other cell types. In juvenile and adult birds, migratory cells are present at a high density, travel in all directions, and make frequent course changes. Notably, these dynamic migration patterns are well fit by a superdiffusive model. Simulations reveal that these superdiffusive dynamics are sufficient to disperse new neurons throughout the song nucleus HVC. These results suggest that superdiffusive migration may underlie the formation and maintenance of nuclear brain structures in the postnatal brain and indicate that transgenic songbirds are a useful resource for future studies into the mechanisms of adult neurogenesis.

Mechanical control of neural plate folding by apical domain alteration

Vertebrate neural tube closure is associated with complex changes in cell shape and behavior, however, the relative contribution of these processes to tissue folding is not well understood. At the onset of Xenopus neural tube folding, we observed alternation of apically constricted and apically expanded cells. This apical domain heterogeneity was accompanied by biased cell orientation along the anteroposterior axis, especially at neural plate hinges, and required planar cell polarity signaling. Vertex models suggested that dispersed isotropically constricting cells can cause the elongation of adjacent cells. Consistently, in ectoderm, cell-autonomous apical constriction was accompanied by neighbor expansion. Thus, a subset of isotropically constricting cells may initiate neural plate bending, whereas a 'tug-of-war' contest between the force-generating and responding cells reduces its shrinking along the body axis. This mechanism is an alternative to anisotropic shrinking of cell junctions that are perpendicular to the body axis. We propose that apical domain changes reflect planar polarity-dependent mechanical forces operating during neural folding.

New tools reveal PCP-dependent polarized mechanics in the cortex and cytoplasm of single cells during convergent extension

Understanding biomechanics of biological systems is crucial for unraveling complex processes like tissue morphogenesis. However, current methods for studying cellular mechanics in vivo are limited by the need for specialized equipment and often provide limited spatiotemporal resolution. Here we introduce two new techniques, Tension by Transverse Fluctuation (TFlux) and in vivo microrheology, that overcome these limitations. They both offer time-resolved, subcellular biomechanical analysis using only fluorescent reporters and widely available microscopes. Employing these two techniques, we have revealed a planar cell polarity (PCP)-dependent mechanical gradient both in the cell cortex and the cytoplasm of individual cells engaged in convergent extension. Importantly, the non-invasive nature of these methods holds great promise for its application for uncovering subcellular mechanical variations across a wide array of biological contexts.

Summary

Non-invasive imaging-based techniques providing time-resolved biomechanical analysis at subcellular scales in developing vertebrate embryos.

Mechanical control of neural plate folding by apical domain alteration

Vertebrate neural tube closure is associated with complex changes in cell shape and behavior, however, the relative contribution of these processes to tissue folding is not well understood. In this study, we evaluated morphology of the superficial cell layer in the Xenopus neural plate. At the stages corresponding to the onset of tissue folding, we observed the alternation of cells with apically constricting and apically expanding apical domains. The cells had a biased orientation along the anteroposterior (AP) axis. This apical domain heterogeneity required planar cell polarity (PCP) signaling and was especially pronounced at neural plate hinges. Vertex model simulations suggested that spatially dispersed isotropically constricting cells cause the elongation of their non-constricting counterparts along the AP axis. Consistent with this hypothesis, cell-autonomous induction of apical constriction in Xenopus ectoderm cells was accompanied by the expansion of adjacent non-constricting cells. Our observations indicate that a subset of isotropically constricting cells can initiate neural plate bending, whereas a 'tug-of-war' contest between the force-generating and responding cells reduces its shrinking along the AP axis. This mechanism is an alternative to anisotropic shrinking of cell junctions that are perpendicular to the body axis. We propose that neural folding relies on PCP-dependent transduction of mechanical signals between neuroepithelial cells.

Somitic mesoderm morphogenesis is necessary for neural tube closure during Xenopus development

Neural tube closure is a fundamental process during vertebrate embryogenesis, which leads to the formation of the central nervous system. Defective neural tube closure leads to neural tube defects which are some of the most common human birth defects. While the intrinsic morphogenetic events shaping the neuroepithelium have been studied extensively, how tissues mechanically coupled with the neural plate influence neural tube closure remains poorly understood. Here, using Xenopus laevis embryos, live imaging in combination with loss of function experiments and morphometric analysis of fixed samples we explore the reciprocal mechanical communication between the neural plate and the somitic mesoderm and its impact on tissue morphogenesis. We show that although somitic mesoderm convergent extension occurs independently from neural plate morphogenesis neural tube closure depends on somitic mesoderm morphogenesis. Specifically, impaired somitic mesoderm remodelling results in defective apical constriction within the neuroepithelium and failure of neural tube closure. Last, our data reveal that mild abnormalities in somitic mesoderm and neural plate morphogenesis have a synergistic effect during neurulation, leading to severe neural tube closure defects. Overall, our data reveal that defective morphogenesis of tissues mechanically coupled with the neural plate can not only drastically exacerbate mild neural tube defects that may arise from abnormalities within the neural tissue but can also elicit neural tube defects even when the neural plate is itself free of inherent defects.

Dimensions of Morphological Integration

Over several generations of evolutionary and developmental biologists, ever since Olson and Miller’s pioneering work of the 1950’s, the concept of “morphological integration” as applied to Gaussian representations $$N(\mu ,\Sigma )$$ N ( μ , Σ ) of morphometric data has been a focus equally of methodological innovation and methodological perplexity. Reanalysis of a century-old example from Sewall Wright shows how some fallacies of distance analysis by correlations can be avoided by careful matching of the distance rosters involved to a different multivariate approach, factor analysis. I reinterpret his example by restoring the information (means and variances) ignored by the correlation matrix, while confirming what Wright called “special size factors” by a different technique, inspection of the concentration matrix $$\Sigma ^{-1}.$$ Σ - 1 . In geometric morphometrics (GMM), data accrue instead as Cartesian coordinates of labelled points; nevertheless, just as in the Wright example, statistical manipulations do better when they reconsider the normalizations that went into the generation of those coordinates. Here information about both $$\mu $$ μ and $$\Sigma ,$$ Σ , the means and the variances/covariances, can be preserved via the Boas coordinates (Procrustes shape coordinates without the size adjustment) that protect the role of size per se as an essential explanatory factor while permitting the analyst to acknowledge the realities of animal anatomy and its trajectories over time or size in the course of an analysis. A descriptive quantity for this purpose is suggested, the correlation of vectorized $$\mu $$ μ against the first eigenvector of $$\Sigma $$ Σ for the Boas coordinates. The paper reanalyzes two GMM data sets from this point of view. In one, the classic Vilmann rodent neurocranial growth data, a description of integration can be aligned with the purposes of evolutionary and developmental biology by a graphical exegesis based mainly in the loadings of the first Boas principal component. There results a multiplicity of morphometric patterns, some homogeneous and others characterized by gradients. In the other, a Vienna data set comprising human midsagittal skull sections mostly sampled along curves, a further integrated feature emerges, thickening of the calvaria, that requires a reparametrization and a modified thin plate spline graphic distinct from the digitized configurations per se. This new GMM protocol fulfills the original thrust of Olson & Miller’s (Evolution 5:325–338, 1951) “$$\rho $$ ρ F-groups,” the alignment of statistical and biological explanatory guidance, while respecting the enormously greater range of morphological descriptors afforded by well-designed landmark/semilandmark configurations.

Distinct spatiotemporal contribution of morphogenetic events and mechanical tissue coupling during Xenopus neural tube closure

Neural tube closure (NTC) is a fundamental process during vertebrate development and is indispensable for the formation of the central nervous system. Here, using Xenopus laevis embryos, live imaging, single-cell tracking, optogenetics and loss-of-function experiments, we examine the roles of convergent extension and apical constriction, and define the role of the surface ectoderm during NTC. We show that NTC is a two-stage process with distinct spatiotemporal contributions of convergent extension and apical constriction at each stage. Convergent extension takes place during the first stage and is spatially restricted at the posterior tissue, whereas apical constriction occurs during the second stage throughout the neural plate. We also show that the surface ectoderm is mechanically coupled with the neural plate and its movement during NTC is driven by neural plate morphogenesis. Finally, we show that an increase in surface ectoderm resistive forces is detrimental for neural plate morphogenesis.

Summary: Detailed characterization of the contribution of distinct morphogenetic processes and mechanical tissue coupling during neural tube closure, a process indispensable for central nervous system formation in vertebrates.

Hinge point emergence in mammalian spinal neurulation

Neurulation is the process in early vertebrate embryonic development during which the neural plate folds to form the neural tube. Spinal neural tube folding in the posterior neuropore changes over time, first showing a median hinge point, then both the median hinge point and dorsolateral hinge points, followed by dorsolateral hinge points only. The biomechanical mechanism of hinge point formation in the mammalian neural tube is poorly understood. Here we employ a mechanical finite element model to study neural tube formation. The computational model mimics the mammalian neural tube using microscopy data from mouse and human embryos. While intrinsic curvature at the neural plate midline has been hypothesized to drive neural tube folding, intrinsic curvature was not sufficient for tube closure in our simulations. We achieved neural tube closure with an alternative model combining mesoderm expansion, nonneural ectoderm expansion, and neural plate adhesion to the notochord. Dorsolateral hinge points emerged in simulations with low mesoderm expansion and zippering. We propose that zippering provides the biomechanical force for dorsolateral hinge point formation in settings where the neural plate lateral sides extend above the mesoderm. Together, these results provide a perspective on the biomechanical and molecular mechanism of mammalian spinal neurulation.

Global analysis of cell behavior and protein dynamics reveals region-specific roles for Shroom3 and N-cadherin during neural tube closure

Failures of neural tube closure are common and serious birth defects, yet we have a poor understanding of the interaction of genetics and cell biology during neural tube closure. Additionally, mutations that cause neural tube defects (NTDs) tend to affect anterior or posterior regions of the neural tube but rarely both, indicating a regional specificity to NTD genetics. To better understand the regional specificity of cell behaviors during neural tube closure, we analyzed the dynamic localization of actin and N-cadherin via high-resolution tissue-level time-lapse microscopy during Xenopus neural tube closure. To investigate the regionality of gene function, we generated mosaic mutations in shroom3, a key regulator or neural tube closure. This new analytical approach elucidates several differences between cell behaviors during cranial/anterior and spinal/posterior neural tube closure, provides mechanistic insight into the function of shroom3, and demonstrates the ability of tissue-level imaging and analysis to generate cell biological mechanistic insights into neural tube closure.

A New Method for Landmark-Based Studies of the Dynamic Stability of Growth, with Implications for Evolutionary Analyses

A matrix manipulation new to the quantitative study of develomental stability reveals unexpected morphometric patterns in a classic data set of landmark-based calvarial growth. There are implications for evolutionary studies. Among organismal biology’s fundamental postulates is the assumption that most aspects of any higher animal’s growth trajectories are dynamically stable, resilient against the types of small but functionally pertinent transient perturbations that may have originated in genotype, morphogenesis, or ecophenotypy. We need an operationalization of this axiom for landmark data sets arising from longitudinal data designs. The present paper introduces a multivariate approach toward that goal: a method for identification and interpretation of patterns of dynamical stability in longitudinally collected landmark data. The new method is based in an application of eigenanalysis unfamiliar to most organismal biologists: analysis of a covariance matrix of Boas coordinates (Procrustes coordinates without the size standardization) against their changes over time. These eigenanalyses may yield complex eigenvalues and eigenvectors (terms involving $$i=\sqrt{-1}$$ i = - 1 ); the paper carefully explains how these are to be scattered, gridded, and interpreted by their real and imaginary canonical vectors. For the Vilmann neurocranial octagons, the classic morphometric data set used as the running example here, there result new empirical findings that offer a pattern analysis of the ways perturbations of growth are attenuated or otherwise modified over the course of developmental time. The main finding, dominance of a generalized version of dynamical stability (negative autoregressions, as announced by the negative real parts of their eigenvalues, often combined with shearing and rotation in a helpful canonical plane), is surprising in its strength and consistency. A closing discussion explores some implications of this novel pattern analysis of growth regulation. It differs in many respects from the usual way covariance matrices are wielded in geometric morphometrics, differences relevant to a variety of study designs for comparisons of development across species.

Neurulation in the posterior region of zebrafish, Danio rerio embryos

The neural tube of amniotes is formed through different mechanisms that take place in the anterior and posterior regions and involve neural plate folding or mesenchymal condensation followed by its cavitation. Meanwhile, in teleost trunk region, the neural plate forms the neural keel, while the lumen develops later. However, the data on neurulation and other morphogenetic processes in the posterior body region in Teleostei remain fragmentary. We proposed that there could be variations in the morphogenetic processes, such as cell shape changes and cell rearrangements, in the posterior region compared to the anterior one at the different stages. Here, we performed morphological and histochemical analyses of morphogenetic processes with an emphasis on neurulation in the zebrafish tail bud (TB) and posterior region. To analyze the posterior expression of sox2 and tbxta we performed whole mount in situ hybridization. We showed that the TB cells of variable shapes and orientation are tightly packed, and the neural and notochord primordia develop first. The shape of the neural primordium undergoes numerous changes as a result of cell rearrangements leading to the development of the neural rod. At the prim-6 stage, the cells of the neural primordium directly form the neural rod. The neuroepithelial cells undergo sequential shape changes. At the stage of the neural rod formation, the apical regions of triangular neuroepithelial cells of the floor plate are enriched in F-actin. The neurocoel development onset is above the apical poles of neuroepithelial cells. The expression domains of sox2 and tbxta become more restricted during the development.

A short tutorial on the Janus-faced logic of differentiation waves and differentiation trees and their evolution

Differentiation waves offer a different perspective on causality in embryogenesis from that of molecular developmental biology. Janus-faced cybernetic logic, with global and local top down/bottom up dynamics, eschews reductionism, is distinct from emergence, and outlines the process theoretically. Most aspects of differentiation waves require further investigation.

From genes to shape during metamorphosis: a history

Metamorphosis (Greek for a state of transcending-form or change-in-shape) refers to a dramatic transformation of an animal's body structure that occurs after development of the embryo or larva in many species. The development of a fly (or butterfly) from a crawling larva (or caterpillar) that forms a pupa (or chrysalis) before eclosing as a flying adult is a classic example of metamorphosis that captures the imagination and has been immortalized in children's books. Powerful genetic experiments in the fruit fly Drosophila melanogaster have revealed how genes can instruct the behaviour of individual cells to control patterns of tissue growth, mechanical force, cell-cell adhesion and cell-matrix adhesion drive morphogenetic change in epithelial tissues. Together, the distribution of mass, force and resistance determines cell shape changes, cell-cell rearrangements, and/or the orientation of cell divisions to generate the final form of the tissue. In organising tissue shape, genes harness the power of self-organisation to determine the collective behaviour of molecules and cells, which can often be reproduced in computer simulations of cell polarity and/or tissue mechanics. This review highlights fundamental discoveries in epithelial morphogenesis made by pioneers who were fascinated by metamorphosis, including D'Arcy Thompson, Conrad Waddington, Dianne Fristrom and Antonio Garcia-Bellido.

Surface contraction waves or cell proliferation waves in the presumptive neurectoderm during amphibian gastrulation: Mexican axolotl versus African clawed frog

This essay represents a critical analysis of the literary data on various types of waves occurring in the amphibian embryos during gastrulation. A surface contraction wave travels through the presumptive neurectoderm during Mexican axolotl gastrulation. This wave coincides temporally and spatially with involution of the inducing chordomesoderm and with the prospective neural plate. By contrast, there is no similar surface contraction wave during African clawed frog gastrulation. However, the clawed frog displays the waves of DNA synthesis and mitosis in the presumptive neurectoderm during gastrulation, whereas no such waves were discovered in axolotl gastrulae. These sets of experimental data are in accordance with the contemporary concept of considerable ontogenetic diversity of the class Amphibia.

French flag gradients and Turing reaction-diffusion versus differentiation waves as models of morphogenesis

The Turing reaction-diffusion model and the French Flag Model are widely accepted in the field of development as the best models for explaining embryogenesis. Virtually all current attempts to understand cell differentiation in embryos begin and end with the assumption that some combination of these two models works. The result may become a bias in embryogenesis in assuming the problem has been solved by these two-chemical substance-based models. Neither model is applied consistently. We review the differences between the French Flag, Turing reaction-diffusion model, and a mechanochemical model called the differentiation wave/cell state splitter model. The cytoskeletal cell state splitter and the embryonic differentiation waves was first proposed in 1987 as a combined physics and chemistry model for cell differentiation in embryos, based on empirical observations on urodele amphibian embryos. We hope that the development of theory can be advanced and observations relevant to distinguishing the embryonic differentiation wave model from the French Flag model and reaction-diffusion equations will be taken up by experimentalists. Experimentalists rely on mathematical biologists for theory, and therefore depend on them for what parameters they choose to measure and ignore. Therefore, mathematical biologists need to fully understand the distinctions between these three models.

Convergent extension in the amphibian, Xenopus laevis

This review is a comprehensive analysis of the cell biology and biomechanics of Convergent Extension in Xenopus.

Cell rearrangement induced by filopodial tension accounts for the late phase of convergent extension in the sea urchin archenteron

George Oster was a pioneer in using mechanical models to interrogate morphogenesis in animal embryos. Convergent extension is a particularly important morphogenetic process to which George Oster gave significant attention. Late elongation of the sea urchin archenteron is a classic example of convergent extension in a monolayered tube, which has been proposed to be driven by extrinsic axial tension due to the activity of secondary mesenchyme cells. Using a vertex-based mechanical model, we show that key features of archenteron elongation can be accounted for by passive cell rearrangement due to applied tension. The model mimics the cell elongation and the Poisson effect (necking) that occur in actual archenterons. We also show that, as predicted by the model, ablation of secondary mesenchyme cells late in archenteron elongation does not result in extensive elastic recoil. Moreover, blocking the addition of cells to the base of the archenteron late in archenteron elongation leads to excessive cell rearrangement consistent with tension-induced rearrangement of a smaller cohort of cells. Our mechanical simulation suggests that responsive rearrangement can account for key features of archenteron elongation and provides a useful starting point for designing future experiments to examine the mechanical properties of the archenteron.

FAT1 cadherin controls neuritogenesis during NTera2 cell differentiation

Fat1 cadherin is broadly expressed throughout the nervous system and has been implicated in neuronal differentiation. Here we examined the functional contribution of FAT1 during neuronal differentiation of the Ntera2 cell line model. FAT1 expression was increased during the retinoic acid (RA)-induced differentiation of NTera2 cells. Depletion of FAT1 with siRNA decreased the number of neurites produced after RA treatment. Moreover, FAT1 silencing also led to decreased Ser127-phosphorylation of YAP along with transcriptional increases in the Hippo target genes CTGF and ANKRD1, suggesting FAT1 alters Hippo signalling during differentiation. In the context of the Ntera2 model, FAT1 is required for efficient neuritogenesis, acting as a regulator of neurite formation during the early stages of differentiation.

Professor Lev Beloussov and the birth of morphomechanics

The first explanations of the mechanisms of development of living organisms were proposed in antiquity. At that time two competing ideas existed, about the strict determination of embryonic structures (we call it the "Hippocrates line") and about the possible formation of structures from the unstructured condition ("Aristotle line"). We can trace the opposition between the "Hippocrates line" and "Aristotle line" from antiquity till the present time. At the end of the XIX century, experimental investigation of the mechanisms of integrity of development had started. In the XX century, the "Aristotle line" finds its expression in the Morphogenetic Field Theory of A.G. Gurwitsch, according to which cells of the organism are integrated in an organic whole. Since the 1970s, mechanical forces and tensions have been considered as integral factors of ontogenesis. One of the most productive scientific teams which worked in this area was the laboratory of Professor L.V. Beloussov from the Lomonossov Moscow State University, Russia. In the 1970s, Lev Beloussov and his colleagues discovered the presence of "passive" and "active" (i.e. metabolically-dependent) mechanical stresses in the tissues of developing organisms, their organization and stage-specific patterns. In 1980-1990 s, a lot of experimental data about the role of the patterns of mechanical stresses in morphogenesis and cell differentiation was accumulated. Based on the experimental data, Professor Beloussov and his colleagues developed a theory of the regulation of the development of living organisms on the basis of the interaction of passive and active mechanical stresses (Belousov-Mittenthal Theory), which forms the basis of a new science - morphomechanics.

Increasing complexity: Mechanical guidance and feedback loops as a basis for self-organization in morphogenesis

The article is devoted to physical views on embryo development as a combination of structurally stable dynamics and symmetry-breaking events in the general process of self-organization. The first corresponds to the deterministic aspect of embryo development. The second type of processes is associated with sudden increase of variability in the periods of symmetry-breaking, which manifests unstable dynamics. The biological basis under these considerations includes chemokinetics (a system of inductors, repressors, and interaction with their next surrounding) and morphomechanics (i.e. mechanotransduction, mechanosensing, and related feedback loops). Although the latter research area is evolving rapidly, up to this time the role of mechanical properties of embryonic tissues and mechano-dependent processes in them are integrated in the general picture of embryo development to a lesser extent than biochemical signaling. For this reason, the present article is mostly devoted to experimental data on morphomechanics in the process of embryo development, also including analysis of its limitations and possible contradictions. The general system of feedback-loops and system dynamics delineated in this review is in large part a repetition of the views of Lev Beloussov, who was one of the founders of the whole areas of morphomechanics and morphodynamics, and to whose memory this article is dedicated.

A general framework dedicated to computational morphogenesis Part I - Constitutive equations

In order to understand living organisms, considerable experimental efforts and resources have been devoted to correlate genes and their expressions with cell, tissue, organ and whole organisms' phenotypes. This data driven approach to knowledge discovery has led to many breakthrough in our understanding of healthy and diseased states, and is paving the way to improve the diagnosis and treatment of diseases. Complementary to this data-driven approach, computational models of biological systems based on first principles have been developed in order to deepen our understanding of the multi-scale dynamics that drives normal and pathological biological functions. In this paper we describe the biological, physical and mathematical concepts that led to the design of a Computational Morphogenesis (CM) platform baptized Generic Modeling and Simulating Platform (GMSP). Its role is to generate realistic 3D multi-scale biological tissues from virtual stem cells and the intended target applications include in virtuo studies of normal and abnormal tissue (re)generation as well as the development of complex diseases such as carcinogenesis. At all space-scales of interest, biological agents interact with each other via biochemical, bioelectrical, and mechanical fields that operate in concert during embryogenesis, growth and adult life. The spatio-temporal dependencies of these fields can be modeled by physics-based constitutive equations that we propose to examine in relation to the landmark biological events that occur during embryogenesis.

The effect of magnetic resonance imaging on neural tube development in an early chicken embryo model

PURPOSE

We aimed to determine whether varying the magnetic field during magnetic resonance imaging would affect the development of chicken embryos and neural tube defects.

METHODS

Following incubation for 24 h, we exposed chicken embryos to varying magnetic fields for 10 min to assess the impact on development. Three magnetic resonance imaging devices were used, and the eggs were divided into four groups: group 1 is exposed to 1 T, group 2 is exposed to 1.5 T, group 3 is exposed to 3 T, and group 4, control group, was not exposed to magnetic field. After MRI exposure, all embryos were again put inside incubator to complete 48 h. "The new technique" was used to open eggs, a stereomicroscope was used for the examination of magnified external morphology, and each embryo was examined according to the Hamburger and Hamilton chicken embryo stages. Embryos who had delayed stages of development are considered growth retarded. Growth retardation criteria do not include small for stage.

RESULTS

Compared with embryos not exposed to a magnetic field, there was a statistically significant increase in the incidence of neural tube closure defects and growth retardation in the embryos exposed to magnetic fields (p < 0.05). However, although the incidence of neural tube closure defects was expected to increase as exposure (tesla level) increased, we found a higher rate of defects in the 1.5-T group compared with the 3-T group. By contrast, the highest incidence of growth retardation was in the 3-T group, which was consistent with our expectation that growth retardation would be more likely as tesla level increased.

CONCLUSIONS

We therefore conclude that the use of magnetic resonance imaging as a diagnostic tool can result in midline closure defects and growth retardation in chicken embryos. We hypothesize that this may also be true for human embryos exposed to MRI. If a pregnant individual is to take an MRI scan, as for lumbar disc disease or any other any other reason, our results indicate that consideration should be given to an avoidance of MRI during pregnancy.

Large, long range tensile forces drive convergence during Xenopus blastopore closure and body axis elongation

Indirect evidence suggests that blastopore closure during gastrulation of anamniotes, including amphibians such as Xenopus laevis, depends on circumblastoporal convergence forces generated by the marginal zone (MZ), but direct evidence is lacking. We show that explanted MZs generate tensile convergence forces up to 1.5 μN during gastrulation and over 4 μN thereafter. These forces are generated by convergent thickening (CT) until the midgastrula and increasingly by convergent extension (CE) thereafter. Explants from ventralized embryos, which lack tissues expressing CE but close their blastopores, produce up to 2 μN of tensile force, showing that CT alone generates forces sufficient to close the blastopore. Uniaxial tensile stress relaxation assays show stiffening of mesodermal and ectodermal tissues around the onset of neurulation, potentially enhancing long-range transmission of convergence forces. These results illuminate the mechanobiology of early vertebrate morphogenic mechanisms, aid interpretation of phenotypes, and give insight into the evolution of blastopore closure mechanisms.

Neural tube closure: cellular, molecular and biomechanical mechanisms

Neural tube closure has been studied for many decades, across a range of vertebrates, as a paradigm of embryonic morphogenesis. Neurulation is of particular interest in view of the severe congenital malformations - 'neural tube defects' - that result when closure fails. The process of neural tube closure is complex and involves cellular events such as convergent extension, apical constriction and interkinetic nuclear migration, as well as precise molecular control via the non-canonical Wnt/planar cell polarity pathway, Shh/BMP signalling, and the transcription factors Grhl2/3, Pax3, Cdx2 and Zic2. More recently, biomechanical inputs into neural tube morphogenesis have also been identified. Here, we review these cellular, molecular and biomechanical mechanisms involved in neural tube closure, based on studies of various vertebrate species, focusing on the most recent advances in the field.

Biomechanical coupling facilitates spinal neural tube closure in mouse embryos

Neural tube (NT) formation in the spinal region of the mammalian embryo involves a wave of "zippering" that passes down the elongating spinal axis, uniting the neural fold tips in the dorsal midline. Failure of this closure process leads to open spina bifida, a common cause of severe neurologic disability in humans. Here, we combined a tissue-level strain-mapping workflow with laser ablation of live-imaged mouse embryos to investigate the biomechanics of mammalian spinal closure. Ablation of the zippering point at the embryonic dorsal midline causes far-reaching, rapid separation of the elevating neural folds. Strain analysis revealed tissue expansion around the zippering point after ablation, but predominant tissue constriction in the caudal and ventral neural plate zone. This zone is biomechanically coupled to the zippering point by a supracellular F-actin network, which includes an actin cable running along the neural fold tips. Pharmacologic inhibition of F-actin or laser ablation of the cable causes neural fold separation. At the most advanced somite stages, when completion of spinal closure is imminent, the cable forms a continuous ring around the neuropore, and simultaneously, a new caudal-to-rostral zippering point arises. Laser ablation of this new closure initiation point causes neural fold separation, demonstrating its biomechanical activity. Failure of spinal closure in pre-spina bifida Zic2Ku mutant embryos is associated with altered tissue biomechanics, as indicated by greater neuropore widening after ablation. Thus, this study identifies biomechanical coupling of the entire region of active spinal neurulation in the mouse embryo as a prerequisite for successful NT closure.

A. D. Mechanics of neurulation: From classical to current perspectives on the physical mechanics that shape, fold, and form the neural tube

Neural tube defects arise from mechanical failures in the process of neurulation. At the most fundamental level, formation of the neural tube relies on coordinated, complex tissue movements that mechanically transform the flat neural epithelium into a lumenized epithelial tube (Davidson, 2012). The nature of this mechanical transformation has mystified embryologists, geneticists, and clinicians for more than 100 years. Early embryologists pondered the physical mechanisms that guide this transformation. Detailed observations of cell and tissue movements as well as experimental embryological manipulations allowed researchers to generate and test elementary hypotheses of the intrinsic and extrinsic forces acting on the neural tissue. Current research has turned toward understanding the molecular mechanisms underlying neurulation. Genetic and molecular perturbation have identified a multitude of subcellular components that correlate with cell behaviors and tissue movements during neural tube formation. In this review, we focus on methods and conceptual frameworks that have been applied to the study of amphibian neurulation that can be used to determine how molecular and physical mechanisms are integrated and responsible for neurulation. We will describe how qualitative descriptions and quantitative measurements of strain, force generation, and tissue material properties as well as simulations can be used to understand how embryos use morphogenetic programs to drive neurulation. Birth Defects Research 109:153-168, 2017. © 2016 Wiley Periodicals, Inc.

Tissue morphodynamics shaping the early mouse embryo

Generation of the elongated vertebrate body plan from the initially radially symmetrical embryo requires comprehensive changes to tissue form. These shape changes are generated by specific underlying cell behaviors, coordinated in time and space. Major principles and also specifics are emerging, from studies in many model systems, of the cell and physical biology of how region-specific cell behaviors produce regional tissue morphogenesis, and how these, in turn, are integrated at the level of the embryo. New technical approaches have made it possible more recently, to examine the morphogenesis of the mouse embryo in depth, and to elucidate the underlying cellular mechanisms. This review focuses on recent advances in understanding the cellular basis for the early fundamental events that establish the basic form of the embryo.

Mechanotransduction During Vertebrate Neurulation

Vertebrate neural tube formation is a complex morphogenetic process, which involves hundreds of genes dynamically coordinating various behaviors in different cell populations of neural tissue. The challenge remains to determine the relative contributions of physical forces and biochemical signaling events to neural tube closure and accompanying cell fate specification. Planar cell polarity (PCP) molecules are prime candidate factors for the production of actomyosin-dependent mechanical signals necessary for morphogenesis. Conversely, physical forces may contribute to the polarized distribution of PCP proteins. Understanding mechanosensory and mechanotransducing properties of diverse molecules should help define the direction and amplitude of physical stresses that are critical for neurulation.

Btk-dependent epithelial cell rearrangements contribute to the invagination of nearby tubular structures in the posterior spiracles of Drosophila

The Drosophila respiratory system consists of two connected organs, the tracheae and the spiracles. Together they ensure the efficient delivery of air-borne oxygen to all tissues. The posterior spiracles consist internally of the spiracular chamber, an invaginated tube with filtering properties that connects the main tracheal branch to the environment, and externally of the stigmatophore, an extensible epidermal structure that covers the spiracular chamber. The primordia of both components are first specified in the plane of the epidermis and subsequently the spiracular chamber is internalized through the process of invagination accompanied by apical cell constriction. It has become clear that invagination processes do not always or only rely on apical constriction. We show here that in mutants for the src-like kinase Btk29A spiracle cells constrict apically but do not complete invagination, giving rise to shorter spiracular chambers. This defect can be rescued by using different GAL4 drivers to express Btk29A throughout the ectoderm, in cells of posterior segments only, or in the stigmatophore pointing to a non cell-autonomous role for Btk29A. Our analysis suggests that complete invagination of the spiracular chamber requires Btk29A-dependent planar cell rearrangements of adjacent non-invaginating cells of the stigmatophore. These results highlight the complex physical interactions that take place among organ components during morphogenesis, which contribute to their final form and function.

Distinct apical and basolateral mechanisms drive planar cell polarity-dependent convergent extension of the mouse neural plate

The mechanisms of tissue convergence and extension (CE) driving axial elongation in mammalian embryos, and in particular, the cellular behaviors underlying CE in the epithelial neural tissue, have not been identified. Here we show that mouse neural cells undergo mediolaterally biased cell intercalation and exhibit both apical boundary rearrangement and polarized basolateral protrusive activity. Planar polarization and coordination of these two cell behaviors are essential for neural CE, as shown by failure of mediolateral intercalation in embryos mutant for two proteins associated with planar cell polarity signaling: Vangl2 and Ptk7. Embryos with mutations in Ptk7 fail to polarize cell behaviors within the plane of the tissue, whereas Vangl2 mutant embryos maintain tissue polarity and basal protrusive activity but are deficient in apical neighbor exchange. Neuroepithelial cells in both mutants fail to apically constrict, leading to craniorachischisis. These results reveal a cooperative mechanism for cell rearrangement during epithelial morphogenesis.

Cell segregation, mixing, and tissue pattern in the spinal cord of the Xenopus laevis neurula

BACKGROUND

During Xenopus laevis neurulation, neural ectodermal cells of the spinal cord are patterned at the same time that they intercalate mediolaterally and radially, moving within and between two cell layers. Curious if these rearrangements disrupt early cell identities, we lineage-traced cells in each layer from neural plate stages to the closed neural tube, and used in situ hybridization to assay gene expression in the moving cells.

RESULTS

Our biotin and fluorescent labeling of deep and superficial cells reveals that mediolateral intercalation does not disrupt cell cohorts; in other words, it is conservative. However, outside the midline notoplate, later radial intercalation does displace superficial cells dorsoventrally, radically disrupting cell cohorts. The tube roof is composed almost exclusively of superficial cells, including some displaced from ventral positions; gene expression in these displaced cells must now be surveyed further. Superficial cells also flank the tube's floor, which is, itself, almost exclusively composed of deep cells.

CONCLUSIONS

Our data provide: (1) a fate map of superficial- and deep-cell positions within the Xenopus neural tube, (2) the paths taken to these positions, and (3) preliminary evidence of re-patterning in cells carried out of one environment and into another, during neural morphogenesis.

Planar morphometry, shear and optimal quasi-conformal mappings

To characterize the diversity of planar shapes in such instances as insect wings and plant leaves, we present a method for the generation of a smooth morphometric mapping between two planar domains which matches a number of homologous points. Our approach tries to balance the competing requirements of a descriptive theory which may not reflect mechanism and a multi-parameter predictive theory that may not be well constrained by experimental data. Specifically, we focus on aspects of shape as characterized by local rotation and shear, quantified using quasi-conformal maps that are defined precisely in terms of these fields. To make our choice optimal, we impose the condition that the maps vary as slowly as possible across the domain, minimizing their integrated squared-gradient. We implement this algorithm numerically using a variational principle that optimizes the coefficients of the quasi-conformal map between the two regions and show results for the recreation of a sample historical grid deformation mapping of D’Arcy Thompson. We also deploy our method to compare a variety of Drosophila wing shapes and show that our approach allows us to recover aspects of phylogeny as marked by morphology.

Neural tube closure: the curious case of shrinking junctions

Your brain and spinal cord began as a flat sheet, which narrowed, elongated, and rolled up to form a tube. A new study identifies a key molecular link underlying vertebrate neural tube formation, connecting planar cell polarity patterning to contraction of specific cell-cell junctions.

Computational models for mechanics of morphogenesis

In the developing embryo, tissues differentiate, deform, and move in an orchestrated manner to generate various biological shapes driven by the complex interplay between genetic, epigenetic, and environmental factors. Mechanics plays a key role in regulating and controlling morphogenesis, and quantitative models help us understand how various mechanical forces combine to shape the embryo. Models allow for the quantitative, unbiased testing of physical mechanisms, and when used appropriately, can motivate new experimentaldirections. This knowledge benefits biomedical researchers who aim to prevent and treat congenital malformations, as well as engineers working to create replacement tissues in the laboratory. In this review, we first give an overview of fundamental mechanical theories for morphogenesis, and then focus on models for specific processes, including pattern formation, gastrulation, neurulation, organogenesis, and wound healing. The role of mechanical feedback in development is also discussed. Finally, some perspectives aregiven on the emerging challenges in morphomechanics and mechanobiology.

Planar cell polarity and the developmental control of cell behavior in vertebrate embryos

Planar cell polarity (PCP), the orientation and alignment of cells within a sheet, is a ubiquitous cellular property that is commonly governed by the conserved set of proteins encoded by so-called PCP genes. The PCP proteins coordinate developmental signaling cues with individual cell behaviors in a wildly diverse array of tissues. Consequently, disruptions of PCP protein functions are linked to defects in axis elongation, inner ear patterning, neural tube closure, directed ciliary beating, and left/right patterning, to name only a few. This review attempts to synthesize what is known about PCP and the PCP proteins in vertebrate animals, with a particular focus on the mechanisms by which individual cells respond to PCP cues in order to execute specific cellular behaviors.

Planar cell polarity links axes of spatial dynamics in neural-tube closure

Neural-tube closure is a critical step of embryogenesis, and its failure causes serious birth defects. Coordination of two morphogenetic processes--convergent extension and neural-plate apical constriction--ensures the complete closure of the neural tube. We now provide evidence that planar cell polarity (PCP) signaling directly links these two processes. In the bending neural plates, we find that a PCP-regulating cadherin, Celsr1, is concentrated in adherens junctions (AJs) oriented toward the mediolateral axes of the plates. At these AJs, Celsr1 cooperates with Dishevelled, DAAM1, and the PDZ-RhoGEF to upregulate Rho kinase, causing their actomyosin-dependent contraction in a planar-polarized manner. This planar-polarized contraction promotes simultaneous apical constriction and midline convergence of neuroepithelial cells. Together our findings demonstrate that PCP signals confer anisotropic contractility on the AJs, producing cellular forces that promote the polarized bending of the neural plate.