Can tissue surface tension drive somite formation?

A chemo-mechanical model of endoderm movements driving elongation of the amniote hindgut

Although mechanical and biochemical descriptions of development are each essential, integration of upstream morphogenic cues with downstream tissue mechanics remains understudied during vertebrate morphogenesis. Here, we developed a two-dimensional chemo-mechanical model to investigate how mechanical properties of the endoderm and transport properties of fibroblast growth factor (FGF) regulate avian hindgut morphogenesis in a coordinated manner. Posterior endoderm cells convert a gradient of FGF ligands into a contractile force gradient, leading to a force imbalance that drives collective cell movements that elongate the forming hindgut tube. We formulated a 2D reaction-diffusion-advection model describing the formation of an FGF protein gradient as a result of posterior displacement of cells transcribing unstable Fgf8 mRNA during axis elongation, coupled with translation, diffusion and degradation of FGF protein. The endoderm was modeled as an active viscous fluid that generates contractile stresses in proportion to FGF concentration. With parameter values constrained by experimental data, the model replicates key aspects of hindgut morphogenesis, suggests that graded isotropic contraction is sufficient to generate large anisotropic cell movements, and provides new insight into how chemo-mechanical coupling across the mesoderm and endoderm coordinates hindgut elongation with axis elongation.

Left-right symmetry of zebrafish embryos requires somite surface tension

The body axis of vertebrate embryos is periodically segmented into bilaterally symmetric pairs of somites^1,2. The anteroposterior length of somites, their position and left-right symmetry are thought to be molecularly determined before somite morphogenesis^3,4. Here we show that, in zebrafish embryos, initial somite anteroposterior lengths and positions are imprecise and, consequently, many somite pairs form left-right asymmetrically. Notably, these imprecisions are not left unchecked and we find that anteroposterior lengths adjust within an hour after somite formation, thereby increasing morphological symmetry. We find that anteroposterior length adjustments result entirely from changes in somite shape without change in somite volume, with changes in anteroposterior length being compensated by corresponding changes in mediolateral length. The anteroposterior adjustment mechanism is facilitated by somite surface tension, which we show by comparing in vivo experiments and in vitro single-somite explant cultures using a mechanical model. Length adjustment is inhibited by perturbation of molecules involved in surface tension, such as integrin and fibronectin. By contrast, the adjustment mechanism is unaffected by perturbations to the segmentation clock, therefore revealing a distinct process that influences morphological segment lengths. We propose that tissue surface tension provides a general mechanism to adjust shapes and ensure precision and symmetry of tissues in developing embryos.

Somite Division and New Boundary Formation by Mechanical Strain

Somitogenesis, the primary segmentation of the vertebrate embryo, is associated with oscillating genes that interact with a wave of cell differentiation. The necessity of cell-matrix adherence and embryonic tension, however, suggests that mechanical cues are also involved. To explicitly investigate this, we applied surplus axial strain to live chick embryos. Despite substantial deformations, the embryos developed normally and somite formation rate was unaffected. Surprisingly, however, we observed slow cellular reorganizations of the most elongated somites into two or more well-shaped daughter somites. In what appeared to be a regular process of boundary formation, somites divided and fibronectin was deposited in between. Cell counts and morphology indicated that cells from the somitocoel underwent mesenchymal-epithelial transition; this was supported by a Cellular Potts model of somite division. Thus, although somitogenesis appeared to be extremely robust, we observed new boundary formation in existing somites and conclude that mechanical strain can be morphologically instructive.

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Live chick embryos develop normally under substantial axial strain (>50%)

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Mature somites divide into daughter somites, and fibronectin is deposited in between

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Mesenchymal cells from the somitocoel transition into epithelial border cells

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Mechanical strain can induce border formation and thus affect morphogenesis

Type IV Pilin Post-Translational Modifications Modulate Material Properties of Bacterial Colonies

Bacterial type 4 pili (T4P) are extracellular polymers that initiate the formation of microcolonies and biofilms. T4P continuously elongate and retract. These pilus dynamics crucially affect the local order, shape, and fluidity of microcolonies. The major pilin subunit of the T4P bears multiple post-translational modifications. By interfering with different steps of the pilin glycosylation and phosphoform modification pathways, we investigated the effect of pilin post-translational modification on the shape and dynamics of microcolonies formed by Neisseria gonorrhoeae. Deleting the phosphotransferase responsible for phosphoethanolamine modification at residue serine 68 inhibits shape relaxations of microcolonies after perturbation and causes bacteria carrying the phosphoform modification to segregate to the surface of mixed colonies. We relate these mesoscopic phenotypes to increased attractive forces generated by T4P between cells. Moreover, by deleting genes responsible for the pilin glycan structure, we show that the number of saccharides attached at residue serine 63 affects the ratio between surface tension and viscosity and cause sorting between bacteria carrying different pilin glycoforms. We conclude that different pilin post-translational modifications moderately affect the attractive forces between bacteria but have severe effects on the material properties of microcolonies.

Molecular Motors Govern Liquidlike Ordering and Fusion Dynamics of Bacterial Colonies

Bacteria can adjust the structure of colonies and biofilms to enhance their survival rate under external stress. Here, we explore the link between bacterial interaction forces and colony structure. We show that the activity of extracellular pilus motors enhances local ordering and accelerates fusion dynamics of bacterial colonies. The radial distribution function of mature colonies shows local fluidlike order. The degree and dynamics of ordering are dependent on motor activity. At a larger scale, the fusion dynamics of two colonies shows liquidlike behavior whereby motor activity strongly affects surface tension and viscosity.

4D imaging reveals stage dependent random and directed cell motion during somite morphogenesis

Somites are paired embryonic segments that form in a regular sequence from unsegmented mesoderm during vertebrate development. Although transient structures they are of fundamental importance as they generate cell lineages of the musculoskeletal system in the trunk such as cartilage, tendon, bone, endothelial cells and skeletal muscle. Surprisingly, very little is known about cellular dynamics underlying the morphological transitions during somite differentiation. Here, we address this by examining cellular rearrangements and morphogenesis in differentiating somites using live multi-photon imaging of transgenic chick embryos, where all cells express a membrane-bound GFP. We specifically focussed on the dynamic cellular changes in two principle regions within the somite, the medial and lateral domains, to investigate extensive morphological transformations. Furthermore, by using quantitative analysis and cell tracking, we capture for the first time a directed movement of dermomyotomal progenitor cells towards the rostro-medial domain of the dermomyotome, where skeletal muscle formation initiates.

Water in the human body: An anesthesiologist's perspective on the connection between physicochemical properties of water and physiologic relevance

The unique structure and multifaceted physicochemical properties of the water molecule, in addition to its universal presence in body compartments, make water a key player in multiple biological processes in human physiology. Since anesthesiologists deal with physiologic processes where water molecules are critical at different levels, and administer medications whose pharmacokinetics and pharmacodynamics depend on interaction with water molecules, we consider that exploration of basic science aspects related to water and its role in physiology and pharmacology is relevant to the practice of anesthesiology. The purpose of this paper is to delineate the physicochemical basis of water that are critical in enabling it to support various homeostatic processes. The role of water in the formation of solutions, modulation of surface tension and in homeostasis of body temperature, acid-base status and osmolarity, is analyzed. Relevance of molecular water interactions to the anesthesiologist is not limited to the realm of physiology and pathophysiology. Deep knowledge of the importance of water in volatile anesthetic effects on neurons opens a window to a new comprehensive understanding of complex cellular mechanisms underlying the practice of anesthesiology.

Timing by rhythms: Daily clocks and developmental rulers

Biological rhythms are widespread, allowing organisms to temporally organize their behavior and metabolism in advantageous ways. Such proper timing of molecular and cellular events is critical to their development and health. This is best understood in the case of the circadian clock that orchestrates the daily sleep/wake cycle of organisms. Temporal rhythms can also be used for spatial organization, if information from an oscillating system can be recorded within the tissue in a manner that leaves a permanent periodic pattern. One example of this is the "segmentation clock" used by the vertebrate embryo to rhythmically and sequentially subdivide its elongating body axis. The segmentation clock moves with the elongation of the embryo, such that its period sets the segment length as the tissue grows outward. Although the study of this system is still relatively young compared to the circadian clock, outlines of molecular, cellular, and tissue-level regulatory mechanisms of timing have emerged. The question remains, however, is it truly a clock? Here we seek to introduce the segmentation clock to a wider audience of chronobiologists, focusing on the role and control of timing in the system. We compare and contrast the segmentation clock with the circadian clock, and propose that the segmentation clock is actually an oscillatory ruler, with a primary function to measure embryonic space.

Mechanics of tissue compaction

During embryonic development, tissues deform by a succession and combination of morphogenetic processes. Tissue compaction is the morphogenetic process by which a tissue adopts a tighter structure. Recent studies characterized the respective roles of cells' adhesive and contractile properties in tissue compaction. In this review, we formalize the mechanical and molecular principles of tissue compaction and we analyze through the prism of this framework several morphogenetic events: the compaction of the early mouse embryo, the formation of the fly retina, the segmentation of somites and the separation of germ layers during gastrulation.

Surface tension in human pathophysiology and its application as a medical diagnostic tool

Introduction: Pathological features of disease appear to be quite different. Despite this diversity, the common feature of various disorders underlies physicochemical and biochemical factors such as surface tension. Human biological fluids comprise various proteins and phospholipids which are capable of adsorption at fluid interfaces and play a vital role in the physiological function of human organs. Surface tension of body fluids correlates directly to the development of pathological states.

Methods: In this review, the variety of human diseases mediated by the surface tension changes of biological phenomena and the failure of biological fluids to remain in their native state are discussed.

Results: Dynamic surface tension measurements of human biological fluids depend on various parameters such as sex, age and changes during pregnancy or certain disease. It is expected that studies of surface tension behavior of human biological fluids will provide additional information and might become useful in medical practice. Theoretical background on surface tension measurement and surface tension values of reference fluids obtained from healthy and sick patients are depicted.

Conclusion: It is well accepted that no single biomarker will be effective in clinical diagnosis. The surface tension measurement combined with routine lab tests may be a novel non-invasive method which can not only facilitate the discovery of diagnostic models for various diseases and its severity, but also be a useful tool for monitoring treatment efficacy. We therefore expect that studies of surface tension behavior of human biological fluids will provide additional useful information in medical practice.

Segment-specific adhesion as a driver of convergent extension

Convergent extension, the simultaneous extension and narrowing of tissues, is a crucial event in the formation of the main body axis during embryonic development. It involves processes on multiple scales: the sub-cellular, cellular and tissue level, which interact via explicit or intrinsic feedback mechanisms. Computational modelling studies play an important role in unravelling the multiscale feedbacks underlying convergent extension. Convergent extension usually operates in tissue which has been patterned or is currently being patterned into distinct domains of gene expression. How such tissue patterns are maintained during the large scale tissue movements of convergent extension has thus far not been investigated. Intriguingly, experimental data indicate that in certain cases these tissue patterns may drive convergent extension rather than requiring safeguarding against convergent extension. Here we use a 2D Cellular Potts Model (CPM) of a tissue prepatterned into segments, to show that convergent extension tends to disrupt this pre-existing segmental pattern. However, when cells preferentially adhere to cells of the same segment type, segment integrity is maintained without any reduction in tissue extension. Strikingly, we demonstrate that this segment-specific adhesion is by itself sufficient to drive convergent extension. Convergent extension is enhanced when we endow our in silico cells with persistence of motion, which in vivo would naturally follow from cytoskeletal dynamics. Finally, we extend our model to confirm the generality of our results. We demonstrate a similar effect of differential adhesion on convergent extension in tissues that can only extend in a single direction (as often occurs due to the inertia of the head region of the embryo), and in tissues prepatterned into a sequence of domains resulting in two opposing adhesive gradients, rather than alternating segments.

The process of convergent extension is a major contributor to the formation of the anterior-posterior body axis in the early embryo. Convergent extension refers to the directed movement of cells that leads to the extension of tissue in one direction and narrowing of the tissue in the perpendicular direction. Often, convergent extension occurs in tissue which already contains distinct domains of gene expression such as segments, and it is unclear how these patterns are maintained despite extensive cell movement. Interestingly, experimental evidence suggests that these tissue patterns may drive rather than be compromised by convergent extension. However, existing computational models aimed at unravelling the mechanisms of convergent extension have thus far only studied the process in homogeneous tissues. With our model, we demonstrate that in a segmented tissue, preferential adhesion of cells to other cells within the same segment type is required to maintain the tissue pattern during convergent extension. Furthermore, such segment-specific adhesion is by itself capable of driving convergent extension.

ONE-DIMENSIONAL MODEL OF SOMITIC CELLS POLARIZATION IN A BISTABILITY WINDOW OF EMBRYONIC MESODERM

The considerations are based on the understanding that somitic cells polarization in bistability window of embryonic (pre-somitic) mesoderm is a dynamical process. It occurs in the form of a polarization wavefront of somite cells spread in anterior–posterior direction of the embryonic mesoderm. It is assumed that a macroscopic cell polarization has a bistable behavior corresponding to the molecular mechanism of bistability window formation. Moreover this type of polarization is supposed to be transmittable to the other cells by contact interaction. At the end, a volume of polarized cells is taken, which is able to create mechanical tension in the volume of nonpolarized neighbor cells and to inhibit their polarization. On this basis we explore the leading aspect of somitogenesis robustness by considering a simple wavefront model of polarization and analyzing its propagation in terms of the standard methods of qualitative theory of differential equations. The obtained theoretical results are interpreted in the context of their possible experimental verification.

Mechanochemical control of mesenchymal condensation and embryonic tooth organ formation

Mesenchymal condensation is critical for organogenesis, yet little is known about how this process is controlled. Here we show that Fgf8 and Sema3f, produced by early dental epithelium, respectively, attract and repulse mesenchymal cells, which cause them to pack tightly together during mouse tooth development. Resulting mechanical compaction-induced changes in cell shape induce odontogenic transcription factors (Pax9, Msx1) and a chemical cue (BMP4), and mechanical compression of mesenchyme is sufficient to induce tooth-specific cell fate switching. The inductive effects of cell compaction are mediated by suppression of the mechanical signaling molecule RhoA, and its overexpression prevents odontogenic induction. Thus, the mesenchymal condensation that drives tooth formation is induced by antagonistic epithelial morphogens that manifest their pattern-generating actions mechanically via changes in mesenchymal cell shape and altered mechanotransduction.

Physics and the canalization of morphogenesis: a grand challenge in organismal biology

Morphogenesis takes place against a background of organism-to-organism and environmental variation. Therefore, fundamental questions in the study of morphogenesis include: How are the mechanical processes of tissue movement and deformation affected by that variability, and in turn, how do the mechanic of the system modulate phenotypic variation? We highlight a few key factors, including environmental temperature, embryo size and environmental chemistry that might perturb the mechanics of morphogenesis in natural populations. Then we discuss several ways in which mechanics-including feedback from mechanical cues-might influence intra-specific variation in morphogenesis. To understand morphogenesis it will be necessary to consider whole-organism, environment and evolutionary scales because these larger scales present the challenges that developmental mechanisms have evolved to cope with. Studying the variation organisms express and the variation organisms experience will aid in deciphering the causes of birth defects.

Cell sorting in development

During the development of multicellular organisms, cell fate specification is followed by the sorting of different cell types into distinct domains from where the different tissues and organs are formed. Cell sorting involves both the segregation of a mixed population of cells with different fates and properties into distinct domains, and the active maintenance of their segregated state. Because of its biological importance and apparent resemblance to fluid segregation in physics, cell sorting was extensively studied by both biologists and physicists over the last decades. Different theories were developed that try to explain cell sorting on the basis of the physical properties of the constituent cells. However, only recently the molecular and cellular mechanisms that control the physical properties driving cell sorting, have begun to be unraveled. In this review, we will provide an overview of different cell-sorting processes in development and discuss how these processes can be explained by the different sorting theories, and how these theories in turn can be connected to the molecular and cellular mechanisms driving these processes.

Adding adhesion to a chemical signaling model for somite formation

Somites are condensations of mesodermal cells that form along the two sides of the neural tube during early vertebrate development. They are one of the first instances of a periodic pattern, and give rise to repeated structures such as the vertebrae. A number of theories for the mechanisms underpinning somite formation have been proposed. For example, in the "clock and wavefront" model (Cooke and Zeeman in J. Theor. Biol. 58:455-476, 1976), a cellular oscillator coupled to a determination wave progressing along the anterior-posterior axis serves to group cells into a presumptive somite. More recently, a chemical signaling model has been developed and analyzed by Maini and coworkers (Collier et al. in J. Theor. Biol. 207:305-316, 2000; Schnell et al. in C. R. Biol. 325:179-189, 2002; McInerney et al. in Math. Med. Biol. 21:85-113, 2004), with equations for two chemical regulators with entrained dynamics. One of the chemicals is identified as a somitic factor, which is assumed to translate into a pattern of cellular aggregations via its effect on cell-cell adhesion. Here, the authors propose an extension to this model that includes an explicit equation for an adhesive cell population. They represent cell adhesion via an integral over the sensing region of the cell, based on a model developed previously for adhesion driven cell sorting (Armstrong et al. in J. Theor. Biol. 243:98-113, 2006). The expanded model is able to reproduce the observed pattern of cellular aggregates, but only under certain parameter restrictions. This provides a fuller understanding of the conditions required for the chemical model to be applicable. Moreover, a further extension of the model to include separate subpopulations of cells is able to reproduce the observed differentiation of the somite into separate anterior and posterior halves.

Limb bud and flank mesoderm have distinct "physical phenotypes" that may contribute to limb budding

Limb bud outgrowth in chicken embryos is initiated during the third day of development by Fibroblast Growth Factor 8 (FGF8) produced by the newly formed apical ectodermal ridge (AER). One of the earliest effects of this induction is a change in the properties of the limb field mesoderm leading to bulging of the limb buds from the body wall. Heintzelman et al. [Heintzelman, K.F., Phillips, H.M., Davis, G.S., 1978. Liquid-tissue behavior and differential cohesiveness during chick limb budding. J. Embryol. Exp. Morphol. 47, 1-15.] suggested that budding of the limbs is caused by a higher liquid-like cohesivity of limb bud tissue compared with flank. We sought additional evidence relevant to this hypothesis by performing direct measurements of the effective surface tension, a measure of relative tissue cohesivity, of 4-day embryonic chicken wing and leg bud mesenchymal tissue, and adjacent flank mesoderm. As predicted, the two types of limb tissues were 1.5- to 2-fold more cohesive than the flank tissue. These differences paralleled cell number and volume density differences: 4-day limb buds had 2- to 2.5-fold as many cells per unit area of tissue as surrounding flank, a difference also seen at 3 days, when limb budding begins. Exposure of flank tissue to exogenous FGF8 for 24 h increased its cell number and raised its cohesivity to limb-like values. Four-day flank tissue exhibited a novel and unique active rebound response to compression, which was suppressed by the drug latrunculin and therefore dependent on an intact actin cytoskeleton. Correspondingly, flank at this stage expressed high levels of alpha-smooth muscle actin (SMA) mRNA and protein and a dense network of microfilaments. Treatment of flank with FGF8 eliminated the rebound response. We term material properties of tissues, such as cohesivity and mechanical excitability, the "physical phenotype", and propose that changes thereof are driving forces of morphogenesis. Our results indicate that two independent aspects of the physical phenotype of flank mesoderm can be converted to a limb-like state in response to treatment with FGF8. The higher tissue cohesivity induced by this effect will cause the incipient limb bud to phase separate from the surrounding flank, while the active mechanical response of the flank could help ensure that the limb bud bulges out from, rather than becoming engulfed by, this less cohesive tissue.

Dynamical patterning modules: physico-genetic determinants of morphological development and evolution

The shapes and forms of multicellular organisms arise by the generation of new cell states and types and changes in the numbers and rearrangements of the various kinds of cells. While morphogenesis and pattern formation in all animal species are widely recognized to be mediated by the gene products of an evolutionarily conserved 'developmental-genetic toolkit', the link between these molecular players and the physics underlying these processes has been generally ignored. This paper introduces the concept of 'dynamical patterning modules' (DPMs), units consisting of one or more products of the 'toolkit' genes that mobilize physical processes characteristic of chemically and mechanically excitable meso- to macroscopic systems such as cell aggregates: cohesion, viscoelasticity, diffusion, spatiotemporal heterogeneity based on lateral inhibition and multistable and oscillatory dynamics. We suggest that ancient toolkit gene products, most predating the emergence of multicellularity, assumed novel morphogenetic functions due to change in the scale and context inherent to multicellularity. We show that DPMs, acting individually and in concert with each other, constitute a 'pattern language' capable of generating all metazoan body plans and organ forms. The physical dimension of developmental causation implies that multicellular forms during the explosive radiation of animal body plans in the middle Cambrian, approximately 530 million years ago, could have explored an extensive morphospace without concomitant genotypic change or selection for adaptation. The morphologically plastic body plans and organ forms generated by DPMs, and their ontogenetic trajectories, would subsequently have been stabilized and consolidated by natural selection and genetic drift. This perspective also solves the apparent 'molecular homology-analogy paradox', whereby widely divergent modern animal types utilize the same molecular toolkit during development by proposing, in contrast to the Neo-Darwinian principle, that phenotypic disparity early in evolution occurred in advance of, rather than closely tracked, genotypic change.

Variation and robustness of the mechanics of gastrulation: the role of tissue mechanical properties during morphogenesis

Diverse mechanisms of morphogenesis generate a wide variety of animal forms. In this work, we discuss two ways that the mechanical properties of embryonic tissues could guide one of the earliest morphogenetic movements in animals, gastrulation. First, morphogenetic movements are a function of both the forces generated by cells and the mechanical properties of the tissues. Second, cells could change their behavior in response to their mechanical environment. Theoretical studies of gastrulation indicate that different morphogenetic mechanisms differ in their inherent sensitivity to tissue mechanical properties. Those few empirical studies that have investigated the mechanical properties of amphibian and echinoderm gastrula-stage embryos indicate that there could be high embryo-to-embryo variability in tissue stiffness. Such high embryo-to-embryo variability would imply that gastrulation is fairly robust to variation in tissue stiffness. Cell culture studies demonstrate a wide variety of cellular responses to the mechanical properties of their microenvironment. These responses are likely to be developmentally regulated, and could either increase or decrease the robustness of gastrulation movements depending on which cells express which responses. Hence both passive physical and mechanoregulatory processes will determine how sensitive gastrulation is to tissue mechanics. Addressing these questions is important for understanding the significance of diverse programs of early development, and how genetic or environmental perturbations influence development. We discuss methods for measuring embryo-to-embryo variability in tissue mechanics, and for experimentally perturbing those mechanical properties to determine the sensitivity of gastrulation to tissue mechanics.

Coordinated action of N-CAM, N-cadherin, EphA4, and ephrinB2 translates genetic prepatterns into structure during somitogenesis in chick

During gastrulation in vertebrates, mesenchymal cells at the anterior end of the presomitic mesoderm (PSM) periodically compact, transiently epithelialize and detach from the posterior PSM to form somites. In the prevailing clock-and-wavefront model of somitogenesis, periodic gene expression, particularly of Notch and Wnt, interacts with an FGF8-based thresholding mechanism to determine cell fates. However, this model does not explain how cell determination and subsequent differentiation translates into somite morphology. In this paper, we use computer simulations of chick somitogenesis to show that experimentally-observed temporal and spatial patterns of adhesive N-CAM and N-cadherin and repulsive EphA4-ephrinB2 pairs suffice to reproduce the complex dynamic morphological changes of somitogenesis in wild-type and N-cadherin (-/-) chick, including intersomitic separation, boundary-shape evolution and sorting of misdifferentiated cells across compartment boundaries. Since different models of determination yield the same, experimentally-observed, distribution of adhesion and repulsion molecules, the patterning is independent of the details of this mechanism.

Mathematical models for somite formation

Somitogenesis is the process of division of the anterior-posterior vertebrate embryonic axis into similar morphological units known as somites. These segments generate the prepattern which guides formation of the vertebrae, ribs and other associated features of the body trunk. In this work, we review and discuss a series of mathematical models which account for different stages of somite formation. We begin by presenting current experimental information and mechanisms explaining somite formation, highlighting features which will be included in the models. For each model we outline the mathematical basis, show results of numerical simulations, discuss their successes and shortcomings and avenues for future exploration. We conclude with a brief discussion of the state of modeling in the field and current challenges which need to be overcome in order to further our understanding in this area.