Mechanotransduction: getting morphogenesis down pat

Embryonic morphogenesis requires the coordination of forces across multiple tissues and their associated extracellular matrices. A new study reports a mechanical feedback loop in the Caenorhabditis elegans embryo between muscle and epidermis that may provide a model for understanding how tissues coordinate morphogenetic events in the embryo.     

Collagen involvement in branching morphogenesis of embryonic lung and salivary gland

The possibility that extracellular collagen is involved in branching morphogenesis of mouse embryo lung and salivary glands has been explored duringin vitro organ culture. Control cultures of both rudiment types contain abundant collagen in extracellular spaces between mesenchymal cells and in the epithelial-mesenchymal interface. Branching morphogenesis of lungs and salivary glands is not perturbed by the presence of β-aminopropionitrile, implying that extracellular collagen cross-linking is not required, but is perturbed by α,α′-dipyridyl orl-azetidine-2-car?ylic acid (LACA), agents reported to interfere with collagen synthesis and secretion. Analysis of the structural and biosynthetic effects of LACA revealed a severe inhibition of collagen synthesis, as monitored by hydroxyproline synthesis, and extracellular collagen accumulation. Cell and tissue integrity was not affected, but a slight inhibition of general protein synthesis, protein accumulation, and epithelial expansion was observed. The strong correlations between collagen biosynthesis, extracellular collagen presence, and branching morphogenesis are consistent with an integral role for collagen in embryonic lung and salivary gland morphogenesis.     

Measuring the multi-scale integration of mechanical forces during morphogenesis

The elaborate changes in morphology of an organism during development are the result of mechanical contributions that are a mixture of those generated locally and those that influence from a distance. We would like to know how chemical and mechanical information is transmitted and transduced, how work is done to achieve robust morphogenesis and why it sometimes fails. We introduce a scheme for separating the influence of two classes of forces. Active intrinsic forces integrate up levels of scale to shape tissues. Counter-currently, extrinsic forces exert influence from higher levels downwards and feed back directly and indirectly upon the intrinsic behaviours. We identify the measurable signatures of different kinds of forces and identify the frontiers where work is most needed.     

Multi-scale mechanics from molecules to morphogenesis

Dynamic mechanical processes shape the embryo and organs during development. Little is understood about the basic physics of these processes, what forces are generated, or how tissues resist or guide those forces during morphogenesis. This review offers an outline of some of the basic principles of biomechanics, provides working examples of biomechanical analyses of developing embryos, and reviews the role of structural proteins in establishing and maintaining the mechanical properties of embryonic tissues. Drawing on examples we highlight the importance of investigating mechanics at multiple scales from milliseconds to hours and from individual molecules to whole embryos. Lastly, we pose a series of questions that will need to be addressed if we are to understand the larger integration of molecular and physical mechanical processes during morphogenesis and organogenesis.     

Mechanical control of tissue morphogenesis during embryological development

Twenty years ago, we proposed a model of developmental control based on tensegrity architecture, in which tissue pattern formation in the embryo is controlled through mechanical interactions between cells and extracellular matrix (ECM) which place the tissue in a state of isometric tension (prestress). The model proposed that local changes in the mechanical compliance of the ECM, for example, due to regional variations in basement membrane degradation beneath growing epithelium, may result in local stretching of the ECM and associated adherent cells, much like a "run-in-a-stocking". Cell growth and function would be controlled locally though physical distortion of the associated cells, or changes in cytoskeletal tension. Importantly, experimental studies have demonstrated that cultured cells can be switched between different fates, including growth, differentiation, apoptosis, directional motility and different stem cell lineages, by modulating cell shape. Experiments in whole embryonic organ rudiments also have confirmed the tight correlation between basement membrane thinning, cell tension generation and new bud and branch formation during tissue morphogenesis and that this process can be inhibited or accelerated by dissipating or enhancing cytoskeletal tension, respectively. Taken together, this work confirms that mechanical forces generated in the cytoskeleton of individual cells and exerted on ECM scaffolds, play a critical role in the sculpting of the embryo.     

Cell traction models for generating pattern and form in morphogenesis

During early development migratory mesenchymal cells navigate to distant sites where they aggregate to form a variety of embryonic organ rudiments. We present here a new model for mesenchymal cell morphogenesis based on the mechanical interaction between motile cells and their extracellular environment. The model is based on two properties of motile cells: (a) they are capable of generating large traction forces which can deform the extracellular matrix through which they move, and (b) the deformations they produce in their environment affect the direction of their movements. We derive field equations which describe the motion of cells in an elastic extracellular matrix and show that these equations can generate a variety of spatial patterns, such as the formations of skin organ primordia, especially feather germs, cartilage condensation patterns which presage bone formation in limb development, and melanocyte density patterns which form animal coat patterns.Support for this work was provided by NSF Grant # MCS-8110557 [GFO]     

Cell confinement controls centrosome positioning and lumen initiation during epithelial morphogenesis

Epithelial organ morphogenesis involves sequential acquisition of apicobasal polarity by epithelial cells and development of a functional lumen. In vivo, cells perceive signals from components of the extracellular matrix (ECM), such as laminin and collagens, as well as sense physical conditions, such as matrix stiffness and cell confinement. Alteration of the mechanical properties of the ECM has been shown to promote cell migration and invasion in cancer cells, but the effects on epithelial morphogenesis have not been characterized. We analyzed the effects of cell confinement on lumen morphogenesis using a novel, micropatterned, three-dimensional (3D) Madin-Darby canine kidney cell culture method. We show that cell confinement, by controlling cell spreading, limits peripheral actin contractility and promotes centrosome positioning and lumen initiation after the first cell division. In addition, peripheral actin contractility is mediated by master kinase Par-4/LKB1 via the RhoA–Rho kinase–myosin II pathway, and inhibition of this pathway restores lumen initiation in minimally confined cells. We conclude that cell confinement controls nuclear–centrosomal orientation and lumen initiation during 3D epithelial morphogenesis.     

Forced to communicate: Integration of mechanical and biochemical signaling in morphogenesis

Morphogenesis is a physical process that requires the generation of mechanical forces to achieve dynamic changes in cell position, tissue shape, and size as well as biochemical signals to coordinate these events. Mechanical forces are also used by the embryo to transmit detailed information across space and detected by target cells, leading to downstream changes in cellular properties and behaviors. Indeed, forces provide signaling information of complementary quality that can both synergize and diversify the functional outputs of biochemical signaling. Here, we discuss recent findings that reveal how mechanical signaling and biochemical signaling are integrated during morphogenesis and the possible context-specific advantages conferred by the interactions between these signaling mechanisms.     

Depletion of FGF acts as a lateral inhibitory factor in lung branching morphogenesis in vitro

Previous studies have shown that the interaction of positive and inhibitory signals plays a crucial role during lung branching morphogenesis. We found that in mesenchyme-free conditions, the lung epithelium exerted a lateral inhibitory effect on the neighbouring epithelium via depletion of fibroblast growth factor 1 (FGF1). Contrary to previous suggestions, bone morphogenetic protein 4 could not substitute for the inhibitory effect. Based on of this observation, we used a reaction-diffusion model of the substrate-depletion type to represent the initial phase of in vitro branching morphogenesis of lung epithelium, with depletion of FGF playing the role of lateral inhibitor. The model was able to account for the effects of the FGF1 concentration, extracellular matrix degradation and different subtypes of FGF on morphogenesis of the lung bud epithelia. These results suggest that the depletion of FGF may be a key regulatory component in initial phase of branching morphogenesis of the lung bud epithelium in vitro.     

The role of convection in the standard fluid-mechanical model for morphogenesis

The effect of convection on reaction-diffusion instabilities in a visco-elastic medium is studied by using the standard continuum theory of a fluid mixture. The medium is assumed to be in local mechanical equilibrium, and convection is generated by pressure forces which arise if the equilibrium density of the medium changes with its composition. A linear stability analysis shows that reaction-diffusion instabilities proceeding from homogeneous steady states at rest are unmodified by induced convection to first order in concentration changes. We suggest that a non-linear analysis would show convection produces no new instabilities, as a linear analysis of inhomogeneous non-convecting stationary states shows that reaction-diffusion growth rates are reduced by convection at long wavelengths and are otherwise unchanged. For applications in embryology, numerical estimates suggest that convection can be ignored in reaction-diffusion mechanisms for pattern formation, and this conclusion is supported by a dimensional analysis.On leave from Department of Physics, Monash University, Clayton, Victoria 3168, Australia     

The mechanics of development: Models and methods for tissue morphogenesis

Embryonic development is a physical process during which groups of cells are sculpted into functional organs. The mechanical properties of tissues and the forces exerted on them serve as epigenetic regulators of morphogenesis. Understanding these mechanobiological effects in the embryo requires new experimental approaches. Here we focus on branching of the lung airways and bending of the heart tube to describe examples of mechanical and physical cues that guide cell fate decisions and organogenesis. We highlight recent technological advances to measure tissue elasticity and endogenous mechanical stresses in real time during organ development. We also discuss recent progress in manipulating forces in intact embryos. Birth Defects Research (Part C) 90:193–202, 2010. © 2010 Wiley‐Liss, Inc.     

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. Birth Defects Research (Part C) 96:132–152, 2012. © 2012 Wiley Periodicals, Inc.     

'Generic' physical mechanisms of morphogenesis and pattern formation

The role of 'generic' physical mechanisms in morphogenesis and pattern formation of tissues is considered. Generic mechanisms are defined as those physical processes that are broadly applicable to living and non-living systems, such as adhesion, surface tension and gravitational effects, viscosity, phase separation, convection and reaction-diffusion coupling. They are contrasted with 'genetic' mechanisms, a term reserved for highly evolved, machine-like, biomolecular processes. Generic mechanisms acting upon living tissues are capable of giving rise to morphogenetic rearrangements of cytoplasmic, tissue and extracellular matrix components, sometimes leading to 'microfingers', and to chemical waves or stripes. We suggest that many morphogenetic and patterning effects are the inevitable outcome of recognized physical properties of tissues, and that generic physical mechanisms that act on these properties are complementary to, and interdependent with genetic mechanisms. We also suggest that major morphological reorganizations in phylogenetic lineages may arise by the action of generic physical mechanisms on developing embryos. Subsequent evolution of genetic mechanisms could stabilize and refine developmental outcomes originally guided by generic effects.     

The extracellular matrix in development and morphogenesis: A dynamic view

The extracellular matrix (ECM) is synthesized and secreted by embryonic cells beginning at the earliest stages of development. Our understanding of ECM composition, structure and function has grown considerably in the last several decades and this knowledge has revealed that the extracellular microenvironment is critically important for cell growth, survival, differentiation and morphogenesis. ECM and the cellular receptors that interact with it mediate both physical linkages with the cytoskeleton and the bidirectional flow of information between the extracellular and intracellular compartments. This review considers the range of cell and tissue functions attributed to ECM molecules and summarizes recent findings specific to key developmental processes. The importance of ECM as a dynamic repository for growth factors is highlighted along with more recent studies implicating the 3-dimensional organization and physical properties of the ECM as it relates to cell signaling and the regulation of morphogenetic cell behaviors. Embryonic cell and tissue generated forces and mechanical signals arising from ECM adhesion represent emerging areas of interest in this field.     

Mutational analysis of Stubble-stubbloid gene structure and function in Drosophila leg and bristle morphogenesis

The Stubble-stubbloid (Sb-sbd) gene is required for ecdysone-regulated epithelial morphogenesis of imaginal tissues during Drosophila metamorphosis. Mutations in Sb-sbd are associated with defects in apical cell shape changes critical for the evagination of the leg imaginal disc and with defects in assembly and extension of parallel actin bundles in growing mechanosensory bristles. The Sb-sbd gene encodes a type II transmembrane serine protease (TTSP). Here we use a Sb-sbd transgenic construct to rescue both bristle and leg morphogenesis defects in Sb-sbd mutations. Molecular characterization of Sb-sbd mutations and rescue experiments with wild-type and modified Sb-sbd transgenic constructs show that the protease domain is required for both leg and bristle functions. Truncated proteins that express the noncatalytic domains without the protease have dominant effects in bristles but not in legs. Leg morphogenesis, but not bristle growth, is sensitive to Sb-sbd overexpression. Antibody localization of the Sb-sbd protein shows apical expression in elongating legs. Sb-sbd protein is found in the base and shaft in budding bristles and then concentrates at the growing tip when bristles are elongating rapidly. We propose a model whereby Sb-sbd helps coordinate proteolytic modification of extracellular matrix attachments with cytoskeletal changes in both legs and bristles.     

A deformation gradient decomposition method for the analysis of the mechanics of morphogenesis

A new finite element model is proposed for the analysis of the mechanical aspects of morphogenesis and tested on the biologically well studied gastrulation phenomenon, in particular ventral furrow invagination of the Drosophila melanogaster embryo. A set of mechanisms are introduced in the numerical model, which lead to the observed deformed shapes. We split the total deformation into two parts: an imposed active deformation, and an elastic deformation superimposed onto the latter. The active deformation simulates the effects of apical constriction and apico-basal elongation. These mechanisms are associated with known gene expressions and so in this way we attempt to bridge the well explored signalling pathways, and their associated phenotypes in a mechanical model. While the former have been studied in depth, much less can be said about the forces they produce and the mechanisms involved. From the numerical results, we are able to test different plausible mechanical hypotheses that generate the necessary folding observed in the invagination process. In particular, we conclude that only certain ratios between both modes (apical constriction and apico-basal elongation) can successfully reproduce the invagination process. The model also supports the idea that this invagination requires the contribution of several mechanisms, and that their redundancy provides the necessary robustness.     

Turing's next steps: the mechanochemical basis of morphogenesis

Nearly 60 years ago, Alan Turing showed theoretically how two chemical species, termed morphogens, diffusing and reacting with each other can generate spatial patterns. Diffusion plays a crucial part in transporting chemical signals through space to establish the length scale of the pattern. When coupled to chemical reactions, mechanical processes - forces and flows generated by motor proteins - can also define length scales and provide a mechanochemical basis for morphogenesis. forces and flows generated by motor proteins - can also define length scales and provide a mechanochemical basis for morphogenesis.     

Apoptotic force: active mechanical function of cell death during morphogenesis

Apoptosis, or programmed cell death, is an essential process for the elimination of unnecessary cells during embryonic development, tissue homeostasis, and certain pathological conditions. Recently, an active mechanical function of apoptosis called apoptotic force has been demonstrated during a tissue fusion process of Drosophila embryogenesis. The mechanical force produced during apoptosis is used not only to force dying cells out from tissues in order to keep tissue integrity, but also to change the morphology of neighboring cells to fill the space originally occupied by the dying cell. Furthermore, the occurrence of apoptosis correlates with tissue movement and tension of the tissue. This finding suggests that apoptotic forces might be harnessed throughout cell death-related morphogenesis; however, this concept remains to be fully investigated. While the investigation of this active mechanical function of apoptosis has just begun, here we summarize the current understandings of this novel function of apoptosis, and discuss some possible developmental processes in which apoptosis may play a mechanical role. The concept of apoptotic force prompts a necessity to rethink the role of programmed cell death during morphogenesis.     

The mechanical basis of cell rearrangement. I. Epithelial morphogenesis during Fundulus epiboly

Many morphogenetic processes are accomplished by coordinated cell rearrangements. These rearrangements are accompanied by substantial shifts in the neighbor relationships between cells. Here we propose a model for studying morphogenesis in epithelial sheets by directed cell neighbor change. Our model describes cell rearrangements by accounting for the balance of forces between neighboring cells within an epithelium. Cell rearrangement and cell shape changes occur when these forces are not in mechanical equilibrium. We will show that cell rearrangement within the epidermal enveloping layer (EVL) of the teleost fish Fundulus during epiboly can be explained solely in terms of the balance of forces generated among constituent epithelial cells. Within a cell, we account for circumferential elastic forces and the force generated by hydrostatic and osmotic pressure. The model treats epithelial cells as two-dimensional polygons where the mechanical forces are applied to the polygonal nodes. A cell node protrudes or contracts when the nodal forces are not in mechanical equilibrium. In an epithelial sheet, adjacent cells share common boundary nodes; in this way, mechanical force is transmitted from cell to cell, mimicking junctional coupling. These junctional nodes can slide, and nodes may appear or disappear, so that the number of polygonal sides is variable. Computer graphics allows us to compare numerical simulations of the model with time-lapse cinemicroscopy of cell rearrangements in the living embryo, and data obtained from fixed and silver stained embryos. By manipulating the mechanical properties of the model cells we can study the conditions necessary to reproduce normal cell behavior during Fundulus epiboly. We find that simple stress relaxation is sufficient to account for cell rearrangements among interior cells of the EVL when they are isotropically contractile. Experimental observations show that the number of EVL marginal cells continuously decreases throughout epiboly. In order for the simulation to reproduce this behavior, cells at the EVL boundary must generate protrusive forces rather than contractile tension forces. Therefore, the simulation results suggest that the mechanical properties of EVL marginal cells at their leading edge must be quite different from EVL interior cells.     

Epidermal growth factor inhibits morphogenesis and cell differentiation in cultured mouse embryonic teeth

Although local epithelial-mesenchymal tissue interactions which are presumably mediated by extracellular matrix molecules are important regulators of tooth morphogenesis and differentiation, our studies have indicated that these developmental processes also depend on circulating molecules. The iron-carrying serum protein transferrin is necessary for the early morphogenesis of mouse tooth in organ culture (A-M. Partanen, I. Thesleff, and P. Ekblom, 1984, Differentiation 27, 59-66). In the present study we have examined the effects of other growth factors on mouse tooth germs grown in a chemically defined medium containing transferrin. Fibroblast growth factor and platelet derived growth factor had no detectable effects but epidermal growth factor (EGF) inhibited dramatically the morphogenesis of teeth, and prevented odontoblast and ameloblast cell differentiation. EGF stimulated cell proliferation in the explants measured as [3H]thymidine incorporation in DNA. However, when the distribution of dividing cells was visualized in autoradiographs, it was observed that cell proliferation was stimulated in the dental epithelium but was inhibited in the dental mesenchyme. The inhibition of cell proliferation in the dental mesenchyme apparently caused the inhibition of morphogenesis. We do not know whether the dental epithelium or mesenchyme was the primary target for the action of EGF in the inhibition of morphogenesis. It is, however, apparent that the response of the dental mesenchymal cells to EGF (inhibition of proliferation) is regulated by their local environment, since EGF enhanced proliferation when these cells were disaggregated and cultured as monolayers. This indicates that the organ culture system where the various embryonic cell lineages are maintained in their original environment corresponds better to the in vivo situation when the roles of exogenous growth factors during development are examined.