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... The intracellular models predict that consistent chirality is an ancient, universal property of single cells (Davison et al., 2016;Naganathan et al., 2016;Raymond et al., 2017;Tamada and Igarashi, 2017;Tee et al., 2015). While this is known to be the case for unicellular ciliates (Frankel, 1991) and slime molds (Dimonte et al., 2016), and increasingly being discovered in snail, nematode, and fruit fly embryos (Callander et al., 2014;Chen et al., 2016;Davison et al., 2016;Hayashi and Murakami, 2001;Inaki et al., 2016;Kuroda et al., 2009;Naganathan et al., 2014;Sato et al., 2015;Schonegg et al., 2014;Taniguchi et al., 2011), it is still often claimed that asymmetry in vertebrates requires a large, ciliated node structure that can support strong fluid flows. Interestingly, several studies have reported that individual cells can exhibit consistent chiral behaviors in culture in the direction of neurite outgrowth, cell:substrate interactions, and docking with neighboring cells (Chen et al., 2012;Frankel, 1991;Heacock and Agranoff, 1977; not certified by peer review) is the author/funder. ...
Preprint
The intracellular model of embryonic left-right (LR) asymmetry proposes that body laterality originates from intrinsic chiral properties of individual cells, and several recent studies identified consistent chirality in the behavior of cells in vitro. Here, we explored one prediction of the intrinsic asymmetry model: that LR asymmetries would be present in a wide range of mammalian cells, manifesting in the form of LR-biased migration toward an attractant. We mined data from published papers on galvanotaxis and chemotaxis and quantitatively analyzed the migration trajectories of adult somatic cells, stem cells, and cancer cells to determine whether they display significant consistent LR biases in their movements toward migration targets. We found that several cell types exhibited LR biases during galvanotaxis and chemotaxis, and that treatments inhibiting cytoskeletal remodeling or targeting ion channel activity both abolished these LR biases. While we cannot conclusively rule out the existence of subtle biasing cues in the apparatus of some of the studies, the analysis of this dataset suggests specific assays and cell types for further investigation into the chiral aspects of intrinsic cell behavior. Funding This work was funded by the American Heart Association Established Investigator grant 0740088N and NIH grants R01-GM077425 (to ML) and NRSA grant 1F32GM087107 (to LNV). M.L. is also supported by the G. Harold and Leila Y. Mathers Charitable Foundation.
... In the case of the body asymmetry, the chirality at the molecular and cellular levels is critical for the formation of asymmetry. Recently, many studies have demonstrated the existence of chirality at a cellular level [73,[86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][101][102]. The cell chirality could be the key intermediate phenotype at the cellular level that bridges the gap between molecular chirality and L-R asymmetry at the tissue or organism levels, which could be also applied to the case of brain asymmetry. ...
Article
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Left–right brain asymmetry is a fundamental property observed across phyla from invertebrates to humans, but the mechanisms underlying its formation are still largely unknown. Rapid progress in our knowledge of the formation of body asymmetry suggests that brain asymmetry might be controlled by the same mechanisms. However, most of the functional brain laterality, including language processing and handedness, does not share common mechanisms with visceral asymmetry. Accumulating evidence indicates that asymmetry is manifested as chirality at the single cellular level. In neurons, the growth cone filopodia at the tips of neurites exhibit a myosin V-dependent, left-helical, and right-screw rotation, which drives the clockwise circular growth of neurites on adhesive substrates. Here, I propose an alternative model for the formation of brain asymmetry that is based on chiral neuronal motility. According to this chiral neuron model, the molecular chirality of actin filaments and myosin motors is converted into chiral neuronal motility, which is in turn transformed into the left–right asymmetry of neural circuits and lateralized brain functions. I also introduce automated, numerical, and quantitative methods to analyze the chirality and the left–right asymmetry that would enable the efficient testing of the model and to accelerate future investigations in this field.
... In fact, biological systems including bacteria (Mendelson and Keener, 1982), slime molds (Dimonte et al., 2016), plants (Hashimoto, 2002), and individual vertebrate cells in culture (Chen et al., ...
Article
While the external vertebrate body plan appears bilaterally symmetrical with respect to anterior-posterior and dorsal-ventral axes, the internal organs are arranged with a striking and invariant left-right asymmetry. This laterality is important for normal body function, as alterations manifest as numerous human birth defect syndromes. The left-right axis is set up very early during embryogenesis by an initial and still poorly understood break in bilateral symmetry, followed by a cascade of molecular events that was discovered 20 years ago in the chick embryo model. This gene regulatory network leads to activation of the pitx2 gene on the left side of the embryo which ultimately establishes asymmetric organogenesis of the heart, gut, brain, and other organs. In this review, we highlight the crucial contributions of the avian model to the discovery of the differential transcriptional cascades operating on the Left and Right sides, as well as to the physiological events operating upstream of asymmetric gene expression. The chick was not only instrumental in the discovery of mechanisms behind left-right patterning, but stands poised to facilitate inroads into the most fundamental aspects that link asymmetry to the rest of evolutionary developmental biology.
... With regard to the initial symmetry-breaking step, it was postulated that the molecular handedness or chirality is converted to a cellular and multicellular asymmetry that finally leads to left-right asymmetry in the organisms 4 . In accordance with this hypothesis, many recent reports demonstrated the existence of chirality at the cellular level [5][6][7][8][9][10][11][12][13][14][15][16] . Cell chirality is emerging as a key geometric property at the intermediate levels that may link the molecular chirality, mostly in cytoskeletons and motor proteins, to the left-right asymmetry at the higher levels 17,18 . ...
Article
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Left-right asymmetry is a fundamental feature of body plans, but its formation mechanisms and roles in functional lateralization remain unclear. Accumulating evidence suggests that left-right asymmetry originates in the cellular chirality. However, cell chirality has not yet been quantitatively investigated, mainly due to the absence of appropriate methods. Here we combine 3D Riesz transform-differential interference contrast (RT-DIC) microscopy and computational kinematic analysis to characterize chiral cellular morphology and motility. We reveal that filopodia of neuronal growth cones exhibit 3D left-helical motion with retraction and right-screw rotation. We next apply the methods to amoeba Dictyostelium discoideum and discover right-handed clockwise cell migration on a 2D substrate and right-screw rotation of subcellular protrusions along the radial axis in a 3D substrate. Thus, RT-DIC microscopy and the computational kinematic analysis are useful and versatile tools to reveal the mechanisms of left-right asymmetry formation and the emergence of lateralized functions.
... Invariant left-right asymmetry is a fundamental aspect of all life, from single cell organisms to plants and animals with complex body plans like humans (Chen et al., 2012;Coutelis et al., 2008;Davison et al., 2016;Dimonte et al., 2016;Gros et al., 2009;Hashimoto, 2002;Kuroda et al., 2009;Naganathan et al., 2014;Petzoldt et al., 2012;Pohl, 2011;Speder et al., 2007;Thitamadee et al., 2002;Wan et al., 2011;Xu et al., 2007;Yost, 1990;Yost, 1991). Consistent orientation of the left-right (LR) axis is a difficult problem for an embryo to solve in a universe that does not macroscopically distinguish left from right, and must be done reliably and accurately to achieve correct organization of internal organ structures. ...
Article
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Laterality is a basic characteristic of all life forms, from single cell organisms to complex plants and animals. For many metazoans, consistent left-right asymmetric patterning is essential for the correct anatomy of internal organs, such as the heart, gut, and brain; disruption of left-right asymmetry patterning leads to an important class of birth defects in human patients. Laterality functions across multiple scales, where early embryonic, subcellular and chiral cytoskeletal events are coupled with asymmetric amplification mechanisms and gene regulatory networks leading to asymmetric physical forces that ultimately result in distinct left and right anatomical organ patterning. Recent studies have suggested the existence of multiple parallel pathways regulating organ asymmetry. Here, we show that an isoform of the Hyperpolarization-activated cyclic-nucleotide gated family of ion channels, HCN4, is important for correct left-right patterning. HCN4 channels are present very early in Xenopus embryos. Blocking HCN channels (Ih current) with pharmacological inhibitors leads to errors in organ situs This effect is only seen when HCN4 channels are blocked early (pre-stage 10) and not by a later block (post-stage 10). Injections of HCN4-DN (dominant-negative) mRNA induces left-right defects only when injected in both blastomeres no later than the 2-cell stage. Analysis of key asymmetric genes' expression showed that the sidedness of Nodal, Lefty, and Pitx2 expression is largely unchanged by HCN4 blockade, despite the randomization of subsequent organ situs, although the area of Pitx2 expression was significantly reduced. Together these data identify a novel, developmental role for HCN4 channels and reveal a new Nodal-Lefty-Pitx2 asymmetric gene expression-independent mechanism upstream of organ positioning during embryonic left-right patterning.
... An interesting question is whether we potentially need to account for directional bias of the organism. Recently, P. polycephalum was found to exhibit chirality [23] in the search phase of the foraging, i.e. a directional preference when the plasmodium is expanding. In contrast to this, our suggested experiment investigates the contraction phase of foraging, i.e. the shrinking of the tube network. ...
Article
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Self-organized mechanisms are frequently encountered in nature and known to achieve flexible, adaptive control and decision-making. Noise plays a crucial role in such systems: It can enable a self-organized system to reliably adapt to short-term changes in the environment while maintaining a generally stable behavior. This is fundamental in biological systems because they must strike a delicate balance between stable and flexible behavior. In the present paper we analyse the role of noise in the decision-making of the true slime mold Physarum polycephalum, an important model species for the investigation of computational abilities in simple organisms. We propose a simple biological experiment to investigate the reaction of P. polycephalum to time-variant risk factors and present a stochastic extension of an established mathematical model for P. polycephalum to analyze this experiment. It predicts that—due to the mechanism of stochastic resonance—noise can enable P. polyce-phalum to correctly assess time-variant risk factors, while the corresponding noise-free system fails to do so. Beyond the study of P. polycephalum we demonstrate that the influence of noise on self-organized decision-making is not tied to a specific organism. Rather it is a general property of the underlying process dynamics, which appears to be universal across a wide range of systems. Our study thus provides further evidence that stochastic resonance is a fundamental component of the decision-making in self-organized macroscopic and microscopic groups and organisms.
... One candidate for a chiral element of physics upstream of asymmetric gene expression is ciliary flow at neurulation [49,73,74]; this model however faces numerous problems, which have been detailed elsewhere [75 -79]. Recent functional data revealed conserved mechanisms in a range of organisms from plants to mammals, which establish asymmetry without a ciliated structure, or long before it forms; indeed, many phyla including some vertebrates determine their LR axis very early after fertilization [80][81][82][83][84][85][86][87][88][89][90]. Even mouse embryos are known to exhibit molecular and functional asymmetries (e.g. ...
Article
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Consistent left–right (LR) asymmetry is a fundamental aspect of the bodyplan across phyla, and errors of laterality form an important class of human birth defects. Its molecular underpinning was first discovered as a sequential pathway of left- and right-sided gene expression that controlled positioning of the heart and visceral organs. Recent data have revised this picture in two important ways. First, the physical origin of chirality has been identified; cytoskeletal dynamics underlie the asymmetry of single-cell behaviour and patterning of the LR axis. Second, the pathway is not linear: early disruptions that alter the normal sidedness of upstream asymmetric genes do not necessarily induce defects in the laterality of the downstream genes or in organ situs . Thus, the LR pathway is a unique example of two fascinating aspects of biology: the interplay of physics and genetics in establishing large-scale anatomy, and regulative (shape-homeostatic) pathways that correct molecular and anatomical errors over time. Here, we review aspects of asymmetry from its intracellular, cytoplasmic origins to the recently uncovered ability of the LR control circuitry to achieve correct gene expression and morphology despite reversals of key ‘determinant’ genes. We provide novel functional data, in Xenopus laevis , on conserved elements of the cytoskeleton that drive asymmetry, and comparatively analyse it together with previously published results in the field. Our new observations and meta-analysis demonstrate that despite aberrant expression of upstream regulatory genes, embryos can progressively normalize transcriptional cascades and anatomical outcomes. LR patterning can thus serve as a paradigm of how subcellular physics and gene expression cooperate to achieve developmental robustness of a body axis. This article is part of the themed issue ‘Provocative questions in left–right asymmetry’.
... By contrast, intracellular cytoskeletal elements are highly conserved; recent data revealed a microtubule protein that was functionally implicated in asymmetry of plants, C. elegans, Xenopus and human cells [10]. One possibility is that asymmetry is an ancient property that predates the origins of multicellularity; even bacteria [16], unicellular ciliates [17] and slime moulds [18] exhibit asymmetry. Satir's paper [19] deftly overviews the many actin filaments and microtubule processes in cells that exhibit chiral properties, and illustrates how helical properties of cytoskeletal elements instruct cell behaviour and subsequent morphogenesis. ...
Article
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Left–right asymmetry is a phenomenon that has a broad appeal—to anatomists, developmental biologists and evolutionary biologists—because it is a morphological feature of organisms that spans scales of size and levels of organization, from unicellular protists, to vertebrate organs, to social behaviour. Here, we highlight a number of important aspects of asymmetry that encompass several areas of biology—cell-level, physiological, genetic, anatomical and evolutionary components—and that are based on research conducted in diverse model systems, ranging from single cells to invertebrates to human developmental disorders. Together, the contributions in this issue reveal a heretofore-unsuspected variety in asymmetry mechanisms, including ancient chirality elements that could underlie a much more universal basis to asymmetry development, and provide much fodder for thought with far reaching implications in biomedical, developmental, evolutionary and synthetic biology. The new emerging theme of binary cell-fate choice, promoted by asymmetric cell division of a deterministic cell, has focused on investigating asymmetry mechanisms functioning at the single cell level. These include cytoskeleton and DNA chain asymmetry—mechanisms that are amplified and coordinated with those employed for the determination of the anterior–posterior and dorsal–ventral axes of the embryo. This article is part of the themed issue ‘Provocative questions in left–right asymmetry’.
... One candidate for a chiral element of physics upstream of asymmetric gene expression is ciliary flow at neurulation [72,98,99]; this model however faces numerous problems which have been detailed elsewhere [100,101,102,103,104]. Recent functional data revealed conserved mechanisms in a range of organisms from plants to mammals, that establish asymmetry without a ciliated structure, or long before it forms; indeed, many phyla including some vertebrates determine their LR axis very early after fertilization [105,106,107,108,109,110,111,112,113,114,115]. Even mouse embryos are known to exhibit molecular and functional asymmetries (e.g., components such as cofilin, which is also asymmetric in cleavage-stage frog embryos) as early as the cleavage stages [107,116,117]. ...
Working Paper
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Consistent left-right asymmetry is a fundamental aspect of the bodyplan across phyla, and errors of laterality form an important class of human birth defects. Its molecular underpinning was first discovered as a sequential pathway of left- and right-sided gene expression that controlled positioning of the heart and visceral organs. Recent data have revised this picture in two important ways. First, the physical origin of chirality has been identified; cytoskeletal dynamics underlie the asymmetry of single cell behavior and of patterning of the left-right axis. Second, the pathway is not linear: early disruptions that alter the normal sidedness of upstream asymmetric genes do not necessarily induce defects in the laterality of the downstream genes or in organ situs. Thus, the LR pathway is a unique example of two fascinating aspects of biology: the interplay of physics and genetics in establishing large-scale anatomy, and regulative (shape-homeostatic) pathways that correct errors of patterning over time. Here, we review aspects of asymmetry from its intracellular, cytoplasmic origins to the recently-uncovered ability of the LR control circuitry to achieve correct gene expression and morphology despite reversals of key ″determinant″ genes. We provide novel functional data, in Xenopus laevis, on conserved elements of the cytoskeleton that drive asymmetry, and repair of downstream gene expression anomalies over developmental time. LR patterning can thus serve as a paradigm of how subcellular physics and gene expression cooperate to achieve developmental robustness of a body axis.
Article
In the context of animal or plant development, we tend to think of cells as small, simple, building blocks, such that complex patterns or shapes can only be constructed from large numbers of cells, with cells in different parts of the organism taking on different fates. However, cells themselves are far from simple, and often take on complex shapes with a remarkable degree of intracellular patterning. How do these patterns arise? As in embryogenesis, the development of structure inside a cell can be broken down into a number of basic processes. For each part of the cell, morphogenetic processes create internal structures such as organelles, which might correspond to organs at the level of a whole organism. Given that mechanisms exist to generate parts, patterning processes are required to ensure that the parts are distributed in the correct arrangement relative to the rest of the cell. Such patterning processes make reference to global polarity axes, requiring mechanisms for axiation which, in turn, require processes to break symmetry. These fundamental processes of symmetry breaking, axiation, patterning, and morphogenesis have been extensively studied in developmental biology but less so at the subcellular level. This review will focus on developmental processes that give eukaryotic cells their complex structures, with a focus on cytoskeletal organization in free-living cells, ciliates in particular, in which these processes are most readily apparent.
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Colloidal devices ‐ in the liquid state ‐, possibly protected from harsh planetary environments by a soft protective deformable skin, represent a totally new paradigm in the field of robotics. They include the autonomy features of conventional robotics, the shape‐changing advantages of soft robotics, and offer new opportunities to science and technology. For these systems, energy management is considered to be the most essential function to guarantee autonomy and operation. This paper describes smart fluids as autonomous systems with energy harvesting / storage capabilities, and provides an initial assessment of existing possibilities that can be leveraged to pursue the emerging topic of Colloidal Autonomous Systems.
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Many developmental processes break left–right (LR) symmetry with a consistent handedness. LR asymmetry emerges early in development, and in many species the primary determinant of this asymmetry has been linked to the cytoskeleton. However, the nature of the underlying chirally asymmetric cytoskeletal processes has remained elusive. In this study, we combine thin-film active chiral fluid theory with experimental analysis of the C. elegans embryo to show that the actomyosin cortex generates active chiral torques to facilitate chiral symmetry breaking. Active torques drive chiral counter-rotating cortical flow in the zygote, depend on myosin activity, and can be altered through mild changes in Rho signaling. Notably, they also execute the chiral skew event at the 4-cell stage to establish the C. elegans LR body axis. Taken together, our results uncover a novel, large-scale physical activity of the actomyosin cytoskeleton that provides a fundamental mechanism for chiral morphogenesis in development.
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Actin-based cellular protrusions are a ubiquitous feature of cell morphology, e.g., filopodia and microvilli, serving a huge variety of functions. Despite this, there is still no comprehensive model for the mechanisms that determine the geometry of these protrusions. We present here a detailed computational model that addresses a combination of multiple biochemical and physical processes involved in the dynamic regulation of the shape of these protrusions. We specifically explore the role of actin polymerization in determining both the height and width of the protrusions. Furthermore, we show that our generalized model can explain multiple morphological features of these systems, and account for the effects of specific proteins and mutations.
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Traditionally, only humans were thought to exhibit brain and behavioural asymmetries, but several studies have revealed that most vertebrates are also lateralized. Recently, evidence of left-right asymmetries in invertebrates has begun to emerge, suggesting that lateralization of the nervous system may be a feature of simpler brains as well as more complex ones. Here I present some examples in invertebrates of sensory and motor asymmetries, as well as asymmetries in the nervous system. I illustrate two cases where an asymmetric brain is crucial for the development of some cognitive abilities. The first case is the nematode C. elegans, which has asymmetric odour sensory neurons and taste perception neurons. In this worm left/right asymmetries are responsible for the sensing of a substantial number of salt ions, and lateralized responses to salt allow the worm to discriminate between distinct salt ions. The second case is the fruit fly D. melanogaster, where the presence of asymmetry in a particular structure of the brain is important in the formation or retrieval of long-term memory. Moreover, I distinguish two distinct patterns of lateralization that occur in both vertebrates and invertebrates: individual-level and population-level lateralization. Theoretical models on the evolution of lateralization suggest that the alignment of lateralization at the population level may have evolved as an evolutionary stable strategy in which individually-asymmetrical organisms must coordinate their behaviour with that of other asymmetrical organisms. This implies that lateralization at the population-level is more likely to have evolved in social rather than in solitary species. I evaluate this new hypothesis with specific focus on insects showing different level of sociality. In particular, I present a series of studies on antennal asymmetries in honeybees and other related species of bees, showing how insects may be extremely useful to test evolutionary hypothesis.
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The plasmodium of the slime mould Physarum polycephalum is a large amoeba-like cell consisting of a dendritic network of tube-like structures (pseudopodia). It changes its shape as it crawls over a plain agar gel and, if food is placed at two different points, it will put out pseudopodia that connect the two food sources. Here we show that this simple organism has the ability to find the minimum-length solution between two points in a labyrinth.
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Ever since cloning the classic iv mutation identified the ‘left-right dynein’ (lrd) gene in mice, most research on body laterality determination has focused on its function in motile cilia at the node embryonic organizer. This model is attractive, as it links chirality of cilia architecture to asymmetry development. However, lrd is also expressed in blastocysts and embryonic stem cells, where it was shown to bias the segregation of recombined sister chromatids away from each other in mitosis. These data suggested that lrd is part of a cellular mechanism that recognizes and selectively segregates sister chromatids based on their replication history: old ‘Watson’ vs. old ‘Crick’ strands. We previously proposed that the mouse left-right axis is established via an asymmetric cell division prior to/or during gastrulation. In this model, left-right dynein selectively segregates epigenetically differentiated sister chromatids harboring a hypothetical ‘left-right axis development 1’ (‘lra1’) gene during the left-right axis establishing cell division. Here, asymmetry development would be ultimately governed by the chirality of the cytoskeleton and the DNA molecule. Our model predicts that randomization of chromatid segregation in lrd mutants should produce embryos with 25% situs solitus, 25% situs inversus, and 50% embryonic death due to heterotaxia and isomerism. Here we confirmed this prediction by using two distinct lrd mutant alleles. Other than lrd, thus far Nodal gene is the most upstream function implicated in visceral organs laterality determination. We next tested whether the Nodal gene constitutes the lra1 gene hypothesized in the model by testing mutant’s effect on 50% embryonic lethality observed in lrd mutants. Since Nodal mutation did not suppress lethality, we conclude that Nodal is not equivalent to the lra1 gene. In summary, we describe the origin of 50% lethality in lrd mutant mice not yet explained by any other laterality-generating hypothesis.
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Many types of embryos' bodyplans exhibit consistently oriented laterality of the heart, viscera, and brain. Errors of left-right patterning present an important class of human birth defects, and considerable controversy exists about the nature and evolutionary conservation of the molecular mechanisms that allow embryos to reliably orient the left-right axis. Here we show that the same mutations in the cytoskeletal protein tubulin that alter asymmetry in plants also affect very early steps of left-right patterning in nematode and frog embryos, as well as chirality of human cells in culture. In the frog embryo, tubulin α and tubulin γ-associated proteins are required for the differential distribution of maternal proteins to the left or right blastomere at the first cell division. Our data reveal a remarkable molecular conservation of mechanisms initiating left-right asymmetry. The origin of laterality is cytoplasmic, ancient, and highly conserved across kingdoms, a fundamental feature of the cytoskeleton that underlies chirality in cells and multicellular organisms.
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A recently developed technique enables quantitative study of the initiation of left-right asymmetry using cells grown on micropatterns with close appositional boundaries. It was found that mammalian cells exhibit either a left or right bias in their migratory behavior, which was determined by cell phenotype, different for certain cancer and normal cells, and dependent on functionality of the actin cytoskeleton. We discuss here the relevance of this simple technique to study of development and birth defects in laterality.
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Left-right (LR) asymmetry (handedness, chirality) is a well-conserved biological property of critical importance to normal development. Changes in orientation of the LR axis due to genetic or environmental factors can lead to malformations and disease. While the LR asymmetry of organs and whole organisms has been extensively studied, little is known about the LR asymmetry at cellular and multicellular levels. Here we show that the cultivation of cell populations on micropatterns with defined boundaries reveals intrinsic cell chirality that can be readily determined by image analysis of cell alignment and directional motion. By patterning 11 different types of cells on ring-shaped micropatterns of various sizes, we found that each cell type exhibited definite LR asymmetry (p value down to 10(-185)) that was different between normal and cancer cells of the same type, and not dependent on surface chemistry, protein coating, or the orientation of the gravitational field. Interestingly, drugs interfering with actin but not microtubule function reversed the LR asymmetry in some cell types. Our results show that micropatterned cell populations exhibit phenotype-specific LR asymmetry that is dependent on the functionality of the actin cytoskeleton. We propose that micropatterning could potentially be used as an effective in vitro tool to study the initiation of LR asymmetry in cell populations, to diagnose disease, and to study factors involved with birth defects in laterality.
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The organization and polarity of actin filaments in neuronal growth cones was studied with negative stain and freeze-etch EM using a permeabilization protocol that caused little detectable change in morphology when cultured nerve growth cones were observed by video-enhanced differential interference contrast microscopy. The lamellipodial actin cytoskeleton was composed of two distinct subpopulations: a population of 40-100-nm-wide filament bundles radiated from the leading edge, and a second population of branching short filaments filled the volume between the dorsal and ventral membrane surfaces. Together, the two populations formed the three-dimensional structural network seen within expanding lamellipodia. Interaction of the actin filaments with the ventral membrane surface occurred along the length of the filaments via membrane associated proteins. The long bundled filament population was primarily involved in these interactions. The filament tips of either population appeared to interact with the membrane only at the leading edge; this interaction was mediated by a globular Triton-insoluble material. Actin filament polarity was determined by decoration with myosin S1 or heavy meromyosin. Previous reports have suggested that the polarity of the actin filaments in motile cells is uniform, with the barbed ends toward the leading edge. We observed that the actin filament polarity within growth cone lamellipodia is not uniform; although the predominant orientation was with the barbed end toward the leading edge (47-56%), 22-25% of the filaments had the opposite orientation with their pointed ends toward the leading edge, and 19-31% ran parallel to the leading edge. The two actin filament populations display distinct polarity profiles: the longer filaments appear to be oriented predominantly with their barbed ends toward the leading edge, whereas the short filaments appear to be randomly oriented. The different length, organization and polarity of the two filament populations suggest that they differ in stability and function. The population of bundled long filaments, which appeared to be more ventrally located and in contact with membrane proteins, may be more stable than the population of short branched filaments. The location, organization, and polarity of the long bundled filaments suggest that they may be necessary for the expansion of lamellipodia and for the production of tension mediated by receptors to substrate adhesion molecules.
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The development of handed asymmetry requires a special mechanism for consistently specifying a difference between left and right sides. This is to be distinguished from both random asymmetry, and from those left/right differences that are mirror symmetrical. We propose a model for the development of handedness in bilateral animals, comprising three components. (i) A process termed conversion, in which a molecular handedness is converted into handedness at the cellular level. A specific model for this process is put forward, based on cell polarity and transport of cellular constituents by a handed molecule. (ii) A mechanism for random generation of asymmetry, which could involve a reaction-diffusion process, so that the concentration of a molecule is higher on one side than the other. The handedness generated by conversion could consistently bias this mechanism to one side. (iii) A tissue-specific interpretation process which responds to the difference between the two sides, and results in the development of different structures on the left and right. There could be direct genetic control of the direction of handedness in this model, most probably through the conversion process. Experimental evidence for the model is considered, particularly the iv mutation in the mouse, which appears to result in loss-of-function in biasing, and so asymmetry is random. The model can explain the abnormal development of handedness observed in bisected embryos of some mammalian, amphibian and sub-vertebrate species. Spiral asymmetry, as seen in spiral cleavage and in ciliates, involves only conversion of molecular asymmetry to the cellular and multicellular level, with no separate interpretation step.
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Left-right asymmetry in plants can be found in helices of stalks, stems and tendrils, and in fan-like petal arrangements. The handedness in these asymmetric structures is often fixed in given species, indicating that genetic factors control asymmetric development. Here we show that dominant negative mutations at the tubulin intradimer interface of alpha-tubulins 4 and 6 cause left-handed helical growth and clockwise twisting in elongating organs of Arabidopsis thaliana. We demonstrate that the mutant tubulins incorporate into microtubule polymers, producing right-handed obliquely oriented cortical arrays, in the root epidermal cells. The cortical microtubules in the mutants had increased sensitivity to microtubule-specific drugs. These results suggest that reduced microtubule stability can produce left-handed helical growth in plants.
Book
This book is devoted to Slime mould Physarum polycephalum, which is a large single cell capable for distributed sensing, concurrent information processing, parallel computation and decentralized actuation. The ease of culturing and experimenting with Physarum makes this slime mould an ideal substrate for real-world implementations of unconventional sensing and computing devices The book is a treatise of theoretical and experimental laboratory studies on sensing and computing properties of slime mould, and on the development of mathematical and logical theories of Physarum behavior. It is shown how to make logical gates and circuits, electronic devices (memristors, diodes, transistors, wires, chemical and tactile sensors) with the slime mould. The book demonstrates how to modify properties of Physarum computing circuits with functional nano-particles and polymers, to interface the slime mould with field-programmable arrays, and to use Physarum as a controller of microbial fuel cells. A unique multi-agent model of slime is shown to serve well as a software slime mould capable for solving problems of computational geometry and graph optimization. The multiagent model is complemented by cellular automata models with parallel accelerations. Presented mathematical models inspired by Physarum include non-quantum implementation of Shor's factorization, structural learning, computation of shortest path tree on dynamic graphs, supply chain network design, p-adic computing and syllogistic reasoning. The book is a unique composition of vibrant and lavishly illustrated essays which will inspire scientists, engineers and artists to exploit natural phenomena in designs of future and emergent computing and sensing devices. It is a 'bible' of experimental computing with spatially extended living substrates, it spanstopics from biology of slime mould, to bio-sensing, to unconventional computing devices androbotics, non-classical logics and music and arts.
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Cellular mechanisms underlying the development of left-right asymmetry in tissues and embryos remain obscure. Here, the development of a chiral pattern of actomyosin was revealed by studying actin cytoskeleton self-organization in cells with isotropic circular shape. A radially symmetrical system of actin bundles consisting of α-actinin-enriched radial fibres (RFs) and myosin-IIA-enriched transverse fibres (TFs) evolved spontaneously into the chiral system as a result of the unidirectional tilting of all RFs, which was accompanied by a tangential shift in the retrograde movement of TFs. We showed that myosin-IIA-dependent contractile stresses within TFs drive their movement along RFs, which grow centripetally in a formin-dependent fashion. The handedness of the chiral pattern was shown to be regulated by α-actinin-1. Computational modelling demonstrated that the dynamics of the RF-TF system can explain the pattern transition from radial to chiral. Thus, actin cytoskeleton self-organization provides built-in machinery that potentially allows cells to develop left-right asymmetry.
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The influence of tool geometry on hole deviation is investigated using four types of deep hole drilling tools, i. e. , a normal type gundrill, a standard type BTA tool, a double-edge gundrill and a multi-edge BTA tool. Two types of workpieces are used to make their influence clear: one has an unsymmetric wall thickness on the right or left side and the other is made up of two materials where the hardness of the bonded plate is lower than that of the base metal. Experimental and theoretical results show that balanced cutting forces on the tool head contribute to decreased hole deviation. However, to some extent, an unbalanced force is necessary for giving accurate hole size. Otherwise, a completely balanced condition of forces generates a slightly oversized hole.
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In multicellular organisms, cell properties, such as shape, size and function are important in morphogenesis and physiological functions. Recently, ‘cellular chirality’ has attracted attention as a cellular property because it can cause asymmetry in the bodies of animals. In recent in vitro studies, the left–right bias of cellular migration and of autonomous arrangement of cells under some specific culture conditions were discovered. However, it is difficult to identify the molecular mechanism underlying their intrinsic chirality because the left–right bias observed to date is subtle or is manifested in the stable orientation of cells. Here, we report that zebrafish (Danio rerio) melanophores exhibit clear cellular chirality by unidirectional counterclockwise rotational movement under isolated conditions without any special settings. The chirality is intrinsic to melanophores because the direction of the cellular rotation was not affected by the type of extracellular matrix. We further found that the cellular rotation was generated as a counter action of the clockwise movement of actin cytoskeleton. It suggested that the mechanism that directs actin cytoskeleton in the clockwise direction is pivotal for determining cellular chirality.
Conference Paper
This paper discusses a project in which wall following and maze navigation algorithms were developed for a simulated robot in a noisy environment. The algorithms were developed using genetic programming and evaluated by determining how well they performed during a series of simulations.
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For centuries, scientists and physicians have been captivated by the consistent left-right (LR) asymmetry of the heart, viscera, and brain. A recent study implicated tubulin proteins in establishing laterality in several experimental models, including asymmetric chemosensory receptor expression in C. elegans neurons, polarization of HL-60 human neutrophil-like cells in culture, and asymmetric organ placement in Xenopus. The same mutations that randomized asymmetry in these diverse systems also affect chirality in Arabidopsis, revealing a remarkable conservation of symmetry-breaking mechanisms among kingdoms. In Xenopus, tubulin mutants only affected LR patterning very early, suggesting that this axis is established shortly after fertilization. This addendum summarizes and extends the knowledge of the cytoskeleton's role in the patterning of the LR axis. Results from many species suggest a conserved role for the cytoskeleton as the initiator of asymmetry, and indicate that symmetry is first broken during early embryogenesis by an intracellular process.
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The body architectures of most multicellular organisms consistently display both symmetry and asymmetry. Here, we discuss some of the available knowledge and open questions on how symmetry and asymmetry appear in several conspicuous plant cells and tissues. We focus, where possible, on the role of genes that participate in the maintenance or the breaking of symmetry and that are directly or indirectly related to the cell cycle, under an organ-centric point of view and with an emphasis on the leaf.
The ability to closely control the directions of holes drilled in rock is important in an increasing range of applications. Although the subject has long been important in oil and gas well drilling, attention is focused here on directional control in other rotary drilling applications including mineral exploration, site investigation and horizontal drilling for probing ahead of tunnels and for coal degasification. Hole direction is controlled by drill-string mechanics and rock bit interaction effects. Anisotropy in the mechanical properties of the rock can influence rock-bit interaction and cause bit deviation. A review of laboratory and field evidence and of the theories advanced to explain bit deviation effects in anisotropic rock shows that, although some useful general conclusions can be drawn, the subject is not at all well understood. It is concluded that to develop a better understanding of the subject, the bit deviation forces induced by rock anisotropy should be measured under a variety of rotary drilling conditions. The data so obtained could be applied in modifications of currently available mathematical models of drill-string mechanics which could be used to predict likely bit deviations or to determine the stabilizing or steering forces required for particular drill-string configurations.
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Understanding how and when the left-right (LR) axis is first established is a fundamental question in developmental biology. A popular model is that the LR axis is established relatively late in embryogenesis, due to the movement of motile cilia and the resultant directed fluid flow during late gastrulation/early neurulation. Yet, a large body of evidence suggests that biophysical, molecular, and bioelectrical asymmetries exist much earlier in development, some as early as the first cell cleavage after fertilization. Alternative models of LR asymmetry have been proposed that accommodate these data, postulating that asymmetry is established due to a chiral cytoskeleton and/or the asymmetric segregation of chromatids. There are some similarities, and many differences, in how these various models postulate the origin and timing of symmetry breaking and amplification, and these events' linkage to the well-conserved subsequent asymmetric transcriptional cascades. This review examines experimental data that lend strong support to an early origin of LR asymmetry, yet are also consistent with later roles for cilia in the amplification of LR pathways. In this way, we propose that the various models of asymmetry can be unified: early events are needed to initiate LR asymmetry, and later events could be utilized by some species to maintain LR-biases. We also present an alternative hypothesis, which proposes that individual embryos stochastically choose one of several possible pathways with which to establish their LR axis. These two hypotheses are both tractable in appropriate model species; testing them to resolve open questions in the field of LR patterning will reveal interesting new biology of wide relevance to developmental, cell, and evolutionary biology.
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Robotics is not a new word. It has attracted people from all phases of life. The failure of a project in any field mostly occurs due to incomplete analysis of problem or improper planning. Such disappointments can be avoided by adapting a proper approach towards solving a given task. The problem of micro-mouse is 30 years old but its importance in the field of robotics is unparalleled, as it requires a complete analysis & proper planning to solve this problem. This paper covers one of the most important areas of robot, “Decision making Algorithm” or inlay-man’s language, “Intelligence”. The environment around the robot is not known, so it must have decision-making capabilities. For starting in the field of micro-mouse it is very difficult to begin with highly sophisticated algorithms. This paper begins with very basic wall follower logic to solve the maze. And gradually improves the algorithm to accurately solve the maze in shortest time with some more intelligence. The Algorithm is developed up to some sophisticated level as Flood fill algorithm.
Article
Turning preferences among 309 white-faced ewes were individually evaluated in an enclosed, artificially lit T-maze, followed by each ewe choosing either a right or left return alley to return to peers. Data recorded included time in the start box, time in the T-maze, exit arm chosen to leave the T-maze, and return alley. Right and left arms of the T-maze were chosen 65.7% and 34.3% of the time, respectively, while right and left return alleys were chosen 32.4% and 67.6%, respectively. Exit arm and return alley were not independently chosen (p<.0001), with observed counts being higher than expected under independence when ewes made the same choice for exit and alley (RR or LL turn patterns) and being lower than expected for alternating choices (RL or LR). Out of the 309 ewes, 28.2% and 30.1% chose RR and LL turn patterns, respectively, while 37.5% chose the RL turn pattern, but only 13 (4.2%) chose the LR turning pattern. Overall, ewes that initially turned right when presented a second turning opportunity had a slight preference to alternate their turning direction, while ewes that initially turned left tended to continue turning left when given another chance to turn. Exit arm and return alley laterality was not related (α=.05) to time of day the test was administered, ewe's age or genetics, most recent liveweight, or most recent shorn fleece weight. The mean time spent in the start box (21 s) was not related to exit arm (p=.947) or return alley (p=.779). Mean time (15 s) spent in the T-maze was not related to exit arm (p =.086) or return alley (p = .952). More research will be required to understand sheep turning laterality and how it can impact working facilities and research equipment.
Article
Left-right (LR) asymmetry is ubiquitous in animal development. Cytoskeletal chirality was recently reported to specify LR asymmetry in embryogenesis, suggesting that LR asymmetry in tissue morphogenesis is coordinated by single- or multi-cell organizers. Thus, to organize LR asymmetry at multiscale levels of morphogenesis, cells with chirality must also be present in adequate numbers. However, observation of LR asymmetry is rarely reported in cultured cells. Using cultured vascular mesenchymal cells, we tested whether LR asymmetry occurs at the single cell level and in self-organized multicellular structures. Using micropatterning, immunofluorescence revealed that adult vascular cells polarized rightward and accumulated stress fibers at an unbiased mechanical interface between adhesive and nonadhesive substrates. Green fluorescent protein transfection revealed that the cells each turned rightward at the interface, aligning into a coherent orientation at 20° relative to the interface axis at confluence. During the subsequent aggregation stage, time-lapse videomicroscopy showed that cells migrated along the same 20° angle into neighboring aggregates, resulting in a macroscale structure with LR asymmetry as parallel, diagonal stripes evenly spaced throughout the culture. Removal of substrate interface by shadow mask-plating, or inhibition of Rho kinase or nonmuscle myosin attenuated stress fiber accumulation and abrogated LR asymmetry of both single-cell polarity and multicellular coherence, suggesting that the interface triggers asymmetry via cytoskeletal mechanics. Examination of other cell types suggests that LR asymmetry is cell-type specific. Our results show that adult stem cells retain inherent LR asymmetry that elicits de novo macroscale tissue morphogenesis, indicating that mechanical induction is required for cellular LR specification.
Article
Some organs in animals display left-right (LR) asymmetry. To better understand LR asymmetric morphogenesis in Drosophila, we studied LR directional rotation of the hindgut epithelial tube. Hindgut epithelial cells adopt a LR asymmetric (chiral) cell shape within their plane, and we refer to this cell behavior as planar cell-shape chirality (PCC). Drosophila E-cadherin (DE-Cad) is distributed to cell boundaries with LR asymmetry, which is responsible for the PCC formation. Myosin ID switches the LR polarity found in PCC and in DE-Cad distribution, which coincides with the direction of rotation. An in silico simulation showed that PCC is sufficient to induce the directional rotation of this tissue. Thus, the intrinsic chirality of epithelial cells in vivo is an underlying mechanism for LR asymmetric tissue morphogenesis.
Article
Consistent laterality is a crucial aspect of embryonic development, physiology, and behavior. While strides have been made in understanding unilaterally expressed genes and the asymmetries of organogenesis, early mechanisms are still poorly understood. One popular model centers on the structure and function of motile cilia and subsequent chiral extracellular fluid flow during gastrulation. Alternative models focus on intracellular roles of the cytoskeleton in driving asymmetries of physiological signals or asymmetric chromatid segregation, at much earlier stages. All three models trace the origin of asymmetry back to the chirality of cytoskeletal organizing centers, but significant controversy exists about how this intracellular chirality is amplified onto cell fields. Analysis of specific predictions of each model and crucial recent data on new mutants suggest that ciliary function may not be a broadly conserved, initiating event in left-right patterning. Many questions about embryonic left-right asymmetry remain open, offering fascinating avenues for further research in cell, developmental, and evolutionary biology.
Article
Defining the three body axes is a central event of vertebrate morphogenesis. Establishment of left-right (L-R) asymmetry in development follows the determination of dorsal-ventral and anterior-posterior (A-P) body axes, although the molecular mechanism underlying precise L-R symmetry breaking in reference to the other two axes is still poorly understood. Here, by removing both Vangl1 and Vangl2, the two mouse homologues of a Drosophila core planar cell polarity (PCP) gene Van Gogh (Vang), we reveal a previously unrecognized function of PCP in the initial breaking of lateral symmetry. The leftward nodal flow across the posterior notochord (PNC) has been identified as the earliest event in the de novo formation of L-R asymmetry. We show that PCP is essential in interpreting the A-P patterning information and linking it to L-R asymmetry. In the absence of Vangl1 and Vangl2, cilia are positioned randomly around the centre of the PNC cells and nodal flow is turbulent, which results in disrupted L-R asymmetry. PCP in mouse, unlike what has been implicated in other vertebrate species, is not required for ciliogenesis, cilium motility, Sonic hedgehog (Shh) signalling or apical docking of basal bodies in ciliated tracheal epithelial cells. Our data suggest that PCP acts earlier than the unidirectional nodal flow during bilateral symmetry breaking in vertebrates and provide insight into the functional mechanism of PCP in organizing the vertebrate tissues in development.
Article
Because cells are sensitive to mechanical forces,microgravity might act on stress-dependent cell changes. Regulation of focal adhesions (FAs) and cytoskeletal activity plays a role in cell maintenance, cell movement,and migration. Human MCF-7 cells were exposed to modeled microgravity (MMG) to test the hypothesis that migration responsiveness to microgravity is associated with cytoskeleton and FA anomalies. MMG acts on MCF-7 cells by disorganizing cytoskeleton filaments (microfilaments and microtubules). Microfilaments in MMG did not display their typical radial array. Likewise, microtubules were disrupted in MCF-7 cells within 4 h of initiation of MMG and were partly reestablished by 48 h. FAs generated inmicrogravity were less mature than those established in controls, shown by reduced FAs number and clustering. In parallel, MMG decreased kinases activity (such as FAK,PYK2, and ILK) of FAs in MCF-7 cells. The expression of both integrinbeta1 and integrinbeta4 were downregulated by MMG. We conclude that cytoskeletal alterations and FAs changes in MMG are concomitant with cell invasion and migration retardation. We suggest that reduced migration response in MCF-7 cells following MMG is linked to changes of cytoskeleton and FAs.
Article
Cilia establish the vertebrate left-right (LR) axis and are integral to the development and function of the kidney, liver, and brain. Left-right asymmetry is established in the ciliated ventral node cells of the mouse. The chiral structure of the cilium provides a reference asymmetry to impose handed LR asymmetric development on the bilaterally symmetric vertebrate embryo. A ciliary mechanism of LR development is evolutionarily conserved, as ciliated organs essential to LR axis formation, called LR organizers, are found in other vertebrates, including rabbit, fish, and Xenopus. Mice with mutations affecting ciliary biogenesis, motility, or sensory function have abnormal LR development and abnormal development of the heart. The axonemal dynein heavy chain left-right dynein (lrd) localizes to the LR organizer and drives counterclockwise movement of node primary cilia. Node primary cilia are an admixture of 9 + 2 and 9 + 0 cilia. Mutations in lrd result in structurally normal, immotile node monocilia. In the mouse, coordinated, directional beating of motile node monocilia at the neural fold stage generates leftward flow of extraembryonic fluid surrounding the node (nodal flow). Nodal flow triggers a rise in intracellular calcium in cells at the left side of the node. The perinodal asymmetric rise in intracellular calcium generated by nodal flow subsequently leads to asymmetric gene expression and morphogenesis.
Article
Consistent left-right (LR) patterning is a clinically important embryonic process. However, key questions remain about the origin of asymmetry and its amplification across cell fields. Planar cell polarity (PCP) solves a similar morphogenetic problem, and although core PCP proteins have yet to be implicated in embryonic LR asymmetry, studies of mutations affecting planar polarity, together with exciting new data in cell and developmental biology, provide a new perspective on LR patterning. Here we propose testable models for the hypothesis that LR asymmetry propagates as a type of PCP that imposes coherent orientation onto cell fields, and that the cue that orients this polarization is a chiral intracellular structure.
Article
Branching network growth patterns, depending on environmental conditions, in plasmodium of true slime mold Physarum polycephalum were investigated. Surprisingly, the patterns resemble those in bacterial colonies even though the biological mechanisms differ greatly. Bacterial colonies are collectives of microorganisms in which individual organisms have motility and interact through nutritious and chemical fields. In contrast, the plasmodium is a giant amoeba-like multinucleated unicellular organism that forms a network of tubular structures through which protoplasm streams. The cell motility of the plasmodium is generated by oscillation phenomena observed in the partial bodies, which interact through the tubular structures. First, we analyze characteristics of the morphology quantitatively, then we abstract local rules governing the growing process to construct a simple network growth model. This model is independent of specific systems, in which only two rules are applied. Finally, we discuss the mechanism of commonly observed biological pattern formations through comparison with the system of bacterial colonies.
Article
When retinal explants from goldfish are grown on a polycation substratum, a marked tendency for directionality of neurite outgrowth is observed. While the direct relevance to nerve growth in vivo is not known, the phenomenon is interpreted as reflecting an inherent helicity of the neurites.
Article
Ciliated protozoa have intrinsically asymmetrical ciliary structures that are asymmetrically arranged over the cell surface. These structures can be arranged in two enantiomorphic configurations, 'right-handed' (RH) and 'left-handed' (LH). Whereas one of these configurations (arbitrarily, RH) is apparently universal in Nature and predominant in the laboratory, mirror-image (RH-LH) doublets and reverse (LH) singlets have been generated and studied in eight different ciliate genera. In all these, the internal asymmetry of individual ciliary structures remains normal even when the asymmetry of arrangement of these structures is reversed. The individual structures may sometimes become inverted (rotationally permuted). LH forms reproduce themselves if they are able to feed, or reorganize periodically before starving to death if they are not. Changes of cellular handedness depend upon unusual geometric configurations and in most cases are unrelated to genic changes. In hypotrich ciliates changes of handedness can be provoked by artificially generated juxtapositions of anterior and posterior cell regions or of right and left cell margins. Reversal of handedness in ciliates can be visualized as a consequence of (re-)establishment of a normal sequence of normally spaced positional values following geometric disturbances created by the experimenter or by the regulating cell.
Article
Characteristics of spatial orientation in T-maze were studied in 1768 Planaria of following types: Dugesia tigrina (sexless and sexual race), Dugesia lugubris, Ijmia tenuis, Bdellacephala punctata. It was shown that from one third to one half of individuals were characterized by asymmetry of movement direction preference. The preference of right turning was typical for Dugesia tigrina; Dugesia lugubris, Ijmia tenuis, Bdellacephala punctata preferred left turning. The asymmetry described is considered as a primitive form of species functional asymmetry.
Article
Ciliates exhibit an asymmetry in arrangement of surface structures around the cell which could be termed handedness. If the usual order of placement of structures defines a 'right-handed' (RH) cell, then a cell with this order reversed would be 'left-handed' (LH). Such LH forms appear to be produced in Tetrahymena thermophila through aberrant reorganization of homopolar doublets back to the singlet condition. Four clones of LH forms were selected and subjected to genetic analysis to test whether this drastic phenotypic alteration resulted from a nuclear genetic change. The results of this analysis indicate that the change in handedness is not due to a genetic change in either the micronucleus or macronucleus. The LH form can, under certain circumstances, revert to the RH form, but typically it propagates itself across both vegetative and sexual generations with similar fidelity. While this analysis does not formally rule out certain possibilities of nuclear genic control involving regulatory elements transmitted through the cytoplasm, when the circumstances of origin and propagation of the LH condition are taken into account direct cortical perpetuation seems far more likely. Here we outline a conceptual framework centred on the idea of longitudinally propagated positional information; the positive evidence supporting this idea as well as further application of the idea itself are presented in the accompanying paper.
Article
The amoebae of the myxomycete Physarum polycephalum are of interest in order to analyze the morphogenesis of the microtubule and microfilament cytoskeleton during cell cycle and flagellation. The amoebal interphase microtubule cytoskeleton consists of 2 distinct levels of organization, which correspond to different physiological roles. The first level is composed of the 2 kinetosomes or centrioles and their associated structures. The anterior kinetosomes forming the anterior and posterior flagella are morphologically distinguishable. Each centriole plays a role in the morphogenesis of its associated satellites and specific microtubule arrays. The 2 distinct centrioles correspond to the 2 successive maturation stages of the pro-centrioles which are built during prophase. The second level of organization consists of a prominent microtubule organizing center (mtoc 1) to which the anterior centriole is attached at least during interphase. The mtoc plays a role in the formation of the mitotic pole. These observations based on ultrastructural and physiological analyses of the amoebal cytoskeleton are now being extended to the biochemical level. The complex formed by the 2 centrioles and the mtoc 1 has been purified without modifying the microtubule-nucleating activity of the mtoc 1. Several microtubule-associated proteins have been characterized by their ability to bind taxol-stabilized microtubules. Their functions (e.g., microtubule assembly, protection of microtubules against dilution or cold treatment, phosphorylating and ATPase activities) are under investigation. These biochemical approaches could allow in vitro analysis of the morphogenesis of the amoebal microtubule cytoskeleton.
Article
The mechanisms that underlie the formation of the left-right embryonic axis in vertebrates are not known. The programmed pattern of cell-type change in fission yeast results from the inheritance of specific chromatids of the parental chromosome. Here, I address how such a model may explain left-right specification of the viscera in mice. The model proposes that DNA replication produces different chromatids, and that these specific chromatids of both homologs are nonrandomly segregated to daughter cells to specify the left-right axis of the embryo. Such a model presents a simple explanation of the interesting phenotype of the newly discovered insertional mutation inv in mice, which causes reversal of the left-right axis, proposing that it is caused by a chromosomal inversion.
Article
Phylogenetic analyses of asymmetry variation offer a powerful tool for exploring the interplay between ontogeny and evolution because (i) conspicuous asymmetries exist in many higher metazoans with widely varying modes of development, (ii) patterns of bilateral variation within species may identify genetically and environmentally triggered asymmetries, and (iii) asymmetries arising at different times during development may be more sensitive to internal cytoplasmic inhomogeneities compared to external environmental stimuli. Using four broadly comparable asymmetry states (symmetry, antisymmetry, dextral, and sinistral), and two stages at which asymmetry appears developmentally (larval and postlarval), I evaluated relations between ontogenetic and phylogenetic patterns of asymmetry variation. Among 140 inferred phylogenetic transitions between asymmetry states, recorded from 11 classes in five phyla, directional asymmetry (dextral or sinistral) evolved directly from symmetrical ancestors proportionally more frequently among larval asymmetries. In contrast, antisymmetry, either as an end state or as a transitional stage preceding directional asymmetry, was confined primarily to postlarval asymmetries. The ontogenetic origin of asymmetry thus significantly influences its subsequent evolution. Furthermore, because antisymmetry typically signals an environmentally triggered asymmetry, the phylogenetic transition from antisymmetry to directional asymmetry suggests that many cases of laterally fixed asymmetries evolved via genetic assimilation.
Article
The study of left-right axis malformations in man and mouse has greatly advanced understanding of the mechanisms regulating vertebrate left-right axis formation. Recently, the roles of the TGF-beta family, Sonic hedgehog and fibroblast growth factor signaling, homeobox genes, and cilia in left-right axis determination have been more clearly defined. The identification of genes and environmental factors affecting left-right axis formation has important implications for understanding human laterality defects.
Article
Single cells and cell culture are very good model for estimation of primary effects of gravitational changes. It is suggested that cell cytoskeleton plays a key role in mechanisms of adaptation to mechanical influences including gravitational ones. Our results demonstrated that cultured cells of human vascular endothelium (correction of endotheliun) are highly sensitive to hypogravity (clinorotation) and respond by significant decrease of cell proliferative activity. Simultaneously it was noted that the formation of confluent monolayer appeared early in cultures exposed to simulated microgravity due to accelerated cells spreading. Long-term hypogravity (several hours or days) leads to significant changes of cell cytoskeleton revealed as microfilament thinning and their redistribution within cell. Such changes were observed only in monolayer cells and not in cell suspensions. Gravitational forces as known to be modificators of cell adhesive ability and determine their mobility. Hypogravity environment stimulated endothelial cell migration in culture: 24-48 hrs pre-exposition to hypogravity significantly increased endothelial cell migration resulting in 2-3-fold acceleration of mechanically injured monolayer repair. Obtained results suggest that the effects of hypogravity on cultured human endothelial cells are, possibly, associated with protein kinase C and/or adenylate cyclase activity and are accompanied by noticeable functional cell changes.
Article
Despite an externally symmetric body plan, the internal viscera of all vertebrates are asymmetric with respect to the left-right body axis. Determination of the handedness of this asymmetry is nonrandom and highly conserved among vertebrates. Errors in patterning along the left-right axis, which occur in about 1 in 10,000 human births, may result in significant morbidity and mortality. During early embryonic development, midline structures, in particular the node, coordinate patterning of the three main embryonic axes: anterior-posterior, dorsal-ventral, and left-right. A current model for specification of the handedness of left-right axis asymmetry invokes the activity of embryonic cilia in the node that create a net leftward flow of extraembryonic fluid. This flow is proposed to provide a signal for subsequent asymmetric gene expression. Signaling from the node defines patterns of asymmetric gene expression on the left and right sides of the embryo. These signals for "left" and "right" are ultimately interpreted by organ primordia during later development. Complex activating and inhibiting interactions involving TGF-beta family members, as well as homeobox transcription factors, mediate these asymmetric patterns of gene expression. The identification of the genes regulating left-right axis patterning in model organisms has resulted in the characterization of human mutations associated with left-right axis malformations.
Article
Space flight and clinostat experiments have been used to alter the influence of gravity. Lower and higher plants, both specialized and non-specialized to gravity percep-tion, have shown through changes in ultrastructure and metabolism (including the intracellular calcium balance) that cells are gravisensitive (Claasen and Spooner, 1994; Halstead and Dutcher, 1987; Kiss, 2000; Kordyum, 1997). In the presented paper, an attempt was made to summarize some experimental data and concepts con-cerning certain cell gravity-sensing systems in the gravi-tational field and their interactions with the changed environment of microgravity basing on the cytoskeleton behavior and Ca 2+ signaling in altered gravity. It was proposed that a distinction be made between cell gravisensing and cell graviperception. Gravisensing is related to cell structure and metabolism stability in the gravitational field and their changes in microgravity. Graviperception is related to the active use of a gravi-tational stimulus by cells, which are specialized to grav-ity perception, for realizing normal plant orientation in space, for growth and vital activity (gravitropism, gravitaxis) (Kordyum and Guikema, 2001). The structure of graviperceptive cells is diverse, but gravisensors are well known. These are statoliths of different types that change their position in the direction of the gravity vector and thus initiate the next steps of the gravitational response. The structural and functional organization of graviperceptive cells is determined gen-etically. In most cell types not specialized for perception of gravity, the primary sensors are not clearly defined. The idea of positional homeostasis (Nace, 1983) was the first to focus attention on the role of the cytoskeleton in cell graviresponse; it also explained the fixed stable position and optimal cell orientation in the gravitational field as a state of mechanical stress of the cytoskeletal elements and those that maintain cell membrane integ-rity. The cytoskeleton is also considered as an integral unspecialized cell gravireceptor (Tairbekov, 1990). A significant role in stability of the cell's spatio-temporal organization in the gravitational field and its gravisens-ing has been attributed to the cytoskeleton in some other concepts on cell gravisensing. These include static stimu-lation (Sievers et al., 1991), passive gravistimulation (Barlow, 1992), protoplast pressure (Wayne et al., 1990), putative tensegrity (Ingber, 1993), and restrained gravi-sensing (Baluska and Hasenstein, 1997). The cytoskel-eton is known to participate in cytoplasmic streaming and cell organelle motion, mitosis, cytokinesis, endo-and exocytosis, as well as in intracellular transport of substances—all activities that are potentially gravity-sensitive through the cytoskeleton (Cipriano, 1993). The intracellular cytoskeleton, the extracellular matrix, and the cytoplasmic membrane are assumed to represent compartments of sufficient macromolecular organiz-ation to be sensitive to gravity-induced phenomena (Claasen and Spooner, 1994). The cytoskeleton and extracellular matrix are indispensable for cellular and developmental processes that are directly or indirectly linked through the cellmembrane acting as an inter-mediary. The most useful models for investigating cell gravi-sensitivity in space flight or on the clinostat are gravi-perceptive cells with statoliths, e.g. root cap statocytes, alga Chara rhizoids, apical cells of moss protonema. The apical and subapical zones of Chara rhizoids contain thin bundles of microfilaments and the basal zone con-tains thicker ones. The gravitropically responsive apical part contains statoliths—compartments filled with crystallites of barium sulfate (Sievers et al., 1991). It was concluded that although the arrangement of micro-tubules is essential for polar cytoplasmic zonation and the functional polar organization of the actin cyto-skeleton, it is not involved in the primary events of * Tel: +38 044 212 3236; fax: +38 044 212 3236.