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Morphomechanical Feedbacks
Abstract
An attempt is made to reconstruct the natural successions of the developmental events on the basis of a common mechanically based trend. It is formulated in terms of a hyper-restoration (HR) hypothesis claiming that embryonic tissue responds to any external deforming force by generating its own one, directed toward the restoration of the initial stress value, but as a rule overshooting it in the opposite side. We give a mathematical formulation of this model, present a number of supporting evidences, and describe several HR-driven feedbacks which may drive forth morphogenesis. We use this approach for reconstructing in greater detail the gastrulation of the embryos from different taxonomic groups. Also, we discuss the application of this model to cytotomy, ooplasmic segregation, and shape complication of tubular rudiments (taking hydroid polyps as examples). In addition, we review the perspectives for applying morphomechanical approach to the problem of cell differentiation.
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The unexpected variants of cytoskeletal behavior that comprises empirical framework were discussed. The elastic and frictional moduli of a variety of cell types over a wide range of time scales were measured. It was found that frictional stresses paralleled the elastic behavior at frequencies below 10 Hz but approached Newtonian viscous behavior at higher frequencies. The analysis shows that the observations were consistent with the hypothesis that the cytoskeleton of the living cell bahaves as a soft glassy material wherein cytoskeletal protein modulate cell mechanical properties mainly by changing an effective temperature of the cytoskeletal matrix.
We have begun a mutational analysis of root morphogenesis in Arabidopsis thaliana. We report here the initial genetic and physiological characterisation of six mutations that affect root growth and development. Three of them (rsw1, rsw2, rsw3) cause extensive radial swelling of the root apex. These mutations are recessive at different loci and show temperature-sensitive expression, such that the roots appear wild type when grown at 18oC but express the mutant phenotype when transferred to 31oC. Following transfer to the restrictive temperature, these three mutations have different kinetic and morphological patterns of radial swelling, and grow at different rates with continued time at high temperature. We believe that these mutations represent three different loci active in the wild type in regulating the shape of the root. We have also characterised two mutations that affect only the root epidermis, causing many epidermal cells to bulge (reb1-1, reb1-2). The two mutations are recessive and are alleles. However, rebl-1 is constitutive whereas reb1-2 is temperature sensitive, only expressing at 33oC. Reb1-2 also causes a deviation from the normal straight growth of the root such that the affected roots grow with sharp bends or meanders. The final mutant reported here is a stunted plant (stp1), in which the root growth rate is approximately 25% of the wild type rate. Moreover, root growth steadily accelerates over 5 days following germination in the wild type but remains constant in stp1, which grows at a constant rate over the same interval.
Organ size is controlled by the concerted action of biochemical and physical processes. Although mechanical forces are known to regulate cell and tissue behavior, as well as organogenesis, the precise molecular events that integrate mechanical and biochemical signals to control these processes are not fully known. The recently delineated Hippo-tumor suppressor network and its two nuclear effectors, YAP and TAZ, shed light on these mechanisms. YAP and TAZ are proto-oncogene proteins that respond to complex physical milieu represented by the rigidity of the extracellular matrix, cell geometry, cell density, cell polarity and the status of the actin cytoskeleton. Here, we review the current knowledge of how YAP and TAZ function as mechanosensors and mechanotransducers. We also suggest that by deciphering the mechanical and biochemical signals controlling YAP/ TAZ function, we will gain insights into new strategies for cancer treatment and organ regeneration.
The hormone auxin plays a crucial role in plant morphogenesis. In the shoot apical meristem, the PIN-FORMED1 (PIN1) efflux carrier concentrates auxin into local maxima in the epidermis, which position incipient leaf or floral primordia. From these maxima, PIN1 transports auxin into internal tissues along emergent paths that pattern leaf and stem vasculature. In Arabidopsis thaliana, these functions are attributed to a single PIN1 protein. Using phylogenetic and gene synteny analysis we identified an angiosperm PIN clade sister to PIN1, here termed Sister-of-PIN1 (SoPIN1), which is present in all sampled angiosperms except for Brassicaceae, including Arabidopsis. Additionally, we identified a conserved duplication of PIN1 in the grasses: PIN1a and PIN1b. In Brachypodium distachyon, SoPIN1 is highly expressed in the epidermis and is consistently polarized toward regions of high expression of the DR5 auxin-signaling reporter, which suggests that SoPIN1 functions in the localization of new primordia. In contrast, PIN1a and PIN1b are highly expressed in internal tissues, suggesting a role in vascular patterning. PIN1b is expressed in broad regions spanning the space between new primordia and previously formed vasculature, suggesting a role in connecting new organs to auxin sinks in the older tissues. Within these regions, PIN1a forms narrow canals that likely pattern future veins. Using a computer model, we reproduced the observed spatio-temporal expression and localization patterns of these proteins by assuming that SoPIN1 is polarized up the auxin gradient, and PIN1a and PIN1b are polarized to different degrees with the auxin flux. Our results suggest that examination and modeling of PIN dynamics in plants outside of Brassicaceae will offer insights into auxin-driven patterning obscured by the loss of the SoPIN1 clade in Brassicaceae.
The formation of body segments (somites) in vertebrate embryos is accompanied by molecular oscillations (segmentation clock).
Interaction of this oscillator with a wave traveling along the body axis (the clock-and-wavefront model) is generally believed
to control somite number, size, and axial identity. Here we show that a clock-and-wavefront mechanism is unnecessary for somite
formation. Non-somite mesoderm treated with Noggin generates many somites that form simultaneously, without cyclic expression
of Notch-pathway genes, yet have normal size, shape, and fate. These somites have axial identity: The Hox code is fixed independently
of somite fate. However, these somites are not subdivided into rostral and caudal halves, which is necessary for neural segmentation.
We propose that somites are self-organizing structures whose size and shape is controlled by local cell-cell interactions.
The modulation of developmental biochemical pathways by mechanical cues is an emerging feature of animal development, but its evolutionary origins have not been explored. Here we show that a common mechanosensitive pathway involving β-catenin specifies early mesodermal identity at gastrulation in zebrafish and Drosophila. Mechanical strains developed by zebrafish epiboly and Drosophila mesoderm invagination trigger the phosphorylation of β-catenin-tyrosine-667. This leads to the release of β-catenin into the cytoplasm and nucleus, where it triggers and maintains, respectively, the expression of zebrafish brachyury orthologue notail and of Drosophila Twist, both crucial transcription factors for early mesoderm identity. The role of the β-catenin mechanosensitive pathway in mesoderm identity has been conserved over the large evolutionary distance separating zebrafish and Drosophila. This suggests mesoderm mechanical induction dating back to at least the last bilaterian common ancestor more than 570 million years ago, the period during which mesoderm is thought to have emerged.
A fundamental problem of morphogenesis is whether it presents itself as a succession of links that are each driven by its own specific cause-effect relationship, or whether all of the links can be embraced by a common law that is possible to formulate in physical terms. We suggest that a common biophysical background for most, if not all, morphogenetic processes is based upon feedback between mechanical stresses (MS) that are imposed to a given part of a developing embryo by its other parts and MS that are actively generated within that part. The latter are directed toward hyper-restoration (restoration with an overshoot) of the initial MS values. We show that under mechanical constraints imposed by other parts, these tendencies drive forth development. To provide specificity for morphogenetic reactions, this feedback should be modulated by long-term parameters and/or initial conditions that are set up by genetic factors. The experimental and model data related to this concept are reviewed.
Graphical abstract
Cortical microtubules (CMTs) are often aligned in a particular direction in individual cells or even in groups of cells and
play a central role in the definition of growth anisotropy. How the CMTs themselves are aligned is not well known, but two
hypotheses have been proposed. According to the first hypothesis, CMTs align perpendicular to the maximal growth direction,
and, according to the second, CMTs align parallel to the maximal stress direction. Since both hypotheses were formulated on
the basis of mainly qualitative assessments, the link between CMT organization, organ geometry, and cell growth is revisited
using a quantitative approach. For this purpose, CMT orientation, local curvature, and growth parameters for each cell were
measured in the growing shoot apical meristem (SAM) of Arabidopsis thaliana. Using this approach, it has been shown that stable CMTs tend to be perpendicular to the direction of maximal growth in cells
at the SAM periphery, but parallel in the cells at the boundary domain. When examining the local curvature of the SAM surface,
no strict correlation between curvature and CMT arrangement was found, which implies that SAM geometry, and presumed geometry-derived
stress distribution, is not sufficient to prescribe the CMT orientation. However, a better match between stress and CMTs was
found when mechanical stress derived from differential growth was also considered.
Root apex of higher plants shows very high sensitivity to environmental stimuli. The root cap acts as the most prominent plant sensory organ; sensing diverse physical parameters such as gravity, light, humidity, oxygen, and critical inorganic nutrients. However, the motoric responses to these stimuli are accomplished in the elongation region. This spatial discrepancy was solved when we have discovered and characterized the transition zone which is interpolated between the apical meristem and the subapical elongation zone. Cells of this zone are very active in the cytoskeletal rearrangements, endocytosis and endocytic vesicle recycling, as well as in electric activities. Here we discuss the oscillatory nature of the transition zone which, together with several other features of this zone, suggest that it acts as some kind of command center. In accordance with the early proposal of Charles and Francis Darwin, cells of this root zone receive sensory information from the root cap and instruct the motoric responses of cells in the elongation zone.
Stem-cell niches in mammalian tissues are often heterogeneous and compartmentalized; however, whether distinct niche locations determine different stem-cell fates remains unclear. To test this hypothesis, here we use the mouse hair follicle niche and combine intravital microscopy with genetic lineage tracing to re-visit the same stem-cell lineages, from their exact place of origin, throughout regeneration in live mice. Using this method, we show directly that the position of a stem cell within the hair follicle niche can predict whether it is likely to remain uncommitted, generate precursors or commit to a differentiated fate. Furthermore, using laser ablation we demonstrate that hair follicle stem cells are dispensable for regeneration, and that epithelial cells, which do not normally participate in hair growth, re-populate the lost stem-cell compartment and sustain hair regeneration. This study provides a general model for niche-induced fate determination in adult tissues.
Many epithelial tissues pack cells into a honeycomb pattern to support their structural and functional integrity. Developmental changes in cell packing geometry have been shown to be regulated by both mechanical and biochemical interactions between cells; however, it is largely unknown how molecular and cellular dynamics and tissue mechanics are orchestrated to realize the correct and robust development of hexagonal cell packing. Here, by combining mechanical and genetic perturbations along with live imaging and Bayesian force inference, we investigate how mechanical forces regulate cellular dynamics to attain a hexagonal cell configuration in the Drosophila pupal wing. We show that tissue stress is oriented towards the proximal-distal axis by extrinsic forces acting on the wing. Cells respond to tissue stretching and orient cell contact surfaces with the stretching direction of the tissue, thereby stabilizing the balance between the intrinsic cell junction tension and the extrinsic force at the cell-population level. Consequently, under topological constraints of the two-dimensional epithelial sheet, mismatches in the orientation of hexagonal arrays are suppressed, allowing more rapid relaxation to the hexagonal cell pattern. Thus, our results identify the mechanism through which the mechanical anisotropy in a tissue promotes ordering in cell packing geometry.
Gastrulation is a dynamic tissue-remodeling process occurring during early development and fundamental to the later organogenesis. It involves both chemical signals and physical factors. Although much is known about the molecular pathways involved, the roles of physical forces in regulating cellular behavior and tissue remodeling during gastrulation have just begun to be explored. Here, we characterized the force generated by the leading edge mesoderm (LEM) that migrates preceding axial mesoderm (AM), and investigated the contribution of LEM during Xenopus gastrulation. First, we constructed an assay system using micro-needle which could measure physical forces generated by the anterior migration of LEM, and estimated the absolute magnitude of the force to be 20-80nN. Second, laser ablation experiments showed that LEM could affect the force distribution in the AM (i.e. LEM adds stretch force on axial mesoderm along anterior-posterior axis). Third, migrating LEM was found to be necessary for the proper gastrulation cell movements and the establishment of organized notochord structure; a reduction of LEM migratory activity resulted in the disruption of mediolateral cell orientation and convergence in AM. Finally, we found that LEM migration cooperates with Wnt/PCP to form proper notochord. These results suggest that the force generated by the directional migration of LEM is transmitted to AM and assists the tissue organization of notochord in vivo independently of the regulation by Wnt/PCP. We propose that the LEM may have a mechanical role in aiding the AM elongation through the rearrangement of force distribution in the dorsal marginal zone.
The relative simplicity of plant developmental systems, having evolved within the universal constraints imposed by the plant cell wall, may allow us to outline a consistent developmental narrative that is not currently possible in the animal kingdom. In this article, I discuss three aspects of the development of the mature form in plants, approaching them in terms of the role played by the biophysics and mechanics of the cell wall during growth. First, I discuss axis extension in terms of a loss of stability-based model of cell wall stress relaxation and I introduce the possibility that cell wall stress relaxation can be modeled as a binary switch. Second, I consider meristem shape and surface conformation as a controlling element in the morphogenetic circuitry of plant organogenesis at the apex. Third, I approach the issue of reproductive differentiation and propose that the multicellular sporangium, a universal feature of land plants, acts as a stress-mechanical lens, focusing growth-induced stresses to create a geometrically precise mechanical singularity that can serve as an inducing developmental signal triggering the initiation of reproductive differentiation. Lastly, I offer these three examples of biophysically integrated control processes as entry points into a narrative that provides an independent, nongenetic context for understanding the evolution of the apoplast and the morphogenetic ontogeny of multicellular land plants.
Amyloplast sedimentation during gravistimulation and organelle movements was studied in living central rootcap cells of Zea mays L. cv. Merit. Cells from sectioned roots were viewed with a horizontally-mounted videomicroscope. The kinetics of gravity-induced amyloplast sedimentation were comparable to those calculated from experiments using fixed material. Individual amyloplasts fell at an average velocity of 5.5 μm min⁻¹; the maximal velocity of fall measured was 18.0 μm min⁻¹. Amyloplasts often rotated, sometimes rose in the cytoplasm, and occasionally underwent sudden rapid movements as fast as 58 μm min⁻¹. Saltations of other organelles were frequently observed. This appears to be the first report of cytoplasmic streaming in the presumptive statocytes of roots.
The leaves of Filipendula ulmaria ‘Aureo-Variegata’ have normal green margins surrounding unmasked yellow centres. Perclinal divisions regularly occur in young epidermisses. Growth inhibition of the hypodermal layer can be found in young and differentiated leaves. Therefore, Filipendula ulmaria ‘Aureo-Variegata’ can be interpreted as a trichimera:
In solution culture the effect of optimal (20 °C) and supra-optimal (30 °C) root-zone temperatures on root growth and root morphology of six potato clones (Solanum tuberosum L spp) was studied Growth was compared with sweet potato (Ipomoea batatas L) and cassava (Manihot esculenta Crantz) Significant genotypical differences in the responses of potato roots to supra-optimal temperatures were observed, indicating the potential for selecting heat tolerant potato clones In both heat tolerant and heat sensitive clones, the size of the root system was reduced by supra-optimal root-zone temperature This was principally a result of decreased number and length of lateral roots The first symptom of heat damage was a reduction in the rate of cell division, followed by cessation of root elongation Bending of the apical root-zone together with the formation of root hairs on the inner (concave) and lateral roots on the outer (convex) side were other symptoms, these coincided with the loss of starch granules in the root-cap
The multinuclear plasmodia of acellular slime molds can reach a size of several m2 and are able to migrate with a velocity of up to 1 cm hr−1. The basis of this rapid locomotion is a permanent translocation of cytoplasm plus nuclei in the form of a shuttle streaming: the endoplasm flows back and forth with a periodicity of ½ to 2 min, especially visible in plasmodial strands which characterize the rear part of a plasmodium. The frontal part of a plasmodium consists of a more or less continuous protoplasmic sheet which also encloses the pathways of an endoplasmic shuttle streaming. This migration polarity of a plasmodium is subject to changes due to, e.g., chemical stimuli.
The generation and analysis of plant chimeras and other genetic mosaics have been used to deduce patterns of cell division and cell fate during plant development and to demonstrate the existence of clonally distinct cell lineages in the shoot meristems of higher plants. Cells derived from these lineages do not have fixed developmental fates but rely on positional information to determine their patterns of division and differentiation. Chimeras with cells that differ genetically for specific developmental processes have been experimentally generated by a variety of methods. This review focuses on studies of intercellular interactions during plant development as well as of the coordination of cells during meristem function and organogenesis. Recent experiments combining mosaic analysis with molecular analysis of developmental mutants have begun to shed light on the nature of the signals involved in these processes and the mechanisms by which they are transmitted and received among cells.
Neurites of PC12 and chick dorsal root ganglion neurons behave as viscoelastic solids in response to applied forces. This passive behavior can be modeled with three mechanical elements; a relatively stiff, undamped spring in series with a Voight element composed of a less stiff spring in parallel with a dashpot. In response to applied tensions greater than 100 microdynes, PC12 cells show lengthening behavior distinct from and in addition to the passive viscoelastic response. We interpret this as "towed growth" (Bray, D. 1984. Dev. Biol. 102:379-389) because the neurites can become twice as long without obvious thinning of the neurite and because in two cases neurite tensions fell below original rest tensions, a result that cannot be obtained with passive viscoelastic elements. The rate of towed growth showed a linear dependence of growth rate with applied tensions in 8 of 12 PC12 neurites exposed to applied tension greater than 100 microdynes. Both PC12 and chick sensory neurons showed evidence of retraction when neurite tensions were suddenly diminished. This response was measured as tension recovery after slackening in chick sensory neurites. In 62% of the cases, tension recovery exceeded and sometimes doubled the preexperimental steady-state tension. Our data indicate that this response is active tension generation by the neurite shaft. We conclude that neurite length is regulated by axial tension in both elongation and retraction. Our data suggest a three-way controller: above some tension set point, the neurite is stimulated to elongate. Below some different, lower tension threshold the neurite is stimulated to retract. Between these two tension thresholds, the neurite responds passively as a viscoelastic solid.
Segmentation is a characteristic feature of the vertebrate body plan. The prevailing paradigm explaining its origin is the ‘clock and wave-front’ model, which assumes that the interaction of a molecular oscillator (clock) with a traveling gradient of morphogens (wave) pre-defines spatial periodicity. While many genes potentially responsible for these processes have been identified, the precise role of molecular oscillations and the mechanism leading to physical separation of the somites remain elusive. In this paper we argue that the periodicity along the embryonic body axis anticipating somitogenesis is controlled by mechanical rather than bio-chemical signaling. Using a prototypical model we show that regular patterning can result from a mechanical instability induced by differential strains developing between the segmenting mesoderm and the surrounding tissues. The main ingredients of the model are the assumptions that cell–cell adhesions soften when overstretched, and that there is an internal length scale defining the cohesive properties of the mesoderm. The proposed mechanism generates a robust number of segments without dependence on genetic oscillations.
After decades focusing on the molecular and genetic aspects of organogenesis, researchers are showing renewed interest in the physical mechanisms that create organs. This review deals with the mechanical processes involved in constructing the heart and brain, concentrating primarily on cardiac looping, shaping of the primitive brain tube, and folding of the cerebral cortex. Recent studies suggest that differential growth drives large-scale shape changes in all three problems, causing the heart and brain tubes to bend and the cerebral cortex to buckle. Relatively local changes in form involve other mechanisms such as differential contraction. Understanding the mechanics of organogenesis is central to determining the link between genetics and the biophysical creation of form and structure.
The plasmalemma of Oocystis apiculata, W. West when freezeetched has been shown to bear granules of several sizes. At the earliest stage of development the outer face of the plasmalemma of the naked autospore has small (8.5 nm diameter) granules aligned in rows, in pairs. These rows are stacked together forming extensive "granule-bands" over the plasmalemma surface. The orientation of these "granule-bands" corresponds exactly to one of the major microfibril directions. Occasionally, the bands are reduced to patches, some of which are at right angles to each other. Banding of granules on the inner plasmalemma face of naked autospores is also seen. During development the plasmalemma is seen to change so that in the final stages it bears reticulate invaginations, the granule bands occurring within them. The significance of the "granulebands" in terms of cellulose microfibril biosynthesis is discussed.
Make Way for the Emerging Rootlet
Plant cells are immobilized by their rigid cells walls, and the root endodermal cell layer maintains a impervious perimeter seal made of an indigestible irregular polymer. Despite these mechanical obstacles, lateral root primordia, which initiate in the deep layers of the root, manage to break through to the surface. Vermeer et al. (p. 178 ; see the cover) used live-tissue imaging and genetics to show that signals are exchanged between the root primordium and the handful of cells overlying it, which then cave in on themselves to open up a channel for the growing root primordium.
In the "regeneration" of a shoot from a leaf of the succulent, Graptopetalum paraguayense E. Walther the first new organs are leaf primordia. The original arrangement of cellulose microfibrils and of microtubules (MTs) in the epidermis of the leaf-forming site is one of parallel, straight lines. In the new primordium both structures still have a congruent arrangement but it is roughly in the form of concentric circles that surround the new cylindrical organ. The regions which undergo the greatest shift in orientation (90°) were studied in detail. Departures from the original cellulose alignment are detected in changes in the polarized-light image. Departures from the original cortical MT arrangement are detected using electron microscopy. The over-all reorganization of the MT pattern is followed by the tally of MT profiles, the various regions being studied in two perpendicular planes of section. This corrects for the difference in efficiency in counting transverse versus longitudinal profiles of MTs. Reorientation takes place sporadically, cell by cell, for both the cellulose microfibrils and the MTs, indicating a coordinated reorientation of the two structures. That MTs and cellulose microfibrils reorient jointly in individual cells was shown by reconstruction of the arrays of cortical MTs in paradermal sections of individual cells whose recent change in the orientation of cellulose deposition had been detected with polarized light. Closeness of the two alignments was also indicated by images where the MT and microfibril alignments co-varied within a single cell. The change-over in alignment of the MTs appears to involve stages where arrays of contrasting orientation co-exist to give a criss-cross image. During this critical reorganization, the frequency of the MTs is high. It falls during subsequent enlargement of the organ. It was found that the rearrangement of the cortical MTs to approximate a series of concentric circles on the residual meristem occurred before the emergence of leaf primordia. Through their apparent influence on microfibril alignments, the changes in MT disposition, described here, have the potential to generate major biophysical changes that accompany organogenesis.
Metastatic outcome is impacted by the biophysical state of the primary tumor cell. To determine if changes in cancer cell biophysical properties facilitate metastasis, we quantified cytoskeletal biophysics in well-characterized human skin, bladder, prostate and kidney cell line pairs that differ in metastatic ability. Using magnetic twisting cytometry with optical detection, cytoskeletal dynamics was observed through spontaneous motion of surface bound marker beads and nonlinear rheology was characterized through large amplitude forced oscillations of probe beads. Measurements of cytoskeletal dynamics and nonlinear rheology differed between strongly and weakly metastatic cells. However, no set of biophysical parameters changed systematically with metastatic ability across all cell lines. Compared to their weakly metastatic counterparts, the strongly metastatic kidney cancer cells exhibited both increased cytoskeletal dynamics and stiffness at large deformation which are thought to facilitate the process of vascular invasion.
Membrane potentials were measured in lateral statocytes of vertically and nonvertically growing roots of Lepidium sativum L. using conventional glass-microelectrode techniques. Statocytes in vertically growing roots showed a stable resting potential of-118±5.9 mV without spontaneous fluctuations. Upon tilting the root 45° from the vertical, an electrical asymmetry was observed. Statocytes on the physically lower side of the root depolarized by approx. 25 mV. This depolarization occurred following a latent period of 8 s reaching a minimum (approx.-93 mV) after 170 s. This depolarization is the earliest event in graviperception ever recorded. After this depolarization, the cell repolarized within 60 s to a potential approx. 10 mV more positive than the original resting potential. Statocytes on the upper flank showed a slow hyperpolarization (t
1/2h=half time for hyperpolarization=168 s) reaching a final, stable potential at a level 10 mV more negative. These effects of gravistimulation were statenchyma-specific, since cells in the cortex and rhizodermis showed no similar effects. The gravi-electrical responses were observed in 25% of all roots tested. Roots which showed no gravi-electrical response had a reduced elongation growth, lacked gravity-induced bending and lacked the typical structural polarity in punctured statocytes. This observed transition from a symmetrical pattern of resting potential in the statenchyma to an asymmetrical pattern following gravistimulation supports the results observed with external current measurements (Behrens et al., Plant Physiol. 70, 1079–1083, 1982) and extends these results to the cellular level and to considerably improved temporal resolution. The asymmetry in the gravi-electrical response extends the graviperception model of Sievers and Volkmann (Planta 102, 160–172, 1972) which comprises an asymmetrical sedimentation of the amyloplasts on the distal endoplasmic reticulum of statocytes. This generates an intraorgan signal which then must be transmitted to the growth zone.
Adhesion of stem cells - like most cells - is not just a membrane
phenomenon. Most tissue cells need to adhere to a ``solid'' for
viability, and over the last decade it has become increasingly clear
that the physical ``elasticity'' of that solid is literally ``felt'' by
cells. Here we show that Mesenchymal Stem Cells (MSCs) specify lineage
and commit to phenotypes with extreme sensitivity to the elasticity
typical of tissues [1]. In serum only media, soft matrices that mimic
brain appear neurogenic, stiffer matrices that mimic muscle are
myogenic, and comparatively rigid matrices that mimic collagenous bone
prove osteogenic. Inhibition of nonmuscle myosin II activity blocks all
elasticity directed lineage specification, which indicates that the
cytoskeleton pulls on matrix through adhesive attachments. Results have
significant implications for `therapeutic' stem cells and have motivated
development of a proteomic-scale method to identify mechano-responsive
protein structures [2] as well as deeper physical studies of matrix
physics [3] and growth factor pathways [4]. [4pt] [1] A. Engler, et al.
Matrix elasticity directs stem cell lineage specification. Cell
(2006).[0pt] [2] C.P. Johnson, et al. Forced unfolding of proteins
within cells. Science (2007).[0pt] [3] A.E.X. Brown, et al. Multiscale
mechanics of fibrin polymer: Gel stretching with protein unfolding and
loss of water. Science (2009).[0pt] [4] D.E. Discher, et al. Growth
factors, matrices, and forces combine and control stem cells. Science
(2009).
Plant organs such as maize (Zea mays L.) coleoptiles are characterized by longitudinal tissue tension, i.e. bulk turgor pressure produces unequal amounts of cell-wall tension in the epidermis (essentially the outer epidermal wall) and in the inner tissues. The fractional amount of turgor borne by the epidermal wall of turgid maize coleoptile segments was indirectly estimated by determining the water potential ψ(*) of an external medium which is needed to replace quantitatively the compressive force of the epidermal wall on the inner tissues. The fractional amount of turgor borne by the walls of the inner tissues was estimated from the difference between -ψ(*) and the osmotic pressure of the cell sap (πi) which was assumed to represent the turgor of the fully turgid tissue. In segments incubated in water for 1 h, -ψ(*) was 6.1-6.5 bar at a πi of 6.7 bar. Both -ψ(*) and πi decreased during auxin-induced growth because of water uptake, but did not deviate significantly from each other. It is concluded that the turgor fraction utilized for the elastic extension of the inner tissue walls is less than 1 bar, i.e. less than 15% of bulk turgor, and that more than 85% of bulk turgor is utilized for counteracting the high compressive force of the outer epidermal wall which, in this way, is enabled to mechanically control elongation growth of the organ. This situation is maintained during auxin-induced growth.
Reversible differentiation was experimentally discovered in a community of modern filamentous cyanobacteria Oscillatoria terebriformis. Splitting of the initially uniform community into differentiated parts (strands, multiradiate aggregates, networks, etc.) occurs only for the duration of a function facilitating the activity of this community as an integral unit. The structures are formed as a result of regrouping of the filaments, without their specialization. A morphologically regulatory system (polygonal network) was found to develop under the impact of extreme factors. The levels of structural organization of filamentous cyanobacteria and multicellular eukaryotes were compared (individual cells in a filament—cell organelles; filaments—individual cells; community—organism), and the similarities and differences in morphogenesis of these groups were analyzed using the data on the embryonic regulation in multicellular eukaryotes. Spatial information in morphogenesis was shown to result not from direct realization of an inherited program but is created by the elements of integral organisms (cells and filaments) in the course of development.
Expansins are plant cell wall-loosening proteins that promote cell growth and are essential for many critical developmental processes and stress responses. The molecular basis for expansin action is uncertain. Recently, it has been proposed that expansins loosen the wall by means of the generation of mobile conformational defects at the surface of cellulose microfibrils. The present work addresses this hypothesis by elaborating three assumptions: (1) microfibril–matrix interfaces cause steep stress gradients on the microfibril surface, (2) stress gradients drive the motion of conformational defects along the microfibril surface toward the microfibril–matrix interfaces, and (3) the approach of the defects to the microfibril–matrix interfaces facilitates the dissociation of matrix polysaccharides from cellulose microfibrils.
Preface Introduction 1. Development of Sponges from the Class Calcarea Bowerbank, 1864 2. Development of Sponges from the Class Hexactinellida Schmidt, 1870 3. Development of sponges from the class Demospongiae Sollas, 1885 4. Development of Homoscleromorpha order Homosclerophorida Dendy, 1905 5. Typization of sponge development and its significance for phylogeny 6. Comparative analysis of individual development in sponges 7. Evolution and individual development of sponges: regularities and directions 8. In place of conclusion: Bauplan and phylotypic stage in Porifera Literature references Figures legends Taxonomic Index
In this chapter, we are providing a brief overview of the tensegral concept as applied to plants. Starting with a short introduction
to the history of the idea of mechanical integration of the cell and the organism, we then discuss the mechanical design of
the plant body. The importance of the mechanical properties of cells, tissues, organs, and their domains is indicated, and
the systems of detection of mechanical stimuli are briefly discussed. Finally, the mechanical integration of plant cells is
presented based on the various aspects of the functioning of the cell wall–plasma membrane–cytoskeleton continuum spanning
the whole cell. The initial stage of knowledge within this area is indicated with special attention paid to different modes
of inter- and intracellular communication as well as the utilization of the continuum to functional organization and integration
of the whole cell.
A B S T R A C T The development of leaves on apically stable, periclinal chimeras was studied in a number of dicot genera. The mutant cell layers of the shoot apex and- the tissues derived from them were as active developmentally as the normal layers. Ontogeny was the same in these chimeras as in nonchimeras, and growth of their leaves can be outlined as follows. Formation of the buttress, the axis, and the lamina of simple dicot leaves were independent events. In each the first growth included derivatives of the apical layers, usually three in number, found in the apex of the shoot and the lateral buds. Most cell divisions in the outer layers (L-I and L-II) were anticlinal relative to the new structures. Therefore, in the proximal regions of the buttress, axis (petiole and midrib), and lamina, the derivative cells of L-I and L-II were usually present in single layers. The rest of the internal tissue was from L-III. As formation of the axis and the lamina proceeded, derivatives of L-II replaced L-III internally in the distal and marginal regions leaving cells of L-III behind. Both the determinate growth of leaves and the pattern of cell divisions at and near the leading edges of growth meant that no cells in the leaf were comparable to the initial cells of the shoot apex. As the lamina extended, there were extensive intercalary cell divisions, both anticlinal and periclinal, so that in any given region of a leaf the layers of internal cells were from either L-II or L-III. At any point along the axis, L-III participated or did not participate in laminar extension. At any given stage in laminar growth either of two sister cells in any internal layer divided either a few times or extensively. The extreme variability in direction and frequency of cell division during leaf development was under an overriding genetic control, which resulted in the normal or typical size, shape and thickness of leaves. IN HIS classical paper on the structure and development of the tobacco leaf, Avery (1933) concluded that the leaf primordium (buttress) was derived from outer apical layers of cells and that, except for the epidermis which was self-perpetuating, growth in length of the primordium (axis) was by divisions of a single subepidermal cell. The origin of the internal tissue of the lamina was likewise described as coming from a single row of subepidermal initial cells, the marginal meristem, which, by anticlinal and periclinal divisions, gave rise to upper, middle, and lower mesophyll. Foster's review (1936) of leaf differentiation discussed apical growth of the leaf axis, laminar growth, and the contribution of the apical layers to the new leaf. He concluded that true apical growth from subepidermal initials ceases relatively early, and that subsequent elongation of the axis included both intercalary cell division and cell enlargement. In Foster's review, all the hypotheses as to the origin of the internal tissue of the lamina suggested that the ultimate source of all cells was divisions of a row of submarginal ini