Absence of furrowing activity following regional cortical tension reduction in sand dollar blastomere and fertilized egg fragment surfaces

The purpose of the present investigation was to test experimentally the possibility that division mechanism establishment at the equator of sand dollar eggs may be a consequence of cortical tension gradients between the equator and the poles. Cytochalasin has been shown to decrease tension at the sea urchin egg surface. The concave ends of cytochalasin D-containing agarose cylinders were held against regions of the surface of Echinarachnius parma blastomeres and enucleated fertilized egg fragments. The ability to interfere with normal furrowing activity was used as a biological indicator of the effectiveness of cytochalasin. When agarose containing 2 microg/mL cytochalasin contacted the equatorial region of the blastomeres resulting from the first cleavage, or the equatorial surfaces of nucleated fertilized egg halves, furrowing was blocked, stalled or delayed, indicating that the concentration of cytochalasin was effective. When the same concentration of cytochalasin was applied to the poles, the cells and nucleated fertilized egg fragments divided in the same way as the controls, indicating that the effectiveness of the cytochalasin did not spread from the poles to the equator and that bisection did not interfere with the division of nucleated fertilized egg fragments. When the same concentration of cytochalasin was applied to diametrically opposed surfaces of enucleated, spherical egg fragments, there was no evidence of furrowing activity between the areas that contacted the cytochalasin or in any other part of the surface. Because of the tension-reducing effect of cytochalasin, a tension gradient existed between the regions affected and unaffected by cytochalasin. The results strongly suggest that establishment of the division mechanism by simple gradients of tension at the surface is unlikely.     

Effects of environmental conditions and space on species turnover for three plant functional groups in Brazilian savannas

Aims Different plant functional groups display diverging responses to the same environmental gradients. Here, we assess the effects of environmental and spatial predictors on species turnover of three functional groups of Brazilian savannas (Cerrado) plants—trees, palms and lianas—across the transition zone between the Cerrado and Amazon biomes in central Brazil.     

Cell mechanical microenvironment for cell volume regulation

Cell volume regulation, as one of the fundamental homeostasis of the cell, is associated with many cellular behaviors and functions. With the increased studies on the effect of environmental mechanical cues on cell volume regulation, the relationship between cell volume regulation and mechanotransduction becomes more and more clear. In this paper, we review the mechanisms and hypotheses by which cell maintains its volume homeostasis both in vivo and in constructed cell mechanical microenvironment (CMM) in vitro. We discuss how the growth-division regulation maintains the volume homeostasis of cells in the cell cycle and how the cell cortex/membrane tension mediates the effect of CMM (i.e., osmotic pressure, matrix stiffness, and mechanical force) on cell volume regulation. We also highlight the roles of cell volume as a perfect integrator of the downstream signals of mechanotransduction from different aspects of CMM and an effective indicator for the mechanical condition that cell confronts. This interdisciplinary perspective can provide new insight into biomechanics and may shed light on bioengineering and pathological research work. We hope this review can facilitate future studies on the investigation of the role of cell volume in mechanotransduction.     

Thermodynamic approach to explain cell adhesion to air-medium interfaces

Cell damage has been observed in suspension cell cultures with air sparging, especially in the absence of any protective additives. This damage is associated with cells adhering to bubbles, and it has been shown that if this adhesion is prevented, cell damage is prevented. This article presents a thermodynamic approach for predicting cell adhesion at the air-medium interface. With this relationship it can be shown that cell-gas adhesion can be prevented by lowering the surface tension of the liquid growth medium through the addition of surface-active protective additives. The thermodynamic relationship describes the change in free energy as a function of the interfacial tensions between the (i) gas and liquid phases, (ii) gas and cell phases, and (iii) liquid and cell phases. Experimental data, along with theoretical and empirical equations, are used to quantify the changes in free energy that predict the process of cell-gas adhesion. The thermodynamic model is nonspecific in nature and, consequently, results are equally valid for all types of cells. (c) 1995 John Wiley & Sons, Inc.     

Effect of mechanical tension on the human dental pulp cells

In this study, we have evaluated the effects of mechanical tension on the proliferation and extracellular matrix (ECM) production of human dental pulp stem cells (DPSCs) using a flexwell system that imposed cyclic mechanical tension at 0.03 Hz with 0, 5, and 8% strains. In the early stage (4 days), DPSCs at 5 and 8% strains had a similar proliferation, which was higher than the control. However, in the late stage (10 days), DPSCs at 8% strain had a higher proliferation than the control and 5% strains. This result clearly demonstrated that DPSC proliferation under tension varied with culture time. In addition, mechanical tension was shown to increase the amount of lactate dehydrogenase (LDH) released during culture. RT-PCR analysis was used to show that mechanical tension also increased collagen and osteopontin expression and decreased α-smooth muscle actin (α-SMA) expression. Furthermore, FACS analysis showed that CD105 expression did not change in all groups but CD 90 expression decreased at 8% strain. In conclusion, our results suggest that an appropriate level of mechanical tension can serve as a potent positive modulator of proliferation, differentiation and ECM production in DPSCs.     

木材细胞壁力学研究进展

基于细胞水平的微观力学研究是探索木材复杂力学行为本质的核心内容。该文系统介绍木材细胞壁力学性能实验表征技术及力学模型的发展,重点说明了我国在这方面取得的进展和成果,在此基础上,明确指出细胞壁力学在现阶段存在的不足,并对其发展方向进行详细阐述,以期丰富和推动我国木材科学理论体系的发展,促进纸浆造纸、纺织材料、复合材料等相关领域的研究进展。     

Bioactive polyacrylamide hydrogels with gradients in mechanical stiffness

We propose a novel, single step method for the production of polyacrylamide hydrogels with a gradient in mechanical properties. In contrast to already existing techniques such as UV photo‐polymerization with photomasks (limited penetration depth) or microfluidic gradient mixers (complex microfluidic chip), this technique is not suffering such limitations. Young's modulus of the hydrogels was varied by changing the total monomer concentration of the hydrogel precursor solution. Using programmable syringe pumps, the total monomer concentration in the solution fed to the hydrogel mold was varied from 16 wt% down to 5 wt% over the feeding time to obtain a gradient in compliance ranging from 150 kPa down to 20 kPa over a length of 10 mm down to 2.5 mm. Polymerization was achieved with the dual initiation system composed of ammonium persulfate and N,N,N′,N′‐tetramethylethylenediamine, which were both fed through separate capillaries to avoid premature polymerization. Functionalized with the model ligand collagen I, the substrates were bioactive and supported the attachment of human foreskin fibroblasts (around 30% of the cells seeded attached after 1 h). A kinetic morphology study on homogeneous hydrogels of different stiffness's indicated that fibroblasts tend to spread to their final size within 2 h on stiff substrates, while the spreading time was much longer (ca. 4–5 h) on soft substrates. These trends were confirmed on hydrogels with compliance gradients, showing well spread fibroblasts on the stiff end of the hydrogel after 2 h, while the cells on the soft end still had small area and rounded morphology. Biotechnol. Bioeng. 2013; 110: 1508–1519. © 2012 Wiley Periodicals, Inc.     

Mechanical Response of Single Plant Cells to Cell Poking： A Numerical Simulation Model

Cell poking is an experimental technique that is widely used to study the mechanical properties of plant cells. A full understanding of the mechanical responses of plant cells to poking force Is helpful for experimental work. The aim of this study was to numerically investigate the stress distribution of the cell wall, cell turgor, and deformation of plant cells in response to applied poking force. Furthermore, the locations damaged during poking were analyzed. The model simulates cell poking, with the cell treated as a spherical, homogeneous, isotropic elastic membrane, filled with incompressible, highly viscous liquid. Equilibrium equations for the contact region and the non-contact regions were determined by using membrane theory. The boundary conditions and continuity conditions for the solution of the problem were found. The forcedeformation curve, turgor pressure and tension of the cell wall under cell poking conditions were obtained. The tension of the cell wall circumference was larger than that of the meridian. In general, maximal stress occurred at the equator around. When cell deformation increased to a certain level, the tension at the poker tip exceeded that of the equator. Breakage of the cell wall may start from the equator or the poker tip, depending on the deformation. A nonlinear model is suitable for estimating turgor, stress, and stiffness, and numerical simulation is a powerful method for determining plant cell mechanical properties.     

Mechanics of spreading cells probed by atomic force microscopy

Cellular adhesion and motility are fundamental processes in biological systems such as morphogenesis and tissue homeostasis. During these processes, cells heavily rely on the ability to deform and supply plasma membrane from pre-existing membrane reservoirs, allowing the cell to cope with substantial morphological changes. While morphological changes during single cell adhesion and spreading are well characterized, the accompanying alterations in cellular mechanics are scarcely addressed. Using the atomic force microscope, we measured changes in cortical and plasma membrane mechanics during the transition from early adhesion to a fully spread cell. During the initial adhesion step, we found that tremendous changes occur in cortical and membrane tension as well as in membrane area. Monitoring the spreading progress by means of force measurements over 2.5 h reveals that cortical and membrane tension become constant at the expense of excess membrane area. This was confirmed by fluorescence microscopy, which shows a rougher plasma membrane of cells in suspension compared with spread ones, allowing the cell to draw excess membrane from reservoirs such as invaginations or protrusions while attaching to the substrate and forming a first contact zone. Concretely, we found that cell spreading is initiated by a transient drop in tension, which is compensated by a decrease in excess area. Finally, all mechanical parameters become almost constant although morphological changes continue. Our study shows how a single cell responds to alterations in membrane tension by adjusting its overall membrane area. Interference with cytoskeletal integrity, membrane tension and excess surface area by administration of corresponding small molecular inhibitors leads to perturbations of the spreading process.     

Do Cell Membranes Flow Like Honey or Jiggle Like Jello?

Cell membranes experience frequent stretching and poking: from cytoskeletal elements, from osmotic imbalances, from fusion and budding of vesicles, and from forces from the outside. Are the ensuing changes in membrane tension localized near the site of perturbation, or do these changes propagate rapidly through the membrane to distant parts of the cell, perhaps as a mechanical mechanism of long-range signaling? Literature statements on the timescale for membrane tension to equilibrate across a cell vary by a factor of ≈10⁶. This study reviews and discusses how apparently contradictory findings on tension propagation in cells can be evaluated in the context of 2D hydrodynamics and poroelasticity. Localization of tension in the cell membrane is likely critical in governing how membrane forces gate ion channels, set the subcellular distribution of vesicle fusion, and regulate the dynamics of cytoskeletal growth. Furthermore, in this study, it is proposed that cells can actively regulate the degree to which membrane tension propagates by modulating the density and arrangement of immobile transmembrane proteins. Also see the video abstract here https://youtu.be/T6K7AIAqqBs .     

密度梯度离心分离大鼠卵巢卵泡膜细胞

探讨用密度梯度离心法快速、有效地分离大鼠卵泡膜细胞.选取23～25 d雌性大鼠卵巢,用Percoll密度梯度离心法将卵泡膜细胞分离纯化,3β-羟基类固醇脱氢酶(3β-hydroxysteroid dehydrogenase,3β-HSD)组织化学染色用于卵泡膜细胞纯度检测.分别用0.1 U/mL和1.0 U/mL卵泡刺激素(Follicle-stimulating hormone,FSH)及黄体生成素(luteinizing hormone,LH)处理细胞,无血清培养48 h后,酶联免疫法检测培养液中雄烯二酮和雌二醇的水平.分离所得细胞中,3β-HSD染色阳性细胞与总细胞数之比大于90％; LH组的雄烯二酮水平显著高于对照组和FSH组(P＜0.05),LH组中1.0 U/mL组的雄烯二酮水平又高于0.1 U/mL组.各组均未检测到雌二醇及孕酮.3β-HSD组织化学染色可快速有效地检验所分离的卵泡膜细胞的纯度,分离所得的卵泡膜细胞可对LH产生反应,且其中几乎没有混杂颗粒细胞.     

不同年龄大鼠大脑皮层神经元胞体与胶质细胞递质氨基酸含量的比较研究

用非连续 Ficoll-蔗糖密度梯度超离心方法分离幼年、成年和老年大鼠大脑皮层的神经元胞体和胶质细胞。观察到以幼年大鼠为材料分离得到的神经元胞体和胶质细胞纯度高;而以成年、老年大鼠为材料分离得到的神经元胞体和胶质细胞则有少量污染。用 Dans 反应-聚酰胺薄膜层析-荧光方法测定神经元胞体和胶质细胞中的 GABA、甘氨酸、谷氨酸、天冬氨酸、牛磺酸含量。发现:(1)幼年与成年大鼠神经元胞体的递质氨基酸含量少于胶质细胞中的含量。老年大鼠的这种差异不显著。(2)由幼年到成年,胶质细胞中的牛磺酸含量显著下降,而神经元胞体中的牛磺酸含量无显著变化。(3)由幼年到成年,神经元胞体与胶质细胞中的“抑制性”与“兴奋性”递质氨基酸总量的比值皆下降50%左右。本文讨论了神经元胞体和胶质细胞的分离纯度及这两类细胞的“抑制性”与“兴奋性”递质氨基酸含量比值的年龄特点。     

Ethylene and growth control in the fringed waterlily (Nymphoides peltata): Stimulation of cell division and interaction with buoyant tension in petioles

The effect of ethylene on petiole growth of the Fringed Waterlily (Nymphoides peltata (S.G. Gmelin) O. Kuntze) changes during leaf ontogeny. During early development (before expansion of laminae), ethylene causes an increase in both cell number and cell size; later in development, promotion of rapid cell expansion is the dominant effect. The early effects may contribute to the accommodation of new leaves to water columns of different depth. The later effects on cell expansion only are shown to contribute to the rapid accommodation of floating leaves when changes in water level submerge the laminae. This kind of accommodation results from an interaction between accumulated ethylene, which increases wall extensibility, and the tension in petioles due to natural buoyancy which, it is suggested, supplements the driving force for cell expansion. Cell age (position) within a petiole and age of the whole petiole influence the growth response to ethylene alone and the amount of extra growth produced by applying tension when ethylene is present. In young petioles, apical cells are highly sensitive to ethylene and tension causes little further growth; older cells in both immature and mature petioles show little response to ethylene unless the petiole is under tension. Young (but not mature) petioles respond slowly to applied tension even in the absence of ethylene. It is concluded that as cells age the driving force for expansion limits increasingly their capacity to respond to the wall-loosening effects of ethylene. Dual sensitivity to ethylene and buoyant tension facilitates rapid accommodation responses but sensitivity of young petioles to tension alone may exclude Nymphoides from habitats where current velocity is appreciable.     

Infection and development of Nosema bombycis (Microsporida: Protozoa) in a cell line of Antheraea eucalypti

Spores of Nosema bombycis derived from diseased insects were highly purified by Urografin density gradient centrifugation. Antheraea eucalypti cells were inoculated with the purified spores primed with 0.1 n KOH solution to start a continuous propagation of N. bombycis in cell culture. The first increase in the number of infected A. eucalypti cells was observed at 48 hr postinoculation, and it was caused by the secondary infective forms of N. bombycis. The secondary infective forms were produced during the course of sporoblast differentiation. The parasites in cell cultures divided synchronously until 36 hr postinoculation. Mature spores were observed initially 6 days postinoculation at 27°C. The infected cultures were subcultured extensively for more than 1 year with the addition of healthy A. eucalypti cells.     

Analysis of cell locomotion on ligand gradient substrates

Directional cell motility plays a key role in many biological processes like morphogenesis, inflammation, wound repair, angiogenesis, immune response, and tumor metastasis. Cells respond to the gradient in surface ligand density by directed locomotion towards the direction of higher ligand density. Theoretical models which address the physical basis underlying the regulatory effect of ligand gradient on cell motility are highly desirable. Predictive models not only contribute to a better understanding of biological processes, but they also provide a quantitative interconnection between cell motility and biophysical properties of the extracellular matrix (ECM) for rational design of biomaterials as scaffolds in tissue engineering. In this work, we consider a one‐dimensional (1D) continuum viscoelastic model to predict the cell velocity in response to linearly increasing density of surface ligands on a substrate. The cell is considered as a 1D linear viscoelastic object with position dependent elasticity due to the variation in actin network density. The cell–substrate interaction is characterized by a frictional force, controlled by the density of ligand–receptor pairs. The generation of contractile stresses is described in terms of kinetic equations for the reactions between actins, myosins, and guanine nucleotide regulatory proteins. The model predictions show a reasonable agreement with experimentally measured cell speeds, considering biologically relevant values for the model parameters. The model predicts a biphasic relationship between cell speed and slope of gradient as well as a maximum limiting speed after a finite migration time. For a given slope of ligand gradient, the onset of the limiting speed appears at longer times for substrates with lower ligand gradients. The model can be applied to the design of biomaterials as scaffolds for guided tissue regeneration as it predicts an optimum range for the slope of ligand gradient. Biotechnol. Bioeng. 2009;103: 424–429. © 2009 Wiley Periodicals, Inc.     

小鼠Kupffer细胞分离及细胞培养

目的 采用在体胶原酶灌注、不连续密度梯度离心、选择性贴壁3步法分离Kupffer细胞（Kupffer cells,KCs）,探讨其在分离小鼠KCs的应用及其对KCs生物活性的影响.方法 根据原位灌注和梯度离心方法不同随机分为4组：无胶原酶原位灌注＋3层梯度离心组（A）、无胶原酶原位灌注＋双层梯度离心组（B）、胶原酶原位灌注＋3层梯度离心组（C）和胶原酶原位灌注＋双层梯度离心组（D）.采用F4/80（BM8）免疫染色及吞墨实验判断细胞纯度和功能、台盼蓝拒染实验判断细胞的活力,探讨不同方法KCs分离的效果及细胞活性.结果 刚分离的KCs细胞近似圆形,接种l h后收获细胞纯度较高,但细胞得率相对较低.培养4 h后KCs得率相对较高,培养28 d仍能存活.免疫荧光可显示分离的为KCs,台盼蓝染色显示各组细胞的活力均在90 %左右,在体胶原酶灌注和双层梯度离心可以增加KCs的得率,双层梯度离心法可以增加分离KCs的纯度.结论 在体胶原酶灌注对提高KCs得率较为重要,在体胶原酶灌注、不连续密度梯度离心、选择性贴壁3步法分离小鼠KCs的的方法简便、高效、稳定,培养的KCs具有良好的细胞生物学性状.     

Spatiotemporal organization and mechanosensory function of podosomes

Podosomes are small, circular adhesions formed by cells such as osteoclasts, macrophages, dendritic cells, and endothelial cells. They comprise a protrusive actin core module and an adhesive ring module composed of integrins and cytoskeletal adaptor proteins such as vinculin and talin. Furthermore, podosomes are associated with an actin network and often organize into large clusters. Recent results from our laboratory and others have shed new light on podosome structure and dynamics, suggesting a revision of the classical “core-ring” model. Also, these studies demonstrate that the adhesive and protrusive module are functionally linked by the actin network likely facilitating mechanotransduction as well as providing feedback between these two modules. In this commentary, we briefly summarize these recent advances with respect to the knowledge on podosome structure and discuss force distribution mechanisms within podosomes and their emerging role in mechanotransduction.     

Interfacial properties of cell culture media with cell-protecting additives

In an effort to identify key rheological properties that contribute to cell protection against shear damage, we have measured surface shear and dilatationai viscosities, dynamic surface tension, foaminess, and foam stability for media containing cell-protecting additives. In a companion article,(18) we found that cell-to-bubble attachment was decreased in media containing Methocel, Pluronic F68, or polyvinyl alcohol (PVA). In medium containing polyethylene glycol (PEG) or potyvinyl-pyrrolidone (PVP), attachment was increased. PEG, PVP, serum (FBS), and serum albumin (BSA) increased the surface viscosity of the air/medium surface (thus, producing a more rigid interface), whereas F68 and PVA lowered it greatly. Foaming experiments showed that Methocel, PEG, PVA, and F68 decreased the foam half-life while FBS, BSA, and PVP were foam stabilizers. Interestingly, the foam stability of CHO cell suspensions decreased significantly for cell concentrations higher than ca. 2 x 10(6) cells/mL. Nonviable CHO cells reduced foam stability further. Dynamic surface tension values of the media tested were found significantly differentfrom their static surface tension values. The interfacial properties measured and the results presented in the companion study suggest that the additives that lower dynamic surface tension the most (Methocel, F68, and PVA) correlate well with reduced cell-to-bubble attachment, and thus, cell protection. Reduced dynamic surface tension with these additives implies faster surfactant adsorption, mobile interfaces, lower surface viscosity, and foam destabilization. Because PEG and PVP resulted in increased cell-to-bubble attachment and had different interfacial properties, a different mechanism (compared with Methocel, PVP, and F68) is apparently responsible for their protective effect. Finally, cell protection offered by FBS and BSA is attributed to the foam stabilization properties provided by these additives. (c) 1995 John Wiley & Sons Inc.