Muscle Dynamics in Fish During Steady Swimming

SYNOPSIS. Recent research in fish locomotion has been dominatedby an interest in the dynamic mechanical properties of the swimmingmusculature. Prior observations have indicated that waves ofmuscle activation travel along the body of an undulating fishfaster than the resulting waves of muscular contraction, suggestingthat the phase relation between the muscle strain cycle andits activation must vary along the body. Since this phase relationis critical in determining how the muscle performs in cycliccontractions, the possibility has emerged that dynamic musclefunction may change with axial position in swimming fish. Quantificationof muscle contractile properties in cyclic contractions relieson in vitro experiments using strain and activation data collectedin vivo. In this paper we discuss the relation between theseparameters and body kinematics. Using videoradiographic datafrom swimming mackerel we demonstrate that red muscle straincan be accurately predicted from midline curvature but not fromlateral displacement. Electromyographic recordings show neuronalactivation patterns that are consistent with red muscle performingnet positive work at all axial positions. The relatively constantcross-section of red muscle along much of the body suggeststhat positive power for swimming is generated fairly uniformlyalong the length of the fish.     

The Dynamics of Microtubules in Cultured Cells

The behavior of microtubules in cultured cells in a cooled matrix after the microinjection of fluorescent tubulin was studied using a frame recording with a digital camcorder. In the cell lamella, the positive ends of individual microtubules extend and shorten at random. The histograms of rate distribution have an almost normal distribution with a mode close to 0. The maximum rate of lengthening and shortening reaches 30 and 50 m/min, respectively. The positive ends of microtubules in PtK cells were in an equilibrium state, while in murine embryonic fibroblasts and Vero cells, they were usually displaced to the cell edge. Free microtubules were present in the cells of all three cultures. In the epithelial cells, they were numerous and relatively stable, while in the fibroblasts, they occurred rarely and were depolymerized at the proximal end. Free microtubules in PtK cells appeared mostly due to spontaneous assembly in the cytoplasm (not in the relationship with the preexisting microtubules) and, more rarely, due to breakage of long microtubules. Separation of microtubules from the centrosome is a very rare event. Unlike positive ends that were characterized by dynamic instability, negative ends were stable and were sometimes depolymerized. When long microtubules were broken, new negative ends were formed that were, as a rule, stable, while in the lamella of fibroblasts (in murine embryonic fibroblasts and Vero cells), new negative ends were immediately depolymerized: free microtubules existed in these cells no more than 1–2 min. A diffusion model has been proposed where the behavior of microtubule ends is considered as unidimensional diffusion. The coefficient of diffusion of positive ends in the epithelial cells is several times less than in the fibroblasts, thus suggesting a higher rate of tubulin metabolism in the fibroblasts as compared to the epithelium. The results obtained indicate that for the exchange of long microtubules, the dynamic instability is not sufficient. In the fibroblasts, their exchange takes place mostly at the expense of depolymerization of the liberating negative ends, which agrees with the previously proposed conveyer hypothesis of microtubule assembly on the centrosome.     

A Computational Model of the Fluid Dynamics of Undulatory and Flagellar Swimming

We present a mathematical model and numerical method designedto study the fluid dynamics of swimming organisms. The fullNavier— Stokes equations are solved in a domain of fluidwithin which an organism undergoing time—dependent motionsis immersed. Of interest are both the dynamics of a single organismand the relationship of its morphology to its motility properties,as well as the collective hydrodynamic interactions of groupsof swimmers with each other and their environment. Biologicalapplications include spermatozoa motility in the reproductivetract, swimming of non-smooth filaments, and collective swimmingof algal cells.     

The Dynamics of Golgi Protein Traffic Visualized in Living Yeast Cells

We describe for the first time the visualization of Golgi membranes in living yeast cells, using green fluorescent protein (GFP) chimeras. Late and early Golgi markers are present in distinct sets of scattered, moving cisternae. The immediate effects of temperature-sensitive mutations on the distribution of these markers give clues to the transport processes occurring. We show that the late Golgi marker GFP-Sft2p and the glycosyltransferases, Anp1p and Mnn1p, disperse into vesicle-like structures within minutes of a temperature shift in sec18, sft1, and sed5 cells, but not in sec14 cells. This is consistent with retrograde vesicular traffic, mediated by the vesicle SNARE Sft1p, to early cisternae containing the target SNARE Sed5p. Strikingly, Sed5p itself moves rapidly to the endoplasmic reticulum (ER) in sec12 cells, implying that it cycles through the ER. Electron microscopy shows that Golgi membranes vesiculate in sec18 cells within 10 min of a temperature shift. These results emphasize the dynamic nature of Golgi cisternae and satisfy the kinetic requirements of a cisternal maturation model in which all resident proteins must undergo retrograde vesicular transport, either within the Golgi complex or from there to the ER, as anterograde cargo advances.     

The Dynamics of a Fish Stock Exploited in Two Fishing Zones

This work presents a specific stock-effort dynamical model. The stocks correspond to two populations of fish moving and growing between two fishery zones. They are harvested by two different fleets. The effort represents the number of fishing boats of the two fleets that operate in the two fishing zones. The bioeconomical model is a set of four ODE's governing the fishing efforts and the stocks in the two fishing areas. Furthermore, the migration of the fish between the two patches is assumed to be faster than the growth of the harvested stock. The displacement of the fleets is also faster than the variation in the number of fishing boats resulting from the investment of the fishing income. So, there are two time scales: a fast one corresponding to the migration between the two patches, and a slow time scale corresponding to growth. We use aggregation methods that allow us to reduce the dimension of the model and to obtain an aggregated model for the total fishing effort and fish stock of the two fishing zones. The mathematical analysis of the model is shown. Under some conditions, we obtain a stable equilibrium, which is a desired situation, as it leads to a sustainable harvesting equilibrium, keeping the stock at exploitable densities.     

Stochastic Dynamics of Hematopoietic Tumor Stem Cells

Many tumors derive from the transformation of normal stem cells into cancer stem-cells that retain their self-renewal capacity. This modern view of cancer has provided a natural explanation for the striking parallels which exist between these two different types of self-renewing cells. Here we develop a simple mathematical model to investigate the implications of this concept regarding the evolution of tumors in the hematopoietic system. Our results unequivocally demonstrate that stochastic effects related to the finite size of the active stem-cell population have a profound influence on the dynamics of cancer evolution. For input parameters compatible with both the natural history of human cancer and mouse models, our results show how stochastic dynamics alone may lead to both remission in some cases and rapid expansion in others.     

The Dynamics of Territory Acquisition: A Model of Two Coexisting Strategies

Two potential strategies for acquiring territories are (1) fighting and taking over a territory from its owner, or (2) waiting until a territory owner dies and then taking its place. In this paper we explore territory acquisition using these two strategies, using a population dynamical model. Factors expected to affect the predominance of each strategy are injury rate, rate of successful territory takeover, birth and death rates on the territory, and birth and death rates while non-territorial. We explore the effects of these parameters on numbers of territorial and nonterritorial fighters and waiters. Waiters predominate when injury rate, birth rate of nonterritorial individuals, and death rate of territory owners are high. Fighters predominate when rate of successful territory takeover, death rate of nonterritorial individuals, and birth rate of territory owners are high. We present supportive evidence for these preditions from the literature.     

Muscles, Elastic Energy, and the Dynamics of Body Stiffness in Swimming Eels

SYNOPSIS. TO investigate the capacity of the myomeric musclesto actively change the stiffness of the body during bending,mid-caudal sections (spanning two to three intervertebral joints)of intact, freshly-killed American eels, Anguilla rostrata,were dynamically bent (3 Hz,+min;4% maximal muscle strain) usingthe whole-body workloop technique. Following unstimulated cycles,the left- and right-sides of the musculature were alternatelystimulated at supra-maximal voltages and at eight differentphases relative to the strain cycle of the muscle. The body'sflexural stiffness (Nm2) increased maximally by a factor ofthree relative to that when the muscles were unstimulated. Thenet external work (J kg^–1) needed to bend the body decreasedmaximally by a factor of seven relative to that when the muscleswere unstimulated. Both of these mechanical features variedsinusoidally with changes in the phase of the stimulus. Stimulusphase of caudal muscle in live swimming eels, taken from otherstudies, leads to the prediction that the caudal muscles increasebody stiffness and produce net positive mechanical work simultaneouslyduring steady forward swimming. The association of increasedbody stiffness and net positive muscle work, and the occurrenceof maximal net muscle power output at a stimulus phase of 325°(as the muscle segment lengthens), suggests that net positivepower is produced, in part, using an elastic strain energy mechanism.     

Dynamics of Receptor-Mediated Nanoparticle Internalization into Endothelial Cells

Nanoparticles offer a promising medical tool for targeted drug delivery, for example to treat inflamed endothelial cells during the development of atherosclerosis. To inform the design of such therapeutic strategies, we develop a computational model of nanoparticle internalization into endothelial cells, where internalization is driven by receptor-ligand binding and limited by the deformation of the cell membrane and cytoplasm. We specifically consider the case of nanoparticles targeted against ICAM-1 receptors, of relevance for treating atherosclerosis. The model computes the kinetics of the internalization process, the dynamics of binding, and the distribution of stresses exerted between the nanoparticle and the cell membrane. The model predicts the existence of an optimal nanoparticle size for fastest internalization, consistent with experimental observations, as well as the role of bond characteristics, local cell mechanical properties, and external forces in the nanoparticle internalization process.     

Evidence for Two Extremes of Ciliary Motor Response in a Single Swimming Microorganism

Because arrays of motile cilia drive fluids for a range of processes, the versatile mechano-chemical mechanism coordinating them has been under scrutiny. The protist Paramecium presents opportunities to compare how groups of cilia perform two distinct functions, swimming propulsion and nutrient uptake. We present how the body cilia responsible for propulsion and the oral-groove cilia responsible for nutrient uptake respond to changes in their mechanical environment accomplished by varying the fluid viscosity over a factor of 7. Analysis with a phenomenological model of trajectories of swimmers made neutrally buoyant with magnetic forces combined with high-speed imaging of ciliary beating reveal that the body cilia exert a nearly constant propulsive force primarily by reducing their beat frequency as viscosity increases. By contrast, the oral-groove cilia beat at a nearly constant frequency. The existence of two extremes of motor response in a unicellular organism prompts unique investigations of factors controlling ciliary beating.     

Evidence for Two Extremes of Ciliary Motor Response in a Single Swimming Microorganism

Because arrays of motile cilia drive fluids for a range of processes, the versatile mechano-chemical mechanism coordinating them has been under scrutiny. The protist Paramecium presents opportunities to compare how groups of cilia perform two distinct functions, swimming propulsion and nutrient uptake. We present how the body cilia responsible for propulsion and the oral-groove cilia responsible for nutrient uptake respond to changes in their mechanical environment accomplished by varying the fluid viscosity over a factor of 7. Analysis with a phenomenological model of trajectories of swimmers made neutrally buoyant with magnetic forces combined with high-speed imaging of ciliary beating reveal that the body cilia exert a nearly constant propulsive force primarily by reducing their beat frequency as viscosity increases. By contrast, the oral-groove cilia beat at a nearly constant frequency. The existence of two extremes of motor response in a unicellular organism prompts unique investigations of factors controlling ciliary beating.     

Cytoplasmic Dynamics Reveals Two Modes of Nucleoid-Dependent Mobility

It has been proposed that forces resulting from the physical exclusion of macromolecules from the bacterial nucleoid play a central role in organizing the bacterial cell, yet this proposal has not been quantitatively tested. To investigate this hypothesis, we mapped the generic motion of large protein complexes in the bacterial cytoplasm through quantitative analysis of thousands of complete cell-cycle trajectories of fluorescently tagged ectopic MS2-mRNA complexes. We find the motion of these complexes in the cytoplasm is strongly dependent on their spatial position along the long axis of the cell, and that their dynamics are consistent with a quantitative model that requires only nucleoid exclusion and membrane confinement. This analysis also reveals that the nucleoid increases the mobility of MS2-mRNA complexes, resulting in a fourfold increase in diffusion coefficients between regions of the lowest and highest nucleoid density. These data provide strong quantitative support for two modes of nucleoid action: the widely accepted mechanism of nucleoid exclusion in organizing the cell and a newly proposed mode, in which the nucleoid facilitates rapid motion throughout the cytoplasm.     

A Complex Pattern of Traveling Stripes Is Produced by Swimming Cells of Bacillus subtilis

Motile cells of Bacillus subtilis inadvertently escaped from the surface of an agar disk that was surrounded by a fluid growth medium and formed a migrating population in the fluid. When viewed from above, the population appeared as a cloud advancing unidirectionally into the fresh medium. The cell population became spontaneously organized into a series of stripes in a region behind the advancing cloud front. The number of stripes increased progressively until a saturation value of stripe density per unit area was reached. New stripes arose at a fixed distance behind the cloud front and also between stripes. The spacing between stripes underwent changes with time as stripes migrated towards and away from the cloud front. The global pattern appeared to be stretched by the advancing cloud front. At a time corresponding to approximately two cell doublings after pattern formation, the pattern decayed, suggesting that there is a maximum number of cells that can be maintained within the pattern. Stripes appear to consist of high concentrations of cells organized in sinking columns that are part of a bioconvection system. Their behavior reveals an interplay between bacterial swimming, bioconvection-driven fluid motion, and cell concentration. A mathematical model that reproduces the development and dynamics of the stripe pattern has been developed.     

两毒素环境中具有分离扩散的种群动力系统的持久性

本文是文「５」在ｎ＝２时更进一步的讨论,模型由一个种群构成,此种群可在两个毒素环境中生存,且种群扩散仅与其所在自下而上环境中的（数量）密度决定,得到了比文「５」更精细的结论。     

The Accumulation and Degradation Dynamics of Cyanophycin in Cyanobacterial Cells Grown in Symbiotic Associations with Plant Tissues and Cells

Five different artificial associations of cyanobacterial cells with the cells or tissues of nightshade and rauwolfia were studied. The associations grown on nitrogen-containing media produced heterocysts. Cyanobacterial cells in the associations retained their ability to take up combined nitrogen from the medium, to store it in the form of cyanophycin granules, and to use them in the process of symbiotic growth. The synthesis and degradation of cyanophycin granules in cyanobacterial cells were more active in the associations than in monocultures. In the symbiotic associations of Chlorogloeopsis fritschii ATCC 27193 with Solanum laciniatum cells and of Nostoc muscorum CALU 304 with the Rauwolfia serpentina callus, heterocysts were produced with a 3- to 30-fold higher cyanophycin content than in pure cyanobacterial cultures. In contrast, in the association of N. muscorum CALU 304 with the Solanum dulcamara callus, heterocysts were produced with a lower cyanophycin content than in the N. muscorum CALU 304 pure culture. The degradation of cyanophycin granules in N. muscorum CALU 304 cells grown in associations with plant tissues or cells was subjected to mathematical analysis. The activation of cyanophycin degradation and heterocyst differentiation in the associations N. muscorum CALU 304–R. serpentinaand C.fritschii–S. laciniatum was accompanied by an enhanced synthesis of the nitrogen-containing alkaloids in plant cells. The data obtained suggest that an integrated system of nitrogen homeostasis can be formed in symbiotic associations. Depending on the growth stage of an association, its plant member can either stimulate the accumulation of combined nitrogen in vegetative cyanobacterial cells in the form of cyanophycin granules, activate their degradation, or initiate the formation of heterocysts independently of the cyanobacterial combined nitrogen deprivation sensing-signaling pathway.