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Density Increases (density + increase)
Selected AbstractsEffects of methylcyclodextrin on lysosomesFEBS JOURNAL, Issue 5 2001Michel Jadot The cholesterol complexing agent methyl-cyclodextrin (MCD) provides an efficient mean for the removal of cholesterol from biological membranes. In order to study the effects of this agent on the lysosomal membrane in situ, we treated HepG2 cells with MCD and studied the effects of this treatment on lysosomes in isolated fractions. We found that lysosomes prepared from treated cells are more sensitive to various membrane perturbing treatments such as: incubation of lysosomes in isotonic glucose, in hypotonic sucrose or in the presence of the lytic agent glycyl- l -phenylalanine 2-naphthylamide. The lysosomal membrane is also less resistant to increased hydrostatic pressure. Centrifugation methods were used to analyse the effect of MCD on lysosomes. Isopycnic centrifugation in sucrose density gradients demonstrates that the drug induces a reversible density increase of the lysosomes. Our study indicates that extracellularly added MCD can modify the properties of the lysosomal membrane in living cells. It suggests that MCD could be an effective tool to modulate the physical properties of lysosomes within intact cells and to monitor the cellular responses to such modifications. [source] Microfabric characteristics and rheological significance of ultra-high-pressure metamorphosed jadeite-quartzite and eclogite from Shuanghe, Dabie Mountains, ChinaJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2010L. WANG Abstract Quantitative analysis of the structural evolution of jadeite-quartzite, a rare ultra-high pressure (UHP) rock type from the Dabie Mountains of eastern China, sheds light on the formation and evolution of UHP orogenic belts worldwide. Geological mapping of the Shuanghe area, where jadeite-quartzites crop out, was carried out to determine the spatial relationships between different UHP rocks within this orogen. The deformation mechanisms of jadeite-quartzite, geodynamical parameters (stress, strain, strain rate), and microstructure including lattice preferred orientation (LPO) were determined from six jadeite-quartzite samples from the Shuanghe area. LPOs of clinopyroxene (jadeite and omphacite), garnet, rutile and quartz from these jadeite-quartzite samples are compared with those of three eclogites preserving different degrees of deformation from the Shuanghe area. Microstructural LPOs of jadeite, omphacite, garnet, rutile and quartz were determined using electron backscattered diffraction (EBSD) analysis. Quartz fabrics were largely recrystallized during late, low-grade stages of deformation, whereas garnet shows no strong LPO patterns. Rutile fabrics show a weak LS fabric along [001]. Jadeite and omphacite show the strongest eclogite facies LPO patterns, suggesting that they may provide important information about mantle deformation patterns and control the rheology of deeply subducted continental crust. Microstructural data show that the jadeite LPO patterns are similar to those of omphacite and vary between L- and S-types, which correlate with prolate and oblate grain shape fabrics (SPO); quartz LPOs are monoclinic. Microstructural analysis using TEM shows that the dominant slip systems of jadeite in one sample are (100)[001], (110)[001] and (1 1 0)1/2[110], while in another sample, no dislocations are observed. Abundant dislocations in quartz were accommodated by the dominant slip system (0001)[110], indicating basal glide and represents regional shearing during the exhumation process. This suggests that dislocation creep is the dominant fundamental deformation mechanism in jadeite under UHP conditions. The protoliths of jadeite-quartzite, metasedimentary rocks from the northern passive continental margin of the Yangtze craton, experienced the same deep subduction and were deformed under similar rheological conditions as other UHP eclogite, marble and paragneiss. Experimental UHP deformation of quartzo-feldspathic gneiss with a chemical composition similar to the bulk continental crust has shown that the formation of a jadeite,stishovite rock is associated with a density increase of the host rock similar to the eclogite conversion from basaltic protoliths. The resulting rock can be denser than the surrounding mantle pyrolite up to depths of 660 km (24 GPa). Thus, processes of deep continental subduction may be better-understood through understanding the rheology and mechanical behaviour of jadeite. Jadeite-quartzites such as those from the Shuanghe may be exhumed remnants of deeply-subducted slabs of continental crust, other parts of which subducted past the ,depth of no return', and remain in the deep mantle. [source] A coupled DEM/CFD analysis of the effect of air on powder flow during die fillingAICHE JOURNAL, Issue 1 2009Y. Guo Abstract Die filling from a stationary shoe in a vacuum and in the presence of air was numerically analyzed using an Eulerian-Lagrangian model, which employs a discrete element method (DEM) for the particles and computational fluid dynamics (CFD) for the air with a two-way air-particle interaction coupling term. Monodisperse and polydisperse powder systems have been simulated to explore the effect of the presence of air on the die filling process. For die filling with monodisperse powders, the influences of particle size and density on the flow behavior were explored. The numerical simulations revealed that the presence of air has a significant impact on the powder flow behavior, especially for systems with smaller and/or lighter particles. Flow has been characterized in terms of a dimensionless mass flow rate, and it has been shown that for die filling in a vacuum this is constant. The flow characteristics for die filling in air can be classified into two regimes. There is an air-inert regime in which the particle size and density are sufficiently large that the effect of air flow becomes negligible, and the dimensionless mass flow rate is essentially identical to that obtained for die filling in a vacuum. There is also an air-sensitive regime, for smaller particle sizes and lower particle densities, in which the dimensionless mass flow rate increases as the particle size and density increase. The effects of particle-size distribution and adhesion on the flow behavior have also been investigated. It was found that, in a vacuum, the dimensionless mass flow rate for polydisperse systems is nearly identical to that for monodisperse systems. In the presence of air, a lower dimensionless mass flow rate is obtained for polydisperse systems compared to monodisperse systems, demonstrating that air effects become more significant. Furthermore, it has been shown that, as expected, the dimensionless mass flow rate decreases as the surface energy increases (i.e., for more cohesive powders). © 2008 American Institute of Chemical Engineers AIChE J, 2009 [source] Effects of Thermal Annealing on the Structure of Ferroelectric Thin FilmsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2006Jiang-Li Cao The effects of thermal annealing on the structure of polycrystalline Pb(Zr0.3Ti0.7)O3 (PZT) ferroelectric thin films prepared by chemical solution deposition on Pt/TiOx electrode stacks were studied using scanning electron microscopy, transmission electron microscopy (TEM), and grazing incidence X-ray specular and diffuse reflectivity of synchrotron radiation. The stratified multilayered structure and element diffusions in the sample were characterized by TEM. Global statistical structural parameters including the density, surface or interface roughness and thickness of each layer in the samples were obtained from fitting the X-ray specular reflectivity using a homogeneous stratified multilayer model of PZT/Pt/TiOx/SiO2. The results showed that the PZT surface and PZT/Pt interface roughness changed slightly during thermal annealing in oxygen at 700°C. By contrast, the density increase of the PZT ceramic and density decrease of the Pt-bottom electrode during annealing were observed. A high density value of the PZT ceramic film after the annealing was found, up to 99.8% of the theoretical value of the corresponding bulk ceramics. The density changes of the PZT and Pt layers were further confirmed by X-ray diffuse reflectivity. The influences of the annealing treatment on the density changes of the PZT and Pt layers were attributed to the further densification of the PZT ceramic and incorporation of light elements such as Zr, Ti and O from the neighboring layers into the Pt layer, respectively, as discussed in correlation with the TEM analyses. [source] Thermal evolution of rotating hybrid starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007Miao Kang ABSTRACT As a neutron star spins down, the nuclear matter is continuously converted into quark matter due to the core density increase, and then latent heat is released. We have investigated the thermal evolution of neutron stars undergoing such deconfinement phase transition. We have taken into account the conversion in the frame of the general theory of relativity. The released energy has been estimated as a function of changed rate of deconfinement baryon number. The numerical solutions to the cooling equation are seen to be very different from those without the heating effect. The results show that neutron stars may be heated to higher temperatures which is well matched with pulsar's data despite the onset of fast cooling in neutron stars with quark matter cores. It is also found that the heating effect has a magnetic field strength dependence. This feature could be particularly interesting for high temperatures of low-field millisecond pulsars at a later stage. The high temperature could fit the observed temperature for PSR J0437,4715. [source] Correlation of Density Pedestal Width and Neutral Penetration LengthCONTRIBUTIONS TO PLASMA PHYSICS, Issue 1-3 2004X. Q. Xu Abstract Pedestal studies in DIII-D and MAST find good correlation between the width of the H-mode density barrier and the neutral penetration length [1, 2]. These results suggest that the width may be set by the combined effects of neutrals and plasma transport. This paper is a report on fluid simulations of boundary plasma using the BOUT code[3] with a neutral source added. Thus both neutral and plasma physics are treated. The plasma transport is self-consistently driven by boundary turbulence due to the resistive X-point mode, while neutrals are described by a simple analytic model. The plasma profiles are evolved on the same time scale as the turbulence for the given heat source from the core plasma and particle source from the neutrals. For prescribed neutral profiles, we find the formation of a density pedestal inside the separatrix in the L-mode even though the calculated plasma diffusion coefficients are almost radially constant and without the formation of a temperature pedestal. These results support the hypothesis that particle fueling can provide the dominant control for the size of the H-mode density barrier. The width of the density barrier decreases as the pedestal density increases which is also consistent with experimental data. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Consequences of hyperconcentrated flow for process-based soil erosion modelling on the Chinese Loess PlateauEARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2006Rudi Hessel Abstract High sediment concentrations in runoff are a characteristic feature of the Chinese Loess Plateau, and are probably caused by factors such as the occurrence of erodible materials on steep slopes, the characteristics of the loess and the harsh climate that results in low plant cover. When sediment concentration increases, fluid density increases, viscosity increases and settling velocity decreases. These effects become increasingly important with increasing concentration and can result in flow behaviour that is quite different from that of clear water flow. Although the net effect of these changes on the flow is not always apparent, erosion models that deal with high sediment concentrations should consider such effects and could include corrections for some of these effects. A case study in a small catchment on the Loess Plateau indicated that sediment concentrations were considerable, and literature data suggested that for such sediment concentrations, corrections for settling velocity, fluid density and viscosity are needed. Furthermore, a number of corrections are necessary to be able to compare field measurements with results of soil erosion models: sediment volume should be subtracted from runoff volume and a density correction is needed to use data from a pressure transducer. For flumes that were used to measure discharge from smaller areas inside the catchment, the measured water level should be corrected by subtracting the sediment level in the flume from the water level, while the sediment volume should also be subtracted from the discharge. Finally, measured concentration should be corrected to give concentration expressed as grams per litre of clear water, since soil erosion models express sediment concentration in this way. Copyright © 2006 John Wiley & Sons, Ltd. [source] A THEORETICAL INVESTIGATION OF SYMPATRIC EVOLUTION OF TEMPORAL REPRODUCTIVE ISOLATION AS ILLUSTRATED BY MARINE BROADCAST SPAWNERSEVOLUTION, Issue 11 2007Maurizio Tomaiuolo Recent theory suggests that frequency-dependent disruptive selection in combination with assortative mating can lead to the establishment of reproductive isolation in sympatry. Here we explore how temporal variation in reproduction might simultaneously generate both disruptive selection and assortative mating, and result in sympatric speciation. The conceptual framework of the model may be applicable to biological systems with negative frequency-dependent selection, such as marine broadcast spawners or systems with pollinator limitation. We present a model that is motivated by recent findings in marine broadcast spawners and is parameterized with data from the Montastraea annularis species complex. Broadcast spawners reproduce via external fertilization and synchronous spawning is required to increase the probability of successful fertilization, but empirical evidence shows that as density increases, so does the risk of polyspermy. Polyspermy is the fusion of multiple sperm with an egg at fertilization, a process that makes the embryo unviable. Synchrony can therefore also act as a source of negative density-dependent disruptive selection. Model analysis shows that the interaction between polyspermy and spawning synchrony can lead to temporal reproductive isolation in sympatry and that, more generally, increased density promotes maintenance of genetic variation. [source] Tumour-associated macrophages and melanoma tumourigenesis: integrating the complexityINTERNATIONAL JOURNAL OF EXPERIMENTAL PATHOLOGY, Issue 3 2006Mahmoud R. Hussein Summary When the body discovers a tumour cell (foreign antigen), several kinds of mechanisms and cells operate in what is called an immune response. The latter has evolved into two mechanisms: non-specific immunity and specific immunity, which are closely linked to and influence each other. The former represents the first line of defence against neoplastic cells. The adaptive (specific) immunity is orchestrated by antigen-specific T and B lymphocytes. The effector cells of innate immunity include granulocytes, macrophages and natural killer cells. Among these cells, macrophages represent the most important part of innate immunity against tumours. Tumour-associated macrophages (TAMs) are important antigen-presenting cells and as such an understanding of their interactions with tumour cells gives insights into novel therapeutic strategies. In tumours, the effect of TAMs is the outcome of their two concomitantly competing interactions: tumour growth reduction and tumour growth promotion. The macrophage (TAMs) content of melanoma ranges from 0 to 30% and their density increases with increasing tumour thickness. The melanoma cells and TAMs seem to interact with each other through the release of soluble factors that either prevent or enhance tumour growth. For instance, syngeneic macrophages from tumour-bearing mice can inhibit melanoma growth in the nude mice more than the control macrophages. Alternatively, metastatic B16 melanoma cells can produce some macrophage cytotoxic substances that help tumour cells not only escape the host immunosurveillance system but also prevent distant metastasis. Together, these observations suggest opposing effects for these soluble factors in melanoma. To date, little is available in the literature about the interactions between TAMs and melanoma cells. This viewpoint not only tries to examine these interactions but also provides relevant speculations. [source] Variable age structure and apparent density dependence in survival of adult ungulatesJOURNAL OF ANIMAL ECOLOGY, Issue 4 2003Marco Festa-Bianchet Summary 1Large herbivores have strongly age-structured populations. Because recruitment often decreases as population density increases, in unexploited populations the proportion of older adults may increase with density. Because survival senescence is typical of ungulates, ignoring density-dependent changes in age structure could lead to apparent density-dependence in adult survival. 2To test for density dependence in adult survival, we used data from three populations that underwent considerable changes in density. Bighorn sheep (Ovis canadensis) on Ram Mountain, Alberta, ranged from 94 to 232, mountain goats (Oreamnos americanus) on Caw Ridge, Alberta, varied from 81 to 147, and estimates of roe deer (Capreolus capreolus) older than 1 year at Chizé, France, ranged from 157 to 569. 3We used recent developments of capture,mark,recapture modelling to assess the response of adult survival to changes in density when age structure was and was not taken into account. 4Survival rates were 10,15% higher during the prime-age stage than during the senescent stage for all sex-species combinations. When adults were pooled into a single age class there was an apparent negative effect of density on female survival in bighorns and roe deer, and negative trends for female mountain goats, male roe deer and male bighorn sheep. When age class was taken into account, there were no significant effects of density on adult survival. Except for male mountain goats, the strength of density dependence was lower when age was taken into account. 5In ungulate populations, age structure is an important determinant of adult survival. Most reports of density dependence in adult survival may have been confounded by changes in age structure. [source] The maintenance of sex: host,parasite coevolution with density-dependent virulenceJOURNAL OF EVOLUTIONARY BIOLOGY, Issue 10 2009C. M. LIVELY Abstract Why don't asexual females replace sexual females in most natural populations of eukaryotes? One promising explanation is that parasites could counter the reproductive advantages of asexual reproduction by exerting frequency-dependent selection against common clones (the Red Queen hypothesis). One apparent limitation of the Red Queen theory, however, is that parasites would seem to be required by theory to be highly virulent. In the present study, I present a population-dynamic view of competition between sexual females and asexual females that interact with co-evolving parasites. The results show that asexual populations have higher carrying capacities, and more unstable population dynamics, than sexual populations. The results also suggest that the spread of a clone into a sexual population could increase the effective parasite virulence as population density increases. This combination of parasite-mediated frequency-dependent selection, and density-dependent virulence, could lead to the coexistence of sexual and asexual reproductive strategies and the long-term persistence of sex. [source] Alcohol Availability and Intimate Partner Violence Among US CouplesALCOHOLISM, Issue 1 2009Christy M. McKinney Objectives:, We examined the relation between alcohol outlet density (the number of alcohol outlets per capita by zip code) and male-to-female partner violence (MFPV) or female-to-male partner violence (FMPV). We also investigated whether binge drinking or the presence of alcohol-related problems altered the relationship between alcohol outlet density and MFPV or FMPV. Methods:, We linked individual and couple sociodemographic and behavioral data from a 1995 national population-based sample of 1,597 couples to alcohol outlet data and 1990 US Census sociodemographic information. We used logistic regression for survey data to estimate unadjusted and adjusted odds ratios between alcohol outlet density and MFPV or FMPV along with 95% confidence intervals (CIs) and p -values. We used a design-based Wald test to derive a p -value for multiplicative interaction to assess the role of binge drinking and alcohol-related problems. Results:, In adjusted analysis, an increase of one alcohol outlet per 10,000 persons was associated with a 1.03-fold increased risk of MFPV (p -value for linear trend = 0.01) and a 1.011-fold increased risk of FMPV (p -value for linear trend = 0.48). An increase of 10 alcohol outlets per 10,000 persons was associated with 34% and 12% increased risk of MFPV and FMPV respectively, though the CI for the association with FMPV was compatible with no increased risk. The relationship between alcohol outlet density and MFPV was stronger among couples reporting alcohol-related problems than those reporting no problems (p -value for multiplicative interaction = 0.01). Conclusions:, We found that as alcohol outlet density increases so does the risk of MFPV and that this relationship may differ for couples who do and do not report alcohol-related problems. Given that MFPV accounts for the majority of injuries related to intimate partner violence, policy makers may wish to carefully consider the potential benefit of limiting alcohol outlet density to reduce MFPV and its adverse consequences. [source] Densification of Si3N4 with LiYO2 AdditiveJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2004Branko Matovic This paper deals with the densification and phase transformation during pressureless sintering of Si3N4 with LiYO2 as the sintering additive. The dilatometric shrinkage data show that the first Li2O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from ,-Si3N4 to ,-Si3N4 taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of ,-Si3N4 to ,-Si3N4 takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li2O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li2O,Y2O3 system can provide very effective low-temperature sintering additives for silicon nitride. [source] The H, luminosity function and star formation rate up to z , 1MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002L. Tresse Abstract We describe ISAAC/ESO-VLT observations of the H,,6563 Balmer line of 33 field galaxies from the Canada,France Redshift Survey (CFRS) with redshifts selected between 0.5 and 1.1. We detect H, in emission in 30 galaxies and compare the properties of this sample with the low-redshift sample of CFRS galaxies at z, 0.2. We find that the H, luminosity, L(H,), is tightly correlated to M(BAB) in the same way for both the low- and high-redshift samples. L(H,) is also correlated to L([O ii],3727), and again the relation appears to be similar at low and high redshifts. The ratio L(lsqb;O ii])/L(H,) decreases for brighter galaxies by as much as a factor of 2 on average. Derived from the H, luminosity function, the comoving H, luminosity density increases by a factor 12 from ,z,= 0.2 to ,z,= 1.3. Our results confirm a strong rise of the star formation rate (SFR) at z < 1.3, proportional to (1 +z)4.1±0.3 (with H0= 50 km s,1 Mpc,1, q0= 0.5). We find an average SFR(2800 Å)/SFR (H,) ratio of 3.2 using the Kennicutt SFR transformations. This corresponds to the dust correction that is required to make the near-ultraviolet data consistent with the reddening-corrected H, data within the self-contained, I -selected CFRS sample. [source] Pollinator aggregative and functional responses to flower density: does pollinator response to patches of plants accelerate at low-densities?OIKOS, Issue 1 2006Tracy S. Feldman Plant reproduction is often reduced at low densities, due to reduced pollinator visitation rates. Recent theory suggests that a disproportionate increase in pollinator visits to patches of plants as heterospecific plant density increases (i.e. if visitation is a sigmoid function of patch density) can rescue sparse populations of a focal plant species from reduced reproductive success or population decline. A field experiment was performed to determine the shape of the pollinator visitation response to patches of differing density of the common weed Brassica rapa. Both the aggregative and functional response for the entire pollinator community were saturating rather than sigmoid, indicating that pollinator response does not accelerate when density increases. The results for the entire pollinator community were consistent among temporal and spatial replicates. Aggregative response curves for specific pollinator taxa were either linear (bombyliid flies) or saturating (syrphid flies, solitary bees, and Lepidoptera). Functional responses for these taxa were saturating (syrphid flies and solitary bees) or flat (bombyliid flies and Lepidoptera). Individual pollinators visited more plants during foraging bouts in high-density patches, but visits per plant decreased. Seeds per fruit and seeds per flower increased with increasing density. There is no evidence that pollinators disproportionately visit denser patches, or that the conditions for this mechanism of pollination facilitation are likely to be met in this generalist pollinator system. [source] Influence of the cross-linking density on the main dielectric relaxation of poly(methyl acrylate) networksPOLYMER ENGINEERING & SCIENCE, Issue 10 2005J.M. Meseguer Dueñas A series of polymer networks of varying cross-linking density was prepared by copolymerization of methyl acrylate and ethyleneglycol dimethacrylate. The aim of this work is to study the influence of cross-linking on the conformational mobility of the polymer chains using dielectric relaxation spectroscopy (DRS) in the temperature range of the main dielectric relaxation. As expected, the temperature range in which glass transition takes place became wider with increasing crosslinking density. DRS results were analyzed using the Havriliak-Negami equation. Master Cole-Cole arcs could be drawn for all the networks. The arcs become more symmetric as cross-linking density increases, as a consequence of the different effect of cross-links on large and small scale mobility. The conformational mobility that produces the main relaxation is drastically reduced when the cross-linking density increases what reduces the relaxation strength, but it also gives a qualitative change of behavior, as shown by the temperature dependence of the relaxation strength. In the loosely cross-linked networks the relaxation strength decreases monotonously as temperature increases, as in the main dielectric relaxation of linear polymers. Nevertheless, in highly cross-linked networks the curve of relaxation strength against temperature presents a maximum. POLYM. ENG. SCI., 45:1336,1342, 2005. © 2005 Society of Plastics Engineers [source] Influence of chemical composition on the rheological behavior of condensation reaction resinsPOLYMER ENGINEERING & SCIENCE, Issue 2 2003M. Doyle In this paper, the chemorheological and dynamic mechanical behavior of melamine-formaldehyde (MF) resins of four different formaldehyde/melamine (F/M) molar ratios (1.25, 1.5 1.75 and 2.00) are investigated. MF resins polymerize via a polycondensation reaction involving formation of up to 10 wt% of H2O on cure. This typically results in rapid and extensive foaming of the resin when it is cured under atmospheric pressure. Experimental adaptation for the foaming behavior of MF resins is used to gather rheological information concerning the curing kinetics and the mechanical response of neat MF resins of different molar ratios. Likewise, the procedures developed allow curing of the resins under atmospheric pressure, hence allowing volatile evacuation as occurs during venting procedures (commonly used during compression molding of MF molding compounds) or as a result of absorption by hydrophilic fillers or substrates. The results show that increased moisture content in the B-stage leads to faster reaction rates and greater foaming. Gelation and vitrification times are identified for each molar ratio, and are found to increase with decreasing molar ratio. The dynamic mechanical behavior of carefully molded neat MF samples of different molar ratios is studied using DMTA. Tg is found to be 200°C for the resin with the lowest formaldehyde content (F/M = 1.25), and around 230°C for the other resins. The storage shear modulus above Tg is studied, and the results show that the crosslink density increases with increasing molar ratio. [source] What happens if density increases?ANIMAL CONSERVATION, Issue 4 2007Conservation implications of population influx into refuges Abstract Sudden catastrophic events like fires, hurricanes, tsunamis, landslides and deforestation increase population densities in habitat fragments, as fleeing animals encroach into these refuges. Such sudden overcrowding will trigger transient fluctuations in population size in the refuges, which may expose refuge populations to an increased risk of extinction. Until recently, detailed information about the operation of density dependence in stage-structured populations, and tools for quantifying the effects of transient dynamics, have not been available, so that exploring the extinction risk of such transient fluctuations has been intractable. Here, we use such recently developed tools to show that extinction triggered by overcrowding can threaten populations in refuges. Apart from situations where density dependence acts on survival, our results indicate that short-lived species may be more at risk than longer-lived species. Because dynamics in local populations may be critical for the preservation of metapopulations and rare species, we argue that this aspect warrants further attention from conservation biologists. [source] Effects of Oxygen Transport on 3-D Human Mesenchymal Stem Cell Metabolic Activity in Perfusion and Static Cultures: Experiments and Mathematical ModelBIOTECHNOLOGY PROGRESS, Issue 4 2005Feng Zhao Human mesenchymal stem cells (hMSCs) have unique potential to develop into functional tissue constructs to replace a wide range of tissues damaged by disease or injury. While recent studies have highlighted the necessity for 3-D culture systems to facilitate the proper biological, physiological, and developmental processes of the cells, the effects of the physiological environment on the intrinsic tissue development characteristics in the 3-D scaffolds have not been fully investigated. In this study, experimental results from a 3-D perfusion bioreactor system and the static culture are combined with a mathematical model to assess the effects of oxygen transport on hMSC metabolism and proliferation in 3-D constructs grown in static and perfusion conditions. Cells grown in the perfusion culture had order of magnitude higher metabolic rates, and the perfusion culture supports higher cell density at the end of cultivation. The specific oxygen consumption rate for the constructs in the perfusion bioreactor was found to decrease from 0.012 to 0.0017 ,mol/106 cells/h as cell density increases, suggesting intrinsic physiological change at high cell density. BrdU staining revealed the noneven spatial distribution of the proliferating cells in the constructs grown under static culture conditions compared to the cells that were grown in the perfusion system. The hypothesis that the constructs in static culture grow under oxygen limitation is supported by higher YL/G in static culture. Modeling results show that the oxygen tension in the static culture is lower than that of the perfusion unit, where the cell density was 4 times higher. The experimental and modeling results show the dependence of cell metabolism and spatial growth patterns on the culture environment and highlight the need to optimize the culture parameters in hMSC tissue engineering [source] Effect of Volume Fraction of Material on Separation by Density Difference in a Liquid-Fluidized Bed of Inert ParticlesCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 7 2010Y. Tatemoto Abstract A liquid-fluidized bed was used to separate a pure material from a mixture. A quantity of relatively large sized material was immersed in an inert-particle fluidized bed and the behavior of materials was examined for different liquid velocities. In particular, the volume fraction of the material was varied and its effect on the separation characteristics was examined. The material floats on the inert-particle fluidized bed when the density of the material is lower than the apparent density of the bed, regardless of the volume fraction of the material. The apparent density of the bed can be adjusted by changing the liquid velocity. The materials in the upper portion of the bed affect the properties of the bed below them, i.e., the void fraction decreases and the apparent density increases in the inert-particle suspension when materials are present in the upper portion of the bed. Therefore, the materials float on the bed although the apparent density of the inert-particle suspension obtained from the case without material is less than the density of the material at a relatively high volume fraction of material. This phenomenon occurs more easily for lighter and smaller materials. This means that small inert particles and low liquid velocities are the optimum operating conditions for the separation. [source] |