118 research outputs found
Generalized Voronoi Tessellation as a Model of Two-dimensional Cell Tissue Dynamics
Voronoi tessellations have been used to model the geometric arrangement of
cells in morphogenetic or cancerous tissues, however so far only with flat
hypersurfaces as cell-cell contact borders. In order to reproduce the
experimentally observed piecewise spherical boundary shapes, we develop a
consistent theoretical framework of multiplicatively weighted distance
functions, defining generalized finite Voronoi neighborhoods around cell bodies
of varying radius, which serve as heterogeneous generators of the resulting
model tissue. The interactions between cells are represented by adhesive and
repelling force densities on the cell contact borders. In addition, protrusive
locomotion forces are implemented along the cell boundaries at the tissue
margin, and stochastic perturbations allow for non-deterministic motility
effects. Simulations of the emerging system of stochastic differential
equations for position and velocity of cell centers show the feasibility of
this Voronoi method generating realistic cell shapes. In the limiting case of a
single cell pair in brief contact, the dynamical nonlinear Ornstein-Uhlenbeck
process is analytically investigated. In general, topologically distinct tissue
conformations are observed, exhibiting stability on different time scales, and
tissue coherence is quantified by suitable characteristics. Finally, an
argument is derived pointing to a tradeoff in natural tissues between cell size
heterogeneity and the extension of cellular lamellae.Comment: v1: 34 pages, 19 figures v2: reformatted 43 pages, 21 figures, 1
table; minor clarifications, extended supplementary materia
Geometric mean extension for data sets with zeros
There are numerous examples in different research fields where the use of the
geometric mean is more appropriate than the arithmetic mean. However, the
geometric mean has a serious limitation in comparison with the arithmetic mean.
Means are used to summarize the information in a large set of values in a
single number; yet, the geometric mean of a data set with at least one zero is
always zero. As a result, the geometric mean does not capture any information
about the non-zero values. The purpose of this short contribution is to review
solutions proposed in the literature that enable the computation of the
geometric mean of data sets containing zeros and to show that they do not
fulfil the `recovery' or `monotonicity' conditions that we define. The standard
geometric mean should be recovered from the modified geometric mean if the data
set does not contain any zeros (recovery condition). Also, if the values of an
ordered data set are greater one by one than the values of another data set
then the modified geometric mean of the first data set must be greater than the
modified geometric mean of the second data set (monotonicity condition). We
then formulate a modified version of the geometric mean that can handle zeros
while satisfying both desired conditions
The role of mathematical modelling in understanding prokaryotic predation
With increasing levels of antimicrobial resistance impacting both human and animal health, novel means of treating resistant infections are urgently needed. Bacteriophages and predatory bacteria such as Bdellovibrio bacteriovorus have been proposed as suitable candidates for this role. Microbes also play a key environmental role as producers or recyclers of nutrients such as carbon and nitrogen, and predators have the capacity to be keystone species within microbial communities. To date, many studies have looked at the mechanisms of action of prokaryotic predators, their safety in in vivo models and their role and effectiveness under specific conditions. Mathematical models however allow researchers to investigate a wider range of scenarios, including aspects of predation that would be difficult, expensive, or time-consuming to investigate experimentally. We review here a history of modelling in prokaryote predation, from simple Lotka-Volterra models, through increasing levels of complexity, including multiple prey and predator species, and environmental and spatial factors. We consider how models have helped address questions around the mechanisms of action of predators and have allowed researchers to make predictions of the dynamics of predatorâprey systems. We examine what models can tell us about qualitative and quantitative commonalities or differences between bacterial predators and bacteriophage or protists. We also highlight how models can address real-world situations such as the likely effectiveness of predators in removing prey species and their potential effects in shaping ecosystems. Finally, we look at research questions that are still to be addressed where models could be of benefit
New, rapid method to measure dissolved silver concentration in silver nanoparticle suspensions by aggregation combined with centrifugation
It is unclear whether the antimicrobial activities of silver nanoparticles (AgNPs) are exclusively mediated by the release of silver ions (Ag(+)) or, instead, are due to combined nanoparticle and silver ion effects. Therefore, it is essential to quantify dissolved Ag in nanosilver suspensions for investigations of nanoparticle toxicity. We developed a method to measure dissolved Ag in Ag(+)/AgNPs mixtures by combining aggregation of AgNPs with centrifugation. We also describe the reproducible synthesis of stable, uncoated AgNPs. Uncoated AgNPs were quickly aggregated by 2Â mM Ca(2+), forming large clusters that could be sedimented in a low-speed centrifuge. At 20,100g, the sedimentation time of AgNPs was markedly reduced to 30Â min due to Ca(2+)-mediated aggregation, confirmed by the measurements of Ag content in supernatants with graphite furnace atomic absorption spectrometry. No AgNPs were detected in the supernatant by UVâVis absorption spectra after centrifuging the aggregates. Our approach provides a convenient and inexpensive way to separate dissolved Ag from AgNPs, avoiding long ultracentrifugation times or Ag(+) adsorption to ultrafiltration membranes. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11051-016-3565-0) contains supplementary material, which is available to authorized users
Repair rather than segregation of damage is the optimal unicellular aging strategy
BACKGROUND: How aging, being unfavourable for the individual, can evolve is one of the fundamental problems of biology. Evidence for aging in unicellular organisms is far from conclusive. Some studies found aging even in symmetrically dividing unicellular species; others did not find aging in the same, or in different, unicellular species, or only under stress. Mathematical models suggested that segregation of non-genetic damage, as an aging strategy, would increase fitness. However, these models failed to consider repair as an alternative strategy or did not properly account for the benefits of repair. We used a new and improved individual-based model to examine rigorously the effect of a range of aging strategies on fitness in various environments. RESULTS: Repair of damage emerges as the best strategy despite its fitness costs, since it immediately increases growth rate. There is an optimal investment in repair that outperforms damage segregation in well-mixed, lasting and benign environments over a wide range of parameter values. Damage segregation becomes beneficial, and only in combination with repair, when three factors are combined: (i) the rate of damage accumulation is high, (ii) damage is toxic and (iii) efficiency of repair is low. In contrast to previous models, our model predicts that unicellular organisms should have active mechanisms to repair damage rather than age by segregating damage. Indeed, as predicted, all organisms have evolved active mechanisms of repair whilst aging in unicellular organisms is absent or minimal under benign conditions, apart from microorganisms with a different ecology, inhabiting short-lived environments strongly favouring early reproduction rather than longevity. CONCLUSIONS: Aging confers no fitness advantage for unicellular organisms in lasting environments under benign conditions, since repair of non-genetic damage is better than damage segregation. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12915-014-0052-x) contains supplementary material, which is available to authorized users
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