Quantifying stretching and rearrangement in epithelial sheet migration

Although understanding the collective migration of cells, such as that seen in epithelial sheets, is essential for understanding diseases such as metastatic cancer, this motion is not yet as well characterized as individual cell migration. Here we adapt quantitative metrics used to characterize the flow and deformation of soft matter to contrast different types of motion within a migrating sheet of cells. Using a Finite-Time Lyapunov Exponent (FTLE) analysis, we find that - in spite of large fluctuations - the flow field of an epithelial cell sheet is not chaotic. Stretching of a sheet of cells (i.e., positive FTLE) is localized at the leading edge of migration. By decomposing the motion of the cells into affine and non-affine components using the metric D$^{2}_{min}$, we quantify local plastic rearrangements and describe the motion of a group of cells in a novel way. We find an increase in plastic rearrangements with increasing cell densities, whereas inanimate systems tend to exhibit less non-affine rearrangements with increasing density.Comment: 21 pages, 7 figures This is an author-created, un-copyedited version of an article accepted for publication in the New Journal of Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at doi:10.1088/1367-2630/15/2/02503

Swarm dynamics may give rise to Lévy flights

“Continuous-time correlated random walks” are now gaining traction as models of scale-finite animal movement patterns because they overcome inherent shortcomings with the prevailing paradigm - discrete random walk models. Continuous-time correlated random walk models are founded on the classic Langevin equation that is driven by purely additive noise. The Langevin equation is, however, changed fundamentally by the smallest of multiplicative noises. The inclusion of such noises gives rise to Lévy flights, a popular but controversial model of scale-free movement patterns. Multiplicative noises have not featured prominently in the literature on biological Lévy flights, being seen, perhaps, as no more than a mathematical contrivance. Here we show how Langevin equations driven by multiplicative noises and incumbent Lévy flights arise naturally in the modelling of swarms. Model predictions find some support in three-dimensional, time-resolved measurements of the positions of individual insects in laboratory swarms of the midge Chironomus riparius. We hereby provide a new window on Lévy flights as models of movement pattern data, linking patterns to generative processes

Lagrangian Structure Functions in Turbulence: A Quantitative Comparison between Experiment and Direct Numerical Simulation

A detailed comparison between data from experimental measurements and numerical simulations of Lagrangian velocity structure functions in turbulence is presented. By integrating information from experiments and numerics, a quantitative understanding of the velocity scaling properties over a wide range of time scales and Reynolds numbers is achieved. The local scaling properties of the Lagrangian velocity increments for the experimental and numerical data are in good quantitative agreement for all time lags. The degree of intermittency changes when measured close to the Kolmogorov time scales or at larger time lags. This study resolves apparent disagreements between experiment and numerics.Comment: 13 RevTeX pages (2 columns) + 8 figures include

Interaction of Supernova Ejecta with Nearby Protoplanetary Disks

The early Solar System contained short-lived radionuclides such as 60Fe (t1/2 = 1.5 Myr) whose most likely source was a nearby supernova. Previous models of Solar System formation considered a supernova shock that triggered the collapse of the Sun's nascent molecular cloud. We advocate an alternative hypothesis, that the Solar System's protoplanetary disk had already formed when a very close (< 1 pc) supernova injected radioactive material directly into the disk. We conduct the first numerical simulations designed to answer two questions related to this hypothesis: will the disk be destroyed by such a close supernova; and will any of the ejecta be mixed into the disk? Our simulations demonstrate that the disk does not absorb enough momentum from the shock to escape the protostar to which it is bound. Only low amounts (< 1%) of mass loss occur, due to stripping by Kelvin-Helmholtz instabilities across the top of the disk, which also mix into the disk about 1% of the intercepted ejecta. These low efficiencies of destruction and injectation are due to the fact that the high disk pressures prevent the ejecta from penetrating far into the disk before stalling. Injection of gas-phase ejecta is too inefficient to be consistent with the abundances of radionuclides inferred from meteorites. On the other hand, the radionuclides found in meteorites would have condensed into dust grains in the supernova ejecta, and we argue that such grains will be injected directly into the disk with nearly 100% efficiency. The meteoritic abundances of the short-lived radionuclides such as 60Fe therefore are consistent with injection of grains condensed from the ejecta of a nearby (< 1 pc) supernova, into an already-formed protoplanetary disk.Comment: 57 pages, 16 figure

Path lengths in turbulence

By tracking tracer particles at high speeds and for long times, we study the geometric statistics of Lagrangian trajectories in an intensely turbulent laboratory flow. In particular, we consider the distinction between the displacement of particles from their initial positions and the total distance they travel. The difference of these two quantities shows power-law scaling in the inertial range. By comparing them with simulations of a chaotic but non-turbulent flow and a Lagrangian Stochastic model, we suggest that our results are a signature of turbulence.Comment: accepted for publication in Journal of Statistical Physic

Effects of dietary cellobiose on the intestinal microbiota and excretion of nitrogen metabolites in healthy adult dogs

In order to evaluate the potential prebiotic effects of cellobiose, 10 healthy adult research beagle dogs received a complete diet containing 0, 0.5 and 1 g cellobiose/kg bodyweight (BW)/day. At the end of each feeding period, faeces, urine and blood of the dogs were collected. The results demonstrated a significant increase of faecal lactate concentrations, indicating a bacterial fermentation of cellobiose in the canine intestine. Along with this, a dose-dependent linear increase of the relative abundance of Lactobacillaceae in the faeces of the dogs was observed (p = 0.014). In addition, a dose-dependent increase (p < 0.05) of Alloprevotella, Bacteroides and Prevotella, and a linear decrease for unidentified Lachnospiraceae (p = 0.011) was observed when cellobiose was added to the diet, although the relative abundance of these genera was low (<1%) among all groups. The faecal pH was not affected by dietary cellobiose. Cellobiose seemed to modulate the excretion of nitrogen metabolites, as lower concentrations of phenol (p = 0.034) and 4-ethylphenol (p = 0.002) in the plasma of the dogs were measured during the supplementation periods. Urinary phenols and indoles, however, were not affected by the dietary supplementation of cellobiose. In conclusion, cellobiose seems to be fermented by the intestinal microbiota of dogs. Although no effect on the faecal pH was detected, the observed increase of microbial lactate production might lower the pH in the large intestine and consecutively modulate the intestinal absorption of nitrogen metabolites. Also, the observed changes of some bacterial genera might have been mediated by increased intestinal lactate concentrations or a higher relative abundance of lactobacilli. Whether these results could be considered as a prebiotic effect and used as a dietetic strategy in diseased animals to improve gut function or hepatic and renal nitrogen metabolism should be evaluated in future studies

The Supernova Triggered Formation and Enrichment of Our Solar System

We investigate the enrichment of the pre-solar cloud core with short lived radionuclides (SLRs), especially 26Al. The homogeneity and the surprisingly small spread in the ratio 26Al/27Al observed in the overwhelming majority of calcium-aluminium-rich inclusions (CAIs) in a vast variety of primitive chondritic meteorites places strong constraints on the formation of the the solar system. Freshly synthesized radioactive 26Al has to be included and well mixed within 20kyr. After discussing various scenarios including X-winds, AGB stars and Wolf-Rayet stars, we come to the conclusion that triggering the collapse of a cold cloud core by a nearby supernova is the most promising scenario. We then narrow down the vast parameter space by considering the pre-explosion survivability of such a clump as well as the cross-section necessary for sufficient enrichment. We employ numerical simulations to address the mixing of the radioactively enriched SN gas with the pre-existing gas and the forced collapse within 20kyr. We show that a cold clump of 10Msun at a distance of 5pc can be sufficiently enriched in 26Al and triggered into collapse fast enough - within 18kyr after encountering the supernova shock - for a range of different metallicities and progenitor masses, even if the enriched material is assumed to be distributed homogeneously in the entire supernova bubble. In summary, we envision an environment for the birth place of the Solar System 4.567Gyr ago similar to the situation of the pillars in M16 nowadays, where molecular cloud cores adjacent to an HII region will be hit by a supernova explosion in the future. We show that the triggered collapse and formation of the Solar System as well as the required enrichment with radioactive 26Al are possible in this scenario.Comment: 12 pages, 8 figures, accepted for publication in ApJ. Resolution of most figures degraded to fit within arXiv size limits. A full resolution version is available at http://www.usm.uni-muenchen.de/~gritschm/Gritschneder_2011_sun.pd

Firearm-Related Pediatric Head Trauma: A Scoping Review.

BACKGROUND: Firearm-related injury is a significant cause of morbidity and mortality in pediatric populations. Despite a disproportionate role in the most morbid outcomes in both traumatic brain injury and firearm-related injury populations, firearm-related traumatic brain injury (frTBI) is an understudied epidemiological entity. There is need to increase understanding and promote interventions that reduce this burden of disease. OBJECTIVE: To assess the evidence characterizing pediatric frTBI to highlight trends and gaps regarding burden of disease and interventions to reduce frTBI. METHODS: We conducted a scoping review under Preferred Reporting Items for Systematic Reviews and Meta-Analyses-Scoping Review (PRISMA-ScR) guidelines on peer-reviewed studies across 5 databases (Medline OVID, EMBASE, Web of Science Legal Collection, PsychINFO, and Academic Search Complete). English studies examining pediatric frTBI epidemiology, prevention, and/or social or legal policy advocacy were included. Articles were excluded if they more generally discussed pediatric firearm-related injury without specific analysis of frTBI. RESULTS: Six studies satisfied inclusion criteria after screening and full-text assessment. Limited studies specifically addressed the burden of disease caused by frTBI. There was an increased risk for both injury and death from frTBI in men, preteenage and teenage youths, minorities, and individuals in firearm-owning households. Further study is required to ascertain if suggested methods of targeted patient screening, firearm-injury prevention counseling, and advocacy of safety-oriented policy tangibly affect rates or outcomes of pediatric frTBI. CONCLUSION: By understanding published epidemiological data and areas of intervention shown to reduce frTBIs, neurosurgeons can become further engaged in public health and prevention rather than strictly treatment after injury