10,734 research outputs found
Correlating Cell Behavior with Tissue Topology in Embryonic Epithelia
Measurements on embryonic epithelial tissues in a diverse range of organisms
have shown that the statistics of cell neighbor numbers are universal in
tissues where cell proliferation is the primary cell activity. Highly
simplified non-spatial models of proliferation are claimed to accurately
reproduce these statistics. Using a systematic critical analysis, we show that
non-spatial models are not capable of robustly describing the universal
statistics observed in proliferating epithelia, indicating strong spatial
correlations between cells. Furthermore we show that spatial simulations using
the Subcellular Element Model are able to robustly reproduce the universal
histogram. In addition these simulations are able to unify ostensibly divergent
experimental data in the literature. We also analyze cell neighbor statistics
in early stages of chick embryo development in which cell behaviors other than
proliferation are important. We find from experimental observation that cell
neighbor statistics in the primitive streak region, where cell motility and
ingression are also important, show a much broader distribution. A non-spatial
Markov process model provides excellent agreement with this broader histogram
indicating that cells in the primitive streak may have significantly weaker
spatial correlations. These findings show that cell neighbor statistics provide
a potentially useful signature of collective cell behavior.Comment: PLoS one 201
A Parametric Study of Radiative Dipole Body Array Coil for 7 Tesla MRI
In this contribution we present numerical and experimental results of a
parametric quantitative study of radiative dipole antennas in a phased array
configuration for efficient body magnetic resonance imaging at 7T via parallel
transmission. For magnetic resonance imaging (MRI) at ultrahigh fields (7T and
higher) dipole antennas are commonly used in phased arrays, particularly for
body imaging targets. This study reveals the effects of dipole positioning in
the array (elevation of dipoles above the subject and inter-dipole spacing) on
their mutual coupling, per and per maximum local
SAR efficiencies as well as the RF-shimming capability. The numerical and
experimental results are obtained and compared for a homogeneous phantom as
well as for a real human models confirmed by in-vivo experiments
The role of the cortical cytoskeleton
We generated Dictyostelium double mutants lacking the two F-actin crosslinking proteins alpha-actinin and gelation factor by inactivating the corresponding genes via homologous recombination. Here we investigated the consequences of these deficiencies both at the single cell level and at the multicellular stage. We found that loss of both proteins severely affected growth of the mutant cells in shaking suspension, and led to a reduction of cell size from 12 microns in wild-type cells to 9 microns in mutant cells. Moreover the cells did not exhibit the typical polarized morphology of aggregating Dictyostelium cells but had a more rounded cell shape, and also exhibited an increased sensitivity towards osmotic shock and a reduced rate of phagocytosis. Development was heavily impaired and never resulted in the formation of fruiting bodies. Expression of developmentally regulated genes and the final developmental stages that were reached varied, however, with the substrata on which the cells were deposited. On phosphate buffered agar plates the cells were able to form tight aggregates and mounds and to express prespore and prestalk cell specific genes. Under these conditions the cells could perform chemotactic signalling and cell behavior was normal at the onset of multicellular development as revealed by time-lapse video microscopy. Double mutant cells were motile but speed was reduced by approximately 30% as compared to wild type. These changes were reversed by expressing the gelation factor in the mutant cells. We conclude that the actin assemblies that are formed and/or stabilized by both F-actin crosslinking proteins have a protective function during osmotic stress and are essential for proper cell shape and motility
Controls on the recent speed-up of Jakobshavn Isbrae, West Greenland
This is the published version, also available here: http://dx.doi.org/10.3189/002214311797409776.Jakobshavn Isbrae, West Greenland, underwent a large, rapid and not well understood change in flow dynamics in 1998, leading to a doubling of its ice discharge rates. We calculate the width-averaged forces controlling flow of Jakobshavn Isbrae in 1995, 2000 and 2005 to elucidate processes responsible for this change in flow speed. In contrast to earlier suggestions, we conclude that the observed acceleration was not caused by the loss of back-stress due to weakening and subsequent break-up of the floating ice tongue alone. Gradients in longitudinal stress are small at all times considered (∼3% of the driving stress) and basal and lateral drag provide resistance to flow. Over the 10 year period considered, the average driving stress increased by 20 kPa, which was balanced by a comparable increase in lateral drag. We surmise that the velocity changes resulted from weakening of the ice in the lateral shear margins and perhaps a change in properties at the bed. Possible mechanisms for weakening of ice in the lateral shear margins include cryo-hydrologic warming of subsurface ice in the ablation zone and hydraulic weakening due to higher water content of ice in the shear margins
Controls on the recent speed-up of Jakobshavn Isbræ, West Greenland
This is the publisher's version, also available electronically from "http://www.ingentaconnect.com/".Jakobshavn Isbrae, West Greenland, underwent a large, rapid and not well understood change in flow dynamics in 1998, leading to a doubling of its ice discharge rates. We calculate the width-averaged forces controlling flow of Jakobshavn Isbrae in 1995, 2000 and 2005 to elucidate processes responsible for this change in flow speed. In contrast to earlier suggestions, we conclude that the observed acceleration was not caused by the loss of back-stress due to weakening and subsequent break-up of the floating ice tongue alone. Gradients in longitudinal stress are small at all times considered (∼3% of the driving stress) and basal and lateral drag provide resistance to flow. Over the 10 year period considered, the average driving stress increased by 20 kPa, which was balanced by a comparable increase in lateral drag. We surmise that the velocity changes resulted from weakening of the ice in the lateral shear margins and perhaps a change in properties at the bed. Possible mechanisms for weakening of ice in the lateral shear margins include cryo-hydrologic warming of subsurface ice in the ablation zone and hydraulic weakening due to higher water content of ice in the shear margins
Modeling Spitzer observations of VV Ser. I. The circumstellar disk of a UX Orionis star
We present mid-infrared Spitzer-IRS spectra of the well-known UX Orionis star
VV Ser. We combine the Spitzer data with interferometric and spectroscopic data
from the literature covering UV to submillimeter wavelengths. The full set of
data are modeled by a two-dimensional axisymmetric Monte Carlo radiative
transfer code. The model is used to test the prediction of (Dullemond et al.
2003) that disks around UX Orionis stars must have a self-shadowed shape, and
that these disks are seen nearly edge-on, looking just over the edge of a
puffed-up inner rim, formed roughly at the dust sublimation radius. We find
that a single, relatively simple model is consistent with all the available
observational constraints spanning 4 orders of magnitude in wavelength and
spatial scales, providing strong support for this interpretation of UX Orionis
stars. The grains in the upper layers of the puffed-up inner rim must be small
(0.01-0.4 micron) to reproduce the colors (R_V ~ 3.6) of the extinction events,
while the shape and strength of the mid-infrared silicate emission features
indicate that grains in the outer disk (> 1-2 AU) are somewhat larger (0.3-3.0
micron). From the model fit, the location of the puffed-up inner rim is
estimated to be at a dust temperature of 1500 K or at 0.7-0.8 AU for small
grains. This is almost twice the rim radius estimated from near-infrared
interferometry. A best fitting model for the inner rim in which large grains in
the disk mid-plane reach to within 0.25 AU of the star, while small grains in
the disk surface create a puffed-up inner rim at ~0.7-0.8 AU, is able to
reproduce all the data, including the near-infrared visibilities. [Abstract
abridged]Comment: 12 pages, accepted for publication in Ap
High-Resolution Spectroscopy in Tr37: Gas Accretion Evolution in Evolved Dusty Disks
Using the Hectochelle multifiber spectrograph, we have obtained
high-resolution (R~34,000) spectra in the Halpha region for a large number of
stars in the 4 Myr-old cluster Tr 37, containing 146 previously known members
and 26 newly identified ones. We present the Halpha line profiles of all
members, compare them to our IR observations of dusty disks (2MASS/JHK + IRAC +
MIPS 24 micron), use the radial velocities as a membership criterion, and
calculate the rotational velocities. We find a good correlation between the
accretion-broadened profiles and the presence of protoplanetary disks, noting
that a small fraction of the accreting stars presents broad profiles with
Halpha equivalent widths smaller than the canonical limit separating CTTS and
WTTS. The number of strong accretors appears to be lower than in younger
regions, and a large number of CTTS have very small accretion rates
(dM/dt<10^{-9} Msun/yr). Taking into account that the spectral energy
distributions are consistent with dust evolution (grain growth/settling) in the
innermost disk, this suggests a parallel evolution of the dusty and gaseous
components. We also observe that about half of the "transition objects" (stars
with no IR excesses at wavelengths shorter than ~6 micron) do not show any
signs of active accretion, whereas the other half is accreting with accretion
rates <10^{-9} Msun/yr. These zero or very low accretion rates reveal important
gas evolution and/or gas depletion in the innermost disk, which could be
related to grain growth up to planetesimal or even planet sizes. Finally, we
examine the rotational velocities of accreting and non accreting stars, finding
no significant differences that could indicate disk locking at these ages.Comment: 51 pages, 13 (reduced resolution) figures, 2 tables. AJ in pres
Role of snow and glacier melt in controlling river hydrology in Liddar watershed (western Himalaya) under current and future climate
This is the publisher's version, also available electronically from http://onlinelibrary.wiley.com/doi/10.1029/2011WR011590/abstract.[1] Snowmelt and icemelt are believed to be important regulators of seasonal discharge of Himalayan rivers. To analyze the long term contribution of snowmelt and glacier/icemelt to river hydrology we apply a water budget model to simulate hydrology of the Liddar watershed in the western Himalaya, India for the 20th century (1901–2010) and future IPCC A1B climate change scenario. Long term (1901–2010) temperature and precipitation data in this region show a warming trend (0.08°C yr−1) and an increase in precipitation (0.28 mm yr−1), with a significant variability in seasonal trends. In particular, winter months have undergone the most warming, along with a decrease in precipitation rates; precipitation has increased throughout the spring. These trends have accelerated the melting and rapid disappearance of snow, causing a seasonal redistribution in the availability of water. Our model results show that about 60% of the annual runoff of the Liddar watershed is contributed from the snowmelt, while only 2% is contributed from glacier ice. The climate trend observed from the 1901 to 2010 time period and its impact on the availability of water will become significantly worse under the IPCC climate change scenarios. Our results suggest that there is a significant shift in the timing and quantity of water runoff in this region of the Himalayas due to snow distribution and melt. With greatly increased spring runoff and its reductions in summer potentially leading to reduced water availability for irrigation agriculture in summer
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