13,649 research outputs found
Cardiovascular ephrinB2 function is essential for embryonic angiogenesis
EphrinB2, a transmembrane ligand of EphB receptor tyrosine kinases, is specifically expressed in arteries. In ephrinB2 mutant embryos, there is a complete arrest of angiogenesis. However, ephrinB2 expression is not restricted to vascular endothelial cells, and it has been proposed that its essential function may be exerted in adjacent mesenchymal cells. We have generated mice in which ephrinB2 is specifically deleted in the endothelium and endocardium of the developing vasculature and heart. We find that such a vascular-specific deletion of ephrinB2 results in angiogenic remodeling defects identical to those seen in the conventional ephrinB2 mutants. These data indicate that ephrinB2 is required specifically in endothelial and endocardial cells for angiogenesis, and that ephrinB2 expression in perivascular mesenchyme is not sufficient to compensate for the loss of ephrinB2 in these vascular cells
Reduction of computer usage costs in predicting unsteady aerodynamic loadings caused by control surface motions: Analysis and results
Results of theoretical and numerical investigations conducted to develop economical computing procedures were applied to an existing computer program that predicts unsteady aerodynamic loadings caused by leading and trailing edge control surface motions in subsonic compressible flow. Large reductions in computing costs were achieved by removing the spanwise singularity of the downwash integrand and evaluating its effect separately in closed form. Additional reductions were obtained by modifying the incremental pressure term that account for downwash singularities at control surface edges. Accuracy of theoretical predictions of unsteady loading at high reduced frequencies was increased by applying new pressure expressions that exactly satisified the high frequency boundary conditions of an oscillating control surface. Comparative computer result indicated that the revised procedures provide more accurate predictions of unsteady loadings as well as providing reduction of 50 to 80 percent in computer usage costs
Persistence in fluctuating environments
Understanding under what conditions interacting populations, whether they be
plants, animals, or viral particles, coexist is a question of theoretical and
practical importance in population biology. Both biotic interactions and
environmental fluctuations are key factors that can facilitate or disrupt
coexistence. To better understand this interplay between these deterministic
and stochastic forces, we develop a mathematical theory extending the nonlinear
theory of permanence for deterministic systems to stochastic difference and
differential equations. Our condition for coexistence requires that there is a
fixed set of weights associated with the interacting populations and this
weighted combination of populations' invasion rates is positive for any
(ergodic) stationary distribution associated with a subcollection of
populations. Here, an invasion rate corresponds to an average per-capita growth
rate along a stationary distribution. When this condition holds and there is
sufficient noise in the system, we show that the populations approach a unique
positive stationary distribution. Moreover, we show that our coexistence
criterion is robust to small perturbations of the model functions. Using this
theory, we illustrate that (i) environmental noise enhances or inhibits
coexistence in communities with rock-paper-scissor dynamics depending on
correlations between interspecific demographic rates, (ii) stochastic variation
in mortality rates has no effect on the coexistence criteria for discrete-time
Lotka-Volterra communities, and (iii) random forcing can promote genetic
diversity in the presence of exploitative interactions.Comment: 25 page
On continuum modeling of sputter erosion under normal incidence: interplay between nonlocality and nonlinearity
Under specific experimental circumstances, sputter erosion on semiconductor
materials exhibits highly ordered hexagonal dot-like nanostructures. In a
recent attempt to theoretically understand this pattern forming process, Facsko
et al. [Phys. Rev. B 69, 153412 (2004)] suggested a nonlocal, damped
Kuramoto-Sivashinsky equation as a potential candidate for an adequate
continuum model of this self-organizing process. In this study we theoretically
investigate this proposal by (i) formally deriving such a nonlocal equation as
minimal model from balance considerations, (ii) showing that it can be exactly
mapped to a local, damped Kuramoto-Sivashinsky equation, and (iii) inspecting
the consequences of the resulting non-stationary erosion dynamics.Comment: 7 pages, 2 Postscript figures, accepted by Phys. Rev. B corrected
typos, few minor change
Resonance energy transfer from a fluorescent dye molecule to plasmon and electron-hole excitations of a metal nanoparticle
We study the distance dependence of the rate of electronic excitation energy
transfer from a dye molecule to a metal nanoparticle. Using the spherical
jellium model, we evaluate the rates corresponding to the excitation of l = 1,
2, and 3 modes of the nanoparticle. Our calculation takes into account both the
electron-hole pair and the plasmon excitations of the nanoparticle. The rate
follows conventional R^-6 dependence at large distances while small deviations
from this behavior are observed at shorter distances. Within the framework of
the jellium model, it is not possible to attribute the experimentally observed
d^-4 dependence of the rate to energy transfer to plasmons or e-h pair
excitations.Comment: 4 figure
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