4,069 research outputs found
The early stages of heart development: insights from chicken embryos
The heart is the first functioning organ in the developing embryo and the detailed understanding of the molecular and cellular mechanisms involved in its formation provides insights into congenital malformations affecting its function and therefore the survival of the organism. Because many developmental mechanisms are highly conserved, it is possible to extrapolate from observations made in invertebrate and vertebrate model organisms to human. This review will highlight the contributions made through studying heart development in avian embryos, particularly the chicken. The major advantage of chick embryos is their accessibility for surgical manipulations and functional interference approaches, both gain- and loss-of-function. In addition to experiments performed in ovo, the dissection of tissues for ex vivo culture, genomic or biochemical approaches, is straightforward. Furthermore, embryos can be cultured for time-lapse imaging, which enables tracking of fluorescently labeled cells and detailed analyses of tissue morphogenesis. Owing to these features, investigations in chick embryos have led to important discoveries, often complementing genetic studies in mouse and zebrafish. As well as including some historical aspects, we cover here some of the crucial advances made in understanding of early heart development using the chicken model
T-Duality for Coset Models
We construct dual Lagrangians for models in two space-time dimensions
for arbitrary Lie groups and . Our approach does not require
choosing coordinates on , and allows for a natural generalization to
Lie-Poisson duality. For the case where the target metric on is induced
from the invariant metric on , the dual system is a gauged Higgs model, with
a nonconstant metric and a coupling to an antisymmetric tensor. The dynamics
for the gauge connection is governed by a -term. Lie-Poisson duality is
relevant once we allow for a more general class of target metrics, as well as
for couplings to an antisymmetric tensor, in the primary theory. Then the dual
theory is written on a group dual to , and the gauge group
(which, in general, is not a subgroup of ) acts nonlinearly on
. The dual system therefore gives a nonlinear realization of a gauge
theory. All dual descriptions are shown to be canonically equivalent to the
corresponding primary descriptions, at least at the level of the current
algebra.Comment: 21 p
Target Space Duality I: General Theory
We develop a systematic framework for studying target space duality at the
classical level. We show that target space duality between manifolds M and
Mtilde arises because of the existence of a very special symplectic manifold.
This manifold locally looks like M x Mtilde and admits a double fibration. We
analyze the local geometric requirements necessary for target space duality and
prove that both manifolds must admit flat orthogonal connections. We show how
abelian duality, nonabelian duality and Poisson-Lie duality are all special
cases of a more general framework. As an example we exhibit new (nonlinear)
dualities in the case M = Mtilde = R^n.Comment: LaTeX, 29 pages, 1 eps figure. Added a couple of references and
corrected a couple of typos. An FAQ that discusses some subtle points may be
found at <http://www.physics.miami.edu/~alvarez/papers/duality/
Broadband X-ray spectral analysis of the Seyfert 1 galaxy GRS 1734-292
We discuss the broadband X-ray spectrum of GRS 1734-292 obtained from
non-simultaneous XMM-Newton and NuSTAR observations, performed in 2009 and
2014, respectively. GRS1734-292 is a Seyfert 1 galaxy, located near the
Galactic plane at . The NuSTAR spectrum ( keV) is dominated by
a primary power-law continuum with and a high-energy
cutoff keV, one of the lowest measured by NuSTAR in a
Seyfert galaxy. Comptonization models show a temperature of the coronal plasma
of keV and an optical depth, assuming a slab
geometry, or a similar temperature and
assuming a spherical geometry. The 2009 XMM-Newton
spectrum is well described by a flatter intrinsic continuum
() and one absorption line due to Fe\textsc{XXV}
K produced by a warm absorber. Both data sets show a modest iron
K emission line at keV and the associated Compton reflection, due
to reprocessing from neutral circumnuclear material
Can Facebook use Induce Well-Being?
[[abstract]]Over the past few decades, the widespread phenomenon of Internet abuse has gained attention from the public, academia, and the media. In a departure from this negative viewpoint, however, researchers and educators have devoted considerable effort in attempting to understand the influence of online communication on people's psychological well-being. This study focuses specifically on Facebook, and proposes a research model to examine the relationships among Facebook use, online social support, general social support, and psychological well-being.
Our results show that using Facebook helped college students to obtain online social support, and that online social support is an extension of general social support. However, although general social support contributes to well-being, online social support appears to have little direct effect on well-being. The relationship between online social support and well-being is mediated through the factor of general social support.[[notice]]補正完畢[[incitationindex]]SSCI[[cooperationtype]]國內[[booktype]]紙本[[booktype]]電子
QCD-scale modified-gravity universe
A possible gluon-condensate-induced modified-gravity model with f(R) \propto
|R|^{1/2} has been suggested previously. Here, a simplified version is
presented using the constant flat-spacetime equilibrium value of the QCD gluon
condensate and a single pressureless matter component (cold dark matter, CDM).
The resulting dynamical equations of a spatially-flat and homogeneous
Robertson-Walker universe are solved numerically. This simple empirical model
allows, in fact, for a careful treatment of the boundary conditions and does
not require a further scaling analysis as the original model did. Reliable
predictions are obtained for several observable quantities of the homogeneous
model universe. In addition, the estimator E_{G}, proposed by Zhang et al. to
search for deviations from standard Einstein gravity, is calculated for linear
sub-horizon matter-density perturbations. The QCD-scale modified-gravity
prediction for E_{G}(z) differs from that of the LambdaCDM model by about \pm
10 % depending on the redshift z.Comment: 24 pages; v7: published versio
The Discovery of Two New Satellites of Pluto
Pluto's first known moon, Charon, was discovered in 1978 (Christy 1978) and
has a diameter about half that of Pluto (Buie 1992,Young 1994, Sicardy 2005),
which makes it larger relative to its primary than any other moon in the Solar
System. Previous searches for other satellites around Pluto have been
unsuccessful (Stern 1991, Stern 1994, Stern 2003), but they were not sensitive
to objects <=150 km in diameter and there are no fundamental reasons why Pluto
should not have more satellites (Stern 1994). Here we report the discovery of
two additional moons around Pluto, provisionally designated S/2005 P1
(hereafter P1) and S/2005 P2 (hereafter P2), which makes Pluto the first Kuiper
belt object (KBO) known to have multiple satellites. These new satellites are
much smaller than Charon (diameter~1200 km), with P1 ranging in diameter from
60-165 km depending on the surface reflectivity, and P2 about 20% smaller than
P1. Although definitive orbits cannot be derived, both new satellites appear to
be moving in circular orbits in the same orbital plane as Charon, with orbital
periods of ~38 days (P1) and ~25 days (P2). The implications of the discovery
of P1 and P2 for the origin and evolution of the Pluto system, and for the
satellite formation process in the Kuiper belt, are discussed in a companion
paper (Stern 2006).Comment: Preprint of a paper accepted for publication in the journal Natur
Robust Neutrino Constraints by Combining Low Redshift Observations with the CMB
We illustrate how recently improved low-redshift cosmological measurements
can tighten constraints on neutrino properties. In particular we examine the
impact of the assumed cosmological model on the constraints. We first consider
the new HST H0 = 74.2 +/- 3.6 measurement by Riess et al. (2009) and the
sigma8*(Omegam/0.25)^0.41 = 0.832 +/- 0.033 constraint from Rozo et al. (2009)
derived from the SDSS maxBCG Cluster Catalog. In a Lambda CDM model and when
combined with WMAP5 constraints, these low-redshift measurements constrain sum
mnu<0.4 eV at the 95% confidence level. This bound does not relax when allowing
for the running of the spectral index or for primordial tensor perturbations.
When adding also Supernovae and BAO constraints, we obtain a 95% upper limit of
sum mnu<0.3 eV. We test the sensitivity of the neutrino mass constraint to the
assumed expansion history by both allowing a dark energy equation of state
parameter w to vary, and by studying a model with coupling between dark energy
and dark matter, which allows for variation in w, Omegak, and dark coupling
strength xi. When combining CMB, H0, and the SDSS LRG halo power spectrum from
Reid et al. 2009, we find that in this very general model, sum mnu < 0.51 eV
with 95% confidence. If we allow the number of relativistic species Nrel to
vary in a Lambda CDM model with sum mnu = 0, we find Nrel =
3.76^{+0.63}_{-0.68} (^{+1.38}_{-1.21}) for the 68% and 95% confidence
intervals. We also report prior-independent constraints, which are in excellent
agreement with the Bayesian constraints.Comment: 19 pages, 6 figures, submitted to JCAP; v2: accepted version. Added
section on profile likelihood for Nrel, improved plot
Male breast cancer
Male breast cancer (MBC) is a rare disease representing less than 1% of all breast cancers (BC) and less than 1% of cancers in men. Age at presentation is mostly in the late 60s. MBC is recognized as an estrogen-driven disease, specifically related to hyperestrogenism. About 20% of MBC patients have family history for BC. Mutations in BRCA1 and, predominantly, BRCA2, account for approximately 10% of MBC cases.
Because of its rarity, MBC is often compared with female BC (FBC). Based on age-frequency distribution, age-specific incidence rate patterns and prognostic factors profiles, MBC is considered similar to late-onset, postmenopausal estrogen/progesterone receptor positive (ER+/PR+) FBC. However, clinical and pathological characteristics of MBC do not exactly overlap FBC. Compared with FBC, MBC has been reported to occur later in life, present at a higher stage, and display lower histologic grade, with a higher proportion of ER+ and PR+ tumors.
Although rare, MBC remains a substantial cause for morbidity and mortality in men, probably because of its occurrence in advanced age and delayed diagnosis. Diagnosis and treatment of MBC generally is similar to that of FBC. Men tend to be treated with mastectomy rather than breast-conserving surgery. The backbone of adjuvant therapy or palliative treatment for advanced disease is endocrine, mostly tamoxifen.
Use of FBC-based therapy led to the observation that treatment outcomes for MBC are worse and that survival rates for MBC do not improve like FBC. These different outcomes may suggest a non-appropriate utilization of treatments and that different underlying pathogenetic mechanisms may exist between male and female BC
X-ray selection of Compton Thick AGN at high redshift
Compton Thick (CT) AGN are a key ingredient of Cosmic X-ray Background (CXB)
synthesis models, but are still an elusive component of the AGN population
beyond the local Universe. Multi-wavelength surveys are the only way to find
them at z > 0.1, and a deep X-ray coverage is crucial in order to clearly
identify them among star forming galaxies. As an example, the deep and wide
COSMOS survey allowed us to select a total of 34 CT sources. This number is
computed from the 64 nominal CT candidates, each counted for its N H
probability distribution function. For each of these sources, rich
multi-wavelength information is available, and is used to confirm their
obscured nature, by comparing the expected AGN luminosity from spectral energy
distribution fitting, with the absorption-corrected X-ray luminosity. While
Chandra is more efficient, for a given exposure, in detecting CT candidates in
current surveys (by a factor ~2), deep XMM-Newton pointings of bright sources
are vital to fully characterize their properties: NH distribution above 10^25
cm^-2, reflection intensity etc., all crucial parameters of CXB models. Since
luminous CT AGN at high redshift are extremely rare, the future of CT studies
at high redshift will have to rely on the large area surveys currently
underway, such as XMM-XXL and Stripe82, and will then require dedicated
follow-up with XMM-Newton, while waiting for the advent of the ESA mission
Athena.Comment: 6 pages, 7 figures, to appear in Astronomische Nachrichten, presented
at the XMM-Newton: The Next Decade conference, ESAC, Madrid, Spain, 9 - 11
May 201
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