244 research outputs found
Bioengineered embryoids mimic post-implantation development in vitro.
The difficulty of studying post-implantation development in mammals has sparked a flurry of activity to develop in vitro models, termed embryoids, based on self-organizing pluripotent stem cells. Previous approaches to derive embryoids either lack the physiological morphology and signaling interactions, or are unconducive to model post-gastrulation development. Here, we report a bioengineering-inspired approach aimed at addressing this gap. We employ a high-throughput cell aggregation approach to simultaneously coax mouse embryonic stem cells into hundreds of uniform epiblast-like aggregates in a solid matrix-free manner. When co-cultured with mouse trophoblast stem cell aggregates, the resulting hybrid structures initiate gastrulation-like events and undergo axial morphogenesis to yield structures, termed EpiTS embryoids, with a pronounced anterior development, including brain-like regions. We identify the presence of an epithelium in EPI aggregates as the major determinant for the axial morphogenesis and anterior development seen in EpiTS embryoids. Our results demonstrate the potential of EpiTS embryoids to study peri-gastrulation development in vitro
High-Resolution Spectroscopy of FUors
High-resolution spectroscopy was obtained of the FUors FU Ori and V1057 Cyg
between 1995 and 2002 with SOFIN at NOT and with HIRES at Keck I. During those
years FU Ori remained about 1 mag. (in B) below its 1938-39 maximum brightness,
but V1057 Cyg (B ~ 10.5 at peak in 1970-71) faded from about 13.5 to 14.9 and
then recovered slightly. Their photospheric spectra resemble a rotating G0 Ib
supergiant, with v_eq sin i = 70 km/s for FU Ori and 55 km/s for V1057 Cyg. As
V1057 Cyg faded, P Cyg structure in Halpha and the IR CaII lines strengthened
and a complex shortward-displaced shell spectrum increased in strength,
disappeared in 1999, and reappeared in 2001. Night-to-night changes in the wind
structure of FU Ori show evidence of sporadic infall. The strength of P Cyg
absorption varied cyclically with a period of 14.8 days, with phase stability
maintained over 3 seasons, and is believed to be the rotation period. The
structure of the photospheric lines also varies cyclically, but with a period
of 3.54 days. A similar variation may be present in V1057 Cyg. As V1057 Cyg has
faded, the emission lines of a pre-existing low-excitation chromosphere have
emerged, so we believe the `line doubling' in V1057 Cyg is produced by these
central emission cores in the absorption lines, not by orbital motion in an
inclined Keplerian disk. No dependence of v_eq sin i on wavelength or
excitation potential was detected in either star, again contrary to expectation
for a self-luminous accretion disk. Nor are critical lines in the near infrared
accounted for by synthetic disk spectra. A rapidly rotating star near the edge
of stability (Larson 1980), can better explain these observations. FUor
eruptions may not be a property of ordinary TTS, but may be confined to a
special subspecies of rapid rotators having powerful quasi-permanent winds.Comment: 41 pages (including 32 figures and 9 tables); ApJ, in press; author
affiliation, figs. 3 and 9 correcte
The effects of disk and dust structure on observed polarimetric images of protoplanetary disks
Imaging polarimetry is a powerful tool for imaging faint circumstellar
material. For a correct analysis of observations we need to fully understand
the effects of dust particle parameters, as well as the effects of the
telescope, atmospheric seeing, and assumptions about the data reduction and
processing of the observed signal. Here we study the major effects of dust
particle structure, size-dependent grain settling, and instrumental properties.
We performed radiative transfer modeling using different dust particle models
and disk structures. To study the influence of seeing and telescope diffraction
we ran the models through an instrument simulator for the ExPo dual-beam
imaging polarimeter mounted at the 4.2m William Herschel Telescope (WHT).
Particle shape and size have a strong influence on the brightness and
detectability of the disks. In the simulated observations, the central
resolution element also contains contributions from the inner regions of the
protoplanetary disk besides the unpolarized central star. This causes the
central resolution element to be polarized, making simple corrections for
instrumental polarization difficult. This effect strongly depends on the
spatial resolution, so adaptive optics systems are needed for proper
polarization calibration. We find that the commonly employed homogeneous sphere
model gives results that differ significantly from more realistic models. For a
proper analysis of the wealth of data available now or in the near future, one
must properly take the effects of particle types and disk structure into
account. The observed signal depends strongly on the properties of these more
realistic models, thus providing a potentially powerful diagnostic. We conclude
that it is important to correctly understand telescope depolarization and
calibration effects for a correct interpretation of the degree of polarization.Comment: Accepted for publication in A&
The magnetic phase diagram of the frustrated spin chain compound linarite, PbCuSO(OH), as seen by neutron diffraction and H-NMR
We report on a detailed neutron diffraction and H-NMR study on the
frustrated spin-1/2 chain material linarite, PbCuSO(OH), where
competing ferromagnetic nearest neighbor and antiferromagnetic next-nearest
neighbor interactions lead to frustration. From the magnetic Bragg peak
intensity studied down to 60 mK, the magnetic moment per Cu atom is obtained
within the whole magnetic phase diagram for axis. Further, we
establish the detailed configurations of the shift of the SDW propagation
vector in phase V with field and temperature. Finally, combining our neutron
diffraction results with those from a low-temperature/high-field NMR study we
find an even more complex phase diagram close to the quasi-saturation field
suggesting that bound two-magnon excitations are the lowest energy excitations
close to and in the quasi-saturation regime. Qualitatively and
semi-quantitatively, we relate such behavior to exchange anisotropy and
contributions from the Dzyaloshinsky-Moriya interaction to affect the magnetic
properties of linarite
Dilution of the magnetic lattice in the Kitaev candidate -RuCl by Rh doping
Magnetic dilution of a well-established Kitaev candidate system is realized
in the substitutional RuRhCl series (). Optimized
syntheses protocols yield uniformly-doped single crystals and polycrystalline
powders that are isostructural to the parental -RuCl as per X-ray
diffraction. The Rh content is accurately determined by the quantitative
energy-dispersive X-ray spectroscopy technique with standards. We determine the
magnetic phase diagram of RuRhCl for in-plane magnetic fields
from magnetization and specific-heat measurements as a function of and
stacking periodicity, and identify the suppression of the magnetic order at towards a disordered phase, which does not show any clear
signature of freezing into a spin glass. Comparing with previous studies on the
substitution series RuIrCl, we propose that chemical pressure
would contribute to the suppression of magnetic order especially in
RuIrCl and that the zigzag magnetic ground state appears to be
relatively robust with respect to the dilution of the
Kitaev----Heisenberg magnetic lattice. We also discovered a slight
dependence of the magnetic properties on thermal cycling, which would be due to
an incomplete structural transition
Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet alpha-RuCl3
Magnetization and high-resolution x-ray diffraction measurements of the
Kitaev-Heisenberg material alpha-RuCl3 reveal a pressure-induced
crystallographic and magnetic phase transition at a hydrostatic pressure of
p=0.2 GPa. This structural transition into a triclinic phase is characterized
by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of
the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose
a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru
links, causing a sharp increase of the antiferromagnetic exchange interactions.
These combined experimental and computational data show that the Kitaev spin
liquid phase in alpha-RuCl3 strongly competes with the crystallization of spin
singlets into a valence bond solid
Malaria transmission pattern resilience to climatic variability is mediated by insecticide-treated nets
<p>Abstract</p> <p>Background</p> <p>Malaria is an important public-health problem in the archipelago of Vanuatu and climate has been hypothesized as important influence on transmission risk. Beginning in 1988, a major intervention using insecticide-treated bed nets (ITNs) was implemented in the country in an attempt to reduce <it>Plasmodium </it>transmission. To date, no study has addressed the impact of ITN intervention in Vanuatu, how it may have modified the burden of disease, and whether there were any changes in malaria incidence that might be related to climatic drivers.</p> <p>Methods and findings</p> <p>Monthly time series (January 1983 through December 1999) of confirmed <it>Plasmodium falciparum </it>and <it>Plasmodium vivax </it>infections in the archipelago were analysed. During this 17 year period, malaria dynamics underwent a major regime shift around May 1991, following the introduction of bed nets as a control strategy in the country. By February of 1994 disease incidence from both parasites was reduced by at least 50%, when at most 20% of the population at risk was covered by ITNs. Seasonal cycles, as expected, were strongly correlated with temperature patterns, while inter-annual cycles were associated with changes in precipitation. Following the bed net intervention, the influence of environmental drivers of malaria dynamics was reduced by 30–80% for climatic forces, and 33–54% for other factors. A time lag of about five months was observed for the qualitative change ("regime shift") between the two parasites, the change occurring first for <it>P. falciparum</it>. The latter might be explained by interspecific interactions between the two parasites within the human hosts and their distinct biology, since <it>P. vivax </it>can relapse after a primary infection.</p> <p>Conclusion</p> <p>The Vanuatu ITN programme represents an excellent example of implementing an infectious disease control programme. The distribution was undertaken to cover a large, local proportion (~80%) of people in villages where malaria was present. The successful coverage was possible because of the strategy for distribution of ITNs by prioritizing the free distribution to groups with restricted means for their acquisition, making the access to this resource equitable across the population. These results emphasize the need to implement infectious disease control programmes focusing on the most vulnerable populations.</p
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