3,025 research outputs found
Critical properties of the one-dimensional spin-1/2 antiferromagnetic Heisenberg model in the presence of a uniform field
In the presence of a uniform field the one-dimensional spin-
antiferromagnetic Heisenberg model develops zero frequency excitations at
field-dependent 'soft mode' momenta. We determine three types of critical
quantities, which we extract from the finite-size dependence of the lowest
excitation energies, the singularities in the static structure factors and the
infrared singularities in the dynamical structure factors at the soft mode
momenta. We also compare our results with the predictions of conformal field
theory.Comment: 12 pages, REVTEX, 7 figures, submitted to Physical Review
VEGF guides angiogenic sprouting utilizing endothelial tip cell filopodia
Vascular endothelial growth factor (VEGF-A) is a major regulator of blood vessel formation and function. it controls several processes in endothelial cells, such as proliferation, survival, and migration, but it is not known how these are coordinately regulated to result in more complex morphogenetic events, such as tubular sprouting, fusion, and network formation. We show here that VEGF-A controls angiogenic sprouting in the early postnatal retina by guiding filopodial extension from specialized endothelial cells situated at the tips of the vascular sprouts. The tip cells respond to VEGF-A only by guided migration; the proliferative response to VEGF-A occurs in the sprout stalks. These two cellular responses are both mediated by agonistic activity of VEGF-A on VEGF receptor 2. Whereas tip cell migration depends on a gradient of VEGF-A, proliferation is regulated by its concentration. Thus, vessel patterning during retinal angiogenesis depends on the balance between two different qualities of the extracellular VEGF-A distribution, which regulate distinct cellular responses in defined populations of endothelial cells
Metamagnetism in the XXZ model with next-to-nearest-neighbor coupling
We investigate groundstate energies and magnetization curves in the one
dimensional XXZ-model with next to nearest neighbour coupling and
anisotropy () at T=0. In between the familiar
ferro- and antiferromagnetic phase we find a transition region -- called
metamagnetic phase -- where the magnetization curve is discontinuous at a
critical field .Comment: LaTeX file (text) + 5 PS files (5 figures
Imaging Transient Blood Vessel Fusion Events by Correlative Volume Electron Microscopy
Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 - August 5, 201
Mean Curvature Flow of Spacelike Graphs
We prove the mean curvature flow of a spacelike graph in of a map from a closed Riemannian
manifold with to a complete Riemannian manifold
with bounded curvature tensor and derivatives, and with
sectional curvatures satisfying , remains a spacelike graph,
exists for all time, and converges to a slice at infinity. We also show, with
no need of the assumption , that if , or if and
, constant, any map is trivially
homotopic provided where
, in case , and
in case . This largely extends some known results for
constant and compact, obtained using the Riemannian structure
of , and also shows how regularity theory on the mean
curvature flow is simpler and more natural in pseudo-Riemannian setting then in
the Riemannian one.Comment: version 5: Math.Z (online first 30 July 2010). version 4: 30 pages:
we replace the condition by the the weaker one .
The proofs are essentially the same. We change the title to a shorter one. We
add an applicatio
Importance of exposure route for behavioural responses in Lumbriculus variegatus Müller (Oligochaeta: Lumbriculida) in short-term exposures to Pb
Abstract Goal, Scope and Background Lumbriculus variegatus Müller (Oligochaeta), a common freshwater sediment-dweller, has frequently been used in toxicokinetic studies, although has been less used in ecotoxicity tests. Methods For the first time the Multispecies Freshwater Biomonitor® (MFB) was applied in a short-term whole-sediment toxicity test. The MFB automatically and quantitatively recorded the spontaneous locomotory behaviour of Lumbriculus variegatus in exposures with two compartments, water and sediment. The study questioned, whether the animals altered their locomotion depending on the compartment which was spiked with lead (Pb). Results and Discussion As in the exposures to Pb-contaminated water/clean sediment, the animals exposed to Pb-contaminated sediment/clean water showed higher activities in intermediate Pb-concentrations. This indicates, that spontaneous locomotory activity is affected by Pb-concentrations at sublethal levels regardless of whether the Pb-concentration is found in the water or in the sediment, because these animals use both environmental compartments simultaneously. However, within the same Pb-levels, the animals showed higher locomotory activity in contaminated water compared with contaminated sediment. This indicates a possible tendency to withdraw from (‘avoidance’) contaminated water into the clean sediment compartment, whereas there was no withdrawal from contaminated sediment into clean water. The latter might be explained by the fact that withdrawal from sediment to water might increase the risk of predation and drift in nature, whereas retracting to sediment might provide shelter. Conclusions The study showed that spontaneous locomotory responses of L. variegatus to Pb depend on whether the water or sediment is contaminated. The study also concluded that the Multispecies Freshwater Biomonitor® can be applied effectively in sediment toxicity testing. Recommendations and Perspectives More emphasis should be given to the interactions of water/sediment in sediment ecotoxicity tests to better simulate field conditions and increase ecological realism in risk assessment, especially as quantitative recording methods exisit
Magnetization cusp singularities of frustrated Kondo necklace model
Magnetization processes of frustrated Kondo necklace model are studied by
means of a density matrix renormalization group (DMRG) method and an elementary
band theory based on a bond-operator formalism. The DMRG calculations clearly
show the cusp singularity in a low-magnetization region () besides
that in a high-magnetization region () which is expected from previous
studies on the magnetization curve of the Majumdar-Ghosh model. An appearance
mechanism of the low-magnetization cusp is interpreted in terms of a
double-well shape of a low-energy band arising from frustrations between
nearest- and next-nearest-neighbor interactions. We also discuss critical
behaviors of magnetization near the cusp and obtain a phase diagram showing
whether the cusp appears in the magnetization curve or not.Comment: 8 pages, 7 figures. to be published in J. Phys. Soc. Jp
Elevated alpha-synuclein caused by SNCA gene triplication impairs neuronal differentiation and maturation in Parkinson's patient-derived induced pluripotent stem cells
We have assessed the impact of α-synuclein overexpression on the differentiation potential and phenotypic signatures of two neural-committed induced pluripotent stem cell lines derived from a Parkinson´s disease patient with a triplication of the human SNCA genomic locus. In parallel, comparative studies were performed on two control lines derived from healthy individuals and lines generated from the patient iPS-derived neuroprogenitor lines infected with a lentivirus incorporating a small hairpin RNA to knock down the SNCA mRNA. The SNCA triplication lines exhibited a reduced capacity to differentiate into dopaminergic or GABAergic neurons and decreased neurite outgrowth and lower neuronal activity compared with control cultures. This delayed maturation phenotype was confirmed by gene expression profiling, which revealed a significant reduction in mRNA for genes implicated in neuronal differentiation such as delta-like homolog 1 (DLK1), gamma-aminobutyric acid type B receptor subunit 2 (GABABR2), nuclear receptor related 1 protein (NURR1), G-protein-regulated inward-rectifier potassium channel 2 (GIRK-2) and tyrosine hydroxylase (TH). The differentiated patient cells also demonstrated increased autophagic flux when stressed with chloroquine. We conclude that a two-fold overexpression of α-synuclein caused by a triplication of the SNCA gene is sufficient to impair the differentiation of neuronal progenitor cells, a finding with implications for adult neurogenesis and Parkinson´s disease progression, particularly in the context of bioenergetic dysfunction.Fil: Oliveira, L. M. A.. Max-Planck-Institut für biophysikalische Chemie; AlemaniaFil: Falomir Lockhart, Lisandro Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Bioquímicas de La Plata "Prof. Dr. Rodolfo R. Brenner". Universidad Nacional de la Plata. Facultad de Ciencias Médicas. Instituto de Investigaciones Bioquímicas de La Plata ; Argentina. Max-Planck-Institut für biophysikalische Chemie; AlemaniaFil: Botelho, M. G.. Max-Planck-Institut für biophysikalische Chemie; Alemania. Universidade Federal do Rio de Janeiro; BrasilFil: Lin, K. H.. Max-Planck-Institut für biophysikalische Chemie; AlemaniaFil: Wales, P.. Universität Göttingen; AlemaniaFil: Koch, J. C.. Universität Göttingen; AlemaniaFil: Gerhardt, Elizabeth. Universität Göttingen; AlemaniaFil: Taschenberger, H.. Max-Planck-Institut für biophysikalische Chemie; AlemaniaFil: Outeiro, T. F.. Universität Göttingen; AlemaniaFil: Lingor, P.. Universität Göttingen; AlemaniaFil: Schüele, B.. The Parkinson’s Institute; Estados UnidosFil: Arndt Jovin, D. J.. Max-Planck-Institut für biophysikalische Chemie; AlemaniaFil: Jovin, T. M.. Max-Planck-Institut für biophysikalische Chemie; Alemani
Artery-vein specification in the zebrafish trunk is pre-patterned by heterogeneous Notch activity and balanced by flow-mediated fine tuning
How developing vascular networks acquire the right balance of arteries, veins and lymphatic vessels to efficiently supply and drain tissues is poorly understood. In zebrafish embryos, the robust and regular 50:50 global balance of intersegmental veins and arteries that form along the trunk, prompts the intriguing question how the organism keeps "count". Previous studies suggest that the ultimate fate of an intersegmental vessel (ISV) is determined by the identity of the approaching secondary sprout emerging from the posterior cardinal vein (PCV). Here, we show that the formation of a balanced trunk vasculature involves an early heterogeneity in endothelial cell (EC) behavior and Notch signaling activity in the seemingly identical primary ISVs that is independent of secondary sprouting and flow. We show that Notch signaling mediates the local patterning of ISVs, and an adaptive flow-mediated mechanism subsequently fine-tunes the global balance of arteries and veins along the trunk. We propose that this dual mechanism provides the adaptability required to establish a balanced network of arteries, veins and lymphatic vessels
Global existence problem in -Gowdy symmetric IIB superstring cosmology
We show global existence theorems for Gowdy symmetric spacetimes with type
IIB stringy matter. The areal and constant mean curvature time coordinates are
used. Before coming to that, it is shown that a wave map describes the
evolution of this system
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