554 research outputs found
Vacuum-UV spectroscopy of interstellar ice analogs. II. Absorption cross-sections of nonpolar ice molecules
Dust grains in cold circumstellar regions and dark-cloud interiors at 10-20 K
are covered by ice mantles. A nonthermal desorption mechanism is invoked to
explain the presence of gas-phase molecules in these environments, such as the
photodesorption induced by irradiation of ice due to secondary ultraviolet
photons. To quantify the effects of ice photoprocessing, an estimate of the
photon absorption in ice mantles is required. In a recent work, we reported the
vacuum-ultraviolet (VUV) absorption cross sections of nonpolar molecules in the
solid phase. The aim was to estimate the VUV-absorption cross sections of
nonpolar molecular ice components, including CH4, CO2, N2, and O2. The column
densities of the ice samples deposited at 8 K were measured in situ by infrared
spectroscopy in transmittance. VUV spectra of the ice samples were collected in
the 120-160 nm (10.33-7.74 eV) range using a commercial microwave-discharged
hydrogen flow lamp. We found that, as expected, solid N2 has the lowest
VUV-absorption cross section, which about three orders of magnitude lower than
that of other species such as O2, which is also homonuclear. Methane (CH4) ice
presents a high absorption near Ly-alpha (121.6 nm) and does not absorb below
148 nm. Estimating the ice absorption cross sections is essential for models of
ice photoprocessing and allows estimating the ice photodesorption rates as the
number of photodesorbed molecules per absorbed photon in the ice.Comment: 9 pages, 6 figures, 7 table
Protein-mediated DNA Loop Formation and Breakdown in a Fluctuating Environment
Living cells provide a fluctuating, out-of-equilibrium environment in which
genes must coordinate cellular function. DNA looping, which is a common means
of regulating transcription, is very much a stochastic process; the loops arise
from the thermal motion of the DNA and other fluctuations of the cellular
environment. We present single-molecule measurements of DNA loop formation and
breakdown when an artificial fluctuating force, applied to mimic a fluctuating
cellular environment, is imposed on the DNA. We show that loop formation is
greatly enhanced in the presence of noise of only a fraction of , yet
find that hypothetical regulatory schemes that employ mechanical tension in the
DNA--as a sensitive switch to control transcription--can be surprisingly robust
due to a fortuitous cancellation of noise effects
Ontogeny of Human IgE-expressing B Cells and Plasma Cells
BACKGROUND: IgEâexpressing (IgE(+)) plasma cells (PCs) provide a continuous source of allergenâspecific IgE that is central to allergic responses. The extreme sparsity of IgE(+) cells in vivo has confined their study almost entirely to mouse models. OBJECTIVE: To characterize the development pathway of human IgE(+) PCs and to determine the ontogeny of human IgE(+) PCs. METHODS: To generate human IgE(+) cells, we cultured tonsil B cells with ILâ4 and antiâCD40. Using FACS and RTâPCR, we examined the phenotype of generated IgE(+) cells, the capacity of tonsil Bâcell subsets to generate IgE(+) PCs and the class switching pathways involved. RESULTS: We have identified three phenotypic stages of IgE(+) PC development pathway, namely (i) IgE(+)germinal centre (GC)âlike B cells, (ii) IgE(+) PCâlike âplasmablastsâ and (iii) IgE(+) PCs. The same phenotypic stages were also observed for IgG1(+) cells. Total tonsil B cells give rise to IgE(+) PCs by direct and sequential switching, whereas the isolated GC Bâcell fraction, the main source of IgE(+) PCs, generates IgE(+) PCs by sequential switching. PC differentiation of IgE(+) cells is accompanied by the downâregulation of surface expression of the short form of membrane IgE (mIgE(S)), which is homologous to mouse mIgE, and the upâregulation of the long form of mIgE (mIgE(L)), which is associated with an enhanced Bâcell survival and expressed in humans, but not in mice. CONCLUSION: Generation of IgE(+) PCs from tonsil GC B cells occurs mainly via sequential switching from IgG. The mIgE(L)/mIgE(S) ratio may be implicated in survival of IgE(+) B cells during PC differentiation and allergic disease
The Cerenkov effect revisited: from swimming ducks to zero modes in gravitational analogs
We present an interdisciplinary review of the generalized Cerenkov emission
of radiation from uniformly moving sources in the different contexts of
classical electromagnetism, superfluid hydrodynamics, and classical
hydrodynamics. The details of each specific physical systems enter our theory
via the dispersion law of the excitations. A geometrical recipe to obtain the
emission patterns in both real and wavevector space from the geometrical shape
of the dispersion law is discussed and applied to a number of cases of current
experimental interest. Some consequences of these emission processes onto the
stability of condensed-matter analogs of gravitational systems are finally
illustrated.Comment: Lecture Notes at the IX SIGRAV School on "Analogue Gravity" in Como,
Italy from May 16th-21th, 201
Video and Synthetic MRI Pre-training of 3D Vision Architectures for Neuroimage Analysis
Transfer learning represents a recent paradigm shift in the way we build
artificial intelligence (AI) systems. In contrast to training task-specific
models, transfer learning involves pre-training deep learning models on a large
corpus of data and minimally fine-tuning them for adaptation to specific tasks.
Even so, for 3D medical imaging tasks, we do not know if it is best to
pre-train models on natural images, medical images, or even synthetically
generated MRI scans or video data. To evaluate these alternatives, here we
benchmarked vision transformers (ViTs) and convolutional neural networks
(CNNs), initialized with varied upstream pre-training approaches. These methods
were then adapted to three unique downstream neuroimaging tasks with a range of
difficulty: Alzheimer's disease (AD) and Parkinson's disease (PD)
classification, "brain age" prediction. Experimental tests led to the following
key observations: 1. Pre-training improved performance across all tasks
including a boost of 7.4% for AD classification and 4.6% for PD classification
for the ViT and 19.1% for PD classification and reduction in brain age
prediction error by 1.26 years for CNNs, 2. Pre-training on large-scale video
or synthetic MRI data boosted performance of ViTs, 3. CNNs were robust in
limited-data settings, and in-domain pretraining enhanced their performances,
4. Pre-training improved generalization to out-of-distribution datasets and
sites. Overall, we benchmarked different vision architectures, revealing the
value of pre-training them with emerging datasets for model initialization. The
resulting pre-trained models can be adapted to a range of downstream
neuroimaging tasks, even when training data for the target task is limited
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