4,565 research outputs found
Identification of dividing, determined sensory neuron precursors in the mammalian neural crest
Sensory and autonomic neurons of the vertebrate peripheral nervous system are derived from the neural crest. Here we use the expression of lineage-specific transcription factors as a means to identify neuronal subtypes that develop in rat neural crest cultures grown in a defined medium. Sensory neurons, identified by expression of the POU-domain transcription factor Brn-3.0, develop from dividing precursors that differentiate within 2 days following emigration from the neural tube. Most of these precursors generate sensory neurons even when challenged with BMP2, a factor that induces autonomic neurogenesis in many other cells in the explants. Moreover, BMP2 fails to prevent expression of the sensory-specific basic helix-loop-helix (bHLH) transcription factors neurogenin1, neurogenin2 and neuroD, although it induces expression of the autonomic-specific bHLH factor MASH1 and the paired homeodomain factor Phox2a in other cells. These data suggest that there are mitotically active precursors in the mammalian neural crest that can generate sensory neurons even in the presence of a strong autonomic-inducing cue. Further characterization of the neurons generated from such precursors indicates that, under these culture conditions, they exhibit a proprioceptive and/or mechanosensory, but not nociceptive, phenotype. Such precursors may therefore correspond to a lineally (Frank, E. and Sanes, J. (1991) Development 111, 895-908) and genetically (Ma, Q., Fode, C., Guillemot, F. and Anderson, D. J. (1999) Genes Dev. 13, in press) distinct subset of early-differentiating precursors of large-diameter sensory neurons identified in vivo
High-resolution antenna near-field imaging and sub-THz measurements with a small atomic vapor-cell sensing element
Atomic sensing and measurement of millimeter-wave (mmW) and THz electric
fields using quantum-optical EIT spectroscopy of Rydberg states in atomic
vapors has garnered significant interest in recent years towards the
development of atomic electric-field standards and sensor technologies. Here we
describe recent work employing small atomic vapor cell sensing elements for
near-field imaging of the radiation pattern of a K-band horn antenna at
13.49 GHz. We image fields at a spatial resolution of and measure
over a 72 to 240 V/m field range using off-resonance AC-Stark shifts of a
Rydberg resonance. The same atomic sensing element is used to measure sub-THz
electric fields at 255 GHz, an increase in mmW-frequency by more than one order
of magnitude. The sub-THz field is measured over a continuous 100 MHz
frequency band using a near-resonant mmW atomic transition
Global Q estimates from antipodal Rayleigh waves
Global average estimates of the group velocity and attenuation of long-period (120ā300 s) Rayleigh waves were made using seismograms from the epicenter's antipode (Īā180Ā°). Focusing at the antipode produced amplified arrivals with favorable signal-to-noise ratios. The high-quality data yielded very stable attenuation values, with excellent agreement between the results from successive Rayleigh arrivals for a single event and between the results for two different events. Lateral heterogeneities in earth structure can cause systematic biasing of attenuation measurements based on antipodal records. The initial, uncorrected results therefore provide a lower bound estimate of global Q. An ellipsoidal perturbation in shape was used to simulate the effects of lateral velocity heterogeneities on Rayleigh wave propagation. Using the agreement of repeated attenuation measurements as a constraint, we estimated both the bias in those measurements and the splitting widths of the Rayleigh modes. At a period of 200 s, the estimated splitting width is 0.30% this agrees closely with calculations by Luh (1974) for an earth model with different continental and oceanic velocity profiles. The estimated bias varied from 30% to zero over the 120- to 260-s band. After correcting for bias, the antipodal Q values range from 108 at 120 s to 188 at 260 s. These Q are within the range of previous measurements but are lower than the mean values from typical great circle studies, implying that the globally averaged upper mantle is slightly more attenuative than has been generally recognized
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Coking-Resistant Sub-Nano Dehydrogenation Catalysts: PtnSnx/SiO2 (n=4, 7)
We present a combined experimental/theoretical study of Pt/SiO and
PtSn/SiO (n = 4, 7) model catalysts for the endothermic
dehydrogenation of hydrocarbons, using the ethylene intermediate as a model
reactant. Supported pure Ptn clusters are found to be highly active toward
dehydrogenation of C2D4, quickly deactivating due to a combination of carbon
deposition and sintering, resulting in loss of accessible Pt sites. Addition of
Sn to Ptn clusters results in the complete suppression of C2D4 dehydrogenation
and carbon deposition, and also stabilizes the clusters against thermal
sintering. Theory shows that both systems have thermal access to a multitude of
cluster structures and adsorbate configurations that form a statistical
ensemble. While Pt4/SiO2 clusters bind ethylene in both di-sigma and pi-bonded
configurations, PtSn/SiO binds C2H4 only in the pi-mode, with
di-sigma bonding suppressed by a combination of electronic and geometric
features of the PtSn clusters. Dehydrogenation reaction profiles on the
accessible cluster isomers were calculated using the climbing image nudged
elastic band (CI-NEB) method
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Preparation of Size- and Composition-Controlled PtnSnx/SiO2 (n=4, 7, 24) Bimetallic Model Catalysts with Atomic Layer Deposition
Open access to novel dual flow chamber technology for in vitro cell mechanotransduction, toxicity and pharamacokinetic studies
<p>Abstract</p> <p>Background</p> <p>A major stumbling block for researchers developing experimental models of mechanotransduction is the control of experimental variables, in particular the transmission of the mechanical forces at the cellular level. A previous evaluation of state of the art commercial perfusion chambers showed that flow regimes, applied to impart a defined mechanical stimulus to cells, are poorly controlled and that data from studies in which different chambers are utilized can not be compared, even if the target stress regimes are comparable.</p> <p>Methods</p> <p>This study provides a novel chamber design to provide both physiologically-based flow regimes, improvements in control of experimental variables, as well as ease of use compared to commercial chambers. This novel design achieves controlled stresses through five gasket designs and both single- and dual-flow regimes.</p> <p>Results</p> <p>The imparted shear stress within the gasket geometry is well controlled. Fifty percent of the entire area of the 10 Ć 21 mm universal gasket (Gasket I, designed to impart constant magnitude shear stresses in the center of the chamber where outcome measures are taken), is exposed to target stresses. In the 8 mm diameter circular area at the center of the chamber (where outcome measures are made), over 92% of the area is exposed to the target stress (Ā± 2.5%). In addition, other gasket geometries provide specific gradients of stress that vary with distance from the chamber inlet. Bench-top testing of the novel chamber prototype shows improvements, in the ease of use as well as in performance, compared to the other commercial chambers. The design of the chamber eliminates flow deviations due to leakage and bubbles and allows actual flow profiles to better conform with those predicted in computational models.</p> <p>Conclusion</p> <p>The novel flow chamber design provides predictable and well defined mechanical forces at the surface of a cell monolayer, showing improvement over previously tested commercial chambers. The predictability of the imparted stress improves both experiment repeatability as well as the accuracy of inter-study comparisons. Carefully controlling the stresses on cells is critical in effectively mimicking <it>in vivo </it>situations. Overall, the improved perfusion flow chamber provides the needed resolution, standardization and <it>in vitro </it>model analogous to <it>in vivo </it>conditions to make the step towards greater use in research and the opportunity to enter the diagnostic and therapeutic market.</p
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