6,256 research outputs found
Indicators of replicative damage in equine tendon fibroblast monolayers
<p>Background:
Superficial digital flexor tendon (SDFT) injuries of horses usually follow cumulative matrix microdamage; it is not known why the reparative abilities of tendon fibroblasts are overwhelmed or subverted. Relevant in vitro studies of this process require fibroblasts not already responding to stresses caused by the cell culture protocols. We investigated indicators of replicative damage in SDFT fibroblast monolayers, effects of this on their reparative ability, and measures that can be taken to reduce it.</p>
<p>Results:
We found significant evidence of replicative stress, initially observing consistently large numbers of binucleate (BN) cells. A more variable but prominent feature was the presence of numerous gammaH2AX (ÎłH2AX) puncta in nuclei, this being a histone protein that is phosphorylated in response to DNA double-stranded breaks (DSBs). Enrichment for injury detection and cell cycle arrest factors (p53 (ser15) and p21) occurred most frequently in BN cells; however, their numbers did not correlate with DNA damage levels and it is likely that the two processes have different causative mechanisms. Such remarkable levels of injury and binucleation are usually associated with irradiation, or treatment with cytoskeletal-disrupting agents.</p>
<p>Both DSBs and BN cells were greatest in subconfluent (replicating) monolayers. The DNA-damaged cells co-expressed the replication markers TPX2/repp86 and centromere protein F. Once damaged in the early stages of culture establishment, fibroblasts continued to express DNA breaks with each replicative cycle. However, significant levels of cell death were not measured, suggesting that DNA repair was occurring. Comet assays showed that DNA repair was delayed in proportion to levels of genotoxic stress.</p>
<p>Conclusions:
Researchers using tendon fibroblast monolayers should assess their âhealthâ using ÎłH2AX labelling. Continued use of early passage cultures expressing initially high levels of ÎłH2AX puncta should be avoided for mechanistic studies and ex-vivo therapeutic applications, as this will not be resolved with further replicative cycling. Low density cell culture should be avoided as it enriches for both DNA damage and mitotic defects (polyploidy). As monolayers differing only slightly in baseline DNA damage levels showed markedly variable responses to a further injury, studies of effects of various stressors on tendon cells must be very carefully controlled.</p>
Detailed Abundances for the Old Population near the Galactic Center: I. Metallicity distribution of the Nuclear Star Cluster
We report the first high spectral resolution study of 17 M giants
kinematically confirmed to lie within a few parsecs of the Galactic Center,
using R=24,000 spectroscopy from Keck/NIRSPEC and a new linelist for the
infrared K band. We consider their luminosities and kinematics, which classify
these stars as members of the older stellar population and the central cluster.
We find a median metallicity of =-0.16 and a large spread from
approximately -0.3 to +0.3 (quartiles). We find that the highest metallicities
are [Fe/H]<+0.6, with most of the stars being at or below the Solar iron
abundance. The abundances and the abundance distribution strongly resembles
that of the Galactic bulge rather than disk or halo; in our small sample we
find no statistical evidence for a dependence of velocity dispersion on
metallicity.Comment: 18 pages, 14 figures, accepted for publication in A
Solution of Orthopositronium lifetime Puzzle
The intrinsic decay rate of orthopositronium formed in powder
is measured using the direct correction method such that the time
dependence of the pick-off annihilation rate is precisely determined. The decay
rate of orthopositronium is found to be , which is consistent with our previous measurements with
about twice the accuracy. Results agree well with the QED
prediction, and also with a result reported very recently using nanoporous
film
Optical Studies of InGaAs/GaAs Multiple Quantum Wells (MQW\u27s) and GaAs/Si Using Novel Scanning Electron Microscopy Based Techniques
The influence of defects on electron-hole plasma transport in nipi-doped in 0.2Ga0.8As/GaAs multiple quantum wells (MQWs) has been studied using a novel technique called electron beam induced absorption modulation (EBIA) imaging. Modulation doped MQW structures exhibit large optical nonlinearities and are important for the development of all-optical spatial light modulators used in optical computing and communication. The electron-hole plasma is generated by a high-energy electron beam in a scanning electron microscope and is used as a probe to study the MQW absorption modulation induced locally by the electron beam. The influence of structural defects on the diffusive transport of carriers is imaged with a ÎŒm-scale resolution. A strong spatial correlation between dark line defects observed in cathodoluminescence (CL) and absorption modulation steps in EBIA reveals the existence of strain-induced band edge fluctuations that are caused by misfit dislocations. The spatial variation of stress in metalorganic chemical vapor deposition grown GaAs/Si has been studied with linearly polarized cathodoluminescence (LPCL). GaAs grown on Si wafers in selective areas (10 to 1000 ÎŒm mesas) were studied. The large difference in thermal expansion coefficient between GaAs and Si results in thermal stress-induced microcracks and dislocations which can occur upon cooling from growth temperatures. Using LPCL, we have determined the spatial distribution of the stress tensor from the polarized CL strain-split peak positions and solutions to the orbital-strain Hamiltonian
Fabrication and characterization of a lithium-glass-based composite neutron detector
A novel composite, scintillating material intended for neutron detection and
composed of small (1.5 mm) cubes of KG2-type lithium glass embedded in a matrix
of scintillating plastic has been developed in the form of a 2.2 in.-diameter,
3.1 in.-tall cylindrical prototype loaded with
lithium glass by mass. The response of the material when exposed to
Cf fission neutrons and various -ray sources has been
studied; using the charge-integration method for pulse shape discrimination,
good separation between neutron and -ray events is observed and
intrinsic efficiencies of and
for Cf fission neutrons
and Co rays are obtained; an upper limit for the sensitivity
to Cs rays is determined to be . The
neutron/ discrimination capabilities are improved in circumstances when
a neutron capture signal in the lithium glass can be detected in coincidence
with a preceding elastic scattering event in the plastic scintillator; with
this coincidence requirement, the intrinsic efficiency of the prototype
detector for Co rays is while its intrinsic efficiency for unmoderated Cf fission
neutrons is . Through use of
subregion-integration ratios in addition to the coincidence requirement, the
efficiency for rays from Co is reduced to while the Cf fission neutron efficiency
becomes .Comment: Final results, figures, and text; published in Nuclear Instruments
and Methods in Physics Research Section
Halos of Spiral Galaxies. III. Metallicity Distributions
(Abriged) We report results of a campaign to image the stellar populations in
the halos of highly inclined spiral galaxies, with the fields roughly 10 kpc
(projected) from the nuclei. We use the F814W (I) and F606W (V) filters in the
Wide Field Planetary Camera 2, on board the Hubble Space telescope. Extended
halo populations are detected in all galaxies. The color-magnitude diagrams
appear to be completely dominated by giant-branch stars, with no evidence for
the presence of young stellar populations in any of the fields. We find that
the metallicity distribution functions are dominated by metal-rich populations,
with a tail extending toward the metal poor end. To first order, the overall
shapes of the metallicity distribution functions are similar to what is
predicted by simple, single-component model of chemical evolution with the
effective yields increasing with galaxy luminosity. However, metallicity
distributions significantly narrower than the simple model are observed for a
few of the most luminous galaxies in the sample. It appears clear that more
luminous spiral galaxies also have more metal-rich stellar halos. The
increasingly significant departures from the closed-box model for the more
luminous galaxies indicate that a parameter in addition to a single yield is
required to describe chemical evolution. This parameter, which could be related
to gas infall or outflow either in situ or in progenitor dwarf galaxies that
later merge to form the stellar halo, tends to act to make the metallicity
distributions narrower at high metallicity.Comment: 20 pages, 8 figures (ApJ, in press
Massive Star Cluster Formation and Destruction in Luminous Infrared Galaxies in GOALS
We present the results of a {\it Hubble Space Telescope} ACS/HRC FUV, ACS/WFC
optical study into the cluster populations of a sample of 22 Luminous Infrared
Galaxies in the Great Observatories All-Sky LIRG Survey. Through integrated
broadband photometry we have derived ages and masses for a total of 484 star
clusters contained within these systems. This allows us to examine the
properties of star clusters found in the extreme environments of LIRGs relative
to lower luminosity star-forming galaxies in the local Universe. We find that
by adopting a Bruzual \& Charlot simple stellar population (SSP) model and
Salpeter initial mass function, the age distribution of clusters declines as
, consistent with the age distribution derived
for the Antennae Galaxies, and interpreted as evidence for rapid cluster
disruption occuring in the strong tidal fields of merging galaxies. The large
number of young clusters identified in the sample also
suggests that LIRGs are capable of producing more high-mass clusters than what
is observed to date in any lower luminosity star-forming galaxy in the local
Universe. The observed cluster mass distribution of is consistent with the canonical -2 power law used to describe the
underlying initial cluster mass function (ICMF) for a wide range of galactic
environments. We interpret this as evidence against mass-dependent cluster
disruption, which would flatten the observed CMF relative to the underlying
ICMF distribution.Comment: 63 pages, 58 Figures, 56 Tables, Accepted for publication in Ap
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