1,773 research outputs found
Epithelial label-retaining cells are absent during tooth cycling in <i>Salmo salar</i> and <i>Polypterus senegalus</i>
The Atlantic salmon (Salmo salar) and African bichir (Polypterus senegalus) are both actinopterygian fish species that continuously replace their teeth without the involvement of a successional dental lamina. Instead, they share the presence of a middle dental epithelium: an epithelial tier enclosed by inner and outer dental epithelium. It has been hypothesized that this tier could functionally substitute for a successional dental lamina and might be a potential niche to house epithelial stem cells involved in tooth cycling. Therefore, in this study we performed a BrdU pulse chase experiment on both species to (1) determine the localization and extent of proliferating cells in the dental epithelial layers, (2) describe cell dynamics and (3) investigate if label-retaining cells are present, suggestive for the putative presence of stem cells. Cells proliferate in the middle dental epithelium, outer dental epithelium and cervical loop at the lingual side of the dental organ to form a new tooth germ. Using long chase times, both in S. salar (eight weeks) and P. senegalus (eight weeks and twelve weeks), we could not reveal the presence of label-retaining cells in the dental organ. Immunostaining of P. senegalus dental organs for the transcription factor Sox2, often used as a stem cell marker, labelled cells in the zone of outer dental epithelium which grades into the oral epithelium (ODE transition zone) and the inner dental epithelium of a successor only. The location of Sox2 distribution does not provide evidence for epithelial stem cells in the dental organ and, more specifically, in the middle dental epithelium. Comparison of S. salar and P. senegalus reveals shared traits in tooth cycling and thus advances our understanding of the developmental mechanism that ensures lifelong replacement
Coulomb Breakup Mechanism of Neutron-Halo Nuclei in a Time-Dependent Method
The mechanism of the Coulomb breakup reactions of the nuclei with
neutron-halo structure is investigated in detail. A time-dependent
Schr\"odinger equation for the halo neutron is numerically solved by treating
the Coulomb field of a target as an external field. The momentum distribution
and the post-acceleration effect of the final fragments are discussed in a
fully quantum mechanical way to clarify the limitation of the intuitive picture
based on the classical mechanics. The theory is applied to the Coulomb breakup
reaction of Be + Pb. The breakup mechanism is found to be
different between the channels of and
, reflecting the underlying structure of Be. The
calculated result reproduces the energy spectrum of the breakup fragments
reasonably well, but explains only about a half of the observed longitudinal
momentum difference.Comment: 15 pages,revtex, 9 figures (available upon request
A Prismatic Analyser concept for Neutron Spectrometers
A development in modern neutron spectroscopy is to avoid the need of large
samples. We demonstrate how small samples together with the right choice of
analyser and detector components makes distance collimation an important
concept in crystal analyser spectrometers. We further show that this opens new
possibilities where neutrons with different energies are reflected by the same
analyser but counted in different detectors, thus improving both energy
resolution and total count rate compared to conventional spectrometers. The
technique can be combined with advanced focusing geometries and with
multiplexing instrument designs. We present a combination of simulations and
data with 3 energies from one analyser. The data was taken on a prototype
installed at PSI, Switzerland, and shows excellent agreement with the
predictions. Typical improvements will be 2 times finer resolution and a factor
1.9 in flux gain compared to a Rowland geometry or 3 times finer resolution and
a factor 3.2 in flux gain compared to a single flat analyser slab
Neutron Halo Isomers in Stable Nuclei and their Possible Application for the Production of Low Energy, Pulsed, Polarized Neutron Beams of High Intensity and High Brilliance
We propose to search for neutron halo isomers populated via -capture
in stable nuclei with mass numbers of about A=140-180 or A=40-60, where the
or neutron shell model state reaches zero binding energy.
These halo nuclei can be produced for the first time with new -beams of
high intensity and small band width ( 0.1%) achievable via Compton
back-scattering off brilliant electron beams thus offering a promising
perspective to selectively populate these isomers with small separation
energies of 1 eV to a few keV. Similar to single-neutron halo states for very
light, extremely neutron-rich, radioactive nuclei
\cite{hansen95,tanihata96,aumann00}, the low neutron separation energy and
short-range nuclear force allows the neutron to tunnel far out into free space
much beyond the nuclear core radius. This results in prolonged half lives of
the isomers for the -decay back to the ground state in the 100
ps-s range. Similar to the treatment of photodisintegration of the
deuteron, the neutron release from the neutron halo isomer via a second,
low-energy, intense photon beam has a known much larger cross section with a
typical energy threshold behavior. In the second step, the neutrons can be
released as a low-energy, pulsed, polarized neutron beam of high intensity and
high brilliance, possibly being much superior to presently existing beams from
reactors or spallation neutron sources.Comment: accepted for publication in Applied Physics
Nuclear halo and its scaling laws
We have proposed a procedure to extract the probability for valence particle
being out of the binding potential from the measured nuclear asymptotic
normalization coefficients. With this procedure, available data regarding the
nuclear halo candidates are systematically analyzed and a number of halo nuclei
are confirmed. Based on these results we have got a much relaxed condition for
nuclear halo occurrence. Furthermore, we have presented the scaling laws for
the dimensionless quantity of nuclear halo in terms of the
analytical expressions of the expectation value for the operator in a
finite square-well potential.Comment: 14 pages, 3 figure
Three-Body Halos. II. from Two- to Three-Body Asymptotics
The large distance behavior of weakly bound three-body systems is
investigated. The Schr\"{o}dinger equation and the Faddeev equations are
reformulated by an expansion in eigenfunctions of the angular part of a
corresponding operator. The resulting coupled set of effective radial equations
are then derived. Both two- and three-body asymptotic behavior are possible and
their relative importance is studied for systems where subsystems may be bound.
The system of two nucleons outside a core is studied numerically in detail and
the character of possible halo structure is pointed out and investigated.Comment: 16 pages, compressed and uuencoded PosrScript file, IFA-94/3
Strategy for ILRI research on feed resources
The importance of feed in increasing livestock productivity and benefits from livestock is stressed. Four major feed resources are identified: pastures, common property resources, forests, and fallow lands; planted forages; crop residues; and concentrate and agricultural by-products. There is a scarcity of quantitative, countrywide data on the current contribution of these resources to the actual feed budget, and the likely trends in future. Systematic mapping of fodder resources is required, and seen as an integral part of feed research. Research on feed resources needs to contribute to poverty alleviation and production of global public goods, but ILRI also needs to have a competitive advantage in the research field, and the research needs to be cost efficient. Partnerships play a key role in producing synergies from diverse research efforts, and in implementation of research results and output-to-outcome processes. Collaboration between crop improvement and livestock research is seen as a key partnership in achieving the feed-related outputs of ILRI’s Medium Term Plan. Although considerable uncertainty exists in terms of quantitative contribution of specific feed resources to overall feed budget, it seems highly probable that crop residue plays, and will continue to play, a central role in sustaining mixed-crop livestock systems in resource-poor areas
Momentum Distributions of Particles from Three--Body Halo Fragmentation: Final State Interactions
Momentum distributions of particles from nuclear break-up of fast three-body
halos are calculated consistently, and applied to Li. The same two-body
interactions between the three particles are used to calculate the ground state
structure and the final state of the reaction processes. We reproduce the
available momentum distributions from Li fragmentation, together with
the size and energy of Li, with a neutron-core relative state containing
a -state admixture of 20\%-30\%. The available fragmentation data strongly
suggest an -state in Li at about 50 keV, and indicate a -state
around 500 keV.Comment: 11 pages (RevTeX), 3 Postscript figures (uuencoded postscript file
attached at the end of the LaTeX file). To be published in Phys. Rev.
Towards a machine-independent transput section
If the transput section of an ALGOL-68 compiler is to be portable, it must be described in such a way that it is clear which aspects are machine-dependent, and which are not. There should be a clear set of primitives underlying the transput. In this report, a description is proposed which can really be used as an implementation model: the transput is described in pseudo-ALGOL 68, except for the underlying primitives, whose semantics are given in some kind of formalized English. The state of this model is by no means definitive, but may serve as a start for further discussion
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