2,595 research outputs found
In situ analysis of neuronal dynamics and positional cues in the patterning of nerve connections
Recently developed imaging techniques permit individual cells to be uniquely labeled and followed over time as development proceeds in intact vertebrate embryos. Small groups of cells in the developing eye rudiment of the frog Xenopus have been labeled with the vital dyes DiI, lysinated fluorescein dextran (LFD) or lysinated rhodamine dextran (LRD). Individual optic axons and their growth cones were clearly visible in the intact living animal using confocal microscopy or epifluorescence microscopy with a low light level video camera and computer-based video image enhancement. To follow the dynamics of single optic nerve fiber terminal arborizations, small groups of cells, or even single retinal ganglion cells, were labeled with DiI, and the resulting labeled optic nerve fibers were imaged using a confocal microscope. The images show a profound alteration in morphology from day to day, demonstrating that optic nerve terminal arborizations are dynamic structures constantly extending and retracting branches. To follow the topography of the developing projection and analyze the cues that guide its formation, small groups of eyebud cells from LFD- and LRD-labeled donor embryos were grafted to an unlabeled host in either a location equivalent to that from which they had been removed (homotopic grafts) or a non-equivalent location (heterotopic grafts). Axons from homotopic grafts projected to the tectum as expected from the adult topography of the retinotectal projection. Dorsoventral topography was present from the time that the optic nerve fibers were observable in the tectum, in agreement with previous work. Nasotemporal topography was subtle or absent for the first few days, and then slowly refined. The importance of positional cues was tested by performing heterotopic eyebud grafts, in which the labeled eyebud cells are grafted to inappropriate places in the host eyebud. The heterotopic grafts appeared to integrate with the ectopic site in the eyebud in a functional manner. They should, therefore, project to the tectum together with their new neighbors if neighbor interactions or activity-based cues were of primary importance in the initial patterning of the map. However, the experiments showed that the axons from heterotopic grafts always behaved in a fashion appropriate to their position of origin in the donor, regardless of their final position in the host. These observations indicate that small groups of eyebud cells (as small as a single cell) possess positional information that plays a dominant role in guiding the optic nerve fibers to their target sites in the tectum
Thermal Infrared Observations of Asteroid (99942) Apophis with Herschel
The near-Earth asteroid (99942) Apophis is a potentially hazardous asteroid.
We obtained far-infrared observations of this asteroid with the Herschel Space
Observatory's PACS instrument at 70, 100, and 160 micron. These were taken at
two epochs in January and March 2013 during a close Earth encounter. These
first thermal measurements of Apophis were taken at similar phase angles before
and after opposition. We performed a detailed thermophysical model analysis by
using the spin and shape model recently derived from applying a 2-period
Fourier series method to a large sample of well-calibrated photometric
observations. We find that the tumbling asteroid Apophis has an elongated shape
with a mean diameter of 375 m (of an equal volume sphere) and a
geometric V-band albedo of 0.30. We find a thermal inertia in
the range 250-800 JmsK (best solution at 600
JmsK), which can be explained by a mixture of low
conductivity fine regolith with larger rocks and boulders of high thermal
inertia on the surface. The thermal inertia, and other similarities with
(25143) Itokawa indicate that Apophis might also have a rubble-pile structure.
If we combine the new size value with the assumption of an Itokawa-like density
and porosity we estimate a mass between 4.4 and 6.2 10 kg which is more
than 2-3 times larger than previous estimates. We expect that the newly derived
properties will influence impact scenario studies and influence the long-term
orbit predictions of Apophis.Comment: Accepted for publication in Astronomy & Astrophysics, 21 pages, 8
figures, 2 table
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Commodity market integration, 1500-2000
This paper provides an introduction to what is known about trends in international commodity market integration during the second half of the second millennium. Throughout, our focus is on intercontinental trade, since it is the emergence of large-scale trade between the continents which has especially distinguished the centuries following the voyages of da Gama and Columbus. This is by no means to imply that intra-European or intra-Asian trade was in any sense less significant It is simply a consequence of the limitations of space
Locked and Unlocked Polygonal Chains in 3D
In this paper, we study movements of simple polygonal chains in 3D. We say
that an open, simple polygonal chain can be straightened if it can be
continuously reconfigured to a straight sequence of segments in such a manner
that both the length of each link and the simplicity of the chain are
maintained throughout the movement. The analogous concept for closed chains is
convexification: reconfiguration to a planar convex polygon. Chains that cannot
be straightened or convexified are called locked. While there are open chains
in 3D that are locked, we show that if an open chain has a simple orthogonal
projection onto some plane, it can be straightened. For closed chains, we show
that there are unknotted but locked closed chains, and we provide an algorithm
for convexifying a planar simple polygon in 3D with a polynomial number of
moves.Comment: To appear in Proc. 10th ACM-SIAM Sympos. Discrete Algorithms, Jan.
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Gyrofluid simulations of collisionless reconnection in the presence of diamagnetic effects
The effects of the ion Larmor radius on magnetic reconnection are
investigated by means of numerical simulations, with a Hamiltonian gyrofluid
model. In the linear regime, it is found that ion diamagnetic effects decrease
the growth rate of the dominant mode. Increasing ion temperature tends to make
the magnetic islands propagate in the ion diamagnetic drift direction. In the
nonlinear regime, diamagnetic effects reduce the final width of the island.
Unlike the electron density, the guiding center density does not tend to
distribute along separatrices and at high ion temperature, the electrostatic
potential exhibits the superposition of a small scale structure, related to the
electron density, and a large scale structure, related to the ion
guiding-center density
Gyrofluid simulations of collisionless reconnection in the presence of diamagnetic effects
The effects of the ion Larmor radius on magnetic reconnection are
investigated by means of numerical simulations, with a Hamiltonian gyrofluid
model. In the linear regime, it is found that ion diamagnetic effects decrease
the growth rate of the dominant mode. Increasing ion temperature tends to make
the magnetic islands propagate in the ion diamagnetic drift direction. In the
nonlinear regime, diamagnetic effects reduce the final width of the island.
Unlike the electron density, the guiding center density does not tend to
distribute along separatrices and at high ion temperature, the electrostatic
potential exhibits the superposition of a small scale structure, related to the
electron density, and a large scale structure, related to the ion
guiding-center density
Quality of Life in Prodromal HD: Qualitative Analyses of Discourse from Participants and Companions
Persons who are at risk for Huntington's Disease (HD) can be tested for the HD gene expansion before symptom onset. People with the gene expansion, but no clinical diagnosis, are in the prodromal phase of HD. This study explored quality of life (QOL) in prodromal HD. Interviews about QOL, conducted with 9 prodromal HD participants and 6 companions, were transcribed. Discourse was coded for emotional valence, content (e.g., coping, spirituality, interpersonal relationships, HD in others, and employment), and time frame (e.g., current, past, and future). Respondents were more positive than negative about the present, which was their major focus. The most common statements were about positive attitudes. Positive statements were made about spirituality, and negative statements were made about HD in other people. Relationships, employment, and coping with HD reflected both positivity and negativity. Participants and companions spoke of the future with different concerns. Applicability of findings to the clinical management of HD are discussed
Gyrofluid simulations of collisionless reconnection in the presence of diamagnetic effects
The effects of the ion Larmor radius on magnetic reconnection are
investigated by means of numerical simulations, with a Hamiltonian gyrofluid
model. In the linear regime, it is found that ion diamagnetic effects decrease
the growth rate of the dominant mode. Increasing ion temperature tends to make
the magnetic islands propagate in the ion diamagnetic drift direction. In the
nonlinear regime, diamagnetic effects reduce the final width of the island.
Unlike the electron density, the guiding center density does not tend to
distribute along separatrices and at high ion temperature, the electrostatic
potential exhibits the superposition of a small scale structure, related to the
electron density, and a large scale structure, related to the ion
guiding-center density
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