4,378 research outputs found
Converting genetic network oscillations into somite spatial pattern
In most vertebrate species, the body axis is generated by the formation of
repeated transient structures called somites. This spatial periodicity in
somitogenesis has been related to the temporally sustained oscillations in
certain mRNAs and their associated gene products in the cells forming the
presomatic mesoderm. The mechanism underlying these oscillations have been
identified as due to the delays involved in the synthesis of mRNA and
translation into protein molecules [J. Lewis, Current Biol. {\bf 13}, 1398
(2003)]. In addition, in the zebrafish embryo intercellular Notch signalling
couples these oscillators and a longitudinal positional information signal in
the form of an Fgf8 gradient exists that could be used to transform these
coupled temporal oscillations into the observed spatial periodicity of somites.
Here we consider a simple model based on this known biology and study its
consequences for somitogenesis. Comparison is made with the known properties of
somite formation in the zebrafish embryo . We also study the effects of
localized Fgf8 perturbations on somite patterning.Comment: 7 pages, 7 figure
A mathematical model for the capillary endothelial cell-extracellular matrix interactions in wound-healing angiogenesis
Angiogenesis, the process by which new blood capillaries grow into a tissue from surrounding parent vessels, is a key event in dermal wound healing, malignant-tumour growth, and other pathologic conditions. In wound healing, new capillaries deliver vital metabolites such as amino acids and oxygen to the cells in the wound which are involved in a complex sequence of repair processes. The key cellular constituents of these new capillaries are endothelial cells: their interactions with soluble biochemical and insoluble extracellular matrix (ECM) proteins have been well documented recently, although the biological mechanisms underlying wound-healing angiogenesis are incompletely understood. Considerable recent research, including some continuum mathematical models, have focused on the interactions between endothelial cells and soluble regulators (such as growth factors). In this work, a similar modelling framework is used to investigate the roles of the insoluble ECM substrate, of which collagen is the predominant macromolecular protein. Our model consists of a partial differential equation for the endothelial-cell density (as a function of position and time) coupled to an ordinary differential equation for the ECM density. The ECM is assumed to regulate cell movement (both random and directed) and proliferation, whereas the cells synthesize and degrade the ECM. Analysis and numerical solutions of these equations highlights the roles of these processes in wound-healing angiogenesis. A nonstandard approximation analysis yields insight into the travel ling-wave structure of the system. The model is extended to two spatial dimensions (parallel and perpendicular to the plane of the skin), for which numerical simulations are presented. The model predicts that ECM-mediated random motility and cell proliferation are key processes which drive angiogenesis and that the details of the functional dependence of these processes on the ECM density, together with the rate of ECM remodelling, determine the qualitative nature of the angiogenic response. These predictions are experimentally testable, and they may lead towards a greater understanding of the biological mechanisms involved in wound-healing angiogenesis
Microarcsecond Radio Imaging using Earth Orbit Synthesis
The observed interstellar scintillation pattern of an intra-day variable
radio source is influenced by its source structure. If the velocity of the
interstellar medium responsible for the scattering is comparable to the
earth's, the vector sum of these allows an observer to probe the scintillation
pattern of a source in two dimensions and, in turn, to probe two-dimensional
source structure on scales comparable to the angular scale of the scintillation
pattern, typically as for weak scattering. We review the theory on
the extraction of an ``image'' from the scintillation properties of a source,
and show how earth's orbital motion changes a source's observed scintillation
properties during the course of a year. The imaging process, which we call
Earth Orbit Synthesis, requires measurements of the statistical properties of
the scintillations at epochs spread throughout the course of a year.Comment: ApJ in press. 25 pages, 7 fig
Modern optical astronomy: technology and impact of interferometry
The present `state of the art' and the path to future progress in high
spatial resolution imaging interferometry is reviewed. The review begins with a
treatment of the fundamentals of stellar optical interferometry, the origin,
properties, optical effects of turbulence in the Earth's atmosphere, the
passive methods that are applied on a single telescope to overcome atmospheric
image degradation such as speckle interferometry, and various other techniques.
These topics include differential speckle interferometry, speckle spectroscopy
and polarimetry, phase diversity, wavefront shearing interferometry,
phase-closure methods, dark speckle imaging, as well as the limitations imposed
by the detectors on the performance of speckle imaging. A brief account is
given of the technological innovation of adaptive-optics (AO) to compensate
such atmospheric effects on the image in real time. A major advancement
involves the transition from single-aperture to the dilute-aperture
interferometry using multiple telescopes. Therefore, the review deals with
recent developments involving ground-based, and space-based optical arrays.
Emphasis is placed on the problems specific to delay-lines, beam recombination,
polarization, dispersion, fringe-tracking, bootstrapping, coherencing and
cophasing, and recovery of the visibility functions. The role of AO in
enhancing visibilities is also discussed. The applications of interferometry,
such as imaging, astrometry, and nulling are described. The mathematical
intricacies of the various `post-detection' image-processing techniques are
examined critically. The review concludes with a discussion of the
astrophysical importance and the perspectives of interferometry.Comment: 65 pages LaTeX file including 23 figures. Reviews of Modern Physics,
2002, to appear in April issu
The unlikely rise of masking interferometry: leading the way with 19th century technology
The exquisite precision delivered by interferometric techniques is rapidly
being applied to more and more branches of optical astronomy. One particularly
successful strategy to obtain structures at the scale of the diffraction limit
is Aperture Masking Interferometry, which is presently experience a golden age
with implementations at a host of large telescopes around the world. This
startlingly durable technique, which turns 144 years old this year, presently
sets the standard for the recovery of faint companions within a few resolution
elements from the core of a stellar point spread function. This invited review
will give a historical introduction and overview of the modern status of the
technique, the science being delivered, and prospects for new advances and
applications.Comment: This is an invited review for SPIE Amsterdam in 2012. It presents a
brief history of masking interferometry, and some thoughts on future
progress. 11 pages, 4 figs, lots of ref
Non-linear Synthesis of Complex Laser Waveforms at Remote Distances
Strong deformation of ultrashort laser pulse shapes is unavoidable when
delivering high intensities at remote distances due to non-linear effects
taking place while propagating. Relying on the reversibility of laser
filamentation, we propose to explicitly design laser pulse shapes so that
propagation serves as a non-linear field synthesizer at a remote target
location. Such an approach allows, for instance, coherent control of molecules
at a remote distance, in the context of standoff detection of pathogens or
explosives.Comment: 17 pages, 6 figure
EMBRACE@Nancay: An Ultra Wide Field of View Prototype for the SKA
A revolution in radio receiving technology is underway with the development
of densely packed phased arrays for radio astronomy. This technology can
provide an exceptionally large field of view, while at the same time sampling
the sky with high angular resolution. Such an instrument, with a field of view
of over 100 square degrees, is ideal for performing fast, all-sky, surveys,
such as the "intensity mapping" experiment to measure the signature of Baryonic
Acoustic Oscillations in the HI mass distribution at cosmological redshifts.
The SKA, built with this technology, will be able to do a billion galaxy
survey. I will present a very brief introduction to radio interferometry, as
well as an overview of the Square Kilometre Array project. This will be
followed by a description of the EMBRACE prototype and a discussion of results
and future plans.Comment: to appear in proceedings of the INFIERI Summer School INtelligent
Signal Processing for FrontIEr Research and Industry, Paris 201
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