10,518 research outputs found
Live imaging of whole mouse embryos during gastrulation : migration analyses of epiblast and mesodermal cells
During gastrulation in the mouse embryo, dynamic cell movements including epiblast invagination and mesodermal layer expansion lead to the establishment of the three-layered body plan. The precise details of these movements, however, are sometimes elusive, because of the limitations in live imaging. To overcome this problem, we developed techniques to enable observation of living mouse embryos with digital scanned light sheet microscope (DSLM). The achieved deep and high time-resolution images of GFP-expressing nuclei and following 3D tracking analysis revealed the following findings: (i) Interkinetic nuclear migration (INM) occurs in the epiblast at embryonic day (E)6 and 6.5. (ii) INM-like migration occurs in the E5.5 embryo, when the epiblast is a monolayer and not yet pseudostratified. (iii) Primary driving force for INM at E6.5 is not pressure from neighboring nuclei. (iv) Mesodermal cells migrate not as a sheet but as individual cells without coordination
Inflationary Scenarios from Branes at Angles
We describe a simple mechanism that can lead to inflation within string-based
brane-world scenarios. The idea is to start from a supersymmetric configuration
with two parallel static Dp-branes, and slightly break the supersymmetry
conditions to produce a very flat potential for the field that parametrises the
distance between the branes, i.e. the inflaton field. This breaking can be
achieved in various ways: by slight relative rotations of the branes with small
angles, by considering small relative velocities between the branes, etc. If
the breaking parameter is sufficiently small, a large number of e-folds can be
produced within the D-brane, for small changes of the configuration in the
compactified directions. Such a process is local, i.e. it does not depend very
strongly on the compactification space nor on the initial conditions. Moreover,
the breaking induces a very small velocity and acceleration, which ensures very
small slow-roll parameters and thus an almost scale invariant spectrum of
metric fluctuations, responsible for the observed temperature anisotropies in
the microwave background. Inflation ends as in hybrid inflation, triggered by
the negative curvature of the string tachyon potential. In this paper we
elaborate on one of the simplest examples: two almost parallel D4-branes in a
flat compactified space.Comment: 29 pages, 9 eps figures, using JHEP3.cls, published in JHE
Knots with small rational genus
If K is a rationally null-homologous knot in a 3-manifold M, the rational
genus of K is the infimum of -\chi(S)/2p over all embedded orientable surfaces
S in the complement of K whose boundary wraps p times around K for some p
(hereafter: S is a p-Seifert surface for K). Knots with very small rational
genus can be constructed by "generic" Dehn filling, and are therefore extremely
plentiful. In this paper we show that knots with rational genus less than 1/402
are all geometric -- i.e. they may be isotoped into a special form with respect
to the geometric decomposition of M -- and give a complete classification. Our
arguments are a mixture of hyperbolic geometry, combinatorics, and a careful
study of the interaction of small p-Seifert surfaces with essential subsurfaces
in M of non-negative Euler characteristic.Comment: 38 pages, 3 figures; version 3 corrects minor typos; keywords: knots,
rational genu
Divergence in right-angled Coxeter groups
Let W be a 2-dimensional right-angled Coxeter group. We characterise such W
with linear and quadratic divergence, and construct right-angled Coxeter groups
with divergence polynomial of arbitrary degree. Our proofs use the structure of
walls in the Davis complex.Comment: This version incorporates the referee's comments. It contains the
complete appendix (which will be abbreviated in the journal version). To
appear in Transactions of the AM
A new approach to upscaling fracture network models while preserving geostatistical and geomechanical characteristics
A new approach to upscaling two-dimensional fracture network models is proposed for preserving geostatistical and geomechanical characteristics of a smaller-scale “source” fracture pattern. First, the scaling properties of an outcrop system are examined in terms of spatial organization, lengths, connectivity, and normal/shear displacements using fractal geometry and power law relations. The fracture pattern is observed to be nonfractal with the fractal dimension D ≈ 2, while its length distribution tends to follow a power law with the exponent 2 < a < 3. To introduce a realistic distribution of fracture aperture and shear displacement, a geomechanical model using the combined finite-discrete element method captures the response of a fractured rock sample with a domain size L = 2 m under in situ stresses. Next, a novel scheme accommodating discrete-time random walks in recursive self-referencing lattices is developed to nucleate and propagate fractures together with their stress- and scale-dependent attributes into larger domains of up to 54 m × 54 m. The advantages of this approach include preserving the nonplanarity of natural cracks, capturing the existence of long fractures, retaining the realism of variable apertures, and respecting the stress dependency of displacement-length correlations. Hydraulic behavior of multiscale growth realizations is modeled by single-phase flow simulation, where distinct permeability scaling trends are observed for different geomechanical scenarios. A transition zone is identified where flow structure shifts from extremely channeled to distributed as the network scale increases. The results of this paper have implications for upscaling network characteristics for reservoir simulation
Approach to a rational rotation number in a piecewise isometric system
We study a parametric family of piecewise rotations of the torus, in the
limit in which the rotation number approaches the rational value 1/4. There is
a region of positive measure where the discontinuity set becomes dense in the
limit; we prove that in this region the area occupied by stable periodic orbits
remains positive. The main device is the construction of an induced map on a
domain with vanishing measure; this map is the product of two involutions, and
each involution preserves all its atoms. Dynamically, the composition of these
involutions represents linking together two sector maps; this dynamical system
features an orderly array of stable periodic orbits having a smooth parameter
dependence, plus irregular contributions which become negligible in the limit.Comment: LaTeX, 57 pages with 13 figure
An introduction to finite type invariants of knots and 3-manifolds defined by counting graph configurations
These introductory lectures show how to define finite type invariants of
links and 3-manifolds by counting graph configurations in 3-manifolds,
following ideas of Witten and Kontsevich. The linking number is the simplest
finite type invariant for 2-component links. It is defined in many equivalent
ways in the first section. As an important example, we present it as the
algebraic intersection of a torus and a 4-chain called a propagator in a
configuration space. In the second section, we introduce the simplest finite
type 3-manifold invariant, which is the Casson invariant (or the
Theta-invariant) of integer homology 3-spheres. It is defined as the algebraic
intersection of three propagators in the same two-point configuration space. In
the third section, we explain the general notion of finite type invariants and
introduce relevant spaces of Feynman Jacobi diagrams. In Sections 4 and 5, we
sketch an original construction based on configuration space integrals of
universal finite type invariants for links in rational homology 3-spheres and
we state open problems. Our construction generalizes the known constructions
for links in the ambient space, and it makes them more flexible. In Section 6,
we present the needed properties of parallelizations of 3-manifolds and
associated Pontrjagin classes, in details.Comment: 68 pages. Change of title, updates and minor reorganization of notes
of five lectures presented in the ICPAM-ICTP research school of Mekn{\`e}s in
May 2012. To appear in the Proceedings of the conference "Quantum topology"
organized by Chelyabinsk State University in July 2014 (Vestnik ChelGU
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