150 research outputs found
The hedgehog pathway and ocular developmental anomalies.
Mutations in effectors of the hedgehog signaling pathway are responsible for a wide variety of ocular developmental anomalies. These range from massive malformations of the brain and ocular primordia, not always compatible with postnatal life, to subtle but damaging functional effects on specific eye components. This review will concentrate on the effects and effectors of the major vertebrate hedgehog ligand for eye and brain formation, Sonic hedgehog (SHH), in tissues that constitute the eye directly and also in those tissues that exert indirect influence on eye formation. After a brief overview of human eye development, the many roles of the SHH signaling pathway during both early and later morphogenetic processes in the brain and then eye and periocular primordia will be evoked. Some of the unique molecular biology of this pathway in vertebrates, particularly ciliary signal transduction, will also be broached within this developmental cellular context
Determining ethylene group disorder levels in -(BEDT-TTF)Cu[N(CN)]Br
We present a detailed structural investigation of the organic superconductor
-(BEDT-TTF)Cu[N(CN)]Br at temperatures from 9 to 300 K.
Anomalies in the dependence of the lattice parameters are associated with a
glass-like transition previously reported at = 77 K. From structure
refinements at 9, 100 and 300 K, the orthorhombic crystalline symmetry, space
group {\it Pnma}, is established at all temperatures. Further, we extract the
dependence of the occupation factor of the eclipsed conformation of the
terminal ethylene groups of the BEDT-TTF molecule. At 300 K, we find 67(2) %,
with an increase to 97(3) % at 9 K. We conclude that the glass-like transition
is not primarily caused by configurational freezing-out of the ethylene groups
Phase diagram of quarter-filled band organic salts, [EDT-TTF-CONMe2]2X, X = AsF6 and Br
An investigation of the P/T phase diagram of the quarter-filled organic
conductors, [EDT-TTF-CONMe2]2X, is reported on the basis of transport and NMR
studies of two members, X=AsF6 and Br of the family. The strongly insulating
character of these materials in the low pressure regime has been attributed to
a remarkably stable charge ordered state confirmed by 13C NMR and the only
existence of 1/4 Umklapp e-e scattering favoring a charge ordering instead of
the 1D Mott localization seen in (TM)2X which are quarter-filled compounds with
dimerization. A non magnetic insulating phase instead of the spin density wave
state is stabilized in the deconfined regime of the phase diagram. This
sequence of phases observed under pressure may be considered as a generic
behavior for 1/4-filled conductors with correlations
Insulator-Metal Transition in One Dimension Induced by Long-Range Electronic Interactions
The effects of a long range electronic potential on a one dimensional
commensurate Charge Density Wave (CDW) state are investigated. Using numerical
techniques it is shown that a transition to a metallic ground state is reached
as the range of the electron-electron repulsion increases. In this metallic
state, the optical conductivity exhibits a large Drude weight. Possible
interpretations of our results are discussed.Comment: 5 pages, Revtex, minor misprints corrected and a reference to earlier
work by V. Emery and C. Noguera adde
Towards a consistent picture for quasi-1D organic superconductors
The electrical resistivity of the quasi-1D organic superconductor (TMTSF)2PF6
was recently measured at low temperature from the critical pressure needed to
suppress the spin-density-wave state up to a pressure where superconductivity
has almost disappeared. This data revealed a direct correlation between the
onset of superconductivity at Tc and the strength of a non-Fermi-liquid linear
term in the normal-state resistivity, going as r(T) = r0 + AT + BT2 at low
temperature, so that A goes to 0 as Tc goes to 0. Here we show that the
contribution of low-frequency antiferromagnetic fluctuations to the
spin-lattice relaxation rate is also correlated with this non-Fermi-liquid term
AT in the resistivity. These correlations suggest that anomalous scattering and
pairing have a common origin, both rooted in the low-frequency
antiferromagnetic fluctuations measured by NMR. A similar situation may also
prevail in the recently-discovered iron-pnictide superconductors.Comment: ISCOM'09 proceedings to be published in Physica
Dbx1-Expressing Cells Are Necessary for the Survival of the Mammalian Anterior Neural and Craniofacial Structures
Development of the vertebrate forebrain and craniofacial structures are intimately linked processes, the coordinated growth of these tissues being required to ensure normal head formation. In this study, we identify five small subsets of progenitors expressing the transcription factor dbx1 in the cephalic region of developing mouse embryos at E8.5. Using genetic tracing we show that dbx1-expressing cells and their progeny have a modest contribution to the forebrain and face tissues. However, their genetic ablation triggers extensive and non cell-autonomous apoptosis as well as a decrease in proliferation in surrounding tissues, resulting in the progressive loss of most of the forebrain and frontonasal structures. Targeted ablation of the different subsets reveals that the very first dbx1-expressing progenitors are critically required for the survival of anterior neural tissues, the production and/or migration of cephalic neural crest cells and, ultimately, forebrain formation. In addition, we find that the other subsets, generated at slightly later stages, each play a specific function during head development and that their coordinated activity is required for accurate craniofacial morphogenesis. Our results demonstrate that dbx1-expressing cells have a unique function during head development, notably by controlling cell survival in a non cell-autonomous manner
Pinning/depinning of crack fronts in heterogeneous materials
The fatigue fracture surfaces of a metallic alloy, and the stress corrosion
fracture surfaces of glass are investigated as a function of crack velocity. It
is shown that in both cases, there are two fracture regimes, which have a well
defined self-affine signature. At high enough length scales, the universal
roughness index 0.78 is recovered. At smaller length scales, the roughness
exponent is close to 0.50. The crossover length separating these two
regimes strongly depends on the material, and exhibits a power-law decrease
with the measured crack velocity , with . The exponents and characterising the dependence of
and upon the pulling force are shown to be close to and
.Comment: 4 pages, latex, and 4 encapsulated postscript figure
X-ray Investigation of the Magneto-elastic Instability of alpha'-NaV2O5
We present an X-ray diffuse scattering study of the pretransitional
structural fluctuations of the magneto-elastic transition in alpha'-NaV2O5.
This transition is characterized by the appearance below Tsp~35K of satellite
reflections at the reduced wave vector (1/2,1/2,1/4). A large regime of
structural fluctuations is measured up to 90 K. These fluctuations are three
dimensional between Tsp and ~50K and quasi-one dimensional above ~60K. At 40 K
the anisotropy ratio is found to be (xib :xia :xic)= (3.8 : 1.8 : 1), which
reveals the importance of transverse interactions in the stabilization of the
low temperature phase. We discuss our results within the framework of recent
theories dealing with the simultaneous occurrence of a charge ordering, a spin
gap and a lattice distortion in this intriguing compound.Comment: Accepted in PRB Rapid.comm. Corrected typos, references added,
figures improve
Linear-T scattering and pairing from antiferromagnetic fluctuations in the (TMTSF)_2X organic superconductors
An exhaustive investigation of metallic electronic transport and
superconductivity of organic superconductors (TMTSF)_2PF_6 and (TMTSF)_2ClO_4
in the Pressure-Temperature phase diagram between T=0 and 20 K and a
theoretical description based on the weak coupling renormalization group method
are reported. The analysis of the data reveals a high temperature domain
(T\approx 20 K) in which a regular T^2 electron-electron Umklapp scattering
obeys a Kadowaki-Woods law and a low temperature regime (T< 8 K) where the
resistivity is dominated by a linear-in temperature component. In both
compounds a correlated behavior exists between the linear transport and the
extra nuclear spin-lattice relaxation due to antiferromagnetic fluctuations. In
addition, a tight connection is clearly established between linear transport
and T_c. We propose a theoretical description of the anomalous resistivity
based on a weak coupling renormalization group determination of
electron-electron scattering rate. A linear resistivity is found and its origin
lies in antiferromagnetic correlations sustained by Cooper pairing via
constructive interference. The decay of the linear resistivity term under
pressure is correlated with the strength of antiferromagnetic spin correlations
and T_c, along with an unusual build-up of the Fermi liquid scattering. The
results capture the key features of the low temperature electrical transport in
the Bechgaard salts
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