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 κ\kappa-(BEDT-TTF)2_2Cu[N(CN)2_2]Br

We present a detailed structural investigation of the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br at temperatures $T$ from 9 to 300 K. Anomalies in the $T$ dependence of the lattice parameters are associated with a glass-like transition previously reported at $T_g$ = 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 $T$ 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

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

Proton relaxation in the superconducting organic solid (BEDT-TTF)2Cu(NCS)2: evidence for relaxation by localized paramagnetic centers

The nonexponential character of the build-up of the nuclear magnetization usually increases on lowering the temperature. Superconducting fluctuations have been offered in a speculative way as an explanation for the nonexponential spin relaxation. We have observed the same relaxation behaviour in our investigation of the proton spin relaxation in (BEDT-TTF)2Cu(NCS)2 which is an organic conductor at room temperature and becomes a superconductor at Tc=10.4 K at ambient pressure. As we noticed during preliminary measurements that the degree of deviation from exponential relaxation depends on the sample under study we decided to do experiments under controlled conditions of sample preparation

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 $\xi_c$ separating these two regimes strongly depends on the material, and exhibits a power-law decrease with the measured crack velocity $\xi_c \propto v^{-\phi}$, with $\phi \simeq 1$. The exponents $\nu$ and $\beta$ characterising the dependence of $\xi_c$ and $v$ upon the pulling force are shown to be close to $\nu \simeq 2$ and $\beta \simeq 2$.Comment: 4 pages, latex, and 4 encapsulated postscript figure

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

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