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911.
te Welscher P Zuniga A Kuijper S Drenth T Goedemans HJ Meijlink F Zeller R 《Science (New York, N.Y.)》2002,298(5594):827-830
Distal limb development and specification of digit identities in tetrapods are under the control of a mesenchymal organizer called the polarizing region. Sonic Hedgehog (SHH) is the morphogenetic signal produced by the polarizing region in the posterior limb bud. Ectopic anterior SHH signaling induces digit duplications and has been suspected as a major cause underlying congenital malformations that result in digit polydactyly. Here, we report that the polydactyly of Gli3-deficient mice arises independently of SHH signaling. Disruption of one or both Gli3 alleles in mouse embryos lacking Shh progressively restores limb distal development and digit formation. Our genetic analysis indicates that SHH signaling counteracts GLI3-mediated repression of key regulator genes, cell survival, and distal progression of limb bud development. 相似文献
912.
Mészáros P 《Science (New York, N.Y.)》2001,291(5501):79-84
Gamma-ray bursts (GRBs) are sudden, intense flashes of gamma rays that, for a few blinding seconds, light up in an otherwise fairly dark gamma-ray sky. They are detected at the rate of about once a day, and while they are on, they outshine every other gamma-ray source in the sky, including the sun. Major advances have been made in the last 3 or 4 years, including the discovery of slowly fading x-ray, optical, and radio afterglows of GRBs, the identification of host galaxies at cosmological distances, and evidence showing that many GRBs are associated with star-forming regions and possibly supernovae. Progress has been made in understanding how the GRB and afterglow radiation arises in terms of a relativistic fireball shock model. These advances have opened new vistas and questions on the nature of the central engine, the identity of their progenitors, the effects of the environment, and their possible gravitational wave, cosmic ray, and neutrino luminosity. The debates on these issues indicate that GRBs remain among the most mysterious puzzles in astrophysics. 相似文献
913.
Zepik H Shavit E Tang M Jensen TR Kjaer K Bolbach G Leiserowitz L Weissbuch I Lahav M 《Science (New York, N.Y.)》2002,295(5558):1266-1269
Differences in the two-dimensional packing arrangements of racemic and enantiomeric crystalline self-assemblies on the water surface of amphiphilic activated analogs of lysine and glutamic acid have been used to prepare oligopeptides of homochiral sequence and oligopeptides of single handedness from chiral nonracemic mixtures. The crystalline structures on the water surface were determined by grazing incidence x-ray diffraction and the diastereomeric composition of the oligopeptides by matrix-assisted laser desorption time-of-flight mass spectrometry with enantio-labeling. These results suggest that reactivity of ordered clusters at interfaces might have played a role in the generation of early homochiral biopolymers. 相似文献
914.
Viscosity of fluids in subduction zones 总被引:2,自引:0,他引:2
The viscosities of aqueous fluids with 10 to 80 weight percent dissolved silicates have been measured at 600 degrees to 950 degrees C and 1.0 to 2.0 gigapascals by in situ observation of falling spheres in the diamond anvil cell. The viscosities at 800 degrees C range from 10(-4) to 10(0.5) pascal seconds. The combination of low viscosities with a favorable wetting angle makes silicate-rich fluid an efficient agent for material transport at low-volume fractions. Our results therefore suggest that there may be a direct relationship between the position of the volcanic front and the onset of complete miscibility between water and silicate melt in the subducting slab. 相似文献
915.
Spahni R Chappellaz J Stocker TF Loulergue L Hausammann G Kawamura K Flückiger J Schwander J Raynaud D Masson-Delmotte V Jouzel J 《Science (New York, N.Y.)》2005,310(5752):1317-1321
The European Project for Ice Coring in Antarctica Dome C ice core enables us to extend existing records of atmospheric methane (CH4) and nitrous oxide (N2O) back to 650,000 years before the present. A combined record of CH4 measured along the Dome C and the Vostok ice cores demonstrates, within the resolution of our measurements, that preindustrial concentrations over Antarctica have not exceeded 773 +/- 15 ppbv (parts per billion by volume) during the past 650,000 years. Before 420,000 years ago, when interglacials were cooler, maximum CH4 concentrations were only about 600 ppbv, similar to lower Holocene values. In contrast, the N2O record shows maximum concentrations of 278 +/- 7 ppbv, slightly higher than early Holocene values. 相似文献
916.
Gérard GAILLARD 《油气储运》2005,24(Z1)
讨论了聚氨酯的化学性质,指出聚氨酯的性能取决于它的一些特性,如附着性、抗冲击性、抗水解能力、抗化学品的性质、阴极剥离及使用寿命等,并对这些性质进行了分析和评价性试验.建议现场涂敷聚氨酯涂层时必须彻底清除掉现有的涂层,并在新涂层涂敷之前进行适当的表面处理.同时还讨论了底层温度对聚氨酯防腐层涂敷的影响. 相似文献
917.
Periodic slow earthquakes from the Cascadia subduction zone 总被引:1,自引:0,他引:1
918.
Vanhollebeke B De Muylder G Nielsen MJ Pays A Tebabi P Dieu M Raes M Moestrup SK Pays E 《Science (New York, N.Y.)》2008,320(5876):677-681
The protozoan parasite Trypanosoma brucei is lysed by apolipoprotein L-I, a component of human high-density lipoprotein (HDL) particles that are also characterized by the presence of haptoglobin-related protein. We report that this process is mediated by a parasite glycoprotein receptor, which binds the haptoglobin-hemoglobin complex with high affinity for the uptake and incorporation of heme into intracellular hemoproteins. In mice, this receptor was required for optimal parasite growth and the resistance of parasites to the oxidative burst by host macrophages. In humans, the trypanosome receptor also recognized the complex between hemoglobin and haptoglobin-related protein, which explains its ability to capture trypanolytic HDLs. Thus, in humans the presence of haptoglobin-related protein has diverted the function of the trypanosome haptoglobin-hemoglobin receptor to elicit innate host immunity against the parasite. 相似文献
919.
Markó IE Stérin S Buisine O Mignani G Branlard P Tinant B Declercq JP 《Science (New York, N.Y.)》2002,298(5591):204-206
The hydrosilylation reaction enables the production of silicon polymers. Platinum-carbene complexes are reported that catalyze the hydrosilylation reaction of alkenes with remarkable efficiency and exquisite selectivity and avoid the formation of platinum colloids. By-products, typically encountered with previous catalytic systems, are suppressed with these platinum derivatives. 相似文献
920.