Habitat and nesting biology of Mountain Plovers in Wyoming

Although previous research has considered habitat associations and breeding biology of Mountain Plovers in Wyoming at discrete sites, no study has considered these attributes at a statewide scale. We located 55 Mountain Plover nests in 6 counties across Wyoming during 2002 and 2003. Nests occurred in 2 general habitat types: grassland and desert-shrub. Mean estimated hatch date was 26 June ( n = 31) in 2002 and 21 June ( n = 24) in 2003. Mean hatch date was not related to latitude or elevation. Hatch success of nests was inferred in 2003 by the presence of eggshell fragments in the nest scrape. Eggs in 14 of 22 (64%) known-fate nests hatched. All grassland sites and 90% of desert sites were host to ungulate grazers, although prairie dogs were absent at 64% of nest sites. Nest plots had less grass coverage and reduced grass height compared with random plots. More than 50% of nests occurred on elevated plateaus. The Mountain Plover's tendency to nest on arid, elevated plateaus further substantiates claims that the bird is also a disturbed- prairie species.     

Strongly linked current flow in polycrystalline forms of the superconductor MgB2

The discovery of superconductivity at 39 K in magnesium diboride, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field-temperature (H-T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered.     

High critical current density and enhanced irreversibility field in superconducting MgB2 thin films

The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.     

Superconductivity in the non-oxide perovskite MgCNi3

The interplay of magnetic interactions, the dimensionality of the crystal structure and electronic correlations in producing superconductivity is one of the dominant themes in the study of the electronic properties of complex materials. Although magnetic interactions and two-dimensional structures were long thought to be detrimental to the formation of a superconducting state, they are actually common features of both the high transition-temperature (Tc) copper oxides and low-Tc material Sr2RuO4, where they appear to be essential contributors to the exotic electronic states of these materials. Here we report that the perovskite-structured compound MgCNi3 is superconducting with a critical temperature of 8 K. This material is the three-dimensional analogue of the LnNi2B2C family of superconductors, which have critical temperatures up to 16 K (ref. 2). The itinerant electrons in both families of materials arise from the partial filling of the nickel d-states, which generally leads to ferromagnetism as is the case in metallic Ni. The high relative proportion of Ni in MgCNi3 suggests that magnetic interactions are important, and the lower Tc of this three-dimensional compound-when compared to the LnNi2B2C family-contrasts with conventional ideas regarding the origins of superconductivity.     

Electron tomography at 2.4-ångström resolution

Transmission electron microscopy is a powerful imaging tool that has found broad application in materials science, nanoscience and biology. With the introduction of aberration-corrected electron lenses, both the spatial resolution and the image quality in transmission electron microscopy have been significantly improved and resolution below 0.5??ngstr?ms has been demonstrated. To reveal the three-dimensional (3D) structure of thin samples, electron tomography is the method of choice, with cubic-nanometre resolution currently achievable. Discrete tomography has recently been used to generate a 3D atomic reconstruction of a silver nanoparticle two to three nanometres in diameter, but this statistical method assumes prior knowledge of the particle's lattice structure and requires that the atoms fit rigidly on that lattice. Here we report the experimental demonstration of a general electron tomography method that achieves atomic-scale resolution without initial assumptions about the sample structure. By combining a novel projection alignment and tomographic reconstruction method with scanning transmission electron microscopy, we have determined the 3D structure of an approximately ten-nanometre gold nanoparticle at 2.4-?ngstr?m resolution. Although we cannot definitively locate all of the atoms inside the nanoparticle, individual atoms are observed in some regions of the particle and several grains are identified in three dimensions. The 3D surface morphology and internal lattice structure revealed are consistent with a distorted icosahedral multiply twinned particle. We anticipate that this general method can be applied not only to determine the 3D structure of nanomaterials at atomic-scale resolution, but also to improve the spatial resolution and image quality in other tomography fields.     

Neuronal and glial markers of the central nervous system

This brief review evaluates the expression of cell-specific markers on differentiated neural cells and, where necessary, on their developing precursors. Within these limitations only the commonly used markers are discussed and those deemed unequivocal are only briefly appraised.     

Common Molecular Mechanisms of Mammary Gland Development and Breast Cancer

The mammary gland undergoes major developmental changes during puberty and pregnancy. It is thought that stem cells drive mammary gland development during puberty and are responsible for tissue maintenance as well as the major growth and remodelling that occurs with every pregnancy. The use of sophisticated cell separation procedures has facilitated the prospective isolation of mammary epithelial stem and differentiated cell subpopulations from the mouse mammary gland, while studies of primary human breast cancers have described sub-populations of tumourigenic cells capable of initiating tumour growth in immuno-compromised mice. These potential tumour 'stem cells' constitute an important therapeutic target population with respect to cancer therapy, as these are likely to be the cells which maintain tumour growth. Understanding the origin of these cells, their relationship to breast cancer subtypes, and how and why they differ from normal breast stem cells will lead to a revolution in tumour understanding, treatment and prevention. (Part of a Multi-author Review).     

Alkaline phosphatase in synchronized human cells

Zusammenfassung Die Aktivität der alkalischen Phosphatase (AP) wurde in mit 5-Aminouracil synchronisierten Suspensionskulturen menschlicher Zellen histochemisch unter Benutzung von -Naphtholphosphat als Substrat und chemisch unter Benutzung vonp-Nitrophenylphosphat als Substrat untersucht. Die histochemischen Untersuchungen weisen auf eine gesteigerte AP-Aktivität in mitotischen und postmitotischen Zellen, insbesondere im Bereich der Kernmembran hin. Im Gegensatz dazu zeigen die Experimente mit synchronisierten Zellen eine weitgehend konstante AP-Aktivität während der gesamten DNS-Synthese, Mitose und unmittelbaren postmitotischen Phase. Die Ergebnisse der histochemischen Untersuchungen sprechen daher eher für eine intrazelluläre Verlagerung des Enzyms mit spezifischer Lokalisation und nicht für eine Steigerung der AP-Aktivität.

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USPHS Postdoctoral Fellow (National Cancer Institute Fellowship No. 2-F2-CA-13, 666-02).     

Loss of superconductivity with the addition of Al to MgB2 and a structural transition in Mg1-x AlxB2

The basic magnetic and electronic properties of most binary compounds have been well known for decades. The recent discovery of superconductivity at 39 K in the simple binary ceramic compound magnesium diboride, MgB2, was therefore surprising. Indeed, this material has been known and structurally characterized since the mid 1950s (ref. 2), and is readily available from chemical suppliers (it is commonly used as a starting material for chemical metathesis reactions). Here we show that the addition of electrons to MgB2, through partial substitution of Al for Mg, results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near an Al content of 10 per cent. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations.