1,720,971 research outputs found
Radiation damage studies in cardiac muscle cells and tissue using microfocused X-ray beams: experiment and simulation
No full textSoft materials are easily affected by radiation damage from intense, focused synchrotron beams, often limiting the use of scanning diffraction experiments to radiation-resistant samples. To minimize radiation damage in experiments on soft tissue and thus to improve data quality, radiation damage needs to be studied as a function of the experimental parameters. Here, the impact of radiation damage in scanning X-ray diffraction experiments on hydrated cardiac muscle cells and tissue is investigated. It is shown how the small-angle diffraction signal is affected by radiation damage upon variation of scan parameters and dose. The experimental study was complemented by simulations of dose distributions for microfocused X-ray beams in soft muscle tissue. As a simulation tool, the Monte Carlo software package EGSnrc was used that is widely used in radiation dosimetry research. Simulations also give additional guidance for a more careful planning of dose distribution in tissue
Multiscale X-Ray Analysis of Biological Cells and Tissues by Scanning Diffraction and Coherent Imaging
Full text linkThe past 70 years of muscle research have profoundly shaped our current understanding of the structure and function of muscle. X-ray diffraction became a key method in its structural analysis and yielded valuable insights into the molecular arrangement of the contraction apparatus. This work employs an extension of the X-ray diffraction methodology, scanning X-ray diffraction, for structural imaging of biological cells and tissue. With this technique periodicites in a structure on the order of several nanometers can be detected and, by raster scanning of the X-ray beam over the sample, images of the nano-structure can be computed. This makes it an ideal method to study e.g. local changes in the usually highly conserved distance between the myosin filaments in muscle cells. This work shows how such experiments can contribute to the understanding of cardiac tissue architecture and in particular to the development of the contraction apparatus of cardiac muscle cells upon cell maturation. This thesis also covers the fruitful combination of scanning X-ray diffraction and X-ray fluorescence microscopy in Parkinson-related biomedical research. While e.g. scanning X-ray diffraction was used to image the lamellar periodicity of the myelin sheath in brain tissue sections, by X-ray fluorescence microscopy we could verify a dishomeostasis of common trace elements in substantia nigra neurons due to Parkinson’s disease. To obtain a more complete picture of the complex, often very hierarchical, structure of biological cells and tissues, holographic X-ray imaging can be used. The holographic approach is used to directly image the electron density of the sample and greatly extends the resolution range covered in a scanning X-ray diffraction experiment. Due to its adjunct contrast, low dose requirement and experimental compatibility, holographic imaging can be advantageously paired with scanning X-ray diffraction. This work shows that, in combination, the two methods can cover three orders of magnitude in resolution, from approximately 10 nm to 10 μm. Lastly, an example is given where holographic imaging could be used to visualize barium aggregates in macrophages, not only in two- but also in three dimensions. To stabilize the macrophages for the holographic recording, the barium-loaded cells were trapped in an optical stretcher
The Dual Functional Reflecting Iris of the Zebrafish
No full textAbstract Many marine organisms have evolved a reflective iris to prevent unfocused light from reaching the retina. The fish iris has a dual function, both to camouflage the eye and serving as a light barrier. Yet, the physical mechanism that enables this dual functionality and the benefits of using a reflective iris have remained unclear. Using synchrotron microfocused diffraction, cryo‐scanning electron microscopy imaging, and optical analyses on zebrafish at different stages of development, it is shown that the complex optical response of the iris is facilitated by the development of high‐order organization of multilayered guanine‐based crystal reflectors and pigments. It is further demonstrated how the efficient light reflector is established during development to allow the optical functionality of the eye, already at early developmental stages.Israel Science Foundation https://doi.org/10.13039/50110000397
The optical stretcher as a tool for single-particle X-ray imaging and diffraction
No full textFor almost half a century, optical tweezers have successfully been used to micromanipulate micrometre and sub-micrometre-sized particles. However, in recent years it has been shown experimentally that, compared with single-beam traps, the use of two opposing and divergent laser beams can be more suitable in studying the elastic properties of biological cells and vesicles. Such a configuration is termed an optical stretcher due to its capability of applying high deforming forces on biological objects such as cells. In this article the experimental capabilities of an optical stretcher as a potential sample delivery system for X-ray diffraction and imaging studies at synchrotrons and X-ray free-electron laser (FEL) facilites are demonstrated. To highlight the potential of the optical stretcher its micromanipulation capabilities have been used to image polymer beads and label biological cells. Even in a non-optimized configuration based on a commercially available optical stretcher system, X-ray holograms could be recorded from different views on a biological cell and the three-dimensional phase of the cell could be reconstructed. The capability of the setup to deform cells at higher laser intensities in combination with, for example, X-ray diffraction studies could furthermore lead to interesting studies that couple structural parameters to elastic properties. By means of high-throughput screening, the optical stretcher could become a useful tool in X-ray studies employing synchrotron radiation, and, at a later stage, femtosecond X-ray pulses delivered by X-ray free-electron lasers
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
X-Ray Structural Analysis of Single Adult Cardiomyocytes: Tomographic Imaging and Microdiffraction
No full textWe present a multiscale imaging approach to characterize the structure of isolated adult murine cardiomyocytes based on a combination of full-field three-dimensional coherent x-ray imaging and scanning x-ray diffraction. Using these modalities, we probe the structure from the molecular to the cellular scale. Holographic projection images on freeze-dried cells have been recorded using highly coherent and divergent x-ray waveguide radiation. Phase retrieval and tomographic reconstruction then yield the three-dimensional electron density distribution with a voxel size below 50 nm. In the reconstruction volume, myofibrils, sarcomeric organization, and mitochondria can be visualized and quantified within a single cell without sectioning. Next, we use microfocusing optics by compound refractive lenses to probe the diffraction signal of the actomyosin lattice. Comparison between recordings of chemically fixed and untreated, living cells indicate that the characteristic lattice distances shrink by ∼10% upon fixation
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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