Femtosecond time-delay X-ray holography

Extremely intense and ultrafast X-ray pulses from free-electron lasers offer unique opportunities to study fundamental aspects of complex transient phenomena in materials. Ultrafast time-resolved methods usually require highly synchronized pulses to initiate a transition and then probe it after a precisely defined time delay. In the X-ray regime, these methods are challenging because they require complex optical systems and diagnostics. Here we propose and apply a simple holographic measurement scheme, inspired by Newton's 'dusty mirror' experiment, to monitor the X-ray-induced explosion of microscopic objects. The sample is placed near an X-ray mirror; after the pulse traverses the sample, triggering the reaction, it is reflected back onto the sample by the mirror to probe this reaction. The delay is encoded in the resulting diffraction pattern to an accuracy of one femtosecond, and the structural change is holographically recorded with high resolution. We apply the technique to monitor the dynamics of polystyrene spheres in intense free-electron-laser pulses, and observe an explosion occurring well after the initial pulse. Our results support the notion that X-ray flash imaging can be used to achieve high resolution, beyond radiation damage limits for biological samples. With upcoming ultrafast X-ray sources we will be able to explore the three-dimensional dynamics of materials at the timescale of atomic motion.     

行星着陆——推进下降过程的建模与控制(英文)

登陆火星或月球时,释放降落伞后进入推进阶段,通过轴向推力倾斜实现水平方向运动,能够与速度矢量(重力)的相反方向一致或与需要的加速度方向保持一致.后一种策略能实现精确着陆,在该策略下,倾斜角(俯仰角和偏转角)与水平加速度成正比.这种策略并未采用以水平加速度为姿态控制目标的递阶导航与控制方案,而是基于角加速度对位置作用的四阶动力学特性设计了一个独特的控制系统.除去倾斜角的非线性因素(垂直制动规定的轴向推力)后,系统综合动力学方程能够实现(准)输入-状态线性化.论文同时表明:一方面,基于参考轨迹(倾斜角和位置)的控制器设计仅能实现部分输入-状态线性化;另一方面,系统的内部稳定性可以证明.即使存在不可镇定的外部干扰动力学模态时,仍能确保系统稳定性.蒙特卡洛仿真验证了闭环控制策略的有效性.     

Feyerabend,truth, and relativisms: Footnotes to the Italian debate

There is a substantial literature on Feyerabend's relativism—including a few papers in this collection—but fewer specific studies of the ways that his writings and ideas have been taken up among the non-academic public. This is odd, given his obvious interest in the lives and concerns of persons who were not ‘intellectuals’—a term that, for him, had a pejorative ring to it. It is also odd, given the abundance of evidence of how Feyerabend's relativism played a role in a specific national and cultural context—namely, contemporary Italian debates about relativism. This paper offers a study of how Feyerabend's ideas have been deployed by Italian intellectuals and cultural commentators—including the current Pope—and critically assesses them.     

Identification of common variants influencing risk of the tauopathy progressive supranuclear palsy

Progressive supranuclear palsy (PSP) is a movement disorder with prominent tau neuropathology. Brain diseases with abnormal tau deposits are called tauopathies, the most common of which is Alzheimer's disease. Environmental causes of tauopathies include repetitive head trauma associated with some sports. To identify common genetic variation contributing to risk for tauopathies, we carried out a genome-wide association study of 1,114 individuals with PSP (cases) and 3,247 controls (stage 1) followed by a second stage in which we genotyped 1,051 cases and 3,560 controls for the stage 1 SNPs that yielded P ≤ 10(-3). We found significant previously unidentified signals (P < 5 × 10(-8)) associated with PSP risk at STX6, EIF2AK3 and MOBP. We confirmed two independent variants in MAPT affecting risk for PSP, one of which influences MAPT brain expression. The genes implicated encode proteins for vesicle-membrane fusion at the Golgi-endosomal interface, for the endoplasmic reticulum unfolded protein response and for a myelin structural component.     

From pluripotency to forebrain patterning: an in vitro journey astride embryonic stem cells

Embryonic stem cells (ESCs) have been used extensively as in vitro models of neural development and disease, with special efforts towards their conversion into forebrain progenitors and neurons. The forebrain is the most complex brain region, giving rise to several fundamental structures, such as the cerebral cortex, the hypothalamus, and the retina. Due to the multiplicity of signaling pathways playing different roles at distinct times of embryonic development, the specification and patterning of forebrain has been difficult to study in vivo. Research performed on ESCs in vitro has provided a large body of evidence to complement work in model organisms, but these studies have often been focused more on cell type production than on cell fate regulation. In this review, we systematically reassess the current literature in the field of forebrain development in mouse and human ESCs with a focus on the molecular mechanisms of early cell fate decisions, taking into consideration the specific culture conditions, exogenous and endogenous molecular cues as described in the original studies. The resulting model of early forebrain induction and patterning provides a useful framework for further studies aimed at reconstructing forebrain development in vitro for basic research or therapy.     

Tumor suppressive pathways in the control of neurogenesis

The generation of specialized neural cells in the developing and postnatal central nervous system is a highly regulated process, whereby neural stem cells divide to generate committed neuronal progenitors, which then withdraw from the cell cycle and start to differentiate. Cell cycle checkpoints play a major role in regulating the balance between neural stem cell expansion and differentiation. Loss of tumor suppressors involved in checkpoint control can lead to dramatic alterations of neurogenesis, thus contributing to neoplastic transformation. Here we summarize and critically discuss the existing literature on the role of tumor suppressive pathways and their regulatory networks in the control of neurogenesis and transformation.     

Scalarolbutenolide,a new sesterterpenoid from the marine spongeSpongia nitens

Summary A new sesterterpenoid, scalarolbutenolide (5), has been isolated from the marine spongeSpongia nitens; its structure, including the absolute stereochemistry, has been established by chemical and spectroscopic studies.This work is a part of the Progetto Finalizzato per l'Oceanografia e i Fondi Marini, C.N.R., Roma.We are indebted to A. Crispino, C. Di Pinto, G. Scognamiglio and R. Turco for their technical assistance.     

Palinurin,a new linear sesterterpene from a marine sponge

Summary The structure of a new linear sesterterpene, palinurin (1), obtained from the marine spongeIrcinia variabilis, was determined by spectral and chemical means.This work was carried out at the Progetto Finalizzato Oceanografia e Fondi Marini, C.N.R., Roma. We thank Professor M. Sarà (University of Genova) for the identification of the sponge, Mr C. Di Pinto for¹H-and¹³C-NMR, Mr G. Scognamiglio for technical assistance.