Stem cells find their niche.

The concept that stem cells are controlled by particular microenvironments known as 'niches' has been widely invoked. But niches have remained largely a theoretical construct because of the difficulty of identifying and manipulating individual stem cells and their surroundings. Technical advances now make it possible to characterize small zones that maintain and control stem cell activity in several organs, including gonads, skin and gut. These studies are beginning to unify our understanding of stem cell regulation at the cellular and molecular levels, and promise to advance efforts to use stem cells therapeutically.     

Chemistry of ion coordination and hydration revealed by a K+ channel-Fab complex at 2.0 A resolution.

Ion transport proteins must remove an ion's hydration shell to coordinate the ion selectively on the basis of its size and charge. To discover how the K+ channel solves this fundamental aspect of ion conduction, we solved the structure of the KcsA K+ channel in complex with a monoclonal Fab antibody fragment at 2.0 A resolution. Here we show how the K+ channel displaces water molecules around an ion at its extracellular entryway, and how it holds a K+ ion in a square antiprism of water molecules in a cavity near its intracellular entryway. Carbonyl oxygen atoms within the selectivity filter form a very similar square antiprism around each K+ binding site, as if to mimic the waters of hydration. The selectivity filter changes its ion coordination structure in low K+ solutions. This structural change is crucial to the operation of the selectivity filter in the cellular context, where the K+ ion concentration near the selectivity filter varies in response to channel gating.     

右中立Beta过程的一些结果

给出了非参数Bayesian 中的Beta 过程的2个性质,并证明了在考虑非参数Bayesian 时,用Beta 过程和Beta-Stacy 过程作先验是等价的.     

A massive cloud of cold atomic hydrogen in the outer Galaxy.

A large fraction of the mass of the interstellar medium in our Galaxy is in the form of warm (103-104 K) and cool (50-100 K) atomic hydrogen (H i) gas. Cold (10-30 K) regions are thought to be dominated by dense clouds of molecular hydrogen. Cold H i is difficult to observe, and therefore our knowledge of its abundance and distribution in the interstellar medium is poor. The few known clouds of cold H i are much smaller in size and mass than typical molecular clouds. Here we report the discovery that the H i supershell GSH139-03-69 is very cold (10 K). It is about 2 kiloparsecs in size and as massive as the largest molecular complexes. The existence of such an immense structure composed of cold atomic hydrogen in the interstellar medium runs counter to the prevailing view that cold gas resides almost exclusively in clouds dominated by molecular hydrogen.     

浅谈公路养护管理存在的问题与对策

公路基本建设事业的快速发展，对公路养护工程提出了更高的要求，公路养护不断遇到新的难题，面对困难和挑战。文章结合本地区公路养护实际工作实践，对公路养护管理存在的问题进行了分析，并探讨了相应对策。     

在量子场弱作用领域中基于微扰论计算μ衰变的寿命

考虑S矩阵元和物理的截面及寿命之间的普遍关系，采用一种变通方法，通过对中微子动量k和k’的积分计算，由μ衰变寿命的观测值和实验值得到弱衰变耦合常数，最后得到μ衰变的寿命r．     

On the complexity of self.

Self peptides bound to self major histocompatibility complex (MHC) molecules have been implicated both in positive and in negative selection of T cells during intrathymic development. We report here that the novel MHC-restricted monoclonal antibody Y-Ae detects the MHC class II bound form of a major self peptide. Y-Ae binds approximately 12% of the relevant MHC class II molecules on self antigen presenting cells. The peptide detected by Y-Ae is one of several major peptides eluted from the MHC molecule. These data suggest that self peptides presented by self MHC class II molecules at densities sufficient to signal a CD4 T cell are of very limited complexity. Furthermore, as Y-Ae stains antigen presenting cells that mediate negative selection but not thymic cortical epithelial cells that drive positive selection, differential expression of self peptide:self MHC class II complexes may be a key feature of intrathymic selection.     

Homeobox gene expression correlated with the bifurcation process of limb cartilage development.

The complex architecture of the limb cartilage pattern probably develops by the sequential segmentation and branching process of precartilaginous cell condensation under the control of positional signalling provided by the zone of polarizing activity (anteroposterior) and the apical ectodermal ridge (proximodistal). This signalling is monitored and interpreted in the mesenchymal cells and induces the position-specific response of subsets of genes. Homeobox genes may be responsible for the interpretation of signalling. A correlation between limb pattern and expression domains of the homeobox genes in the upstream region of Hox/Chox-4 has been proposed. We have analysed the spatial expression pattern of the Chox-1 genes during development of chick limb buds. In contrast to genes in Hox/Chox-4 expressed coordinately along the anteroposterior axis, homeobox genes in Chox-1 have unique and mutually exclusive expression domains along the proximodistal axis. We report here that the expression domains of the Chox-1 genes are closely related to the segmental structure of cartilage along the proximodistal axis, whereas the expression domains of the Chox-4 genes are related to the cartilage branching pattern.