Adult pancreatic beta-cells are formed by self-duplication rather than stem-cell differentiation

How tissues generate and maintain the correct number of cells is a fundamental problem in biology. In principle, tissue turnover can occur by the differentiation of stem cells, as is well documented for blood, skin and intestine, or by the duplication of existing differentiated cells. Recent work on adult stem cells has highlighted their potential contribution to organ maintenance and repair. However, the extent to which stem cells actually participate in these processes in vivo is not clear. Here we introduce a method for genetic lineage tracing to determine the contribution of stem cells to a tissue of interest. We focus on pancreatic beta-cells, whose postnatal origins remain controversial. Our analysis shows that pre-existing beta-cells, rather than pluripotent stem cells, are the major source of new beta-cells during adult life and after pancreatectomy in mice. These results suggest that terminally differentiated beta-cells retain a significant proliferative capacity in vivo and cast doubt on the idea that adult stem cells have a significant role in beta-cell replenishment.     

The formation of a massive protostar through the disk accretion of gas

The formation of low-mass stars like our Sun can be explained by the gravitational collapse of a molecular cloud fragment into a protostellar core and the subsequent accretion of gas and dust from the surrounding interstellar medium. Theoretical considerations suggest that the radiation pressure from the protostar on the in-falling material may prevent the formation of stars above ten solar masses through this mechanism, although some calculations have claimed that stars up to 40 solar masses can in principle be formed via accretion through a disk. Given this uncertainty and the fact that most massive stars are born in dense clusters, it was suggested that high-mass stars are the result of the runaway merging of intermediate-mass stars. Here we report observations that clearly show a massive star being born from a large rotating accretion disk. The protostar has already assembled about 20 solar masses, and the accretion process is still going on. The gas reservoir of the circumstellar disk contains at least 100 solar masses of additional gas, providing sufficient fuel for substantial further growth of the forming star.     

Infall of gas as the formation mechanism of stars up to 20 times more massive than the Sun

Theory predicts and observations confirm that low-mass stars (like the Sun) in their early life grow by accreting gas from the surrounding material. But for stars approximately 10 times more massive than the Sun (approximately 10M(o)), the powerful stellar radiation is expected to inhibit accretion and thus limit the growth of their mass. Clearly, stars with masses >10M(o) exist, so there must be a way for them to form. The problem may be solved by non-spherical accretion, which allows some of the stellar photons to escape along the symmetry axis where the density is lower. The recent detection of rotating disks and toroids around very young massive stars has lent support to the idea that high-mass ( > 8M(o)) stars could form in this way. Here we report observations of an ammonia line towards a high-mass star forming region. We conclude that the gas is falling inwards towards a very young star of approximately 20M(o), in line with theoretical predictions of non-spherical accretion.     

Detectable radio flares following gravitational waves from mergers of binary neutron stars

Mergers of neutron-star/neutron-star binaries are strong sources of gravitational waves. They can also launch subrelativistic and mildly relativistic outflows and are often assumed to be the sources of short γ-ray bursts. An electromagnetic signature that persisted for weeks to months after the event would strengthen any future claim of a detection of gravitational waves. Here we present results of calculations showing that the interaction of mildly relativistic outflows with the surrounding medium produces radio flares with peak emission at 1.4 gigahertz that persist at detectable (submillijansky) levels for weeks, out to a redshift of 0.1. Slower subrelativistic outflows produce flares detectable for years at 150 megahertz, as well as at 1.4 gigahertz, from slightly shorter distances. The radio transient RT 19870422 (ref. 11) has the properties predicted by our model, and its most probable origin is the merger of a compact neutron-star/neutron-star binary. The lack of radio detections usually associated with short γ-ray bursts does not constrain the radio transients that we discuss here (from mildly relativistic and subrelativistic outflows) because short γ-ray burst redshifts are typically >0.1 and the appropriate timescales (longer than weeks) have not been sampled.     

四部分类是学术分类而非图书分类--兼与黄晏妤先生商榷

针对四部分类是图书分类而非学术分类的说法，从四部分类的演变历史、社会功能、学术价值和式微结局等四个方面，论证了四部分类的实质是学术分类而非图书分类的观点。     

Fossiliferous Lana'i deposits formed by multiple events rather than a single giant tsunami

Giant tsunamis, generated by submarine landslides in the Hawaiian Islands, have been thought to be responsible for the deposition of chaotic gravels high on the southern coastal slopes of the islands of Lana'i and Moloka'i, Hawaii. Here we investigate this hypothesis, using uranium-thorium dating of the Hulopoe gravel (on Lana'i) and a study of stratigraphic relationships, such as facies changes and hiatuses, within the deposit. The Hulopoe gravel contains corals of two age groups, representing marine isotope stages 5e and 7 (approximately 135,000 and 240,000 years ago, respectively), with significant geographical and stratigraphic ordering. We show that the Hulopoe gravel was formed by multiple depositional events, separated by considerable periods of time, thus invalidating the main premise of the 'giant wave' hypothesis. Instead, the gravels were probably deposited during interglacial periods (when sea level was relatively high) by typical Hawaiian shoreline processes such as seasonal wave patterns, storm events and possibly 'normal' tsunamis, and reached their present height by uplift of Lana'i.     

The critical role of disks in the formation of high-mass stars

Although massive stars (commonly defined as those in excess of about eight solar masses, or with initial luminosities of a thousand times the solar luminosity or more) have an enormous impact on the galactic environment, how they form has been a mystery. The solution probably involves the existence of accretion disks. Rotational motions have been found in the gas surrounding young high-mass stars, which suggests that non-spherical accretion could be the fundamental ingredient of the massive-star formation recipe.     

近代图书馆的先驱是古代书院而非藏书楼

在对书院、藏书楼、学校和图书馆的本质属性与功能特征进行辨析的基础上,提出就本质而言,近现代图书馆的先驱是书院而非藏书楼,书院精神的继承发扬者是图书馆而非学校.     

The formation of the first low-mass stars from gas with low carbon and oxygen abundances

The first stars in the Universe are predicted to have been much more massive than the Sun. Gravitational condensation, accompanied by cooling of the primordial gas via molecular hydrogen, yields a minimum fragmentation scale of a few hundred solar masses. Numerical simulations indicate that once a gas clump acquires this mass it undergoes a slow, quasi-hydrostatic contraction without further fragmentation; lower-mass stars cannot form. Here we show that as soon as the primordial gas--left over from the Big Bang--is enriched by elements ejected from supernovae to a carbon or oxygen abundance as small as approximately 0.01-0.1 per cent of that found in the Sun, cooling by singly ionized carbon or neutral oxygen can lead to the formation of low-mass stars by allowing cloud fragmentation to smaller clumps. This mechanism naturally accommodates the recent discovery of solar-mass stars with unusually low iron abundances (10(-5.3) solar) but with relatively high (10(-1.3) solar) carbon abundance. The critical abundances that we derive can be used to identify those metal-poor stars in our Galaxy with elemental patterns imprinted by the first supernovae. We also find that the minimum stellar mass at early epochs is partially regulated by the temperature of the cosmic microwave background.     

住宅小区后山崩塌成因机制及稳定性评价

该小区地点位于宜宾市旧州组团岷江北路以东,总征地面积约为40 896m2,建设用地面积约为37 010m2,拟建房屋5栋,拟入住居民5 000余人。由于建筑区域后方存在一个高于65～105m,长约300m的危岩崩塌区,会影响到建筑小区房屋建筑安全和居民生命财产安全。在基于对本地区的工程地质条件和危岩控制结构面的调查,阐述了危岩变形破坏的机制和模式,并结合相应的稳定性评价给出相应的防治措施。     

A microenvironment,rather than chemical,initiates the cardiomyogenic differentiation of marrow stromal cells

To investigate the potential of cardiomyogenic differentiation of rat hone marrow stromal cells (MSCs), they were exposed to 5-azacytidine treatments (single/repeat) at varying concentrations (3, 5, 10μmol/L) and the fates of the cells were analyzed by immunocytochemistry, Western blot and the reporter gene of enhanced cyan fluorescent protein (ECFP) under the control of ventricular myosin light chain 2 (MLC2v) promoter. MSCs were also cocultured with cardiomyocytes for periods up to 16 days, and the expression of cardiac myosin heavy chain(MHC) and troponin Ⅰ (Tn I) proteins was analyzed. After the induction with 5-azacytidine, neither spontaneously beating ceils nor myotubes were found; MHC and Tn I proteins were also undetectable and no ECFP-positive MSCs were detected. But when cocultured with cardiomyocytes, spontaneously contracting MSCs were observed and cardiac specific proteins could be detected. The results proved that the novel effects of 5-azacytldine on the cardiomyogenic differentiation of MSCs should be questioned and a direct intercellular communication with cardiomyocytes is necessary for MSCs to differentiate into cardiomyocytes.     

星的盘状分布

在广义相对论框架内 ,运用相对论形式的无碰撞Boltzmann方程和Schwarzshild度规 ,求得了中心为Schwarzshild黑洞的星的盘状分布 .计算结果表明盘状分布时的星数衰减比只有径向速度时的球状分布慢     

二次引力超导研究

引入二次引力场电磁型张量,将线性二次引力场方程转化为麦克斯韦方程组形式,并利用该方程组推导出二次引力超导方程,研究结果表明,二次引力超导与Einstein引力超导Shielding效应存在差异,该差异由二次引力理论的曲率平方项引起.     

旋转混合星的结构

利用微扰方法研究旋转混合星的结构.应用基于相对论平均场理论的硬和软的两种强子物质状态方程以及将s夸克质量作为自由参量的夸克物质的MIT袋模型来构造混合星模型.研究了旋转对于星体内部结构的影响,并近一步对比和分析不同模型星体结构改变的不同.结果显示s夸克质量对于旋转星体的结构和性质有较大的影响.     

地层塌陷区段埋地管道变形与应力分析

已有文献都是基于Winkler地基梁模型来研究塌陷以及悬空等对管道安全的影响,还没有采用其他力学分析模型的报道.分别采用Winkler和理想弹塑性地基梁模型,研究地层塌陷对管道安全的影响.建立了管土相互作用的几何大变形力学分析模型,采用非线性理论对力学模型求解.分析结果表明:在管道的屈服应力较小的情况下,为简化工程计算可采用Winkler模型计算临界塌陷区长度,较弹塑性模型简单,便于工程应用;当地层塌陷区长度较小时,为简化计算可采用Winkler模型分析管道的变形与应力;弹塑性模型可忽略土弹簧刚度对管道分析结果的影响,而Winkler模型则要考虑土弹簧刚度对其的影响.给出了使管道失效的临界地层塌陷区长度,所得结果可供工程设计参考.     

小金县三关桥村崩塌堆积体形成机制及稳定性研究

崩塌是一种破坏力极强的地质灾害,5·12汶川地震和4.20芦山地震后,在碎裂状结构岩体中产生了大量的这种地质灾害。以小金县三关桥村崩塌堆积体为研究对象,在充分分析崩塌形成机制的基础上,运用GEO-SLOPE软件对该崩塌堆积体稳定性进行计算分析。崩塌形成机制分为5个阶段:应力重分布卸荷回弹阶段—裂隙发展岩体碎裂化阶段—地震作用下地形导致加速度放大阶段—裂隙扩张连通阶段—岩体脱离物源,最终产生崩塌;有限元分析表明崩塌堆积体在天然工况下稳定,在加速度峰值为0.15g地震作用下开始失稳。分析结果可为其治理提供相关依据和参考。     

Interleukin-23 rather than interleukin-12 is the critical cytokine for autoimmune inflammation of the brain

Interleukin-12 (IL-12) is a heterodimeric molecule composed of p35 and p40 subunits. Analyses in vitro have defined IL-12 as an important factor for the differentiation of naive T cells into T-helper type 1 CD4+ lymphocytes secreting interferon-gamma (refs 1, 2). Similarly, numerous studies have concluded that IL-12 is essential for T-cell-dependent immune and inflammatory responses in vivo, primarily through the use of IL-12 p40 gene-targeted mice and neutralizing antibodies against p40. The cytokine IL-23, which comprises the p40 subunit of IL-12 but a different p19 subunit, is produced predominantly by macrophages and dendritic cells, and shows activity on memory T cells. Evidence from studies of IL-23 receptor expression and IL-23 overexpression in transgenic mice suggest, however, that IL-23 may also affect macrophage function directly. Here we show, by using gene-targeted mice lacking only IL-23 and cytokine replacement studies, that the perceived central role for IL-12 in autoimmune inflammation, specifically in the brain, has been misinterpreted and that IL-23, and not IL-12, is the critical factor in this response. In addition, we show that IL-23, unlike IL-12, acts more broadly as an end-stage effector cytokine through direct actions on macrophages.     

星图并的匹配唯一性

本文利用匹配多项式和伴随多项式的性质，证明了一类星图的并是匹配唯一的。