A role for Gbx2 in repression of Otx2 and positioning the mid/hindbrain organizer.

The mid/hindbrain (MHB) junction can act as an organizer to direct the development of the midbrain and anterior hindbrain. In mice, Otx2 is expressed in the forebrain and midbrain and Gbx2 is expressed in the anterior hindbrain, with a shared border at the level of the MHB organizer. Here we show that, in Gbx2-/- mutants, the earliest phenotype is a posterior expansion of the Otx2 domain during early somite stages. Furthermore, organizer genes are expressed at the shifted Otx2 border, but not in a normal spatial relationship. To test whether Gbx2 is sufficient to position the MHB organizer, we transiently expressed Gbx2 in the caudal Otx2 domain and found that the Otx2 caudal border was indeed shifted rostrally and a normal appearing organizer formed at this new Otx2 border. Transgenic embryos then showed an expanded hindbrain and a reduced midbrain at embryonic day 9.5-10. We propose that formation of a normal MHB organizer depends on a sharp Otx2 caudal border and that Gbx2 is required to position and sharpen this border.     

The molecular nature of the zebrafish tail organizer

Based on grafting experiments, Mangold and Spemann showed the dorsal blastopore lip of an amphibian gastrula to be able to induce a secondary body axis. The equivalent of this organizer region has been identified in different vertebrates including teleosts. However, whereas the graft can induce ectopic head and trunk, endogenous and ectopic axes fuse in the posterior part of the body, raising the question of whether a distinct organizer region is necessary for tail development. Here we reveal, by isochronic and heterochronic transplantation, the existence of a tail organizer deriving from the ventral margin of the zebrafish embryo, which is independent of the dorsal Spemann organizer. Loss-of-function experiments reveal that bone morphogenetic protein (BMP), Nodal and Wnt8 signalling pathways are required for tail development. Moreover, stimulation of naive cells by a combination of BMP, Nodal and Wnt8 mimics the tail-organizing activity of the ventral margin and induces surrounding tissues to become tail. In contrast to induction of the vertebrate head, known to result from the triple inhibition of BMP, Nodal and Wnt, here we show that induction of the tail results from the triple stimulation of BMP, Nodal and Wnt8 signalling pathways.     

Tom22 is a multifunctional organizer of the mitochondrial preprotein translocase.

Mitochondrial preproteins are imported by a multisubunit translocase of the outer membrane (TOM), including receptor proteins and a general import pore. The central receptor Tom22 binds preproteins through both its cytosolic domain and its intermembrane space domain and is stably associated with the channel protein Tom40 (refs 11-13). Here we report the unexpected observation that a yeast strain can survive without Tom22, although it is strongly reduced in growth and the import of mitochondrial proteins. Tom22 is a multifunctional protein that is required for the higher-level organization of the TOM machinery. In the absence of Tom22, the translocase dissociates into core complexes, representing the basic import units, but lacks a tight control of channel gating. The single membrane anchor of Tom22 is required for a stable interaction between the core complexes, whereas its cytosolic domain serves as docking point for the peripheral receptors Tom20 and Tom70. Thus a preprotein translocase can combine receptor functions with distinct organizing roles in a multidomain protein.     

基于涨跌停规则的股票期权定价

借鉴Merton的跳跃扩散模型的思想,根据中国股市的具体情况,用含有Poisson过程的Ito-Skorohod随机微分方程描述股票价格的运动.利用鞅定价技巧(风险中性方法)推导出考虑涨跌停规则的跳跃扩散的股票期权在t时刻的价格公式,并利用随机模拟和中心极限定理讨论如何具体计算股票期权的价格.     

极限存在准则的应用

极限存在准则不但给出了判定函数极限存在的方法,同时也给出了求极限的方法.阐述如何利用极限存在准则来求函数极限.     

Scaling effects in caudal fin propulsion and the speed of ichthyosaurs.

Four unrelated groups of large cruising vertebrates (tunas, whales, lamnid sharks and parvipelvian ichthyosaurs) evolved tuna-shaped (thunniform) body plans. Stringent physical constraints, imposed by the surrounding fluids, are probably responsible for this example of evolutionary convergence. Here I present a mathematical model of swimming kinematics and fluid mechanics that specifies and quantifies such constraints, and test the model with empirical data. The test shows quantitatively that morphology, kinematics, and physiology indeed covary tightly in large cruisers. The model enables calculations of optimal cruising speed from external measurements, and also predicts that wide caudal fin spans, typical of thunniform swimmers, are necessary for large cruisers. This finding is contrary to a popular yet rather teleological view that thunniform tails were selected for their high aspect ratios that increased propulsive efficiency. I also show by calculation that Stenopterygius, a Jurassic ichthyosaur, probably had optimal cruising speeds and basal metabolic rates similar to living tunas.     

页岩气双二维水平井极限延伸能力研究

页岩气藏的开发多采用水平井,水平段的延伸能力直接影响到页岩气藏的综合开发效益。双二维水平井具有施工难度小、邻井碰撞风险低、作业费用低等优点。根据长宁页岩气的钻井实际情况,分析了钻机承载能力、钻柱安全系数、地层承压能力及额定泵压等因素对水平井延伸能力的影响,推导了相应计算模型。以长宁已钻页岩气井为基础,计算了不同影响因素随水平段长度的变化。研究表明,双二维水平井的延伸能力主要受地层承压能力和额定泵压影响,钻井施工中应选择合理的钻井液密度和排量,降低水平段岩屑床高度,采取适当的堵漏措施,选择高额定泵压的钻井泵。     

半稳定环的不存在性

本文将半稳定环存在的必要条件加以变形,使之含有一个可供选择的任意常数。利用这个新的必要条件得到若干半稳定环不存在的判别法,其中包括在单个奇点外围不存在半稳定环的条件,在两个奇点外围不同时存在半稳定环的条件,以及在两个奇点外围均不存在半稳定环的条件。     

等价无穷小的极限定理

求极限时,正确使用等价无穷小代换,可以简化计算.在求两个无穷小之比的极限时,若分子及分母满足一定的条件,可将分子、分母用等价无穷小来代换.并进一步给出求极限时,若因式中某个因子是两个无穷小之和、差时,可用等价无穷小来代换的条件;给出了求幂指函数的极限时,其底和指数可分别用它相应的等价无穷小代换的条件及相关的一些结论.     

ANXA2对Caco2细胞凋亡的影响

为研究膜联蛋白A2(ANXA2)基因与人结直肠癌Caco2细胞凋亡之间的关系,采用RNAi技术沉默Caco2细胞的ANXA2表达后,利用流式细胞仪分析细胞凋亡水平,并利用激光共聚焦显微镜和透射电子显微镜对凋亡细胞进行形态学检测。流式细胞仪检测结果显示,ANXA2表达被抑制后细胞线粒体正常膜电位发生改变(P0.05),线粒体结构受损;同时,形态学检测结果表明,ANXA2表达被抑制后细胞出现了较为明显的凋亡样特征。结果提示:ANXA2在人结直肠癌Caco2细胞的高表达能一定程度地促进细胞生长,抑制ANXA2表达可以诱导细胞出现一定程度的凋亡样形态特征,但不足以引起细胞广泛而明显的凋亡;推测ANXA2表达水平并不是唯一影响瘤细胞凋亡的因素。该结果支持关于ANXA2具有作为肿瘤靶向诊断、治疗靶分子潜能的推论。     

Axial patterning in cephalochordates and the evolution of the organizer

The organizer of the vertebrate gastrula is an important signalling centre that induces and patterns dorsal axial structures. Although a topic of long-standing interest, the evolutionary origin of the organizer remains unclear. Here we show that the gastrula of the cephalochordate amphioxus expresses dorsal/ventral (D/V) patterning genes (for example, bone morphogenetic proteins (BMPs), Nodal and their antagonists) in patterns reminiscent of those of their vertebrate orthlogues, and that amphioxus embryos, like those of vertebrates, are ventralized by exogenous BMP protein. In addition, Wnt-antagonists (for example, Dkks and sFRP2-like) are expressed anteriorly, whereas Wnt genes themselves are expressed posteriorly, consistent with a role for Wnt signalling in anterior/posterior (A/P) patterning. These results suggest evolutionary conservation of the mechanisms for both D/V and A/P patterning of the early gastrula. In light of recent phylogenetic analyses placing cephalochordates basally in the chordate lineage, we propose that separate signalling centres for patterning the D/V and A/P axes may be an ancestral chordate character.     

突破NAT限制的方法浅析

就目前广泛使用的网络地址映射(NAT)技术的限制做了一些分析,就突破这些限制的方法做了一些阐述。     

极限2-周期连分式的一种新算法

加速收敛在连分式理论中占有重要的地位,对连分式进行加速收敛最常用的方法是选择合适的修正因子。如果连分式是极限k-周期的,则修正因子序列也应是k-周期的,这就使对于k≥2的周期连分式其修正因子的选取较为困难;文章借助了连分式的压缩技术,针对极限2-周期连分式推导出一种新算法,使修正因子的选取变得简单;数值例子表明新算法使连分式的收敛更快、精度更高。     

如何用数列极限定义证明数列极限问题

以数列极限为例,详细阐述了用极限定义证明极限存在的三种常用的方法:基本方法、适当放大法、条件放大法,以及在应用这些方法时应注意的一些主要问题,从而强化对极限概念的理解·     

一类奇次微分系统的极限环的存在性

对奇次微分系统·ｘ＝ － ｙ＋ δｘ＋ ａｘｙ＋ ｂｘ２ｎ＋１ ,·ｙ＝ ｘ 进行定性分析,运用Ｎ．Ｌｅｖｉｓｏｎ,Ｏ．Ｋ．Ｓｍｉｔｈ 定理和А．В．Драгидёв定理得到其在一定条件下极限环的存在性,并借助对称方法、特殊曲线以及构造Ｄｕｌａｃ 函数得到其在一定条件下的极限环不存在性     

细菌培养模型极限环的唯一性

分析了一类细菌培养模型,证明了在一定参数范围内极限环是唯一的。     

李普希茨函数族的纲与诱导极限

本文利用FH空间理论 ,研究了定义在Banach空间球上的李普希茨函数族的纲与诱导极限     

极限平衡法对某工程滑坡的计算与分析

由于黄土的结构比较松散,粘聚力、内摩擦角等土力学参数偏小,在强降雨等外部因素的影响下,极易发生滑坡事故。本文以某黄土地区的工程滑坡为研究对象,采用极限平衡法,对该滑坡三个剖面进行了总抗滑力、总下滑力及稳定性系数的计算。通过分析计算结果,对三个剖面的稳定性现状及可能出现的剪出口位置进行了预测,为滑坡治理方案的制定提供了依据。也为同类工程的分析与计算提供了经验。     

一类多项式系统的极限环

讨论了多项式系统dx/dt=ydy/dt=(a1x2x-x)a-1·[a1x2-x+(a2x2+δ)y]的极限环,其中α为正的奇数,得到了极限环存在与不存在的若干条件