首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   167篇
  免费   33篇
  国内免费   10篇
  210篇
  2023年   4篇
  2022年   3篇
  2021年   6篇
  2020年   3篇
  2019年   1篇
  2017年   1篇
  2016年   5篇
  2015年   4篇
  2014年   7篇
  2013年   6篇
  2012年   12篇
  2011年   8篇
  2010年   9篇
  2009年   14篇
  2008年   7篇
  2007年   11篇
  2006年   3篇
  2005年   9篇
  2004年   1篇
  2003年   9篇
  2002年   5篇
  2001年   5篇
  2000年   8篇
  1999年   5篇
  1998年   8篇
  1997年   3篇
  1996年   3篇
  1995年   1篇
  1994年   2篇
  1993年   3篇
  1991年   4篇
  1989年   2篇
  1988年   3篇
  1987年   2篇
  1986年   2篇
  1985年   1篇
  1984年   1篇
  1983年   3篇
  1982年   5篇
  1981年   3篇
  1980年   2篇
  1979年   1篇
  1978年   2篇
  1977年   5篇
  1976年   3篇
  1973年   1篇
  1972年   1篇
  1968年   1篇
  1966年   2篇
排序方式: 共有210条查询结果,搜索用时 15 毫秒
1.
A new class of procedures, previously shown to permit the isolation of pure oligodendroglia from whole rat cerebrum, has been applied with equal or greater success for the bulk isolation of this cell type from bovine white matter. Thus, the generality of this approach has been demonstrated. The bovine preparations have a purity of greater than 90% intact, phase-bright oligodendroglia and are obtained in a yield of 8 x 10(6) cells per gram of white matter. Within 1 day it is possible to obtain a preparation containing 60 mg of protein from a single cell type. These cells show a higher degree of ultrastructural preservation of all cytoplasmic constituents than previously obtained. The values for protein (33 pg/cell), DNA (5.4 pg/cell), and lipid (5-6 pg/cell) are very similar to those obtained with an earlier procedure. The cell lipids are rich in galactolipid, which comprises 20% of the total. The activity of the "myelin-specific" enzyme, 2',3'-cyclic nucleotide 3'-phosphohydrolase (EC 3.1.4.37), is 4.7 mumol/min/mg protein, similar to that obtained previously for isolated oligodendroglia and about 25-40% of that found in myelin. The activity of 5'-nucleotidase (EC 3.1.3.5) in the cells is about 10% of that in myelin or white matter.  相似文献   
2.
A role for Wiskott-Aldrich syndrome protein (WASP) in chemotaxis to various agents has been demonstrated in monocyte-derived cell types. Although WASP has been shown to be activated by multiple mechanisms in vitro, it is unclear how WASP is regulated in vivo. A WASP biosensor (WASPbs), which uses intramolecular fluorescence resonance energy transfer to report WASP activation in vivo, was constructed, and following transfection of macrophages, activation of WASPbs upon treatment with colony-stimulating factor-1 (CSF-1) was detected globally as early as 30 s and remained localized to protrusive regions at later time points. Similar results were obtained when endogenous WASP activation was determined using conformation-sensitive antibodies. In vivo CSF-1-induced WASP activation was fully Cdc42-dependent. Activation of WASP in response to treatment with CSF-1 was also shown to be phosphatidylinositol 3-kinase-dependent. However, treatment with the Src family kinase inhibitors PP2 or SU6656 or disruption of the major tyrosine phosphorylation site of WASPbs (Y291F mutation) did not reduce the level of CSF-1-induced WASP activation. Our results indicate that WASP activation downstream of CSF-1R is phosphatidylinositol 3-kinase- and Cdc42-dependent consistent with an involvement of these molecules in macrophage migration. However, although tyrosine phosphorylation of WASP has been proposed to stimulate WASP activity, we found no evidence to indicate that this occurs in vivo.Macrophages, terminally differentiated cells of the mononuclear phagocytic lineage, are found throughout the body and play important roles in normal tissue development and immune defense. However, in certain circumstances, excessive recruitment of macrophages has been shown to participate in the progression of several diseases, inflammatory (rheumatoid arthritis) or metabolic (atherosclerosis), as well as in tumor progression (13). Importantly expression of colony-stimulating factor-1 (CSF-1),4 the most pleiotropic macrophage growth factor, has been correlated with the progression of these disease states (for a review, see Ref. 4). Inhibition of undesirable macrophage recruitment to specific sites in response to CSF-1 is therefore an attractive goal for therapies (5).In addition to stimulating survival, proliferation, and differentiation of monocytes and macrophages, CSF-1 is also a potent chemotactic factor inducing the migration of these cell types (for a review, see Ref. 4). CSF-1 stimulation leads to the rapid production of F-actin-rich protrusions and the spreading and migration of macrophages (4). All CSF-1 effects are mediated through its tyrosine kinase receptor (CSF-1R), which upon activation leads to phosphorylation of tyrosine residues in a number of signaling molecules. Downstream molecules essential for macrophage migration in response to CSF-1 include phosphatidylinositol 3-kinase (PI3K) isoforms β and δ (6, 7). PI3K may potentially regulate migration through the activation of guanine nucleotide exchange factor activity to Rac1 and Cdc42, which are required for CSF-1-elicited protrusions (8, 9) and chemotaxis (10). The major means by which Rac and Cdc42 regulate the Arp2/3 complex is through the Wiskott-Aldrich syndrome protein/Wiskott-Aldrich syndrome verprolin-homologous (WASP/WAVE) family of proteins (11). A Rac1-IRSp53-Abi1-WAVE2 complex has been shown to mediate CSF-1-induced macrophage motility (12, 13), and a unique role for WASP in macrophage chemotaxis to CSF-1, formylmethionylleucylphenylalanine, MCP-1, and MIP-1α has been demonstrated (14, 15). WASP is a hematopoietic cell-specific regulator of Arp2/3-dependent actin remodeling. The catalytically active domain of WASP lies in its C terminus, which is conserved among all WASP/WAVE proteins and contains a VCA (verprolin homology, cofilin-like, and acidic region) domain capable of activating the Arp2/3 complex. The other domains found in WASP can regulate, directly or indirectly, the activity of its VCA domain (for a review, see Ref. 16). Both WASP and N-WASP bind activated Cdc42 through their GTPase-binding domain, which is believed to cause a structural transition that results in dissociation of the intramolecular contacts leaving the VCA domain accessible for Arp2/3 binding (17, 18). In addition, biochemical studies have revealed that several signaling molecules, including WASP-interacting SH3 protein, WASP-interacting protein, Grb2, phosphoinositides, and Src family kinases, activate N-WASP (for reviews, see Refs. 16 and 19). Phosphorylation of WASP has also been proposed to activate Arp2/3-mediated actin polymerization in vitro (2022).Recently different probes have been developed that detect a conformational change in N-WASP and therefore reflect its activation (2325). Using either a fluorescence resonance energy transfer (FRET)-based biosensor that detects a conformational change in N-WASP (23, 24) or antibodies that can only bind to the open conformation of N-WASP (25), N-WASP has been shown to be activated in response to epidermal growth factor in HEK293 cells and in MTLn3 carcinoma cells. This activity has been temporally localized to subcellular compartments important for carcinoma cell chemotaxis and invasion (24). We have adapted these approaches to explore the signal transduction pathways responsible for the activation of WASP in vivo.  相似文献   
3.
【背景】杨树溃疡病是一种主要由葡萄座腔菌引起的杨树枝干病害,危害严重。前期从杨树中分离到一株内生拮抗细菌N6-34,研究表明该菌株拮抗效果好,对多种植物病原菌均有较强的拮抗作用。【目的】对拮抗细菌N6-34产生的抗菌活性物质进行分离纯化,并鉴定了活性物质组分的结构。【方法】通过硫酸铵盐析、甲醇抽提、分子筛、高效液相色谱等方法分离纯化N6-34菌株的抗菌活性物质,并对其进行结构鉴定。【结果】N6-34菌株发酵液经多步分离纯化,共获得14个组分,其中有13个组分具有抗菌活性,经一级质谱分析,获得了13种抗菌活性组分的分子量;经二级质谱分析,将13种抗菌活性物质鉴定为Fengycin A或Fengycin B的同系物或同分异构体。【结论】从N6-34菌株发酵液中分离获得了13种抗菌成分,为杨树溃疡病的生物防治提供了理论依据。  相似文献   
4.
旨在利用CRISPR/Cas9技术构建敲除花生四烯5-脂氧合酶基因(Arachidonate 5-lipoxygenase gene,ALOX5)的重组质粒。设计合成3对靶向敲除ALOX5第六外显子的sgRNA,将其分别插入到CRISPR/Cas9质粒骨架pX458载体中,转化感受态大肠杆菌DH5α后挑取克隆,通过测序评估重组质粒是否构建成功。将构建好的重组质粒转染293T细胞,在荧光显微镜下观察转染效果,挑取转染成功的细胞,用试剂盒提取转染细胞基因组DNA,PCR扩增含敲除位点的DNA片段,用测序技术获得核苷酸序列,用DNAStar软件分析转染细胞中ALOX5基因敲除情况。测序结果表明2对双链sgRNA寡核苷酸已插入质粒,且序列正确,靶向ALOX5基因的重组质粒pX458-sgRNAs-ALOX5构建成功。其在293T细胞中的转染效率约为50%,用一代测序法未检测到sgRNAs的切割效果。初步表明利用CRISPR/Cas9技术成功构建靶向ALOX5基因的重组质粒pX458-sgRNAs-ALOX5。  相似文献   
5.
目的对实验动物皮肤病原真菌2种培养方法进行了比较。方法将采集到的3只皮肤真菌感染病兔样品经由沙氏平皿法和沙氏试管斜面培养法分别进行培养。结果在3只真菌感染病兔中应用试管斜面法我们只检测到1例皮肤病原真菌阳性,而采用沙氏平皿法3例阳性全部检出。结论结合临床检测经验,我们认为本研究的沙氏平皿法优于沙氏试管斜面法,在实验动物皮肤病原真菌常规检测中具有推广应用价值。  相似文献   
6.
Bone turnover markers (BTMs) are classified as either formation or resorption markers. Their concentrations in blood or urine of adults are considered to reflect the rate of bone remodelling and may be of use in the management of patients with bone disease. Major inter-method differences exist for BTMs, and harmonisation of methods is currently being pursued at an international level. Based on published data, this article describes age- and sex-specific Australian consensus reference intervals for adults for serum procollagen type I amino-terminal propeptide (s-PINP) and serum β-isomerised carboxy-terminal cross-linking telopeptide of type I collagen (s-CTX).  相似文献   
7.
The distribution of Na+ pump sites (Na+-K+-ATPase) in the secretory epithelium of the avian salt gland was demonstrated by freeze-dry autoradiographic analysis of [(3)H] ouabain binding sites. Kinetic studies indicated that near saturation of tissue binding sites occurred when slices of salt glands from salt-stressed ducks were exposed to 2.2 μM ouabain (containing 5 μCi/ml [(3)H]ouabain) for 90 min. Washing with label-free Ringer's solution for 90 min extracted only 10% of the inhibitor, an amount which corresponded to ouabain present in the tissue spaces labeled by [(14)C]insulin. Increasing the KCl concentration of the incubation medium reduced the rate of ouabain binding but not the maximal amount bound. In contrast to the low level of ouabain binding to salt glands of ducks maintained on a freshwater regimen, exposure to a salt water diet led to a more than threefold increase in binding within 9-11 days. This increase paralleled the similar increment in Na+-K+-ATPase activity described previously. [(3)H]ouabain binding sites were localized autoradiographically to the folded basolateral plasma membrane of the principal secretory cells. The luminal surfaces of these cells were unlabeled. Mitotically active peripheral cells were also unlabeled. The cell-specific pattern of [(3)H]ouabain binding to principal secretory cells and the membrane-specific localization of binding sites to the nonluminal surfaces of these cells were identical to the distribution of Na+-K+-ATPase as reflected by the cytochemical localization of ouabain-sensitive and K+-dependent nitrophenyl phosphatase activity. The relationship between the nonluminal localization of Na+-K+-ATPase and the possible role of the enzyme n NaCl secretion is considered in the light of physiological data on electrolyte transport in salt glands and other secretory epithelia.  相似文献   
8.
A puzzling population-genetic phenomenon widely reported in allozyme surveys of marine bivalves is the occurrence of heterozygote deficits relative to Hardy-Weinberg expectations. Possible explanations for this pattern are categorized with respect to whether the effects should be confined to protein-level assays or are genomically pervasive and expected to be registered in both protein- and DNA-level assays. Anonymous nuclear DNA markers from the American oyster were employed to reexamine the phenomenon. In assays based on the polymerase chain reaction (PCR), two DNA-level processes were encountered that can lead to artifactual genotypic scorings: (a) differential amplification of alleles at a target locus and (b) amplification from multiple paralogous loci. We describe symptoms of these complications and prescribe methods that should generally help to ameliorate them. When artifactual scorings at two anonymous DNA loci in the American oyster were corrected, Hardy-Weinberg deviations registered in preliminary population assays decreased to nonsignificant values. Implications of these findings for the heterozygote-deficit phenomenon in marine bivalves, and for the general development and use of PCR-based assays, are discussed.   相似文献   
9.
10.

Background

Vitamin D is associated with lung function in cross-sectional studies, and vitamin D inadequacy is hypothesized to play a role in the pathogenesis of chronic obstructive pulmonary disease. Further data are needed to clarify the relation between vitamin D status, genetic variation in vitamin D metabolic genes, and cross-sectional and longitudinal changes in lung function in healthy adults.

Methods

We estimated the association between serum 25-hydroxyvitamin D [25(OH)D] and cross-sectional forced expiratory volume in the first second (FEV1) in Framingham Heart Study (FHS) Offspring and Third Generation participants and the association between serum 25(OH)D and longitudinal change in FEV1 in Third Generation participants using linear mixed-effects models. Using a gene-based approach, we investigated the association between 241 SNPs in 6 select vitamin D metabolic genes in relation to longitudinal change in FEV1 in Offspring participants and pursued replication of these findings in a meta-analyzed set of 4 independent cohorts.

Results

We found a positive cross-sectional association between 25(OH)D and FEV1 in FHS Offspring and Third Generation participants (P = 0.004). There was little or no association between 25(OH)D and longitudinal change in FEV1 in Third Generation participants (P = 0.97). In Offspring participants, the CYP2R1 gene, hypothesized to influence usual serum 25(OH)D status, was associated with longitudinal change in FEV1 (gene-based P < 0.05). The most significantly associated SNP from CYP2R1 had a consistent direction of association with FEV1 in the meta-analyzed set of replication cohorts, but the association did not reach statistical significance thresholds (P = 0.09).

Conclusions

Serum 25(OH)D status was associated with cross-sectional FEV1, but not longitudinal change in FEV1. The inconsistent associations may be driven by differences in the groups studied. CYP2R1 demonstrated a gene-based association with longitudinal change in FEV1 and is a promising candidate gene for further studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12931-015-0238-y) contains supplementary material, which is available to authorized users.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号