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   6827篇
  免费   481篇
  国内免费   1篇
  7309篇
  2024年   6篇
  2023年   40篇
  2022年   115篇
  2021年   181篇
  2020年   138篇
  2019年   163篇
  2018年   209篇
  2017年   160篇
  2016年   281篇
  2015年   449篇
  2014年   437篇
  2013年   571篇
  2012年   650篇
  2011年   623篇
  2010年   340篇
  2009年   310篇
  2008年   389篇
  2007年   369篇
  2006年   333篇
  2005年   301篇
  2004年   257篇
  2003年   211篇
  2002年   234篇
  2001年   41篇
  2000年   28篇
  1999年   40篇
  1998年   53篇
  1997年   28篇
  1996年   34篇
  1995年   23篇
  1994年   28篇
  1993年   22篇
  1992年   27篇
  1991年   16篇
  1990年   15篇
  1989年   14篇
  1988年   12篇
  1987年   13篇
  1985年   9篇
  1984年   13篇
  1983年   6篇
  1982年   11篇
  1981年   8篇
  1980年   13篇
  1979年   9篇
  1978年   6篇
  1977年   11篇
  1976年   5篇
  1974年   6篇
  1973年   14篇
排序方式: 共有7309条查询结果,搜索用时 10 毫秒
21.
Abstract In newly invaded communities, interspecific competition is thought to play an important role in determining the success of the invader and its impact on the native community. In southern Australia, the native Polistes humilis was the predominant social wasp prior to the arrival of the exotic Vespula germanica (Hymenoptera: Vespidae). Both species forage for similar resources (water, pulp, carbohydrate and protein prey), and concerns have arisen about potential competition between them. The aim of this study was to identify the protein foods that these wasps feed on. As many prey items are masticated by these wasps to the degree that they cannot be identified using conventional means, morphological identification was complemented by sequencing fragments of the mitochondrial 16S rRNA gene. GenBank searches using blast and phylogenetic analyses were used to identify prey items to at least order level. The results were used to construct complete prey inventories for the two species. These indicate that while P. humilis is restricted to feeding on lepidopteran larvae, V. germanica collects a variety of prey of invertebrate and vertebrate origin. Calculated values of prey overlap between the two species are used to discuss the implications of V. germanica impacting on P. humilis. Results obtained are compared to those gained by solely 'conventional' methods, and the advantages of using DNA-based taxonomy in ecological studies are emphasized.  相似文献   
22.
23.
We mapped the genetic influences for type 1 diabetes (T1D), using 2,360 single-nucleotide polymorphism (SNP) markers in the 4.4-Mb human major histocompatibility complex (MHC) locus and the adjacent 493 kb centromeric to the MHC, initially in a survey of 363 Swedish T1D cases and controls. We confirmed prior studies showing association with T1D in the MHC, most significantly near HLA-DR/DQ. In the region centromeric to the MHC, we identified a peak of association within the inositol 1,4,5-triphosphate receptor 3 gene (ITPR3; formerly IP3R3). The most significant single SNP in this region was at the center of the ITPR3 peak of association (P=1.7 x 10(-4) for the survey study). For validation, we typed an additional 761 Swedish individuals. The P value for association computed from all 1,124 individuals was 1.30 x 10(-6) (recessive odds ratio 2.5; 95% confidence interval [CI] 1.7-3.9). The estimated population-attributable risk of 21.6% (95% CI 10.0%-31.0%) suggests that variation within ITPR3 reflects an important contribution to T1D in Sweden. Two-locus regression analysis supports an influence of ITPR3 variation on T1D that is distinct from that of any MHC class II gene.  相似文献   
24.
Abstract

Systemic sclerosis (SSc) is a multisystem autoimmune disease: characterised from the clinical side by progressive vasculopathy and fibrosis of the skin and different organs and from the biochemical side by fibroblast deregulation with excessive production of collagen and increased expression of nicotinamide adenine dinucleotide phosphate oxidase 4 (NOX4). The latter contributes to an overproduction of reactive oxygen species that through an autocrine loop maintains NOX4 in a state of activation. Reactive oxygen and nitrogen species are implicated in the origin and perpetuation of several clinical manifestations of SSc having vascular damage in common; attempts to dampen oxidative and nitrative stress through different agents with antioxidant properties have not translated into a sustained clinical benefit. Objective of this narrative review is to describe the origin and clinical implications of oxidative and nitrative stress in SSc, with particular focus on the central role of NOX4 and its interactions, to re-evaluate the antioxidant approaches so far used to limit disease progression, to appraise the complexity of antioxidant treatment and to touch on novel pathways elements of which may represent specific treatment targets in the not so distant future.  相似文献   
25.
26.
The p53 protein is an important factor of many intra- and extracellular processes. This protein regulates the repair of cellular DNA and induces apoptosis. It is also responsible for the regulation of the senescence and the cell entering the subsequent stages of the cellular cycle. The protein p53 is also involved in inhibiting angiogenesis and the induction of oxidative shock. In our study, we examined the activity of p53 protein in the uterine epithelial cells in rats treated with cladribine. Its action is mainly based on apoptosis induction. We compared the activity of p53 protein in cells with a high apoptosis index and in cells with active repair mechanisms and high proliferation index. We observed stronger p53 protein expression in the epithelial cells of the materials taken 24 h after the last dose of 2-CdA associated with the active process of apoptosis and inhibition of proliferation. After 4 weeks from the last dose of cladribine, the stronger expression of p53 protein was associated with both the existing changes in the cell's genome, the effects of the ongoing repair mechanisms, as well as the high proliferation activity.  相似文献   
27.

Background aims

Cell therapy with autologous mesenchymal stromal cells (MSCs) in patients with spinal cord injury (SCI) is beginning, and the search for its better clinical application is an urgent need.

Methods

We present a phase 2 clinical trial in patients with chronic SCI who received three intrathecal administrations of 100 x 106 MSCs and were followed for 10 months from the first administration. Efficacy analysis was performed on nine patients, and safety analysis was performed on 11 patients. Clinical scales, urodynamic, neurophysiological and neuroimaging studies were performed previous to treatment and at the end of the follow-up.

Results

The treatment was well-tolerated, without any adverse event related to MSC administration. Patients showed variable clinical improvement in sensitivity, motor power, spasms, spasticity, neuropathic pain, sexual function or sphincter dysfunction, regardless of the level or degree of injury, age or time elapsed from the SCI. In the course of follow-up three patients, initially classified as ASIA A, B and C, changed to ASIA B, C and D, respectively. In urodynamic studies, at the end of follow-up, 66.6% of the patients showed decrease in postmicturition residue and improvement in bladder compliance. At this time, neurophysiological studies showed that 55.5% of patients improved in somatosensory or motor-evoked potentials, and that 44.4% of patients improved in voluntary muscle contraction together with infralesional active muscle reinnervation.

Conclusions

The present guideline for cell therapy is safe and shows efficacy in patients with SCI, mainly in recovery of sphincter dysfunction, neuropathic pain and sensitivity.  相似文献   
28.
Production of official statistics frequently requires expert judgement to evaluate and reconcile data of unknown and varying quality from multiple and potentially conflicting sources. Moreover, exceptional events may be difficult to incorporate in modelled estimates. Computational logic provides a methodology and tools for incorporating analyst''s judgement, integrating multiple data sources and modelling methods, ensuring transparency and replicability, and making documentation computationally accessible. Representations using computational logic can be implemented in a variety of computer-based languages for automated production. Computational logic complements standard mathematical and statistical techniques and extends the flexibility of mathematical and statistical modelling. A basic overview of computational logic is presented and its application to official statistics is illustrated with the WHO & UNICEF estimates of national immunization coverage.  相似文献   
29.
ABSTRACT: BACKGROUND: D-Tagatose is a natural monosaccharide which can be used as a low-calorie sugar substitute in food, beverages and pharmaceutical products. It is also currently being tested as an anti-diabetic and obesity control drug. D-Tagatose is a rare sugar, but it can be manufactured by the chemical or enzymatic isomerization of D-galactose obtained by a beta-D-galactosidase-catalyzed hydrolysis of milk sugar lactose and the separation of D-glucose and D-galactose. L-Arabinose isomerases catalyze in vitro the conversion of D-galactose to D-tagatose and are the most promising enzymes for the large-scale production of D-tagatose. RESULTS: In this study, the araA gene from psychrotolerant Antarctic bacterium Arthrobacter sp. 22c was isolated, cloned and expressed in Escherichia coli. The active form of recombinant Arthrobacter sp. 22c L-arabinose isomerase consists of six subunits with a combined molecular weight of approximately 335 kDa. The maximum activity of this enzyme towards D-galactose was determined as occurring at 52[DEGREE SIGN]C; however, it exhibited over 60% of maximum activity at 30[DEGREE SIGN]C. The recombinant Arthrobacter sp. 22c L-arabinose isomerase was optimally active at a broad pH range of 5 to 9. This enzyme is not dependent on divalent metal ions, since it was only marginally activated by Mg2+, Mn2+ or Ca2+ and slightly inhibited by Co2+ or Ni2+. The bioconversion yield of D-galactose to D-tagatose by the purified L-arabinose isomerase reached 30% after 36 h at 50[DEGREE SIGN]C. In this study, a recombinant Pichia pastoris yeast strain secreting beta-D-galactosidase Arthrobacter chlorophenolicus was also constructed. During cultivation of this strain in a whey permeate, lactose was hydrolyzed and D-glucose was metabolized, whereas D-galactose was accumulated in the medium. Moreover, cultivation of the P. pastoris strain secreting beta-D-galactosidase in a whey permeate supplemented with Arthrobacter sp. 22c L-arabinose isomerase resulted in a 90% yield of lactose hydrolysis, the complete utilization of D-glucose and a 30% conversion of D-galactose to D-tagatose. CONCLUSIONS: The method developed for the simultaneous hydrolysis of lactose, utilization of D-glucose and isomerization of D-galactose using a P. pastoris strain secreting beta-D-galactosidase and recombinant L-arabinose isomerase seems to offer an interesting alternative for the production of D-tagatose from lactose-containing feedstock.  相似文献   
30.
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