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   7322篇
  免费   594篇
  国内免费   2篇
  7918篇
  2023年   35篇
  2022年   83篇
  2021年   196篇
  2020年   137篇
  2019年   151篇
  2018年   219篇
  2017年   175篇
  2016年   246篇
  2015年   331篇
  2014年   384篇
  2013年   507篇
  2012年   555篇
  2011年   581篇
  2010年   341篇
  2009年   318篇
  2008年   385篇
  2007年   433篇
  2006年   386篇
  2005年   338篇
  2004年   324篇
  2003年   310篇
  2002年   267篇
  2001年   129篇
  2000年   109篇
  1999年   92篇
  1998年   67篇
  1997年   48篇
  1996年   35篇
  1995年   48篇
  1994年   45篇
  1993年   49篇
  1992年   61篇
  1991年   50篇
  1990年   54篇
  1989年   39篇
  1988年   42篇
  1987年   32篇
  1986年   41篇
  1985年   37篇
  1984年   28篇
  1983年   26篇
  1982年   26篇
  1981年   23篇
  1980年   15篇
  1979年   13篇
  1978年   10篇
  1977年   17篇
  1976年   17篇
  1974年   10篇
  1970年   9篇
排序方式: 共有7918条查询结果,搜索用时 15 毫秒
981.
982.
Previously, we showed that insulin growth factor (IGF)-1 binding protein-3 (IGFBP-3), independent of IGF-1, reduces pathological angiogenesis in a mouse model of the oxygen-induced retinopathy (OIR). The current study evaluates novel endothelium-dependent functions of IGFBP-3 including blood retinal barrier (BRB) integrity and vasorelaxation. To evaluate vascular barrier function, either plasmid expressing IGFBP-3 under the regulation of an endothelial-specific promoter or a control plasmid was injected into the vitreous humor of mouse pups (P1) and compared to the non-injected eyes of the same pups undergoing standard OIR protocol. Prior to sacrifice, the mice were given an injection of horseradish peroxidase (HRP). IGFBP-3 plasmid-injected eyes displayed near-normal vessel morphology and enhanced vascular barrier function. Further, in vitro IGFBP-3 protects retinal endothelial cells from VEGF-induced loss of junctional integrity by antagonizing the dissociation of the junctional complexes. To assess the vasodilatory effects of IGFBP-3, rat posterior cerebral arteries were examined in vitro. Intraluminal IGFBP-3 decreased both pressure- and serotonin-induced constrictions by stimulating nitric oxide (NO) release that were blocked by L-NAME or scavenger receptor-B1 neutralizing antibody (SRB1-Ab). Both wild-type and IGF-1-nonbinding mutant IGFBP-3 (IGFBP-3NB) stimulated eNOS activity/NO release to a similar extent in human microvascular endothelial cells (HMVECs). NO release was neither associated with an increase in intracellular calcium nor decreased by Ca(2+)/calmodulin-dependent protein kinase II (CamKII) blockade; however, dephosphorylation of eNOS-Thr(495) was observed. Phosphatidylinositol 3-kinase (PI3K) activity and Akt-Ser(473) phosphorylation were both increased by IGFBP-3 and selectively blocked by the SRB1-Ab or PI3K blocker LY294002. In conclusion, IGFBP-3 mediates protective effects on BRB integrity and mediates robust NO release to stimulate vasorelaxation via activation of SRB1. This response is IGF-1- and calcium-independent, but requires PI3K/Akt activation, suggesting that IGFBP-3 has novel protective effects on retinal and systemic vasculature and may be a therapeutic candidate for ocular complications such as diabetic retinopathy.  相似文献   
983.

Background

Phylum Porifera includes ∼8,500 valid species distributed world-wide in aquatic ecosystems ranging from ephemeral fresh-water bodies to coastal environments and the deep-sea. The taxonomy and systematics of sponges is complicated, and morphological identification can be both time consuming and erroneous due to phenotypic convergence and secondary losses, etc. DNA barcoding can provide sponge biologists with a simple and rapid method for the identification of samples of unknown taxonomic membership. The Sponge Barcoding Project (www.spongebarcoding.org), the first initiative to barcode a non-bilaterian metazoan phylum, aims to provide a comprehensive DNA barcode database for Phylum Porifera.

Methodology/Principal Findings

∼7,400 sponge specimens have been extracted, and amplification of the standard COI barcoding fragment has been attempted for approximately 3,300 museum samples with ∼25% mean amplification success. Based on this comprehensive sampling, we present the first report on the workflow and progress of the sponge barcoding project, and discuss some common pitfalls inherent to the barcoding of sponges.

Conclusion

A DNA-barcoding workflow capable of processing potentially large sponge collections has been developed and is routinely used for the Sponge Barcoding Project with success. Sponge specific problems such as the frequent co-amplification of non-target organisms have been detected and potential solutions are currently under development. The initial success of this innovative project have already demonstrated considerable refinement of sponge systematics, evaluating morphometric character importance, geographic phenotypic variability, and the utility of the standard barcoding fragment for Porifera (despite its conserved evolution within this basal metazoan phylum).  相似文献   
984.

Background

The CUPID (Cultural and Psychosocial Influences on Disability) study was established to explore the hypothesis that common musculoskeletal disorders (MSDs) and associated disability are importantly influenced by culturally determined health beliefs and expectations. This paper describes the methods of data collection and various characteristics of the study sample.

Methods/Principal Findings

A standardised questionnaire covering musculoskeletal symptoms, disability and potential risk factors, was used to collect information from 47 samples of nurses, office workers, and other (mostly manual) workers in 18 countries from six continents. In addition, local investigators provided data on economic aspects of employment for each occupational group. Participation exceeded 80% in 33 of the 47 occupational groups, and after pre-specified exclusions, analysis was based on 12,426 subjects (92 to 1018 per occupational group). As expected, there was high usage of computer keyboards by office workers, while nurses had the highest prevalence of heavy manual lifting in all but one country. There was substantial heterogeneity between occupational groups in economic and psychosocial aspects of work; three- to five-fold variation in awareness of someone outside work with musculoskeletal pain; and more than ten-fold variation in the prevalence of adverse health beliefs about back and arm pain, and in awareness of terms such as “repetitive strain injury” (RSI).

Conclusions/Significance

The large differences in psychosocial risk factors (including knowledge and beliefs about MSDs) between occupational groups should allow the study hypothesis to be addressed effectively.  相似文献   
985.

Introduction

Micronutrient deficiency is observed in heart failure patients. Taurine, for example, represents 50% of total free amino acids in the heart, and in vivo studies have linked taurine deficiency with cardiomyopathy.

Methods

Thirty-four male Wistar rats (body weight = 100 g) were weighed and randomly assigned to one of two groups: Control (C) or taurine-deficient (T (-)). Beta-alanine at a concentration of 3% was added to the animals’ water to induce taurine deficiency in the T (-) group. On day 30, the rats were individually submitted to echocardiography; morphometrical and histopathological evaluation and metalloproteinase activity, oxidative stress and inflammation evaluation were performed. Tissue samples were collected to determine the taurine concentration in the heart.

Results

Taurine deficiency led to decreases in: ventricular wall thickness, left ventricle dry weight, myocyte sectional area, left ventricle posterior wall thickness and ventricular geometry. With regard to heart function, the velocity of the A wave, the ratio between the E and A wave, the ejection fraction, fractional shortening and cardiac output values were decreased in T (-) rats, suggesting abnormal diastolic and systolic function. Increased fibrosis, inflammation and increased activation of metalloproteinases were not observed. Oxidative stress was increased in deficient animals.

Conclusions

These data suggest that taurine deficiency promotes structural and functional cardiac alterations with unique characteristics.  相似文献   
986.
Halichondrid sponges play a pivotal role in the classification of demosponges as changes in their classification has had direct consequences for the classification of Demospongiae. Historically, the systematics of halichondrids has been unstable. During the 1950s, the order was divided into two subclasses, which were based on empirical and assumed reproductive data. Subsequent morphological and biochemical analyses postulated the re-merging of halichondrid families, but recent molecular data indicate their polyphyly. Here we review the classification history of halichondrid taxa, compare it with the current and predominantly ribosomal molecular data, and support the new phylogenetic hypotheses with mitochondrial data from DNA barcoding.  相似文献   
987.
988.
Background aimsWe have recently shown that thymoglobulin (TG) efficiently expands cytokine-induced killer (CIK) cells in combination with interferon (IFN)-γ and interleukin (IL)-2 (ITG2 protocol). It is presently unknown whether the infusion of autologous immune effector cells generated by TG, IFN-γ and IL-2 is feasible and safeMethodsFive patients with advanced and/or refractory solid tumors were enrolled in the present phase I/II study. Peripheral blood mononuclear cells (PBMC) collected by leukapheresis were stimulated under good manufacturing practice (GMP)-conditions with IFN-γ, followed by TG and IL-2. After 2–3 weeks in culture, a median of 4.65 × 106 immune effector cells per kilogram of recipient's body weight was obtained and infused intravenously. The median time from enrollment into the study to infusion of the expanded CIK cells was 30 daysResultsITG2 efficiently expanded immune effector cells that comprised both conventional natural killer (NK) cells and CD3+ CD16+ CD56+ CIK cells. One patient with advanced melanoma died because of disease progression before the infusion of CIK cells. The target dose of at least 2.5 × 106 CIK cells/kg of recipient's body weight was reached in four out of five evaluable patients. CIK cells were administered intravenously without any measurable toxicity. In vitro, CIK cells exerted lytic activity against cervical cancer cells. The median survival was 4.5 months (range 1–13) from the first infusion of CIK cells.ConclusionsThis study has highlighted the feasibility and safety of the administration of CIK cells generated with the ITG2 protocol. Whether CIK cells can help control disease burden in patients with advanced malignancies will be determined in future clinical trials.  相似文献   
989.
990.
Background and aimComplications of diabetes comprise the leading cause of death in Mexico. We aimed to describe the characteristics of management and achievement of therapeutic targets in Mexican patients with diabetes mellitus.MethodsWe analyzed data from 2642 Mexican patients with type 1 (T1D, n = 203, 7.7%) and type 2 diabetes (T2D, n = 2439, 92.3%) included in the third wave of the International Diabetes Management Practices Study.ResultsOf T2D patients, 63% were on oral glucose-lowering drugs (OGLD) exclusively (mostly metformin), 11% on insulin, 22% on OGLD plus insulin, and 4% on diet and exercise exclusively. T2D patients on insulin were more likely to be trained on diabetes, but they were older, had worse control, longer disease duration and more chronic complications than patients on OGLD only. Glycated hemoglobin (HbA1c) < 7% was achieved by 21% and 37% of T1D and T2D patients, respectively. Only 5% of T1D and 3% of T2D attained the composite target of HbA1c < 7%, blood pressure < 130/80 mmHg and low-density lipoprotein cholesterol < 100 mg/dl. T1D patients had less macrovascular but more microvascular complications, compared with T2D patients. Late complications increased with disease duration, so that about 80% of patients after 20 years of diagnosis have at least one late complication. Reaching the target HbA1c < 7% was associated with a reduced number of microvascular but not with less macrovascular complications.ConclusionA great proportion of these Mexican patients with diabetes did not reach therapeutic targets. Insulin was used mostly in complicated cases with advanced disease.  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