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   485篇
  免费   29篇
  2023年   1篇
  2022年   6篇
  2021年   13篇
  2020年   6篇
  2019年   2篇
  2018年   10篇
  2017年   8篇
  2016年   13篇
  2015年   26篇
  2014年   27篇
  2013年   52篇
  2012年   56篇
  2011年   44篇
  2010年   24篇
  2009年   35篇
  2008年   23篇
  2007年   14篇
  2006年   28篇
  2005年   18篇
  2004年   21篇
  2003年   19篇
  2002年   16篇
  2001年   2篇
  2000年   2篇
  1999年   2篇
  1998年   5篇
  1997年   5篇
  1996年   3篇
  1995年   3篇
  1994年   1篇
  1993年   4篇
  1992年   1篇
  1989年   3篇
  1988年   1篇
  1986年   1篇
  1985年   1篇
  1983年   1篇
  1982年   4篇
  1981年   2篇
  1980年   2篇
  1979年   2篇
  1975年   1篇
  1971年   1篇
  1963年   1篇
  1961年   1篇
  1960年   1篇
  1957年   1篇
  1951年   1篇
排序方式: 共有514条查询结果,搜索用时 265 毫秒
91.
We tested the effect of the presence in the culture medium of chitosan, vanadyl sulfate or methyl jasmonate on growth and ginsenoside production of three stable hairy root lines of Panax ginseng C.A. Meyer showing different morphological phenotypes C-M, HR-M and T-M. The response depended upon line phenotype, specificity of the elicitor and the stage of growth at which the lines were treated. The highest ginsenoside yield was found when methyl jasmonate was added during the progressive deceleration growth phase (on day 25 of culture). In this case, the ginsenoside content reached at the end of the culture (day 28) by root lines C-M, HR-M and T-M was, respectively, 2, 1.8 and 4 times higher than the highest content achieved, also at the end of the culture, by the corresponding untreated root lines. Under the same conditions, the ginsenoside content in the presence of vanadyl sulfate also increased considerably, while with chitosan it clearly decreased. The ginsenoside pattern in response to the presence of the elicitors is also considered.  相似文献   
92.
The aim of this study was to elucidate how the spatial scale and the set of variables included influence our ability to detect the effects of different types of pollution on the biota. Using variance partitioning analysis, we assessed the individual importance of a set of environmental factors (eutrophication and organic pollution) versus metal level pollution on the community structure of diatom assemblages at different spatial scales. At regional scale, environmental factors did not explain more of the variance compared to the watershed study. The results of the watershed scale field survey indicate that diatom community composition was influenced by low metal concentrations, but this pattern was only observed by the inclusion of biofilm metal concentration data. We recommend the analysis of metal traces in the water phase and the biota (fluvial biofilms) as well as the use of the Diffusive Gradient in Thin films (DGT) technique to characterize low metal level pollution in freshwater systems. Handling editor: Judit Padisák  相似文献   
93.
The number and frequency of susceptibility alleles at loci associated to most psychiatric disorders is largely unknown, in spite of its relevance for the design of studies aiming to find these alleles. Both, common polymorphisms and rare mutations may contribute to the genetic susceptibility to complex psychiatric disorders, being the relative relevance of each type of variation currently under debate. Here, we confirmed the existence of a common protective haplotype against schizophrenia at the dopamine D3 receptor (DRD3) gene, by replication and pooled analysis with previous data (Mantel–Haenszel χ2 P value = 0.00227; OR = 0.79, 95% CI 0.68–0.92, based on 794 cases and 1,078 controls from three independent populations of European origin). This protective haplotype is at very low frequency in Sub-Saharan Africans (median 0.06) and at intermediate frequencies in other populations (median 0.25). We also revealed, by examining the patterns of linkage disequilibrium around this gene, that the protective haplotype has reached high frequency in non-African populations due to selection acting, most probably, on a linked functional polymorphism, the non-synonymous single nucleotide polymorphism Ser9Gly (rs6280), also at DRD3. Thus, this finding shows that the natural selection may play a role in the existence of common alleles conferring different susceptibility to schizophrenia. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   
94.
Thermodynamic solubility is a decisive physicochemical property in drug development. The Chasing Equilibrium method offers an alternative to the classical procedures to measure the solubility of compounds with acid–base properties. The method is fast and yields accurate results. In this work, the solubility of several compounds including acids and bases was determined through the Chasing Equilibrium approach. A study of experimental conditions in terms of sample weight was performed to measure solubilities. The study shows that only a limited range of weights, depending on the nature and solubility of the compounds, is adequate to obtain reliable results.  相似文献   
95.

Background  

Major conflict between mitochondrial and nuclear genes in estimating species relationships is an increasingly common finding in animals. Usually this is attributed to incomplete lineage sorting, but recently the possibility has been raised that hybridization is important in generating such phylogenetic patterns. Just how widespread ancient and/or recent hybridization is in animals and how it affects estimates of species relationships is still not well-known.  相似文献   
96.

Introduction

Cardiovascular disease (CVD) is a major cause of premature mortality among Systemic lupus erythematosus (SLE) patients. Many studies have measured and evaluated risk factors for premature subclinical atherosclerosis, but few studies are prospective and few have evaluated risk factors for hard endpoints, i.e. clinically important cardiovascular events (CVE). We investigated the impact of traditional and lupus associated risk factors for the first ever CVE in a longitudinal cohort of SLE patients.

Methods

A total of 182 SLE patients (mean age 43.9 years) selected to be free of CVE were included. Cardiovascular and autoimmune biomarkers were measured on samples collected after overnight fasting at baseline. Clinical information was collected at baseline and at follow up. End point was the first ever CVE (ischemic heart, cerebrovascular or peripheral vascular disease or death due to CVD). Impact of baseline characteristics/biomarkers on the risk of having a first CVE was evaluated with Cox regression.

Results

Follow up was 99.5% after a mean time of 8.3 years. Twenty-four patients (13%) had a first CVE. In age-adjusted Cox regression, any positive antiphospholipid antibody (aPL), elevated markers of endothelial activation (von Willebrand factor (vWf), soluble vascular cellular adhesion molecule-1 (sVCAM-1)) and fibrinogen predicted CVEs. Of SLE manifestations, arthritis, pleuritis and previous venous occlusion were positively associated with future CVEs while thrombocytopenia was negatively associated. Among traditional risk factors only age and smoking were significant predictors. In a multivariable Cox regression model age, any positive aPL, vWf and absence of thrombocytopenia were all predictors of the first CVE.

Conclusions

In addition to age, positive aPL, biomarkers indicating increased endothelial cell activity/damage, and absence of thrombocytopenia were independent predictors of CVEs in this prospective study. Our results indicate that activation of the endothelium and the coagulation system are important features in SLE related CVD. Furthermore, we observed that the risk of CVEs seems to differ between subgroups of SLE patients.  相似文献   
97.
Background: Rheumatoid arthritis (RA) is a chronic inflammatory joint disorder that may adversely affect health-related quality of life (HRQoL) both in established and early disease.Objectives: In patients with recent onset (<12 months) of RA, this extension of a previous study assessed HRQoL and the effect of disease activity over time.Methods: Consecutive patients with recent onset of RA between March 1996 and November 1998 were followed for 6 years at the Department of Rheumatology of the University Hospital of Umeå in Sweden. Patients were requested to complete the 36-item Short Form (SF-36) Health Survey at 0, 24, 48, and 72 months. Gender differences were examined, and correlations between the SF-36 scales (with higher scores indicating better HRQoL) and data reflecting disease activity were analyzed.Results: Fifty-one patients, 34 women and 17 men (mean age, 50.6 years; range, 20–78 years), participated in the study; in all, 41 patients completed the SF-36 at both 0 and 72 months. At inclusion (0 months), women reported significantly higher scores for physical role functioning, bodily pain, and social functioning compared with men (all, P < 0.05). At 72 months compared with 0 months, women reported significantly better mental health (P < 0.05), whereas men reported significantly better physical role functioning (P < 0.05), bodily pain (P < 0.01), mental health (P < 0.01), and vitality (P < 0.01). Additionally, at 72 months, the entire patient group rated physical role functioning and social functioning (both, P < 0.05), bodily pain and vitality (both, P < 0.01), and mental health (P < 0.001) as significantly better compared with the inclusion assessment. Overall improvement with time was significantly better for men than for women (P < 0.05). There were limited correlations between SF-36 point disease activity parameters and the SF-36 scores at 0 months (erythrocyte sedimentation rate vs physical functioning, mental health [both, P < 0.05], and bodily pain [P < 0.01]; 28-joint Disease Activity Score vs bodily pain [P < 0.05] and emotional role functioning [P < 0.01]) and at 72 months (C-reactive protein vs physical role functioning [P < 0.05]). Most of the physical subscales at inclusion correlated with the physical component summary (PCS) of the SF-36 questionnaire at 6 years.Conclusions: At disease onset, women with early RA reported better HRQoL than did their male counterparts. After 6 years, women and especially men both experienced better HRQoL, and no significant gender differences remained in any of the SF-36 scales or values for disease activity parameters. The PCS score at disease onset was the best predictor of the PCS score after 6 years.  相似文献   
98.
River management practices have altered the hydrological regimes of many rivers and also altered the availability of regeneration niches for riparian species. We investigated the impact of changed hydrological regimes on the sex ratios and the Spatial Segregation of the Sexes (SSS) in the dioecious species Salix myrsinifolia Salisb.–phylicifolia L. and S. lapponum L. by studying the free-flowing Vindel River and the regulated Ume River in northern Sweden. We surveyed sex ratios of these species in 12 river reaches on the Vindel River and in 17 reaches on the Ume River. In addition, we surveyed the sex and location above mean river stage of 1,002 individuals across both river systems to investigate the SSS of both species. Cuttings were collected from male and female individuals of S. myrsinifolia–phylicifolia from both rivers and subjected to four different water table regimes in a greenhouse experiment to investigate growth response between the sexes. We found an M/F sex ratio in both river systems similar to the regional norm of 0.62 for S. myrsinifolia–phylicifolia and of 0.42 for S. lapponum. We found no evidence of SSS in either the free-flowing Vindel River or the regulated Ume River. In the greenhouse experiment, hydrological regime had a significant effect on shoot and root dry weight and on root length. Significantly higher shoot dry weights were found in females than in males and significantly different shoot and root dry weights were found between cuttings taken from the two rivers. We concluded that changed hydrological regimes are likely to alter dimensions of the regeneration niche and therefore to influence sex ratios and SSS at an early successional stage, making it difficult to find clear spatial patterns once these species reach maturity and can be sexed.  相似文献   
99.
Nerve fibers accompany blood and lymphatic vessels all over the body. An extensive amount of knowledge has been obtained with regard to tumor angiogenesis and tumor lymphangiogenesis, yet little is known about the potential biological effects of "neoneurogenesis". Cancer cells can exploit the advantage of the factors released by the nerve fibers to generate a positive microenvironment for cell survival and proliferation. At the same time, they can stimulate the formation of neurites by secreting neurotrophic factors and axon guidance molecules. The neuronal influence on the biology of a neoplasm was initially described several decades ago. Since then, an increasing amount of experimental evidence strongly suggests the existence of reciprocal interactions between cancer cells and nerves in humans. Moreover, researchers have been able to demonstrate a crosstalk between cancer cells and nerve fibers as a strategy for survival. Despite all these evidence, a lot remains to be done in order to clarify the role of neurotransmitters, neuropeptides, and their associated receptor-initiated signaling pathways in the development and progression of cancer, and response to therapy. A global-wide characterization of the neurotransmitters or neuropeptides present in the tumor microenvironment would provide insights into the real biological influences of the neuronal tissue on tumor progression. This review is intended to discuss our current understanding of neurosignaling in cancer and its potential implications on cancer prevention and therapy. The review will focus on the soluble factors released by cancer cells and nerve endings, their biological effects and their potential relevance in the treatment of cancer.  相似文献   
100.
Perkinsus marinus (Phylum Perkinsozoa) is a protozoan parasite that has devastated natural and farmed oyster populations in the USA, significantly affecting the shellfish industry and the estuarine environment. The other two genera in the phylum, Parvilucifera and Rastrimonas, are parasites of microeukaryotes. The Perkinsozoa occupies a key position at the base of the dinoflagellate branch, close to its divergence from the Apicomplexa, a clade that includes parasitic protista, many harbouring a relic plastid. Thus, as a taxon that has also evolved toward parasitism, the Perkinsozoa has attracted the attention of biologists interested in the evolution of this organelle, both in its ultrastructure and the conservation, loss or transfer of its genes. A review of the recent literature reveals mounting evidence in support of the presence of a relic plastid in P. marinus, including the presence of multimembrane structures, characteristic metabolic pathways and proteins with a bipartite N-terminal extension. Further, these findings raise intriguing questions regarding the potential functions and unique adaptation of the putative plastid and/or plastid genes in the Perkinsozoa. In this review we analyse the above-mentioned evidence and evaluate the potential future directions and expected benefits of addressing such questions. Given the rapidly expanding molecular/genetic resources and methodological toolbox for Perkinsus spp., these organisms should complement the currently established models for investigating plastid evolution within the Chromalveolata.  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