全文获取类型
收费全文 | 56221篇 |
免费 | 17352篇 |
国内免费 | 6篇 |
专业分类
73579篇 |
出版年
2023年 | 213篇 |
2022年 | 255篇 |
2021年 | 736篇 |
2020年 | 2549篇 |
2019年 | 4058篇 |
2018年 | 4254篇 |
2017年 | 4412篇 |
2016年 | 4981篇 |
2015年 | 5549篇 |
2014年 | 5178篇 |
2013年 | 6189篇 |
2012年 | 4541篇 |
2011年 | 3813篇 |
2010年 | 4274篇 |
2009年 | 2771篇 |
2008年 | 2575篇 |
2007年 | 2083篇 |
2006年 | 2161篇 |
2005年 | 1944篇 |
2004年 | 1900篇 |
2003年 | 1766篇 |
2002年 | 1688篇 |
2001年 | 987篇 |
2000年 | 1045篇 |
1999年 | 592篇 |
1998年 | 251篇 |
1997年 | 148篇 |
1996年 | 166篇 |
1995年 | 182篇 |
1994年 | 149篇 |
1993年 | 147篇 |
1992年 | 192篇 |
1991年 | 160篇 |
1990年 | 146篇 |
1989年 | 130篇 |
1988年 | 105篇 |
1987年 | 75篇 |
1986年 | 89篇 |
1985年 | 106篇 |
1984年 | 109篇 |
1983年 | 73篇 |
1982年 | 86篇 |
1981年 | 62篇 |
1980年 | 69篇 |
1979年 | 51篇 |
1978年 | 47篇 |
1977年 | 49篇 |
1976年 | 68篇 |
1975年 | 46篇 |
1974年 | 51篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Hervé Seitz 《Current biology : CB》2010,20(3):R108-R110
92.
Fabián Michelangeli Dalia M. Sulcas Marie-Christine Ruiz 《Cell and tissue research》1987,250(2):413-419
Summary This study is concerned with electron-microscopic observations on endocrine or paracrine cells in the fundic gastric mucosa of the bullfrog. Also, an attempt was made to identify the histamine-releasing cells involved in the secretagogue response. At least three distinct endocrine-like cell types were found. The classification is based on the appearance of secretory granules and other organelles, and the relationship of endocrine-like cells with other cells in the tissue. The amphibian endocrine-like cells resemble the ECL, D and EC cells of mammals. Type-I (ECL) cells showed degranulation after repeated stimulation with tetragastrin (TG), acetylcholine (ACh) and K+ depolarizing solution, all of which release histamine. 相似文献
93.
R.F. Spurney S.D. Mayros D. Collins P. Ruiz P.E. Klotman T. Coffman 《Prostaglandins & other lipid mediators》1990,39(2)
Cyclosporine A (CyA) nephorotoxicity is associated with impaired renal hemodynamic funtion and increased production of the vasoconstrictor eicosanoid thromboxane A2 (TxA2). In CyA toxic rats, renal dysfunction cna be partially reversed by inhibitors of thromoboxane sysnthase. However, interpretation of these results is complicated since inhibitance of thromboxane synthase may cause accumulation of prostaglandin endoperoxides that can act as partial agonists at the TxA2 receptor and may blunt the efficacy of treatment. Furthermore, these endoperoxides may be used as substrate for production of vasodilator prostaglandins causing beneficial effects on hemodynamics which are independent of thromboxane inhibition. To more specially examine the role of TxA2 in CyA toxicity, we investigated the effects of the thromboxane receptor antagonist GR32191 on renal hemodynamics in a rat model of CyA nephrotoxicity. In this model, administration of CyA resulted in a significant decrease in glomerular filtration rate (GFR) 2.85±0.26 [CyA] vs 6.82±0.96 ml/min/kg [vehicle]; p<0.0005) and renal blood flow (RBF) (21.6±2.31 [CyA] vs 31.8±3.60 ml/min/kg [vehicle]; p<0.025). Renal vascular resistance (RVR) was significantly higher in rats given CyA compared to animals treated with CyA vehicle (5.32±0.55 [cyCyA] vs 3.54±0.24 mm Hg/min/ml/kg [vehicle]; p<0.05). These hemodynamic alterations were associated with a significant increase in urinary excretion of unmetabolized, “native” thromboxane B2 (TxB2 (103±18 [CyA] vs 60±16 pg/hour [vehicle]; p<0.05). Only minimal histomorphologic changes were apparent by light microscopic examination of kidneys from both CyA and vehicle treated animals. However, with immunoperoxidase staining, a significantly greater number of cells experssing the rat common leukocyte antigen was found in the renal interstitium of rats given CyA*. There was no detectable increase in monocytes/macrophages in the kidneys of CyA toxic animals. In rats treated with CyA, intraarterial infusion of GR32191 at maximally tolerated doses significanlty increased GFR and RBD, and decreased RVR. Although both RBF and RVR were restored to levels not different from controls, GFR remained significantly reduced following administration of GR32191. These data suggest that the potent vasoconstrictor TxA2 plays an important role in mediating renal dysfunction in CyA nephrotoxicity. However, other factors may be important in producing nephrotoxicity associated with CyA. 相似文献
94.
95.
Johnathan L Meaders Erica F Geers Belen Fernandez‐Garcia Marvin E Tanenbaum 《The EMBO journal》2012,31(21):4179-4190
The microtubule motor protein kinesin‐5 (Eg5) provides an outward force on centrosomes, which drives bipolar spindle assembly. Acute inhibition of Eg5 blocks centrosome separation and causes mitotic arrest in human cells, making Eg5 an attractive target for anti‐cancer therapy. Using in vitro directed evolution, we show that human cells treated with Eg5 inhibitors can rapidly acquire the ability to divide in the complete absence of Eg5 activity. We have used these Eg5‐independent cells to study alternative mechanisms of centrosome separation. We uncovered a pathway involving nuclear envelope (NE)‐associated dynein that drives centrosome separation in prophase. This NE‐dynein pathway is essential for bipolar spindle assembly in the absence of Eg5, but also functions in the presence of full Eg5 activity, where it pulls individual centrosomes along the NE and acts in concert with Eg5‐dependent outward pushing forces to coordinate prophase centrosome separation. Together, these results reveal how the forces are produced to drive prophase centrosome separation and identify a novel mechanism of resistance to kinesin‐5 inhibitors. 相似文献
96.
Steven I. Higgins Robert B. O’Hara Olga Bykova Michael D. Cramer Isabelle Chuine Eva‐Maria Gerstner Thomas Hickler Xavier Morin Michael R. Kearney Guy F. Midgley Simon Scheiter 《Journal of Biogeography》2012,39(12):2132-2145
Aim To develop a physiologically based model of the plant niche for use in species distribution modelling. Location Europe. Methods We link the Thornley transport resistance (TTR) model with functions which describe how the TTR’s model parameters are influenced by abiotic environmental factors. The TTR model considers how carbon and nutrient uptake, and the allocation of these assimilates, influence growth. We use indirect statistical methods to estimate the model parameters from a high resolution data set on tree distribution for 22 European tree species. Results We infer, from distribution data and abiotic forcing data, the physiological niche dimensions of 22 European tree species. We found that the model fits were reasonable (AUC: 0.79–0.964). The projected distributions were characterized by a false positive rate of 0.19 and a false negative rate 0.12. The fitted models are used to generate projections of the environmental factors that limit the range boundaries of the study species. Main conclusions We show that physiological models can be used to derive physiological niche dimensions from species distribution data. Future work should focus on including prior information on physiological rates into the parameter estimation process. Application of the TTR model to species distribution modelling suggests new avenues for establishing explicit links between distribution and physiology, and for generating hypotheses about how ecophysiological processes influence the distribution of plants. 相似文献
97.
Background
Eosinophilia plays the major role in the pathogenesis of asthma and correlates with the up‐regulation of eotaxin, which, together with interleukin (IL)‐5, is important for differentiation, chemo‐attraction, degranulation, and survival of eosinophils in local tissue. In a previous study, we found that administration of lentivirus‐delivered short hairpin RNA (shRNA) to suppress the expression of IL‐5 inhibited airway inflammation. The present study aimed to investigate the role of eotaxin shRNA and the synergistic effect of eotaxin and IL‐5 shRNAs on airway inflammation in an ovalbumin (OVA)‐induced murine model of asthma.Methods
Lentivirus‐delivered shRNAs were used to suppress the expression of eotaxin and/or IL‐5 in local tissue in an OVA‐induced murine asthma model.Results
Intra‐tracheal administration of lentivirus containing eotaxin shRNA expressing cassette (eoSEC3.3) efficiently moderated the characteristics of asthma, including airway hyper‐responsiveness, cellular infiltration of lung tissues, and eotaxin and IL‐5 levels in bronchio‐alveolar lavage fluid. Administration of lentiviruses expressing IL‐5 or eotaxin shRNAs (IL5SEC4 + eoSEC3.3) also moderated the symptoms of asthma in a mouse model.Conclusions
Local delivery of lentiviruses expressing IL‐5 and eotaxin shRNAs provides a potential tool in moderating airway inflammation and also has the potential for developing clinical therapy based on the application of shRNAs of chemokines and cytokines involved in T helper 2 cell inflammation and eosinophilia. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献98.
How often will elections end in landslides? What is the probability for a head-to-head race? Analyzing ballot results from several large countries rather anomalous and yet unexplained distributions have been observed. We identify tactical voting as the driving ingredient for the anomalies and introduce a model to study its effect on plurality elections, characterized by the relative strength of the feedback from polls and the pairwise interaction between individuals in the society. With this model it becomes possible to explain the polarization of votes between two candidates, understand the small margin of victories frequently observed for different elections, and analyze the polls'' impact in American, Canadian, and Brazilian ballots. Moreover, the model reproduces, quantitatively, the distribution of votes obtained in the Brazilian mayor elections with two, three, and four candidates. 相似文献
99.
Jyri Seppla Matti Melanen Ilmo Menp Sirkka Koskela Jyrki Tenhunen Marja‐Riitta Hiltunen 《Journal of Industrial Ecology》2005,9(4):117-130
The concept of eco-efficiency is commonly referred to as a business link to sustainable development. In this article, ecoefficiency is examined at a regional level as an approach to promoting the competitiveness of economic activities in the Finnish Kymenlaakso region and mitigating their harmful impacts on the environment. The aim is to develop appropriate indicators for monitoring changes in the eco-efficiency of the region. A starting point is to produce indicators for the environmental and economic dimensions of regional development and use them for measuring regional eco-efficiency. The environmental impact indicators are based on a life-cycle assessment method, producing different types of environmental impact indicators: pressure indicators (e.g., emissions of CO2 ), impact category indicators (e.g., CO2 equivalents in the case of climate change), and a total impact indicator (aggregating different impact category indicator results into a single value). Environmental impact indicators based on direct material input, total material input, and total material requirement of the Kymenlaakso region are also assessed. The economic indicators used are the gross domestic product, the value added, and the output of the main economic sectors of Kymenlaakso. In the eco-efficiency assessment, the economic and environmental impact indicators are monitored in the same graph. In a few cases eco-efficiency ratios can also be calculated (the economic indicators are divided by the environmental indicators). Output (= value added + intermediate consumption) is used as an economic indicator related to the environmental impact indicators, which also cover the upstream processes of the region's activities. In the article, we also discuss the strengths and weaknesses of using the different environmental impact indicators. 相似文献
100.