全文获取类型
收费全文 | 5246篇 |
免费 | 507篇 |
国内免费 | 1篇 |
专业分类
5754篇 |
出版年
2023年 | 44篇 |
2022年 | 110篇 |
2021年 | 141篇 |
2020年 | 84篇 |
2019年 | 129篇 |
2018年 | 129篇 |
2017年 | 121篇 |
2016年 | 163篇 |
2015年 | 272篇 |
2014年 | 275篇 |
2013年 | 305篇 |
2012年 | 408篇 |
2011年 | 398篇 |
2010年 | 226篇 |
2009年 | 185篇 |
2008年 | 265篇 |
2007年 | 238篇 |
2006年 | 208篇 |
2005年 | 203篇 |
2004年 | 172篇 |
2003年 | 161篇 |
2002年 | 176篇 |
2001年 | 97篇 |
2000年 | 81篇 |
1999年 | 86篇 |
1998年 | 38篇 |
1997年 | 33篇 |
1996年 | 33篇 |
1995年 | 25篇 |
1994年 | 28篇 |
1993年 | 22篇 |
1992年 | 55篇 |
1991年 | 43篇 |
1990年 | 42篇 |
1989年 | 49篇 |
1988年 | 38篇 |
1987年 | 31篇 |
1986年 | 32篇 |
1985年 | 45篇 |
1984年 | 25篇 |
1983年 | 35篇 |
1982年 | 35篇 |
1981年 | 22篇 |
1980年 | 25篇 |
1979年 | 29篇 |
1977年 | 27篇 |
1975年 | 23篇 |
1973年 | 31篇 |
1972年 | 26篇 |
1970年 | 23篇 |
排序方式: 共有5754条查询结果,搜索用时 15 毫秒
101.
Baticic L Detel D Kucic N Buljevic S Pugel EP Varljen J 《Journal of cellular biochemistry》2011,112(11):3322-3333
Causal connections between dipeptidyl peptidase IV, also known as CD26 molecule (DPP IV/CD26) and inflammatory bowel disease (IBD) have been shown, but mechanisms of these interactions are unclear. Our hypothesis was that DPP IV/CD26 could affect the neuroimmune response during inflammatory events. Therefore, we aimed to evaluate its possible role and the relevance of the gut-brain axis in a model of IBD in mice. Trinitrobenzenesulfonic acid-induced (TNBS) colitis was induced in CD26-deficient (CD26(-/-) ) and wild-type (C57BL/6) mice. Pathohistological and histomorphometrical measurements were done. Concentrations and protein expressions of DPP IV/CD26 substrates neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) were determined. Concentrations of IL-6 and IL-10 were evaluated. Investigations were conducted at systemic and local levels. Acute inflammation induced increased serum NPY concentrations in both mice strains, more enhanced in CD26(-/-) mice. Increased NPY concentrations were found in colon and brain of C57BL/6 mice, while in CD26(-/-) animals only in colon. VIP and IL-6 serum and tissue concentrations were increased in both mice strains in acute inflammation, more pronouncedly in CD26(-/-) mice. IL-10 concentrations, after a decrease in serum of both mice strains, increased promptly in CD26(-/-) mice. Decreased IL-10 concentration was found in brain of C57BL/6 mice, while it was increased in colon of CD26(-/-) mice in acute inflammation. DPP IV/CD26 deficiency affects the neuroimmune response at systemic and local levels during colitis development and resolution in mice. Inflammatory changes in the colon reflected on investigated parameters in the brain, suggesting an important role of the gut-brain axis in IBD pathogenesis. 相似文献
102.
Extracellular invertase is an essential component of cytokinin-mediated delay of senescence 总被引:24,自引:0,他引:24 下载免费PDF全文
Balibrea Lara ME Gonzalez Garcia MC Fatima T Ehness R Lee TK Proels R Tanner W Roitsch T 《The Plant cell》2004,16(5):1276-1287
Leaf senescence is the final stage of leaf development in which the nutrients invested in the leaf are remobilized to other parts of the plant. Whereas senescence is accompanied by a decline in leaf cytokinin content, exogenous application of cytokinins or an increase of the endogenous concentration delays senescence and causes nutrient mobilization. The finding that extracellular invertase and hexose transporters, as the functionally linked enzymes of an apolasmic phloem unloading pathway, are coinduced by cytokinins suggested that delay of senescence is mediated via an effect on source-sink relations. This hypothesis was further substantiated in this study by the finding that delay of senescence in transgenic tobacco (Nicotiana tabacum) plants with autoregulated cytokinin production correlates with an elevated extracellular invertase activity. The finding that the expression of an extracellular invertase under control of the senescence-induced SAG12 promoter results in a delay of senescence demonstrates that effect of cytokinins may be substituted by these metabolic enzymes. The observation that an increase in extracellular invertase is sufficient to delay leaf senescence was further verified by a complementing functional approach. Localized induction of an extracellular invertase under control of a chemically inducible promoter resulted in ectopic delay of senescence, resembling the naturally occurring green islands in autumn leaves. To establish a causal relationship between cytokinins and extracellular invertase for the delay of senescence, transgenic plants were generated that allowed inhibition of extracellular invertase in the presence of cytokinins. For this purpose, an invertase inhibitor was expressed under control of a cytokinin-inducible promoter. It has been shown that senescence is not any more delayed by cytokinin when the expression of the invertase inhibitor is elevated. This finding demonstrates that extracellular invertase is required for the delay of senescence by cytokinins and that it is a key element of the underlying molecular mechanism. 相似文献
103.
Kimberly R. Jordan Rodabe N. Amaria Oscar Ramirez Eryn B. Callihan Dexiang Gao Michelle Borakove Elizabeth Manthey Virginia F. Borges Martin D. McCarter 《Cancer immunology, immunotherapy : CII》2013,62(11):1711-1722
Myeloid-derived suppressor cells are increased in the peripheral blood of advanced-stage cancer patients; however, no studies have shown a correlation of these immunosuppressive cells with clinical outcomes in melanoma patients. We characterized the frequency and suppressive function of multiple subsets of myeloid-derived suppressor cells in the peripheral blood of 34 patients with Stage IV melanoma, 20 patients with Stage I melanoma, and 15 healthy donors. The frequency of CD14+ MDSCs (Lin? CD11b+ HLA-DR? CD14+ CD33+) and CD14? MDSCs (Lin? CD11b+ HLA-DR? CD14? CD33+) was increased in the peripheral blood of Stage IV melanoma patients relative to healthy donors. The frequency of CD14+ and CD14? MDSCs correlated with each other and with the increased frequency of regulatory T cells, but not with classically defined monocytes. CD14? MDSCs isolated from the peripheral blood of Stage IV melanoma patients suppressed T cell activation more than those isolated from healthy donors, and the frequency of these cells correlated with disease progression and decreased overall survival. Our study provides the first evidence that the frequency of CD14? MDSCs negatively correlates with clinical outcomes in advanced-stage melanoma patients. These data indicate that suppressive MDSCs should be considered as targets for future immunotherapies. 相似文献
104.
Permafrost thaw resulting from climate warming may dramatically change the succession and carbon dynamics of northern ecosystems.
To examine the joint effects of regional temperature and local species changes on peat accumulation following thaw, we studied
peat accumulation across a regional gradient of mean annual temperature (MAT). We measured aboveground net primary production
(AGNPP) and decomposition over 2 years for major functional groups and used these data to calculate a simple index of net
annual aboveground peat accumulation. In addition, we collected cores from six adjacent frozen and thawed bog sites to document
peat accumulation changes following thaw over the past 200 years. Aboveground biomass and decomposition were more strongly
controlled by local succession than regional climate. AGNPP for some species differed between collapse scars and associated
permafrost plateaus and was influenced by regional MAT. A few species, such as Picea mariana trees on frozen bogs and Sphagnum mosses in thawed bogs, sequestered a disproportionate amount of peat; in addition, changes in their abundance following thaw
changed peat accumulation. 210Pb-dated cores indicated that peat accumulation doubles following thaw and that the accumulation rate is affected by historical
changes in species during succession. Peat accumulation in boreal peatlands following thaw was controlled by a complex mix
of local vegetation changes, regional climate, and history. These results suggest that northern ecosystems may show responses
more complex than large releases of carbon during transient warming.
Received 8 August 2000; accepted 12 January 2001. 相似文献
105.
106.
Marissa J. Schafer Daniel L. Mazula Ashley K. Brown Thomas A. White Elizabeth Atkinson Vesselina M. Pearsall Zaira Aversa Grace C. Verzosa Leslie A. Smith Aleksey Matveyenko Jordan D. Miller Nathan K. LeBrasseur 《Aging cell》2019,18(4)
Aging and obesity increase multimorbidity and disability risk, and determining interventions for reversing healthspan decline is a critical public health priority. Exercise and time‐restricted feeding (TRF) benefit multiple health parameters when initiated in early life, but their efficacy and safety when initiated at older ages are uncertain. Here, we tested the effects of exercise versus TRF in diet‐induced obese, aged mice from 20 to 24 months of age. We characterized healthspan across key domains: body composition, physical, metabolic, and cardiovascular function, activity of daily living (ADL) behavior, and pathology. We demonstrate that both exercise and TRF improved aspects of body composition. Exercise uniquely benefited physical function, and TRF uniquely benefited metabolism, ADL behavior, and circulating indicators of liver pathology. No adverse outcomes were observed in exercised mice, but in contrast, lean mass and cardiovascular maladaptations were observed following TRF. Through a composite index of benefits and risks, we conclude the net healthspan benefits afforded by exercise are more favorable than those of TRF. Extrapolating to obese older adults, exercise is a safe and effective option for healthspan improvement, but additional comprehensive studies are warranted before recommending TRF. 相似文献
107.
108.
109.
The present investigation was made to evaluate biologically relevant quinones as possible catalysts in the generation of hydroxyl radicals from hydrogen peroxide and superoxide radicals. ESR spectra demonstrated that menadione, plastoquinone, and ubiquinone derivatives could all be reduced to their semiquinone forms by electron transfer from superoxide radicals. Reductive homolytic cleavage of H2O2 was observed to be dependent upon the presence of semiquinones in the reaction medium. Our data strongly support the concept that quinones normally involved in physiological processes may also play a role as catalysts of the Haber-Weiss reaction in the biological generation of hydroxyl radicals. 相似文献
110.