全文获取类型
收费全文 | 10621篇 |
免费 | 834篇 |
国内免费 | 3篇 |
专业分类
11458篇 |
出版年
2024年 | 5篇 |
2023年 | 60篇 |
2022年 | 173篇 |
2021年 | 263篇 |
2020年 | 152篇 |
2019年 | 206篇 |
2018年 | 252篇 |
2017年 | 236篇 |
2016年 | 325篇 |
2015年 | 558篇 |
2014年 | 605篇 |
2013年 | 779篇 |
2012年 | 979篇 |
2011年 | 890篇 |
2010年 | 597篇 |
2009年 | 509篇 |
2008年 | 667篇 |
2007年 | 722篇 |
2006年 | 663篇 |
2005年 | 544篇 |
2004年 | 543篇 |
2003年 | 446篇 |
2002年 | 432篇 |
2001年 | 74篇 |
2000年 | 58篇 |
1999年 | 79篇 |
1998年 | 93篇 |
1997年 | 73篇 |
1996年 | 59篇 |
1995年 | 59篇 |
1994年 | 30篇 |
1993年 | 51篇 |
1992年 | 32篇 |
1991年 | 32篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 15篇 |
1987年 | 7篇 |
1986年 | 11篇 |
1985年 | 18篇 |
1984年 | 16篇 |
1983年 | 13篇 |
1982年 | 17篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1974年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Penolazzi L Lambertini E Tavanti E Torreggiani E Vesce F Gambari R Piva R 《Cell biology international》2008,32(2):320-325
We have used cytokine protein array to analyze the secretion of cytokines from an osteoblastic clone derived from human umbilical cord blood mesenchymal stem cells (MSCs) cultured in an osteogenic differentiation medium. The analysis demonstrated the unexpected ability of osteoblast committed cells and their early progenitors to produce significant amounts of a range of soluble immune mediators without in vitro exposure to clinically relevant bacterial pathogens. The cells were expanded and their osteogenic potential analyzed over 45 days of culture was revealed by the expression of osteoblast-specific markers (alkaline phosphatase and Runx2), and by matrix mineralization. Over this culture period, the cells secreted particularly high levels of IL-8, MCP-1 and VEGF, but did not express IL-2, IL-7, IL-17, eotaxin, G-CSF and IFN-gamma. These findings should encourage the use of human umbilical cord blood as a potential stem cells source for bone regeneration. 相似文献
82.
Wood anatomy and carbon‐isotope discrimination support long‐term hydraulic deterioration as a major cause of drought‐induced dieback 下载免费PDF全文
Elena Pellizzari J. Julio Camarero Antonio Gazol Gabriel Sangüesa‐Barreda Marco Carrer 《Global Change Biology》2016,22(6):2125-2137
Hydraulic impairment due to xylem embolism and carbon starvation are the two proposed mechanisms explaining drought‐induced forest dieback and tree death. Here, we evaluate the relative role played by these two mechanisms in the long‐term by quantifying wood‐anatomical traits (tracheid size and area of parenchyma rays) and estimating the intrinsic water‐use efficiency (iWUE) from carbon isotopic discrimination. We selected silver fir and Scots pine stands in NE Spain with ongoing dieback processes and compared trees showing contrasting vigour (declining vs nondeclining trees). In both species earlywood tracheids in declining trees showed smaller lumen area with thicker cell wall, inducing a lower theoretical hydraulic conductivity. Parenchyma ray area was similar between the two vigour classes. Wet spring and summer conditions promoted the formation of larger lumen areas, particularly in the case of nondeclining trees. Declining silver firs presented a lower iWUE than conspecific nondeclining trees, but the reverse pattern was observed in Scots pine. The described patterns in wood anatomical traits and iWUE are coherent with a long‐lasting deterioration of the hydraulic system in declining trees prior to their dieback. Retrospective quantifications of lumen area permit to forecast dieback in declining trees 2–5 decades before growth decline started. Wood anatomical traits provide a robust tool to reconstruct the long‐term capacity of trees to withstand drought‐induced dieback. 相似文献
83.
Rotifer Nervous System Visualized by FMRFamide and 5-HT
Immunocytochemistry and Confocal Laser Scanning Microscopy 总被引:6,自引:4,他引:6
Elena?A.?Kotikova Olga?I.?Raikova Maria?ReuterEmail author Margaretha?K. S.?Gustafsson 《Hydrobiologia》2005,546(1):239-248
The characteristic ecology of floodplain lakes is in part due to their relatively strong water-level fluctuations. We analyzed the factors determining water-level fluctuations in 100 floodplain lakes (during non-flooded conditions) in the active floodplains of the Lower Rhine in the Netherlands. Furthermore, we explored the relationship between water-level fluctuations and macrophyte species richness, and analyzed the suitability of artificially created lakes for macrophyte vegetation. During non-flooded conditions along the Rhine, lake water-level fluctuations are largely driven by groundwater connection to the river. Hence, water-level fluctuations are largest in lakes close to the main channel in strongly fluctuating sectors of the river and smallest in isolated lakes. Additionally, water-level fluctuations are usually small in old lakes, mainly due to reduced groundwater hydraulic conductivity resulting from accumulated clay and silt on the bottom. Species richness of floating-leaved and emergent macrophytes was reduced at both small and large water-level fluctuations, whereas species richness of submerged macrophytes was reduced at small water-level fluctuations only. In addition, species richness of submerged macrophytes was higher in lakes that experienced drawdown, whereas no similar pattern was detected for floating-leaved and emergent macrophytes. The decline in amplitude of lake water-level with lake age implies that the number of hydrologically dynamic lakes will decrease over time. Therefore, we suggest that excavation of new lakes is essential to conserve the successional sequence of floodplain water bodies including conditions of high biodiversity. Shallow, moderately isolated, lakes with occasional bottom exposure have the highest potential for creating macrophyte-rich floodplain lakes along large lowland rivers. The water-level regime of such lakes can in part be designed, through choice of the location along the river, the distance away from the river and the depth profile of the lake. 相似文献
84.
Arbuscular Mycorrhizal Symbiosis with Arundo donax Decreases Root Respiration and Increases Both Photosynthesis and Plant Biomass Accumulation 下载免费PDF全文
Antònia Romero‐Munar Miquel Ribas‐Carbó Jaume Flexas Elena Baraza Igor Florez‐Sarasa Alisdair Robert Fernie Javier Gulías 《Plant, cell & environment》2017,40(7):1115-1126
The effect of arbuscular mycorrhiza (AM) symbiosis on plant growth is associated with the balance between costs and benefits. A feedback regulation loop has been described in which the higher carbohydrate cost to plants for AM symbiosis is compensated by increases in their photosynthetic rates. Nevertheless, plant carbon balance depends both on photosynthetic carbon uptake and respiratory carbon consumption. The hypothesis behind this research was that the role of respiration in plant growth under AM symbiosis may be as important as that of photosynthesis. This hypothesis was tested in Arundo donax L. plantlets inoculated with Rhizophagus irregularis and Funneliformis mosseae. We tested the effects of AM inoculation on both photosynthetic capacity and in vivo leaf and root respiration. Additionally, analyses of the primary metabolism and ion content were performed in both leaves and roots. AM inoculation increased photosynthesis through increased CO2 diffusion and electron transport in the chloroplast. Moreover, respiration decreased only in AM roots via the cytochrome oxidase pathway (COP) as measured by the oxygen isotope technique. This decline in the COP can be related to the reduced respiratory metabolism and substrates (sugars and tricarboxylic acid cycle intermediates) observed in roots. 相似文献
85.
Eph receptor signalling casts a wide net on cell behaviour 总被引:11,自引:0,他引:11
Pasquale EB 《Nature reviews. Molecular cell biology》2005,6(6):462-475
Eph receptor tyrosine kinases mould the behaviour of many cell types by binding membrane-anchored ligands, ephrins, at sites of cell-cell contact. Eph signals affect both of the contacting cells and can produce diverse biological responses. New models explain how quantitative variations in the densities and signalling abilities of Eph receptors and ephrins could account for the different effects that are elicited on axon guidance, cell adhesion and cell migration during development, homeostasis and disease. 相似文献
86.
Human Cytomegalovirus UL131-128 Genes Are Indispensable for Virus Growth in Endothelial Cells and Virus Transfer to Leukocytes 下载免费PDF全文
87.
Kakajan Komurov Jen‐Te Tseng Melissa Muller Elena G Seviour Tyler J Moss Lifeng Yang Deepak Nagrath Prahlad T Ram 《Molecular systems biology》2012,8(1)
Dynamic interactions between intracellular networks regulate cellular homeostasis and responses to perturbations. Targeted therapy is aimed at perturbing oncogene addiction pathways in cancer, however, development of acquired resistance to these drugs is a significant clinical problem. A network‐based computational analysis of global gene expression data from matched sensitive and acquired drug‐resistant cells to lapatinib, an EGFR/ErbB2 inhibitor, revealed an increased expression of the glucose deprivation response network, including glucagon signaling, glucose uptake, gluconeogenesis and unfolded protein response in the resistant cells. Importantly, the glucose deprivation response markers correlated significantly with high clinical relapse rates in ErbB2‐positive breast cancer patients. Further, forcing drug‐sensitive cells into glucose deprivation rendered them more resistant to lapatinib. Using a chemical genomics bioinformatics mining of the CMAP database, we identified drugs that specifically target the glucose deprivation response networks to overcome the resistant phenotype and reduced survival of resistant cells. This study implicates the chronic activation of cellular compensatory networks in response to targeted therapy and suggests novel combinations targeting signaling and metabolic networks in tumors with acquired resistance. 相似文献
88.
Serban DE 《Roumanian archives of microbiology and immunology》2011,70(3):134-140
The normal intestinal microflora (microbiota) represents a complex, dynamic, and diverse collection of microorganisms, which usually inhabit the gastrointestinal tract. Normally, between this flora and the human host a mutually beneficial long-term symbiotic relationship is established, where the host contributes essential nutrients necessary for the survival of the microbiota and the latter fulfils multiple roles in host nutrition and development. Several achievements have recently converged to renew interest in studying the normal gut microbiota: the development of molecular methods of studying the microbial communities, the improved understanding of host-microbe interactions in health and disease, and the potential for therapeutic manipulation of the microbiota. We present recent data concerning the molecular technologies of studying the microbiota and new findings regarding the composition of the normal flora. We underline the beneficial activities of the gut flora on the human host. We emphasize the recent findings in the alterations of the microbiota in various medical conditions (celiac disease, irritable bowel syndrome, obesity, colorectal cancer, allergic disorders, and especially inflammatory bowel diseases). The results of these new studies suggest that changes of the microbiota could be linked to the etiopathogenesis of these diseases. These outstanding findings could be used for further diagnostic tools and/or therapy. 相似文献
89.
MAK-V/Hunk is a scantily characterized AMPK-like protein kinase. Recent findings identified MAK-V as a pro-survival and anti-apoptotic protein and revealed its role in embryonic development as well as in tumorigenesis and metastasis. However molecular mechanisms of MAK-V action and regulation of its activity remain largely unknown. We identified Nedd4 as an interaction partner for MAK-V protein kinase. However, this HECT-type E3 ubiquitin ligase is not involved in the control of MAK-V degradation by the ubiquitin-proteasome system that regulates MAK-V abundance in cells. However, Nedd4 in an ubiquitin ligase-independent manner rescued developmental defects in Xenopus embryos induced by MAK-V overexpression, suggesting physiological relevance of interaction between MAK-V and Nedd4. This identifies Nedd4 as the first known regulator of MAK-V function. 相似文献
90.
The trunk canal of fishes is contained within a series of lateral line (LL) scales. To categorise LL scale structural types, and determine their distribution, an analysis of original data was undertaken using light and scanning electron microscopy in combination with a literature survey from over 1,000 species representative of most orders of bony fishes. Our categorisation of LL scales is based on the relationship between the tube, or ossified trunk canal segment, and associated scale. Tubular‐Scalar LL scales consist of a distinguishable tube and elasmoid scale in scale pockets. Four types occur only in species with elasmoid scales. Integrated LL scales do not develop in scale pockets, and their tube is enclosed or extended by a non‐elasmoid scale or spines. Integrated 1 and 2 LL scales co‐occur with ganoid and calcidermoid scales, and Integrated 3 LL scales occur when common scales are absent or elasmoid. Tubular LL scales are tubes only, occurring mainly in scaleless species or with calcidermoid and elasmoid scales. Non‐Tubular LL scales are composed only of a scale, co‐occurring mainly with cycloid scales. There is consistency of LL scale type in many orders, families and genera and the presence of different types within taxa can be meaningful. 相似文献