全文获取类型
收费全文 | 6246篇 |
免费 | 701篇 |
国内免费 | 3篇 |
专业分类
6950篇 |
出版年
2021年 | 73篇 |
2019年 | 61篇 |
2017年 | 77篇 |
2016年 | 116篇 |
2015年 | 175篇 |
2014年 | 201篇 |
2013年 | 237篇 |
2012年 | 311篇 |
2011年 | 284篇 |
2010年 | 181篇 |
2009年 | 171篇 |
2008年 | 250篇 |
2007年 | 237篇 |
2006年 | 220篇 |
2005年 | 227篇 |
2004年 | 196篇 |
2003年 | 201篇 |
2002年 | 218篇 |
2001年 | 218篇 |
2000年 | 184篇 |
1999年 | 187篇 |
1998年 | 103篇 |
1997年 | 78篇 |
1996年 | 64篇 |
1995年 | 73篇 |
1994年 | 54篇 |
1993年 | 66篇 |
1992年 | 136篇 |
1991年 | 113篇 |
1990年 | 101篇 |
1989年 | 116篇 |
1988年 | 105篇 |
1987年 | 94篇 |
1986年 | 103篇 |
1985年 | 91篇 |
1984年 | 63篇 |
1983年 | 74篇 |
1982年 | 66篇 |
1981年 | 52篇 |
1980年 | 65篇 |
1979年 | 63篇 |
1977年 | 52篇 |
1975年 | 63篇 |
1974年 | 67篇 |
1973年 | 64篇 |
1972年 | 73篇 |
1970年 | 63篇 |
1969年 | 57篇 |
1968年 | 55篇 |
1967年 | 58篇 |
排序方式: 共有6950条查询结果,搜索用时 0 毫秒
941.
Anthony D. Vaudo James D. Ellis Garth A. Cambray Martin Hill 《Journal of Insect Conservation》2012,16(4):601-611
Livestock farming in the Eastern Cape, South Africa is a common land use practice that has affected the biodiversity of plants and animals in the region negatively. Indigenous populations of wild honey bee (Apis mellifera) colonies also may suffer. Recently, farmers in the Eastern Cape have been converting their farms to game reserves as ecotourism attractions and nature conservation lands. Consequently, the goal of this research was to determine if land use habits (livestock farming and conversion to game reserves) in the Eastern Cape affect honey bee colony population density and colony strength parameters. A series of indices were developed to compare the relative population densities of colonies in two or more areas by counting the number of foraging bees and number of bee lines established at feeding stations. Wild colonies on farms and reserves were located and sampled to determine land use effects on colony strength parameters including total area of comb in the colony, the area of comb containing stored honey, pollen, and brood, adult bee population, weight per bee, and the colony nest cavity volume ratio. When viewed collectively, the data indicated that land use practices have affected honey bee nesting dynamics in the Eastern Cape. Trends in the data suggested that colonies nesting on the reserves may occur in greater densities than those nesting on livestock farms, though they do not appear to be healthier. Hopefully, this work will be continued since honey bee conservation in areas where they are native is crucial to the health of agriculture and whole ecosystems globally. 相似文献
942.
Dull RO Cluff M Kingston J Hill D Chen H Hoehne S Malleske DT Kaur R 《American journal of physiology. Lung cellular and molecular physiology》2012,302(9):L816-L828
Lung endothelial cells respond to changes in vascular pressure through mechanotransduction pathways that alter barrier function via non-Starling mechanism(s). Components of the endothelial glycocalyx have been shown to participate in mechanotransduction in vitro and in systemic vessels, but the glycocalyx's role in mechanosensing and pulmonary barrier function has not been characterized. Mechanotransduction pathways may represent novel targets for therapeutic intervention during states of elevated pulmonary pressure such as acute heart failure, fluid overload, and mechanical ventilation. Our objective was to assess the effects of increasing vascular pressure on whole lung filtration coefficient (K(fc)) and characterize the role of endothelial heparan sulfates in mediating mechanotransduction and associated increases in K(fc). Isolated perfused rat lung preparation was used to measure K(fc) in response to changes in vascular pressure in combination with superimposed changes in airway pressure. The roles of heparan sulfates, nitric oxide, and reactive oxygen species were investigated. Increases in capillary pressure altered K(fc) in a nonlinear relationship, suggesting non-Starling mechanism(s). nitro-l-arginine methyl ester and heparanase III attenuated the effects of increased capillary pressure on K(fc), demonstrating active mechanotransduction leading to barrier dysfunction. The nitric oxide (NO) donor S-nitrosoglutathione exacerbated pressure-mediated increase in K(fc). Ventilation strategies altered lung NO concentration and the K(fc) response to increases in vascular pressure. This is the first study to demonstrate a role for the glycocalyx in whole lung mechanotransduction and has important implications in understanding the regulation of vascular permeability in the context of vascular pressure, fluid status, and ventilation strategies. 相似文献
943.
Oliver Windram Priyadharshini Madhou Stuart McHattie Claire Hill Richard Hickman Emma Cooke Dafyd J. Jenkins Christopher A. Penfold Laura Baxter Emily Breeze Steven J. Kiddle Johanna Rhodes Susanna Atwell Daniel J. Kliebenstein Youn-sung Kim Oliver Stegle Karsten Borgwardt Cunjin Zhang Alex Tabrett Roxane Legaie Jonathan Moore B?rbel Finkenstadt David L. Wild Andrew Mead David Rand Jim Beynon Sascha Ott Vicky Buchanan-Wollaston Katherine J. Denby 《The Plant cell》2012,24(9):3530-3557
944.
The mechanism responsible for the decrease in vitamin D status (i.e., plasma or serum 25-hydroxyvitamin D (25(OH)D) concentration) during inflammatory stress is unknown in humans. Interferon (IFN)-γ is an inflammatory cytokine that regulates vitamin D metabolism in isolated immune cells, but data suggesting this regulation exists in vivo is lacking. The purpose of this study, therefore, was to associate circulating IFN-γ perturbations with 25(OH)D and 1,25-dihydroxyvitamin D (1,25(OH)D) alterations during inflammatory stress in young adults recovering from anterior cruciate ligament (ACL) reconstruction. Plasma 25(OH)D, 1,25(OH)D and IFN-γ concentrations were measured in fasting blood draw samples obtained from twelve-male patients pre-surgery and 90-m, 3-d and 7-d post-surgery. 25(OH)D decreased significantly (p<0.05) after surgery, and strikingly, tended to inversely correlate (r=-0.32, p=0.058) with IFN-γ changes from pre- to post- (i.e., 90-m, 3-d, and 7-d) surgery. Additionally, 1,25(OH)D (r=0.37, p<0.05) and the 1,25(OH)D-to-25(OH)D ratio (r=0.52, p<0.05) changes from pre- to post- (i.e., 90-m, 3-d, and 7-d) surgery correlated with those of IFN-γ. These are the first reported in vivo findings suggesting that the 25(OH)D decrease and conversion to 1,25(OH)D increase with increasing IFN-γ in the circulation. We conclude that IFN-γ contributes to the decrease in vitamin D and the conversion of vitamin D to its active hormonal form in the circulation during inflammatory insult in humans. 相似文献
945.
Helen Sharp Andrew Pickles Michael Meaney Kate Marshall Florin Tibu Jonathan Hill 《PloS one》2012,7(10)
Animal studies find that prenatal stress is associated with increased physiological and emotional reactivity later in life, mediated via fetal programming of the HPA axis through decreased glucocorticoid receptor (GR) gene expression. Post-natal behaviours, notably licking and grooming in rats, cause decreased behavioural indices of fear and reduced HPA axis reactivity mediated via increased GR gene expression. Post-natal maternal behaviours may therefore be expected to modify prenatal effects, but this has not previously been examined in humans. We examined whether, according to self-report, maternal stroking over the first weeks of life modified associations between prenatal depression and physiological and behavioral outcomes in infancy, hence mimicking effects of rodent licking and grooming. From a general population sample of 1233 first time mothers recruited at 20 weeks gestation we drew a stratified random sample of 316 for assessment at 32 weeks based on reported inter-partner psychological abuse, a risk to child development. Of these 271 provided data at 5, 9 and 29 weeks post delivery. Mothers reported how often they stroked their babies at 5 and 9 weeks. At 29 weeks vagal withdrawal to a stressor, a measure of physiological adaptability, and maternal reported negative emotionality were assessed. There was a significant interaction between prenatal depression and maternal stroking in the prediction of vagal reactivity to a stressor (p = .01), and maternal reports of infant anger proneness (p = .007) and fear (p = .043). Increasing maternal depression was associated with decreasing physiological adaptability, and with increasing negative emotionality, only in the presence of low maternal stroking. These initial findings in humans indicate that maternal stroking in infancy, as reported by mothers, has effects strongly resembling the effects of observed maternal behaviours in animals, pointing to future studies of the epigenetic, physiological and behavioral effects of maternal stroking. 相似文献
946.
Background
A recent trial with PCV-7 in a rural Gambian community showed reduced vaccine-type pneumococcal carriage in fully vaccinated compared with control communities. We measured pneumococcal polysaccharide antibody concentrations in this trial to understand further the mechanisms underlying the observed changes.Methods
A single-blind, cluster-randomized (by village) trial was conducted in 21 Gambian villages. In 11 villages, all residents received PCV-7 (Vaccine group); in 10 control villages only children <30 months old or those born during the study received PCV-7. Subjects over the age of 30 months resident in vaccine villages received a single dose of PCV-7 whilst those in control villages received a single dose of a serogroup C meningococcal conjugate vaccine. Serum antibody concentrations against specific pneumococcal polysaccharides were measured in approximately 200 age-stratified subjects before, 4–6, 12 and 24 months following vaccination.Results
Baseline pneumococcal antibody concentrations were generally high and increased with age up to 10 years. One dose of PCV-7 increased geometric mean antibody concentrations (GMC) in vaccinated versus control villages for vaccine serotypes 6B and 18C, and 4 and 18C, in the young (under 5 years) and older age groups (5+ years) respectively. There were significantly higher proportions of subjects in the vaccinated than in the control communities with an antibody concentration believed to protect against carriage (>5.0 µg/mL) for all but serotype 9V of the PCV-7 serotypes in the older group, but not in the younger age group.Conclusion
Higher antibodies in vaccinated communities provide an explanation for the lower pneumococcal carriage rates in fully vaccinated compared to control communities.Trial Registration
Controlled-Trials.com ISRCTN51695599 51695599. 相似文献947.
KS Hill I Gaziova L Harrigal YA Guerra S Qiu SK Sastry T Arumugam CD Logsdon LA Elferink 《PloS one》2012,7(7):e40420
At diagnosis, the majority of pancreatic cancer patients present with advanced disease when curative resection is no longer feasible and current therapeutic treatments are largely ineffective. An improved understanding of molecular targets for effective intervention of pancreatic cancer is thus urgent. The Met receptor tyrosine kinase is one candidate implicated in pancreatic cancer. Notably, Met is over expressed in up to 80% of invasive pancreatic cancers but not in normal ductal cells correlating with poor overall patient survival and increased recurrence rates following surgical resection. However the functional role of Met signaling in pancreatic cancer remains poorly understood. Here we used RNA interference to directly examine the pathobiological importance of increased Met signaling for pancreatic cancer. We show that Met knockdown in pancreatic tumor cells results in decreased cell survival, cell invasion, and migration on collagen I in vitro. Using an orthotopic model for pancreatic cancer, we provide in vivo evidence that Met knockdown reduced tumor burden correlating with decreased cell survival and tumor angiogenesis, with minimal effect on cell growth. Notably, we report that Met signaling regulates the secretion of the pro-angiogenic chemokine interleukin-8/CXCL8. Our data showing that the interleukin-8 receptors CXCR1 and CXCR2 are not expressed on pancreatic tumor cells, suggests a paracrine mechanism by which Met signaling regulates interleukin-8 secretion to remodel the tumor microenvironment, a novel finding that could have important clinical implications for improving the effectiveness of treatments for pancreatic cancer. 相似文献
948.
A major focus of stem cell research is the generation of neurons that may then be implanted to treat neurodegenerative diseases. However, a picture is emerging where astrocytes are partners to neurons in sustaining and modulating brain function. We therefore investigated the functional properties of NT2 derived astrocytes and neurons using electrophysiological and calcium imaging approaches. NT2 neurons (NT2Ns) expressed sodium dependent action potentials, as well as responses to depolarisation and the neurotransmitter glutamate. NT2Ns exhibited spontaneous and coordinated calcium elevations in clusters and in extended processes, indicating local and long distance signalling. Tetrodotoxin sensitive network activity could also be evoked by electrical stimulation. Similarly, NT2 astrocytes (NT2As) exhibited morphology and functional properties consistent with this glial cell type. NT2As responded to neuronal activity and to exogenously applied neurotransmitters with calcium elevations, and in contrast to neurons, also exhibited spontaneous rhythmic calcium oscillations. NT2As also generated propagating calcium waves that were gap junction and purinergic signalling dependent. Our results show that NT2 derived astrocytes exhibit appropriate functionality and that NT2N networks interact with NT2A networks in co-culture. These findings underline the utility of such cultures to investigate human brain cell type signalling under controlled conditions. Furthermore, since stem cell derived neuron function and survival is of great importance therapeutically, our findings suggest that the presence of complementary astrocytes may be valuable in supporting stem cell derived neuronal networks. Indeed, this also supports the intriguing possibility of selective therapeutic replacement of astrocytes in diseases where these cells are either lost or lose functionality. 相似文献
949.
Pollen tube growth is central to the sexual reproduction of plants and is a longstanding model for cellular tip growth. For rapid tip growth, cell wall deposition and hardening must balance the rate of osmotic water uptake, and this involves the control of turgor pressure. Pressure contributes directly to both the driving force for water entry and tip expansion causing thinning of wall material. Understanding tip growth requires an analysis of the coordination of these processes and their regulation. Here we develop a quantitative physiological model which includes water entry by osmosis, the incorporation of cell wall material and the spreading of that material as a film at the tip. Parameters of the model have been determined from the literature and from measurements, by light, confocal and electron microscopy, together with results from experiments made on dye entry and plasmolysis in Lilium longiflorum. The model yields values of variables such as osmotic and turgor pressure, growth rates and wall thickness. The model and its predictive capacity were tested by comparing programmed simulations with experimental observations following perturbations of the growth medium. The model explains the role of turgor pressure and its observed constancy during oscillations; the stability of wall thickness under different conditions, without which the cell would burst; and some surprising properties such as the need for restricting osmotic permeability to a constant area near the tip, which was experimentally confirmed. To achieve both constancy of pressure and wall thickness under the range of conditions observed in steady-state growth the model reveals the need for a sensor that detects the driving potential for water entry and controls the deposition rate of wall material at the tip. 相似文献
950.