全文获取类型
收费全文 | 15734篇 |
免费 | 1202篇 |
国内免费 | 3篇 |
专业分类
16939篇 |
出版年
2023年 | 79篇 |
2022年 | 166篇 |
2021年 | 313篇 |
2020年 | 157篇 |
2019年 | 229篇 |
2018年 | 276篇 |
2017年 | 252篇 |
2016年 | 486篇 |
2015年 | 755篇 |
2014年 | 881篇 |
2013年 | 1009篇 |
2012年 | 1400篇 |
2011年 | 1347篇 |
2010年 | 890篇 |
2009年 | 840篇 |
2008年 | 1098篇 |
2007年 | 999篇 |
2006年 | 926篇 |
2005年 | 827篇 |
2004年 | 823篇 |
2003年 | 781篇 |
2002年 | 710篇 |
2001年 | 121篇 |
2000年 | 96篇 |
1999年 | 126篇 |
1998年 | 154篇 |
1997年 | 117篇 |
1996年 | 108篇 |
1995年 | 80篇 |
1994年 | 84篇 |
1993年 | 103篇 |
1992年 | 61篇 |
1991年 | 59篇 |
1990年 | 38篇 |
1989年 | 51篇 |
1988年 | 42篇 |
1987年 | 41篇 |
1986年 | 36篇 |
1985年 | 30篇 |
1984年 | 27篇 |
1983年 | 47篇 |
1982年 | 30篇 |
1981年 | 35篇 |
1980年 | 21篇 |
1979年 | 12篇 |
1978年 | 20篇 |
1977年 | 29篇 |
1976年 | 18篇 |
1975年 | 15篇 |
1974年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 21 毫秒
21.
Birgitta Lindqvist Bianca B. Jütte Luca Love Wlaa Assi Julie Roux Anders Snnerborg Tugsan Tezil Eric Verdin J. Peter Svensson 《PLoS pathogens》2022,18(6)
The reservoir of latently HIV-1 infected cells is heterogeneous. To achieve an HIV-1 cure, the reservoir of activatable proviruses must be eliminated while permanently silenced proviruses may be tolerated. We have developed a method to assess the proviral nuclear microenvironment in single cells. In latently HIV-1 infected cells, a zinc finger protein tethered to the HIV-1 promoter produced a fluorescent signal as a protein of interest came in its proximity, such as the viral transactivator Tat when recruited to the nascent RNA. Tat is essential for viral replication. In these cells we assessed the proviral activation and chromatin composition. By linking Tat recruitment to proviral activity, we dissected the mechanisms of HIV-1 latency reversal and the consequences of HIV-1 production. A pulse of promoter-associated Tat was identified that contrasted to the continuous production of viral proteins. As expected, promoter H3K4me3 led to substantial expression of the provirus following T cell stimulation. However, the activation-induced cell cycle arrest and death led to a surviving cell fraction with proviruses encapsulated in repressive chromatin. Further, this cellular model was used to reveal mechanisms of action of small molecules. In a proof-of-concept study we determined the effect of modifying enhancer chromatin on HIV-1 latency reversal. Only proviruses resembling active enhancers, associated with H3K4me1 and H3K27ac and subsequentially recognized by BRD4, efficiently recruited Tat upon cell stimulation. Tat-independent HIV-1 latency reversal of unknown significance still occurred. We present a method for single cell assessment of the microenvironment of the latent HIV-1 proviruses, used here to reveal how T cell stimulation modulates the proviral activity and how the subsequent fate of the infected cell depends on the chromatin context. 相似文献
22.
23.
The Role of Influenza A Virus Hemagglutinin Residues 226 and 228 in Receptor Specificity and Host Range Restriction 总被引:18,自引:8,他引:18 下载免费PDF全文
Angela Vines Krisna Wells Mikhail Matrosovich Maria R. Castrucci Toshihiro Ito Yoshihiro Kawaoka 《Journal of virology》1998,72(9):7626-7631
Influenza A viruses can be isolated from a variety of animals, but their range of hosts is restricted. For example, human influenza viruses do not replicate in duck intestine, the major replication site of avian viruses in ducks. Although amino acids at positions 226 and 228 of hemagglutinin (HA) of the H3 subtype are known to be important for this host range restriction, the contributions of specific amino acids at these positions to restriction were not known. Here, we address this issue by generating HAs with site-specific mutations of a human virus that contain different amino acid residues at these positions. We also let ducks select replication-competent viruses from a replication-incompetent virus containing a human virus HA by inoculating animals with 1010.5 50% egg infectious dose of the latter virus and identified a mutation in the HA. Our results showed that the Ser-to-Gly mutation at position 228, in addition to the Leu-to-Gln mutation at position 226 of the HA of the H3 subtype, is critical for human virus HA to support virus replication in duck intestine. 相似文献
24.
25.
Julie C. Williamson Ed C. Rowe Paul W. Hill Mark A. Nason David L. Jones John R. Healey 《Restoration Ecology》2011,19(2):194-204
Hard rock quarries are commonly located close to national parks and special areas of conservation and are generally regarded as visually intrusive. Consequently, restoration strategies that effectively accelerate natural plant regeneration processes are required. Slate waste tips present extreme conditions for plant establishment with multiple potential limiting factors (e.g., lack of organic matter, nutrients, and poor water retention). In this study, we investigated ecological strategies to accelerate natural regeneration at the largest slate quarry in Europe. A field experiment was conducted to assess ecosystem restoration using a contrasting set of native woody species. Treatments included amendments of waste tips with: polyacrylamide gel to increase water‐holding capacity; mineral fertilizer to increase nutrient supply; and two treatments that increased both (organic waste or boulder clay addition). Ecosystem recovery was evaluated through above‐ and below‐ground productivity (plant and microbial, respectively) and soil analyses. Neither increasing nutrient supply (with mineral fertilizer) nor water‐holding capacity (with polyacrylamide gel) was sufficient, alone, to improve plant establishment. However, both boulder clay and organic waste amendment significantly enhanced plant growth. There was a marked positive interaction in the effects on tree growth of the amendment with organic waste and boulder clay. Large interactions occurred between tree species and substrate amendments. The growth of N2‐fixing species was strongly favored over non‐fixers where there was no addition of material increasing soil nitrogen supply, whereas the growth advantage of pioneer species over non‐pioneers was greatest with fertilizer, organic waste, or clay additions. Organic waste addition had the greatest positive impact on soil processes. 相似文献
26.
Comparison of vascular smooth muscle cells from adult human,monkey and rabbit in primary culture and in subculture 总被引:27,自引:0,他引:27
Julie H. Chamley Gordon R. Campbell Janet D. McConnell Prof. Dr. rer. nat. Ute Gröschel-Stewart 《Cell and tissue research》1977,177(4):503-522
Summary A method is presented for growing large numbers of pure isolated smooth muscle cells from adult human, monkey, and rabbit blood vessels in primary culture.In the first few days in culture these cells closely resembled those in vivo and could be induced to contract with angiotensin II, noradrenaline and mechanical stimulation. They stained intensely with antibodies against smooth muscle actin and myosin. Fibroblasts and endothelial cells did not stain with these antibodies thereby allowing the purity of each batch of cultures to be monitored. This was consistently found to be better than 99%. The smooth muscle cells modified or dedifferentiated after about 9 days in culture to morphologically resemble fibroblasts. At this stage cells could no longer be induced to contract and did not stain with the myosin antibodies. Intense proliferation of these cells soon resulted in a confluent monolayer being formed at which stage some differentiated characteristics returned. The modification or dedifferentiation process could be inhibited by the presence of a feeder layer of fibroblasts or endothelial cells, or the addition of cAMP to the culture medium.Smooth muscle cells which had migrated from explants in primary culture, and cells in subculture, had morphological and functional properties of dedifferentiated cells at all times.The advantages of differentiated rather than dedifferentiated smooth muscle cells in culture for the study of mitogenic agents in atherosclerosis is discussed.The authors wish to thank Professor H.H. Bentall of the Royal Postgraduate Medical School, Hammersmith Hospital, London, for making available human material, and Dr. S. Zeki of Department of Anatomy, University College London for material from monkeys. We are also extremely grateful to Professor G. Burnstock for the use of his laboratory facilitiesHolder of a John Halliday Travelling Fellowship from the Life Insurance Medical Research Fund of Australia and New ZealandResearch Fellow with the National Heart Foundation of AustraliaSupported by the Deutsche Forschungsgemeinschaft 相似文献
27.
ACCLIMATION TO SPECTRAL IRRADIANCE IN ALGAE 总被引:3,自引:0,他引:3
28.
29.
Brain Cytochrome Oxidase in Alzheimer''s Disease 总被引:5,自引:0,他引:5
Stephen J. Kish Catherine Bergeron Ali Rajput† Slobodan Dozic‡ Frank Mastrogiacomo Li-Jan Chang Julie M. Wilson Linda M. DiStefano Jose N. Nobrega§ 《Journal of neurochemistry》1992,59(2):776-779
A recent demonstration of markedly reduced (-50%) activity of cytochrome oxidase (CO; complex 4), the terminal enzyme of the mitochondrial enzyme transport chain, in platelets of patients with Alzheimer's disease (AD) suggested the possibility of a systemic and etiologically fundamental CO defect in AD. To determine whether a CO deficiency occurs in AD brain, we measured the activity of CO in homogenates of autopsied brain regions of 19 patients with AD and 30 controls matched with respect to age, postmortem time, sex, and, as indices of agonal status, brain pH and lactic acid concentration. Mean CO activity in AD brain was reduced in frontal (-26%: p less than 0.01), temporal (-17%; p less than 0.05), and parietal (-16%; not significant, p = 0.055) cortices. In occipital cortex and putamen, mean CO levels were normal, whereas in hippocampus, CO activity, on average, was nonsignificantly elevated (20%). The reduction of CO activity, which is tightly coupled to neuronal metabolic activity, could be explained by hypofunction of neurons, neuronal or mitochondrial loss, or possibly by a more primary, but region-specific, defect in the enzyme itself. The absence of a CO activity reduction in all of the examined brain areas does not support the notion of a generalized brain CO abnormality. Although the functional significance of a 16-26% cerebral cortical CO deficit in human brain is not known, a deficiency of this key energy-metabolizing enzyme could reduce energy stores and thereby contribute to the brain dysfunction and neurodegenerative processes in AD. 相似文献
30.
Ulrich Membe Femoe Hermine Boukeng Jatsa Valentin Greigert Julie Brunet Catherine Cannet Mrim Christian Kenfack Nestor Gipwe Feussom Joseph Bertin Kadji Fassi Emilenne Tienga Nkondo Ahmed Abou-Bacar Alexander Wilhelm Pfaff Thophile Dimo Pierre Kamtchouing Louis-Albert Tchuem Tchuent 《PLoS neglected tropical diseases》2022,16(4)
BackgroundOne of the considerable challenges of schistosomiasis chemotherapy is the inefficacy of praziquantel (PZQ) at the initial phase of the infection. Immature schistosomes are not susceptible to PZQ at the curative dose. Here, we investigated the efficacy of different PZQ regimens administered during the initial stage of Schistosoma mansoni infection in mice.Methodology/Principal findingsTwo months-old mice were individually infected with 80 S. mansoni cercariae and divided into one infected-untreated control group (IC) and four PZQ-treated groups: PZQ at 100 mg/kg/day for five consecutive days (group PZQ1), PZQ at 100 mg/kg/day for 28 days (group PZQ2), PZQ at 18 mg/kg/day for 28 days (group PZQ3) and a single dose of PZQ at 500 mg/kg (group PZQ4). The treatment started on day one post-infection (p.i), and each group of mice was divided into two subgroups euthanized on day 36 or 56 p.i, respectively. We determined the mortality rate, the parasitological burden, the hepatic and intestinal granulomas, the serum levels of Th-1, Th-2, and Th-17 cytokines, and gene expression. The treatment led to a significant (p < 0.001) reduction of worm burden and egg counts in the intestine and liver in groups PZQ2 and PZQ3. On 56th day p.i, there was a significant reduction (p < 0.001) of the number and volume of the hepatic granulomas in groups PZQ2 and PZQ3 compared to group PZQ1 or PZQ4. Moreover, in group PZQ3, the serum levels of IFN-γ, TNF-α, IL-13, and IL-17 and their liver mRNA expressions were significantly reduced while IL-10 and TGF-β gene expression significantly increased. The highest mortality rate (81.25%) was recorded in group PZQ2.Conclusion/SignificanceThis study revealed that the administration of PZQ at 18 mg/kg/day for 28 consecutive days was the optimal effective posology for treating S. mansoni infection at the initial stage in a murine model. 相似文献