全文获取类型
收费全文 | 6245篇 |
免费 | 577篇 |
国内免费 | 3篇 |
专业分类
6825篇 |
出版年
2023年 | 28篇 |
2022年 | 62篇 |
2021年 | 122篇 |
2020年 | 65篇 |
2019年 | 85篇 |
2018年 | 109篇 |
2017年 | 98篇 |
2016年 | 162篇 |
2015年 | 277篇 |
2014年 | 317篇 |
2013年 | 402篇 |
2012年 | 430篇 |
2011年 | 472篇 |
2010年 | 281篇 |
2009年 | 265篇 |
2008年 | 386篇 |
2007年 | 377篇 |
2006年 | 312篇 |
2005年 | 333篇 |
2004年 | 311篇 |
2003年 | 315篇 |
2002年 | 309篇 |
2001年 | 46篇 |
2000年 | 30篇 |
1999年 | 50篇 |
1998年 | 76篇 |
1997年 | 49篇 |
1996年 | 53篇 |
1995年 | 39篇 |
1994年 | 44篇 |
1993年 | 47篇 |
1992年 | 37篇 |
1991年 | 34篇 |
1990年 | 27篇 |
1989年 | 29篇 |
1988年 | 22篇 |
1987年 | 29篇 |
1983年 | 27篇 |
1982年 | 35篇 |
1981年 | 25篇 |
1980年 | 24篇 |
1979年 | 26篇 |
1978年 | 30篇 |
1976年 | 20篇 |
1974年 | 23篇 |
1973年 | 21篇 |
1972年 | 26篇 |
1970年 | 20篇 |
1967年 | 28篇 |
1966年 | 20篇 |
排序方式: 共有6825条查询结果,搜索用时 0 毫秒
121.
Gareth Betts Hazel Poyntz Elena Stylianou Arturo Reyes-Sandoval Matthew Cottingham Adrian Hill Helen McShane 《PloS one》2012,7(12)
The Bacillus Calmette - Guerin (BCG) vaccine provides a critical but limited defense against Mycobacterium tuberculosis (M.tb). More than 60 years after the widespread introduction of BCG, there is an urgent need for a better vaccine. A large body of pre-clinical research continues to support ongoing clinical trials to assess whether viral vectors expressing M.tb antigens that are shared by BCG and M.tb, can be used alongside BCG to enhance protection. A major focus involves using multiple unique viral vectors to limit anti-vector immunity and thereby enhance responses to the insert antigen delivered. The successful introduction of viral vector vaccines to target M.tb and other pathogens will be reliant on reducing the costs when using multiple vectors and inhibiting the development of unwanted anti-vector responses that interfere with the response to insert antigen. This study examines methods to reduce the logistical costs of vaccination by mixing different viral vectors that share the same insert antigen in one vaccine; and whether combining different viral vectors reduces anti-vector immunity to improve immunogenicity to the insert antigen. Here we show that a homologous prime-boost regimen with a mixture of MVA (Modified Vaccinia virus Ankara) and Ad5 (human adenovirus type 5) vectors both expressing Ag85A in a single vaccine preparation is able to reduce anti-vector immunity, compared with a homologous prime-boost regimen with either vector alone. However, the level of immunogenicity induced by the homologous mixture remained comparable to that induced with single viral vectors and was less immunogenic than a heterologous Ad5 prime-MVA-boost regimen. These findings advance the understanding of how anti-vector immunity maybe reduced in viral vector vaccination regimens. Furthermore, an insight is provided to the impact on vaccine immunogenicity from altering vaccination methods to reduce the logistical demands of using separate vaccine preparations in the field. 相似文献
122.
Peter F. T. Vaughan David F. Kaye Helen L. Reeve Stephen G. Ball Chris Peers 《Journal of neurochemistry》1993,60(6):2159-2166
Abstract: Dimethylphenylpiperazinium iodide (a nicotinic agonist) evokes noradrenaline release from human neuroblastoma SH-SY5Y cells that have been pretreated with 12- O -tetradecanoylphorbol 13-acetate for 8 min. This effect of dimethylphenylpiperazinium iodide was inhibited by 1 μ M mecamylamine but not by 1 μ M atropine, which suggests that SH-SY5Y cells express nicotinic receptors coupled to the release of noradrenaline. Dimethylphenylpiperazinium iodide-evoked release was enhanced by 5 μ M Bay K 8644 (an L-type calcium agonist) and inhibited by 1 μ M nifedipine. Dimethylphenylpiperazinium iodide depolarised SH-SY5Y cells and enhanced the level of intracellular calcium in cells loaded with fura 2. The effects of dimethylphenylpiperazinium iodide on noradrenaline release, depolarisation, and intracellular calcium levels were all inhibited by 1 μ M desmethylimipramine. The results of this study show that nicotinic receptors in SH-SY5Y cells stimulate noradrenaline release by activation of L-type calcium channels. 相似文献
123.
James D. Glover Lorna Taylor Adrian Sherman Caroline Zeiger-Poli Helen M. Sang Michael J. McGrew 《PloS one》2013,8(11)
In this work, we describe a single piggyBac transposon system containing both a tet-activator and a doxycycline-inducible expression cassette. We demonstrate that a gene product can be conditionally expressed from the integrated transposon and a second gene can be simultaneously targeted by a short hairpin RNA contained within the transposon, both in vivo and in mammalian and avian cell lines. We applied this system to stably modify chicken primordial germ cell (PGC) lines in vitro and induce a reporter gene at specific developmental stages after injection of the transposon-modified germ cells into chicken embryos. We used this vector to express a constitutively-active AKT molecule during PGC migration to the forming gonad. We found that PGC migration was retarded and cells could not colonise the forming gonad. Correct levels of AKT activation are thus essential for germ cell migration during early embryonic development. 相似文献
124.
125.
The “paranoiac” mutants of Paramecium aurelia show prolonged backward swimming in solutions containing Na+, unlike wild-type paramecia, which jerk back and forth in Na+ solutions. The paranoiac mutants in Na+ solutions also show large losses of cellular K+ and large influxes of Na+. Three different paranoiac mutants all show similar defects in ion regulation but to different degrees. Wild-type Paramecium, in contrast, shows no Na+-dependent loss of cellular K+ and a much smaller Na+ influx. In K+-containing solutions, there is no difference between wild-type and paranoiac paramecia with respect to their cellular K+ content.The Na+ influx, the K+ loss, and the duration of backward swimming are all proportional to the extracellular Na+ concentration. Electrophysiologically, the backward swimming of the paranoiac mutants corresponds to a prolonged depolarization of the membrane potential, while the backward jerks of wild-type Paramecium correspond to a series of transient depolarizations. We propose that the large Na+ influxes and the large K+ effluxes in paranoiacs occur during the periods of backward swimming, while the membrane is depolarized. 相似文献
126.
127.
128.
Helen Farrah Janette Pilarski Pamela Waller William F. Pickering 《Chemical Speciation and Bioavailability》2013,25(2-3):53-63
AbstractThe effect of drying temperature and oxidation on the level of exchangeable ammonium ion found in sediments has been examined using samples collected from along a polluted creek and from shallow lake bays. The sediments were dried at temperatures between 20°C and 100°C (either in air or under a nitrogen atmosphere), and the ammonium ion content was extracted into 0.1 M KCl prior to analysis using an ion selective electrode. Exposure to air during the drying stage usually resulted in lower ammonium values, while increasing the drying temperature altered the amount of displaceable (i.e. available) ammonium ion extracted, generally in an upward direction. The amount detected (5–25 μ g?1) varied between sites, and surface sediment values differed from the 10–50 cm core material results. The pH of the extracts varied with the drying temperature used, indicating that the heating process promoted some chemical changes in the test samples. The study has demonstrated that in nutrient level surveys, the analytical data produced can depend greatly on the sample preparation procedure selected. It also indicated the type of changes which could occur when dredged sediments are land dumped. 相似文献
129.
Cellulin granules, the polysaccharide inclusions found uniquely in oomycetous fungi of the order Leptomitales, were isolated from Apodachlya sp. The granules were prepared free of cell wall and cytoplasmic contaminants. Biochemical analyses and X-ray diffraction studies demonstrated that the granules were composed of 60% chitin and 39% glucan consisting of β-1,3-and β-1,6-linked glucose units. A protein content of only 0.1% was attributed to an insignificant amount of cytoplasmic contamination. Isolated granules and those in situ showed no apparent differences in their microscopic form. 相似文献
130.
Helen Schoch-Bodmer 《Protoplasma》1936,25(1):337-371
Ohne ZusammenfassungHerrn Dr. A. Frey-Wyssling, Zürich, bin ich für die kritische Durchsicht des Manuskriptes, Herrn Dr. E. Funk, St. Gallen, für Hilfe bei der Volumenberechnung des Protoplasten, Herrn Dr. E. Müller, Schaffhausen, für Beschaffung von Glaskammern, Herrn Dr. 0. Widmer, St. Gallen, für die Herstellung von Zuckerlösungen, Herrn Dr. E. Wieser, St. Gallen, für die mikrophotographischen Aufnahmen und Herrn Prof. Dr, M. Wildi, St. Gallen, für die Durchsicht der englischen Zusammenfassung zu großem Dank verpflichtet. 相似文献