全文获取类型
收费全文 | 2923篇 |
免费 | 215篇 |
国内免费 | 2篇 |
专业分类
3140篇 |
出版年
2021年 | 30篇 |
2020年 | 21篇 |
2018年 | 21篇 |
2017年 | 24篇 |
2016年 | 60篇 |
2015年 | 84篇 |
2014年 | 97篇 |
2013年 | 141篇 |
2012年 | 138篇 |
2011年 | 144篇 |
2010年 | 109篇 |
2009年 | 116篇 |
2008年 | 143篇 |
2007年 | 140篇 |
2006年 | 129篇 |
2005年 | 116篇 |
2004年 | 122篇 |
2003年 | 104篇 |
2002年 | 131篇 |
2001年 | 56篇 |
2000年 | 48篇 |
1999年 | 41篇 |
1998年 | 41篇 |
1997年 | 39篇 |
1996年 | 32篇 |
1995年 | 35篇 |
1994年 | 27篇 |
1993年 | 22篇 |
1992年 | 30篇 |
1991年 | 22篇 |
1990年 | 24篇 |
1989年 | 35篇 |
1988年 | 26篇 |
1987年 | 22篇 |
1985年 | 25篇 |
1984年 | 29篇 |
1983年 | 17篇 |
1982年 | 27篇 |
1981年 | 31篇 |
1980年 | 15篇 |
1979年 | 18篇 |
1978年 | 17篇 |
1977年 | 22篇 |
1976年 | 20篇 |
1973年 | 14篇 |
1968年 | 15篇 |
1963年 | 15篇 |
1960年 | 16篇 |
1951年 | 14篇 |
1927年 | 13篇 |
排序方式: 共有3140条查询结果,搜索用时 15 毫秒
81.
Visual search for a target object can be facilitated by the repeated presentation of an invariant configuration of nontargets (‘contextual cueing’). Here, we tested adaptation of learned contextual associations after a sudden, but permanent, relocation of the target. After an initial learning phase targets were relocated within their invariant contexts and repeatedly presented at new locations, before they returned to the initial locations. Contextual cueing for relocated targets was neither observed after numerous presentations nor after insertion of an overnight break. Further experiments investigated whether learning of additional, previously unseen context-target configurations is comparable to adaptation of existing contextual associations to change. In contrast to the lack of adaptation to changed target locations, contextual cueing developed for additional invariant configurations under identical training conditions. Moreover, across all experiments, presenting relocated targets or additional contexts did not interfere with contextual cueing of initially learned invariant configurations. Overall, the adaptation of contextual memory to changed target locations was severely constrained and unsuccessful in comparison to learning of an additional set of contexts, which suggests that contextual cueing facilitates search for only one repeated target location. 相似文献
82.
QTL mapping of Fusarium head blight resistance in three related durum wheat populations 总被引:2,自引:0,他引:2
Noemie Prat Camille Guilbert Ursa Prah Elisabeth Wachter Barbara Steiner Thierry Langin Olivier Robert Hermann Buerstmayr 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》2017,130(1):13-27
Key message
The QTL Fhb1 was successfully introgressed and validated in three durum wheat populations. The novel germplasm and the QTL detected will support improvement of Fusarium resistance in durum wheat.Abstract
Durum wheat (Triticum durum Desf.) is particularly susceptible to Fusarium head blight (FHB) and breeding for resistance is hampered by limited genetic variation within this species. To date, resistant sources are mainly available in a few wild relative tetraploid wheat accessions. In this study, the effect of the well-known hexaploid wheat (Triticum aestivum L.) quantitative trait locus (QTL) Fhb1 was assessed for the first time in durum wheat. Three F7-RIL mapping populations of about 100 lines were developed from crosses between the durum wheat experimental line DBC-480, which carries an Fhb1 introgression from Sumai-3, and the European T. durum cultivars Karur, Durobonus and SZD1029K. The RILs were evaluated in field experiments for FHB resistance in three seasons using spray inoculation and genotyped with SSR as well as genotyping-by-sequencing markers. QTL associated with FHB resistance were identified on chromosome arms 2BL, 3BS, 4AL, 4BS, 5AL and 6AS at which the resistant parent DBC-480 contributed the positive alleles. The QTL on 3BS was detected in all three populations centered at the Fhb1 interval. The Rht-B1 locus governing plant height was found to have a strong effect in modulating FHB severity in all populations. The negative effect of the semi-dwarf allele Rht-B1b on FHB resistance was compensated by combining with Fhb1 and additional resistance QTL. The successful deployment of Fhb1 in T. durum was further substantiated by assessing type 2 resistance in one population. The efficient introgression of Fhb1 represents a significant step forward for enhancing FHB resistance in durum wheat.83.
A survey was conducted to determine the levels of fumonisins B1 and B2 in corn and corn-based products available in Colombia
for human and animal consumption. A total of 120 samples were analyzed by acetonitrile-water extraction, cleanup with a strong-anion-exchange
column, and liquid chromatography with o-phthaldialdehyde-2-mercaptoethanol derivatization and fluorescence detection. The
samples of corn and corn-based products for animal intake were taken at different feed manufacturing plants, whereas the samples
used for human foods where purchased from local retail stores. The number of positive samples for fumonisin B1 was 20.0% higher
in corn and corn-based products for animal intake (75.0%) than in corn and corn-based products for human consumption (55.0%).
The levels of fumonisin B1 were also higher in corn and corn-based products for animal intake (mean = 694 μg/kg; range = 32–2964
μg/kg), than in corn and corn-based products for human intake (mean = 218 μg/kg; range = 24–2170 μg/ kg). The incidence and
levels of fumonisin B2 were lower than those for fumonisin B1. Corn and corn-based products for animal consumption had an
incidence of fumonisin B2 of 58.3%, with a mean value of 283 μg/kg, and a range of 44–987 μg/kg. The incidence of fumonisin
B2 in corn-based products for human intake was 35.0%, with a mean value of 118 μg/kg and a range of 21–833 μg/kg. The highest
incidence and levels of fumonisins were found in samples of hominy feed, with concentrations ranging from 86 to 2964 μg/kg
fumonisin B1 and 57 to 987 μg/kg fumonisin B2. 相似文献
84.
The most important but still unresolved problem in bioelectromagnetics is the interaction of weak electromagnetic fields (EMFs) with living cells. Thermal and other types of noise pose restrictions in cell detection of weak signals. As a consequence, some extant experimental results that indicate low-intensity field effects cannot be accounted for, and this renders the results themselves questionable. One way out of this dead end is to search for possible mechanisms of signal amplification. In this paper, we discuss a general mechanism in which a weak signal is amplified by system noise itself. This mechanism was discovered several years ago in physics and is known, in its simplest form, as a stochastic resonance. It was shown that signal amplification may exceed a factor of 1000, which renders existing estimations of EMF thresholds highly speculative. The applicability of the stochastic resonance concept to cells is discussed particularly with respect to the possible role of the cell membrane in the amplification process. © 1994 Wiley-Liss, Inc. 相似文献
85.
In Drosophila, vast numbers of cells undergo apoptosis during normal development. In addition, excessive apoptosis can be induced in response to a variety of stress or injury paradigms, including DNA damage, oxidative stress, nutrient deprivation, unfolded proteins and mechanical tissue damage. Two of the most commonly used methods to label apoptotic cells in Drosophila are terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) for fixed tissues and acridine orange (AO) staining for live embryos or tissues. Here, we describe protocols for labeling apoptotic cells in Drosophila embryos and adult male gonads. Slightly modified protocols can also be applied for other Drosophila tissues. The AO protocol is quick, simple and allows real-time imaging of doomed cells in live tissues. However, it is difficult to combine with conventional counterstains or Ab labeling. On the other hand, this functionality is readily afforded by the TUNEL protocol, which permits the detection of apoptotic cells in fixed tissues. These staining procedures can be completed in 1-2 d. 相似文献
86.
Endosomal translocation of vertebrate DNA activates dendritic cells via TLR9-dependent and -independent pathways 总被引:14,自引:0,他引:14
Yasuda K Yu P Kirschning CJ Schlatter B Schmitz F Heit A Bauer S Hochrein H Wagner H 《Journal of immunology (Baltimore, Md. : 1950)》2005,174(10):6129-6136
TLRs discriminate foreign from self via their specificity for pathogen-derived invariant ligands, an example being TLR9 recognizing bacterial unmethylated CpG motifs. In this study we report that endosomal translocation of CpG DNA via the natural endocytotic pathway is inefficient and highly saturable, whereas endosomal translocation of DNA complexed to the cationic lipid N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium methylsulfate (DOTAP) is not. Interestingly, DOTAP-mediated enhanced endosomal translocation of otherwise nonstimulatory vertebrate DNA or of certain noncanonical CpG motifs triggers robust dendritic cell activation in terms of both up-regulation of CD40/CD69 and cytokine production, such as type I IFN and IL-6. We report that the stimulatory activity of phosphorothioated noncanonical CpG oligodeoxynucleotides is TLR9 dependent, whereas phosphodiester DNA, such as vertebrate DNA, in addition trigger TLR9-independent pathways. We propose that the inefficiency of the natural route for DNA internalization hinders low affinity TLR9 ligands in endosomes to reach threshold concentrations required for TLR9 activation. Endosomal compartmentalization of TLR9 may thus reflect an evolutionary strategy to avoid TLR9 activation by self-DNA. 相似文献
87.
88.
Interactions of donor sources and media influence the histo‐morphological quality of full‐thickness skin models 下载免费PDF全文
Julia Lange Frederik Weil Christoph Riegler Florian Groeber Silke Rebhan Szymon Kurdyn Miriam Alb Hermann Kneitz Götz Gelbrich Heike Walles Stephan Mielke 《Biotechnology journal》2016,11(10):1352-1361
Human artificial skin models are increasingly employed as non‐animal test platforms for research and medical purposes. However, the overall histopathological quality of such models may vary significantly. Therefore, the effects of manufacturing protocols and donor sources on the quality of skin models built‐up from fibroblasts and keratinocytes derived from juvenile foreskins is studied. Histo‐morphological parameters such as epidermal thickness, number of epidermal cell layers, dermal thickness, dermo‐epidermal adhesion and absence of cellular nuclei in the corneal layer are obtained and scored accordingly. In total, 144 full‐thickness skin models derived from 16 different donors, built‐up in triplicates using three different culture conditions were successfully generated. In univariate analysis both media and donor age affected the quality of skin models significantly. Both parameters remained statistically significant in multivariate analyses. Performing general linear model analyses we could show that individual medium‐donor‐interactions influence the quality. These observations suggest that the optimal choice of media may differ from donor to donor and coincides with findings where significant inter‐individual variations of growth rates in keratinocytes and fibroblasts have been described. Thus, the consideration of individual medium‐donor‐interactions may improve the overall quality of human organ models thereby forming a reproducible test platform for sophisticated clinical research. 相似文献
89.
McClure KJ Maher M Wu N Chaplan SR Eckert WA Lee DH Wickenden AD Hermann M Allison B Hawryluk N Breitenbucher JG Grice CA 《Bioorganic & medicinal chemistry letters》2011,21(18):5197-5201
The discovery of a series of novel, potent, and selective blockers of the cyclic nucleotide-modulated channel HCN1 is disclosed. Here we report an SAR study around a series of selective blockers of the HCN1 channel. Utilization of a high-throughput VIPR assay led to the identification of a novel series of 2,2-disubstituted indane derivatives, which had moderate selectivity and potency at HCN1. Optimization of this hit led to the identification of the potent, 1,1-disubstituted cyclohexane HCN1 blocker, 2-ethoxy-N-((1-(4-isopropylpiperazin-1-yl)cyclohexyl)methyl)benzamide. The work leading to the discovery of this compound is described herein. 相似文献
90.
Lima Mda R Nogueira RM Schatzmayr HG de Filippis AM Limonta D dos Santos FB 《PLoS neglected tropical diseases》2011,5(5):e1147