全文获取类型
收费全文 | 18761篇 |
免费 | 1793篇 |
国内免费 | 41篇 |
专业分类
20595篇 |
出版年
2022年 | 160篇 |
2021年 | 296篇 |
2020年 | 195篇 |
2019年 | 244篇 |
2018年 | 302篇 |
2017年 | 273篇 |
2016年 | 444篇 |
2015年 | 787篇 |
2014年 | 795篇 |
2013年 | 1065篇 |
2012年 | 1298篇 |
2011年 | 1258篇 |
2010年 | 796篇 |
2009年 | 700篇 |
2008年 | 1004篇 |
2007年 | 1001篇 |
2006年 | 913篇 |
2005年 | 907篇 |
2004年 | 898篇 |
2003年 | 833篇 |
2002年 | 904篇 |
2001年 | 350篇 |
2000年 | 337篇 |
1999年 | 354篇 |
1998年 | 255篇 |
1997年 | 224篇 |
1996年 | 197篇 |
1995年 | 177篇 |
1994年 | 160篇 |
1993年 | 176篇 |
1992年 | 254篇 |
1991年 | 206篇 |
1990年 | 208篇 |
1989年 | 191篇 |
1988年 | 190篇 |
1987年 | 157篇 |
1986年 | 146篇 |
1985年 | 167篇 |
1984年 | 151篇 |
1983年 | 105篇 |
1982年 | 140篇 |
1981年 | 133篇 |
1980年 | 110篇 |
1979年 | 132篇 |
1978年 | 125篇 |
1977年 | 112篇 |
1976年 | 99篇 |
1975年 | 71篇 |
1974年 | 75篇 |
1973年 | 91篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
In this work, incorporation of plasmid DNA, pre-complexed with PEI, into polyelectrolyte multilayers has been studied to further develop platforms for local gene delivery. Polyplex embedding in synthetic and naturally degradable architectures was efficient for transfection of human hepato-cellular carcinoma cells. 相似文献
3.
L-myo-Inositol-1-phosphate synthase has been found to have at least a 5-fold preference for the beta-anomer of its natural substrate D-Glc-6-P. The alpha-anomer appears to be an inhibitor of the reaction and may be converted to product as well. As well as showing an enzymatic preference for the equatorial C-1 hydroxyl of D-Glc-6-P, our results suggest that it is the pyranose form of D-Glc-6-P that binds to the enzyme and that ring-opening is an enzymatic step. We have also found D-2-dGlc-6-P, D-2-F-2-dGlc-6-P, and D-Man-6-P each to be both competitive inhibitors and substrates that are converted to inositol phosphates by the synthase. D-Allose-6-P is a weak inhibitor of the enzyme, but not a substrate. D-Gal-6-P is neither substrate nor inhibitor. Thus the specificity of the synthase with respect to single position epimers of D-Glc-6-P increases in the order C1 less than C2 much less than C3 less than C4. 相似文献
4.
5.
T. E. Graedel Julian Allwood Jean‐Pierre Birat Matthias Buchert Christian Hagelüken Barbara K. Reck Scott F. Sibley Guido Sonnemann 《Journal of Industrial Ecology》2011,15(3):355-366
The recycling of metals is widely viewed as a fruitful sustainability strategy, but little information is available on the degree to which recycling is actually taking place. This article provides an overview on the current knowledge of recycling rates for 60 metals. We propose various recycling metrics, discuss relevant aspects of recycling processes, and present current estimates on global end‐of‐life recycling rates (EOL‐RR; i.e., the percentage of a metal in discards that is actually recycled), recycled content (RC), and old scrap ratios (OSRs; i.e., the share of old scrap in the total scrap flow). Because of increases in metal use over time and long metal in‐use lifetimes, many RC values are low and will remain so for the foreseeable future. Because of relatively low efficiencies in the collection and processing of most discarded products, inherent limitations in recycling processes, and the fact that primary material is often relatively abundant and low‐cost (which thereby keeps down the price of scrap), many EOL‐RRs are very low: Only for 18 metals (silver, aluminum, gold, cobalt, chromium, copper, iron, manganese, niobium, nickel, lead, palladium, platinum, rhenium, rhodium, tin, titanium, and zinc) is the EOL‐RR above 50% at present. Only for niobium, lead, and ruthenium is the RC above 50%, although 16 metals are in the 25% to 50% range. Thirteen metals have an OSR greater than 50%. These estimates may be used in considerations of whether recycling efficiencies can be improved; which metric could best encourage improved effectiveness in recycling; and an improved understanding of the dependence of recycling on economics, technology, and other factors. 相似文献
6.
The difficulty in predicting the consequences of interactions between different cytokine networks has increased with the expansion
of the T helper (Th) cell universe and the discovery of numerous B lymphocyte-derived cytokines. Consequently, it is now difficult
to conceptualize a straightforward view of the contribution of these disturbances to the pathogenesis of primary Sj?gren's
syndrome (SS). Th1 cells, which produce interferon-γ and IL-2, and Th17 cells, which make IL-17 and TNF-α, have been cast
in the leading roles of the play. However, the complex role of T-cell subsets in SS is accentuated by the reciprocal effects
of Th17 cells and regulatory T cells found in salivary glands of SS patients. Furthermore, B lymphocyte polarization into
type-1 B effector (Be1) and Be2 cells and B-cell modulating factors of the TNF family, most notably the B-cell-activating
factor (BAFF), and their prominent role in SS are additional complicating factors. Whereas Th17 cells orchestrate autoreactive
germinal centers, local BAFF would repress the generation of Th17 cells. Such new insights into interconnected cytokines in
primary SS may lead to new treatments for these patients. 相似文献
7.
The ultrastructure of the interstitial cells of Cajal (ICC) in the oesophagus of the monkey resembled that described in the oesophagus of other mammalian species but differed in their paucity and almost lack of smooth endoplasmic reticulum, caveolae and filaments. The plasmalemma of the ICC was in close contact (20- to 30-nm gaps) with that of smooth muscle cells. This may occasionally take the form of a desmosome, but gap junctions have not been observed. Vesiculated axon profiles, containing large granular or agranular vesicles were in close contact (20- to 30-nm gaps) with the plasmalemma of ICC. In a few vesiculated profiles a presynaptic density could be recognized. The intercalation of the ICC between the vesiculated axon profiles and the smooth muscle cells suggest a role in oesophageal motility. Between 3 and 21 days following bilateral vagotomy some ICC showed regressive changes such as increased electron density and shrinkage of the cytoplasm, crowding of the organelles and dissolution of the nuclear chromatin material. Axon profiles in the vicinity of the affected ICC contained glycogen granules suggesting injury. In late stages, the number of ICC and smooth muscle contacts was reduced. The results suggest that the vagus nerves exert a trophic influence on the ICC and that the intercellular relationships between ICC and smooth muscle cells possess a degree of plasticity. It is tentatively suggested that these vagal effects may be mediated via the oesophageal myenteric ganglia. 相似文献
8.
Ultrastructure of the epithelial cells of the ventral prostate from the hopping mouse Notomys alexis
The large ventral prostate of the hopping mouse has abundant secretory units whose epithelial cells vary in height and which often have nuclei in the apical region of the cell. TEM observations indicated two epithelial cell types in which some unusual features occurred. Type A cells had granular endoplasmetic reticulum (GER) whose membranes often formed intracytoplasmic confronting cisternae. Type B cells had more fragmented and vesiculated GER with sparse ribosomes and less frequently also intracytoplasmic confronting confronting cisternae. In the latter cells, two types of granules were found, one of which was derived from the Golgi and the other possibly directly from the GER. Type A cells only had one type of granule present. A highly convoluted membrane was also found at the basal region in many of the cells. The significance of these unusual ultrastructural features has yet to be ascertained. 相似文献
9.
The fully enclosed Taiaro lagoon is hypersaline (42.5 psu) and non-tidal; constant salinity and water level result from strong
evaporation balanced by low percolation through the lagoon floor. Seawater can flow over the atoll rim during exceptionally
high seas and may then replenish lagoonal communities with propagules of oceanic origin. The distinctive water chemistry of
the lagoon suggests a possible way of identifying these immigrants. We established this potential by analysing stable isotopes
of carbon and oxygen in the recent growth layers of otoliths of two adult reef fishes, Chaetodon ulietensis and Acanthurus triostegus, collected from both sides of the atoll rim. Fish from the two locations were discriminated by their isotopic signatures,
suggesting that analysis of the microchemical signatures deposited during the larval development could be used in future work
to determine which individuals and species complete their life-cycles in this unusual lagoon.
Accepted: 28 August 1997 相似文献
10.
Monitoring signal transduction in cancer: cDNA microarray for semiquantitative analysis. 总被引:1,自引:0,他引:1
H B Hsieh R A Lersch D E Callahan S Hayward M Wong O H Clark H U Weier 《The journal of histochemistry and cytochemistry》2001,49(8):1057-1058
This study targeted the development of a novel microarray tool to allow rapid determination of the expression levels of 58 different tyrosine kinase (tk) genes in small tumor samples. The goals were to define a reference probe for multi-sample comparison and to investigate the variability and reproducibility of the image acquisition and RT-PCR procedures. The small number of tk genes on our arrays enabled us to define a reference probe by artificially mixing all genes on the arrays. Such a probe provided contrast reference for comparative hybridization of control and sample DNA and enabled cross-comparison of more than two samples against one another. Comparison of signals generated from multiple scanning eliminated the concern of photo bleaching and scanner intrinsic noise. Tests performed with breast, thyroid, and prostate cancer samples yielded distinctive patterns and suggest the feasibility of our approach. Repeated experiments indicated reproducibility of such arrays. Up- or downregulated genes identified by this rapid screening are now being investigated with techniques such as in situ hybridization. 相似文献