全文获取类型
收费全文 | 9373篇 |
免费 | 51篇 |
国内免费 | 37篇 |
出版年
2023年 | 13篇 |
2022年 | 98篇 |
2021年 | 223篇 |
2020年 | 118篇 |
2019年 | 192篇 |
2018年 | 342篇 |
2017年 | 329篇 |
2016年 | 579篇 |
2015年 | 768篇 |
2014年 | 628篇 |
2013年 | 1111篇 |
2012年 | 635篇 |
2011年 | 480篇 |
2010年 | 706篇 |
2009年 | 578篇 |
2008年 | 501篇 |
2007年 | 455篇 |
2006年 | 435篇 |
2005年 | 294篇 |
2004年 | 104篇 |
2003年 | 60篇 |
2002年 | 39篇 |
2001年 | 49篇 |
2000年 | 167篇 |
1999年 | 71篇 |
1998年 | 61篇 |
1997年 | 56篇 |
1996年 | 32篇 |
1995年 | 21篇 |
1994年 | 15篇 |
1993年 | 16篇 |
1992年 | 25篇 |
1991年 | 19篇 |
1990年 | 13篇 |
1989年 | 16篇 |
1988年 | 9篇 |
1987年 | 15篇 |
1986年 | 16篇 |
1985年 | 20篇 |
1984年 | 10篇 |
1983年 | 10篇 |
1982年 | 9篇 |
1981年 | 11篇 |
1980年 | 12篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1974年 | 9篇 |
1973年 | 7篇 |
1971年 | 7篇 |
1935年 | 6篇 |
排序方式: 共有9461条查询结果,搜索用时 15 毫秒
81.
Keith Paige Melanie Palomares Patricia A. D’Amore Susan J. Braunhut 《In vitro cellular & developmental biology. Animal》1991,27(2):151-157
Summary The growth of the endothelial cell (EC) is tightly regulated throughout the body. Many factors have been implicated in modulating
EC growth including diffusible compounds, cell-to-cell interactions, and the extracellular matrix (ECM). Retinol, or vitamin
A alcohol, has recently been shown to inhibit the growth of bovine capillary ECs, in vitro. Retinoids are known to modify
ECM in other cell systems, and pure ECM components have been shown to effect EC growth rates. We, therefore, examined the
role of the matrix in the retinol-induced inhibition of ECs. Cell-free matrices from control and vitamin A-treated ECs were
prepared by removing cells with EGTA treatment after 7 d of culture. Matrix proteins were analyzed by solubilizing the matrices
in 5M quanidine-HCl and performing Western blot analysis using specific antibodies to matrix proteins. In isolating the ECM, we
observed that retinol-treated cultures of ECs were resistant to EGTA removal; retinol-treated ECs required twice the exposure
time to EGTA to detach from their matrix than did controls cells. Western blot analysis of matrix proteins derived from control
and retinol-treated EC cultures demonstrated a 1.6-fold increase in lamininβ chains and a 2.5-fold increase in fibronectin in the ECM of retinol-treated EC compared to control cell matrix. Functional
properties of these matrices were assessed by plating control and Day 6 retinol-treated ECs onto the matrices and measuring
attachment and growth by determining cell numbers at 24, 72, and 144 h. These studies revealed that control cells attached
in greatest numbers to a control matrix whereas retinol-treated ECs preferentially attached to a matrix derived from retinol-treated
cells. Furthermore, control ECs which grew rapidly on a control matrix were growth inhibited on a retinol-derived matrix.
These data indicate that vitamin A treatment of ECs effects both their phenotype and influences the composition and the functional
properties of their underlying ECM. These studies also demonstrate that alterations of the matrix are at least in part responsible
for the growth inhibition of EC by retinol. 相似文献
82.
Kim B. Saunders Patricia A. D’Amore 《In vitro cellular & developmental biology. Animal》1992,28(7-8):521-528
Summary Heterotypic cell-cell interactions appear to be involved in the control of development and function in a wide variety of tissues.
In the vasculature, endothelial cells and mural cells (smooth muscle cells or pericytes) make frequent contacts, suggesting
a role for intercellular interactions in the regulation of vascular growth and function. We have previously grown endothelial
cells and mural cells together in mixed cultures and found that heterocellular contact led to endothelial growth inhibition.
However, this mixed culture system does not lend itself to the examination of the effects of contact on the phenotype of the
individual cell types. We have therefore developed a co-culture system in which cells can be co-cultured across a porous membrane,
permitting intercellular contact while maintaining pure cell populations. Co-culture of endothelial cells and smooth muscle
cells across membranes with pore sizes of 0.02, 0.4, 0.6, and 0.8μm maintained the two cell types as homogeneous populations, whereas smooth muscle cells migrated across the membrane through
pores of 2.0μm. Vascular cell co-culture across membranes with 0.8-μm pores resulted the inhibition of endothelial cell proliferation and the generation of conditioned media which inhibited
endothelial cell growth. The arrangement of the cells in this co-culture system mimics thein vivo orientation of vascular cells in which mural cells are separated from the abluminal surface of the endothelium by a fenestrated
internal elastic lamina or basement membrane. Because this co-culture system maintains separable populations of cells in contact
or close proximity allowing for biochemical and molecular analyses of pure populations, it should prove useful for the study
of cell-cell interactions in a variety of systems. 相似文献
83.
F Janiak V A Dell J K Abrahamson B S Watson D L Miller A E Johnson 《Biochemistry》1990,29(18):4268-4277
The ubiquity of elongation factor Tu (EF-Tu)-dependent conformational changes in amino-acyl-tRNA (aa-tRNA) and the origin of the binding energy associated with aa-tRNA.EF-Tu.GTP ternary complex formation have been examined spectroscopically. Fluorescein was attached covalently to the 4-thiouridine base at position 8 (s4U-8) in each of four elongator tRNAs (Ala, Met-m, Phe, and Val). Although the probes were chemically identical, their emission intensities in the free aa-tRNAs differed by nearly 3-fold, indicating that the dyes were in different environments and hence that the aa-tRNAs had different tertiary structures near s4U-8. Upon association with EF-Tu.GTP, the emission intensities increased by 244%, 57%, or 15% for three aa-tRNAs due to a change in tRNA conformation; the fourth aa-tRNA exhibited no fluorescence change upon binding to EF-Tu.GTP. Despite the great differences in the emission intensities of the free aa-tRNAs and in the magnitudes of their EF-Tu-dependent intensity increases, the emission intensity per aa-tRNA molecule was nearly the same (within 9% of the average) for the four aa-tRNAs when bound to EF-Tu-GTP. Thus, the binding of EF-Tu.GTP induced or selected a tRNA conformation near s4U-8 that was very similar, and possibly the same, for each aa-tRNA species. It therefore appears that EF-Tu functions, at least in part, by minimizing the conformational diversity in aa-tRNAs prior to their beginning the recognition and binding process at the single decoding site on the ribosome. Since an EF-Tu-dependent fluorescence change was also observed with fluorescein-labeled tRNA(Phe), the protein-dependent structural change is effected by direct interactions between EF-Tu and the tRNA and does not require the aminoacyl group. The Kd of the tRNA(Phe).EF-Tu.GTP ternary complex was determined, at equilibrium, to be 2.6 microM by the ability of the unacylated tRNA to compete with fluorescent Phe-tRNA for binding to the protein. Comparison of this Kd with that of the Phe-tRNA ternary complex showed that in this case the aminoacyl moiety contributed 4.3 kcal/mol toward ternary complex formation at 6 degrees C but that the bulk of the binding energy in the ternary complex was derived from direct protein-tRNA interactions.(ABSTRACT TRUNCATED AT 400 WORDS) 相似文献
84.
Nobuhiko Suzuki Yasuhisa Kunimi Seiji Uematsu Ken’ichi Kobayashi 《Population Ecology》1980,22(2):273-283
Summary The changes in spatial distribution pattern during larval stage of the fall webworm,Hyphantria cunea were quantitatively investigated in the field experimental populations.
The female adult deposits eggs as a cluster and the hatchlings make a compact colonial-web. In this period, the all-or-none
type mortality which is characteristic in gregarious insect species was occasionary recognized before spinning a compact colonial-web.
Once making a compact colonial-web, the larvae feed the leaves in the colonial-web up to about 5th instar. In this period,
the movement of larvae occurred due to the local food shortage in a colonial-web and the expansion of colonial-web. As the
larvae developed, the colonial-web was separated into several small groups. These larvae began to disperse about 5th instar.
In this period, the local food shortage seems to be an important trigger for the larval dispersal. The mean concentration
of larvae on leaves abruptly decreased, and finally the larvae became solitary at the 6th or 7th instars.
The dispersal process in later larval stage is not necessarily due to the complete food shortage. The dispersal prior to the
occurrence of food shortage may be a safety mechanism to protect the larvae from the food shortage. 相似文献
85.
Shay O’Farrell Brian E. Luckhurst Stephen J. Box Peter J. Mumby 《Coral reefs (Online)》2016,35(2):421-425
Parrotfishes are an ecologically and commercially important teleost group whose grazing contributes to maintaining coral-dominated states on hermatypic reefs. However, overfishing has skewed sex ratios of Atlantic parrotfishes because fishing has disproportionate impacts on larger individuals, and males are generally larger than females. Whether protection from fishing may allow sex ratios to return to equilibrium is unknown, as fishing can induce irreversible ecological and/or evolutionary shifts. Bermuda banned trap fishing in 1990, creating a unique opportunity to analyse long-term responses of Atlantic parrotfishes to release from fishing. We found that sex ratios of four common parrotfishes were initially skewed, with male proportions ranging from 0.04 to 0.18. However, male proportions rebounded within 3–4 yr, equilibrating at values ranging from 0.36 to 0.54, similar to those reported at unfished sites in the region. Our results are encouraging for regional efforts to recover lost grazing function by restoring overfished herbivore populations. 相似文献
86.
Structures of novel sialylated O-linked oligosaccharides isolated from human erythrocyte glycophorins 总被引:2,自引:0,他引:2
M Fukuda M Lauffenburger H Sasaki M E Rogers A Dell 《The Journal of biological chemistry》1987,262(25):11952-11957
The O-linked oligosaccharides attached to human erythrocyte glycophorins were extensively characterized. In addition to the previously described disialylated tetrasaccharide, NeuNAc alpha 2----3Gal beta 1----3 (Neu-NAc alpha 2----6)GalNAcOH and monosialylated trisaccharide, NeuNAc alpha 2----3Gal beta 1----3GalNAcOH, novel trisialylated oligosaccharides were isolated. Methylation analysis, fast atom bombardment-mass spectrometry, and enzymatic degradation were used to elucidate the following novel structures: formula; see text: These results suggest that O-linked oligosaccharides with a disialosyl group, NeuNAc alpha 2----8NeuNAc alpha 2----, may be present in various tissues. 相似文献
87.
88.
S. Stephenson A. A. Chariton M. P. Holley M. O’Sullivan M. R. Gillings G. C. Hose 《Geomicrobiology journal》2013,30(7):623-634
Groundwater biota are particularly sensitive to environmental perturbations such as groundwater contamination. The diversity of prokaryotic and eukaryotic biota has been examined along a gradient of chlorinated hydrocarbon (CHC) contamination in the Botany Sands, an urban coastal sand-bed aquifer (Sydney, Australia). Molecular techniques were used to analyze the richness and composition of prokaryote and eukaryote assemblages using 16S and 18S rDNA, respectively. Taxon richness did not change significantly along the gradient for either prokaryotes or eukaryotes; however, significant shifts in assemblage composition were evident for both groups. Assemblage changes were most strongly correlated with concentrations of the major CHC, cis-1,2-dichloroethene, but the concentrations of a number of the contaminants were also correlated, making it difficult to infer if effects were due to any particular contaminant. The presence of cis-1,2-dichloroethene and other secondary ethenes suggests in situ breakdown of the primary CHCs via natural attenuation. The current focus of management of the Botany aquifer is to stop the contaminant plume reaching the adjoining estuary. This approach is clearly justified given the changes evident in the microbial assemblages in the groundwater, which are a likely consequence of the contamination. 相似文献
89.
90.
Yu. S. Akishev A. V. Dem’yanov V. B. Karal’nik M. V. Pan’kin N. I. Trushkin 《Plasma Physics Reports》2001,27(2):164-171
Periodic pulsations of the active current component are revealed experimentally in transversely homogeneous barrier discharges in helium at small values of the parameter Pd (below 500 torr mm) and moderate frequencies of the applied voltage (f < 100 kHz). The frequency of the current pulsations is higher than the frequency of the well-studied pulsations in a transversely inhomogeneous streamer barrier discharge in air by a factor of approximately 100. Numerical calculations show that the physical nature of the observed pulsations can be explained in terms of the negative differential resistance of the cathode fall region, which occupies essentially the entire interelectrode gap in each half-period of the applied voltage. 相似文献