全文获取类型
收费全文 | 19754篇 |
免费 | 1315篇 |
国内免费 | 2篇 |
专业分类
21071篇 |
出版年
2023年 | 150篇 |
2022年 | 292篇 |
2021年 | 571篇 |
2020年 | 412篇 |
2019年 | 498篇 |
2018年 | 587篇 |
2017年 | 561篇 |
2016年 | 834篇 |
2015年 | 1194篇 |
2014年 | 1307篇 |
2013年 | 1584篇 |
2012年 | 1866篇 |
2011年 | 1669篇 |
2010年 | 1045篇 |
2009年 | 904篇 |
2008年 | 1087篇 |
2007年 | 1069篇 |
2006年 | 1010篇 |
2005年 | 809篇 |
2004年 | 766篇 |
2003年 | 684篇 |
2002年 | 548篇 |
2001年 | 192篇 |
2000年 | 180篇 |
1999年 | 169篇 |
1998年 | 110篇 |
1997年 | 100篇 |
1996年 | 84篇 |
1995年 | 71篇 |
1994年 | 73篇 |
1993年 | 57篇 |
1992年 | 61篇 |
1991年 | 50篇 |
1990年 | 42篇 |
1989年 | 43篇 |
1988年 | 27篇 |
1987年 | 34篇 |
1986年 | 20篇 |
1985年 | 25篇 |
1984年 | 27篇 |
1983年 | 27篇 |
1982年 | 17篇 |
1981年 | 18篇 |
1980年 | 23篇 |
1979年 | 24篇 |
1978年 | 23篇 |
1977年 | 21篇 |
1974年 | 14篇 |
1973年 | 18篇 |
1972年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
22.
Dr J. O. Lopes D. V. Pereira L. A. Streher A. A. Fenalte S. H. Alves J. P. Benevenga 《Mycopathologia》1995,130(2):89-92
A case of cutaneous zygomycosis caused byAbsidia coryabifera in a leukemic patient submitted to chemotherapy is reported. The lesion was located on the little finger of the right hand and probably resulted from a latent osteomyelitis. It progressed to form extensive necrotic area. No systemic infection was detected and the lesion did not appear to be associated with any trauma. 相似文献
23.
In mixed culture of Lactobacillus hilgardii X1B and Leuconostoc oenos X2L, isolated from Argentinian wines, an amensalistic growth response was observed: Leuconostoc oenos did not grow, and after 24 h of incubation at 30°C no viable cells were detected. In pure and mixed cultures, Lactobacillus hilgardii produced hydrogen peroxide early in the growth cycle, reaching the maximum at 24 h. The values of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) for the action of hydrogen peroxide on the growth of Leuconostoc oenos were: 4.08g ml-1 and 17.00 g ml-1 respectively. 相似文献
24.
T. Macedo C. A. Fontes Ribeiro D. Cotrim P. Tavares M. T. Morgadinho M. Caramona M. T. Nunes Vicente L. Rodrigues M. G. Cardoso M. L. Keating 《Molecular neurobiology》1995,11(1-3):21-29
This work evaluated in a population of heroin and heroin plus cocaine human addicts:
- Norepinephrine (NE), epinephrine (Epi), and 3-methoxy-4-hydroxyphenylglycol (MHPG) (the principal metabolite of brain NE) plasma levels;
- Monoamine oxidase (MAO) activity; and
- 3H-imipramine specific binding to the amine carrier in platelets.
25.
Marcela F. Lopes Vânia Lúcia C. Merquior JoséMauro Peralta Lúcia M. Teixeira 《FEMS immunology and medical microbiology》1995,12(3-4):205-211
Abstract Exosubstances (cohemolysins) produced by Streptococcus agalactiae (CAMP-factor) and Streptococcus uberis (Uberis-factor) showing hemolytic synergism with β-lysin produced by Staphylococcus aureus were compared. Cohemolytic activity was evaluated in the supernatants of bacterial cultures, before and after ammonium sulfate precipitation. Sheep erythrocytes sensitized with β-lysin were used as substrate. The assays were performed in microtiter plates and results were expressed as cohemolytic units/ml. Maximum cohemolytic activity was detected, respectively, after 8 h and 14 h of growth in Columbia broth in S. uberis and S. agalactiae cultures. Cohemolytic activities of both microorganisms showed similarities when submitted to various physical and chemical treatments. They were significantly decreased by heating at 60°C and 100°C, or in presence of trypsin, and were abolished in the presence of Tween 20. Activities were found to be stable in crude supernatants and concentrated preparations maintained at −20°C for 3 months. Differences were related to levels of activity and kinetics of detection during the growth cycle. The results indicate the proteic nature, at least in part, of the Uberis factor. Analysis by PAGE in the presence or absence of SDS allowed us to correlate Uberis activity with a protein band with apparent molecular mass of 42 kDa, while CAMP activity was associated with a protein band of 27 kDa. 相似文献
26.
27.
Amblyomma americanum: characterization of salivary prostaglandins E2 and F2 alpha by RP-HPLC/bioassay and gas chromatography-mass spectrometry. 总被引:5,自引:0,他引:5
Secretagogue-induced saliva of the tick, Amblyomma americanum (L.) was fractionated by reversed-phase-high-performance liquid chromatography (RP-HPLC) and bioassayed in smooth muscle preparations. Material with retention times of authentic prostaglandin E2 (PGE2) and prostaglandin F2 alpha (PGF2 alpha) were found to cause contraction of preparations of rat colon and rat stomach strips. Gas chromatography-mass spectra of selected ions of both HPLC-purified fractions confirmed the existence of PGE2 and PGF2 alpha. Bioassay of individual samples obtained from ticks stimulated to salivate with pilocarpine, dopamine + theophiline, or dopamine + theophiline + GABA indicated that all these secretagogues induced similar amounts of prostaglandin secretion, averaging 469 ng PGE2/ml. These pharmacological doses of prostaglandin are hypothesized to assist in tick feeding by inducing vasodilation and/or other pharmacological events in their hosts. 相似文献
28.
Miguel López-Estepa Ana Ardá Martin Savko Adam Round William E. Shepard Marta Bruix Miquel Coll Francisco J. Fernández Jesús Jiménez-Barbero M. Cristina Vega 《PloS one》2015,10(4)
Cyclic N6-threonylcarbamoyladenosine (‘cyclic t6A’, ct6A) is a non-thiolated hypermodification found in transfer RNAs (tRNAs) in bacteria, protists, fungi and plants. In bacteria and yeast cells ct6A has been shown to enhance translation fidelity and efficiency of ANN codons by improving the faithful discrimination of aminoacylated tRNAs by the ribosome. To further the understanding of ct6A biology we have determined the high-resolution crystal structures of CsdL/TcdA in complex with AMP and ATP, an E1-like activating enzyme from Escherichia coli, which catalyzes the ATP-dependent dehydration of t6A to form ct6A. CsdL/TcdA is a dimer whose structural integrity and dimer interface depend critically on strongly bound K+ and Na+ cations. By using biochemical assays and small-angle X-ray scattering we show that CsdL/TcdA can associate with tRNA with a 1:1 stoichiometry and with the proper position and orientation for the cyclization of t6A. Furthermore, we show by nuclear magnetic resonance that CsdL/TcdA engages in transient interactions with CsdA and CsdE, which, in the latter case, involve catalytically important residues. These short-lived interactions may underpin the precise channeling of sulfur atoms from cysteine to CsdL/TcdA as previously characterized. In summary, the combination of structural, biophysical and biochemical methods applied to CsdL/TcdA has afforded a more thorough understanding of how the structure of this E1-like enzyme has been fine tuned to accomplish ct6A synthesis on tRNAs while providing support for the notion that CsdA and CsdE are able to functionally interact with CsdL/TcdA. 相似文献
29.
Hernández-Montiel HL Meléndez-Herrera E Cepeda-Nieto AC Mejía-Viggiano C Larriva-Sahd J Guthrie S Varela-Echavarría A 《Developmental biology》2003,255(1):99-112
We have addressed the control of longitudinal axon pathfinding in the developing hindbrain, including the caudal projections of reticular and raphe neurons. To test potential sources of guidance signals, we assessed axon outgrowth from embryonic rat hindbrain explants cultured in collagen gels at a distance from explants of midbrain-hindbrain boundary (isthmus), caudal hindbrain, or cervical spinal cord. Our results showed that the isthmus inhibited caudally directed axon outgrowth by 80% relative to controls, whereas rostrally directed axon outgrowth was unaffected. Moreover, caudal hindbrain or cervical spinal cord explants did not inhibit caudal axons. Immunohistochemistry for reticular and raphe neuronal markers indicated that the caudal, but not the rostral projections of these neuronal subpopulations were inhibited by isthmic explants. Companion studies in chick embryos showed that, when the hindbrain was surgically separated from the isthmus, caudal reticulospinal axon projections failed to form and that descending pioneer axons of the medial longitudinal fasciculus (MLF) play an important role in the caudal reticulospinal projection. Taken together, these results suggest that diffusible chemorepellent or nonpermissive signals from the isthmus and substrate-anchored signals on the pioneer MLF axons are involved in the caudal direction of reticulospinal projections and might influence other longitudinal axon projections in the brainstem. 相似文献
30.
Oxidative stress in pregnancy and fertility pathologies 总被引:1,自引:0,他引:1
Oxidative stress designates the state of imbalance between reactive oxygen species (ROS) production and antioxidant levels. In a healthy placenta, there is an increase in ROS production, due to formation of new tissues and inherent metabolism, but this is balanced by higher levels of antioxidants. However, this balance is lost in some situations, with a consequent increase in oxidative stress levels. Oxidative stress has been implicated in several placental disorders and pregnancy pathologies. The present review intends to summarize what is known about the relationship between oxidative stress and well-known pregnancy disorders. 相似文献