首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   96413篇
  免费   619篇
  国内免费   887篇
  97919篇
  2022年   27篇
  2021年   53篇
  2020年   40篇
  2019年   39篇
  2018年   11865篇
  2017年   10712篇
  2016年   7513篇
  2015年   691篇
  2014年   413篇
  2013年   437篇
  2012年   4412篇
  2011年   12983篇
  2010年   12103篇
  2009年   8328篇
  2008年   9917篇
  2007年   11546篇
  2006年   413篇
  2005年   630篇
  2004年   1094篇
  2003年   1155篇
  2002年   884篇
  2001年   319篇
  2000年   216篇
  1999年   70篇
  1998年   56篇
  1997年   62篇
  1996年   51篇
  1995年   38篇
  1994年   45篇
  1993年   63篇
  1992年   77篇
  1991年   69篇
  1990年   56篇
  1989年   53篇
  1988年   74篇
  1987年   51篇
  1986年   34篇
  1985年   41篇
  1984年   56篇
  1983年   48篇
  1981年   42篇
  1980年   27篇
  1979年   32篇
  1978年   31篇
  1976年   25篇
  1974年   37篇
  1972年   264篇
  1971年   289篇
  1965年   24篇
  1962年   29篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Many studies have been performed to assess the potential utility of natural products as immunomodulatory agents to enhance host responses against infection or to ameliorate immune-based pathologies. To determine whether eriodictyol has immunomodulatory effects and clarify which types of immune effector cells are stimulated in vitro, we investigated the stimulatory effect of eriodictyol on spleen cells isolated from BALB/c mice. Eriodictyol significantly stimulated splenocyte proliferation. However, only B lymphocytes (not T lymphocytes) could be stimulated by eriodictyol in a dose-related manner. Studies assessing potential effect of eriodictyol on innate immunity reported that eriodictyol enhanced significantly the killing activity of natural killer (NK) cells, T lymphocytes, and macrophages. We also demonstrated that eriodictyol inhibited nitric oxide (NO) production and lysosomal enzyme activity in murine peritoneal macrophages cultured ex-vivo, suggesting a potential anti-inflammatory effect in situ. Eriodictyol revealed also a cellular anti-oxidant activity in splenocytes and macrophages. Furthermore, eriodictyol increased catalase activity in spleen cells. From this data, it can be concluded that eriodictyol exhibited an immunomodulatory effect that could be ascribed in part to a cytoprotective effect related to its anti-oxidant activity.  相似文献   
992.

Background

Takotsubo cardiomyopathy often presents with the clinical signs of ST-elevation myocardial infarction (STEMI). The increase in scientific publications addressing this relatively rare condition may result in higher awareness and diagnosis of takotsubo cardiomyopathy.

Aim

To assess the observed prevalence per year of takotsubo cardiomyopathy in a large registry of patients with STEMI, during a 12-year inclusion period.

Method

All patients presenting with STEMI at a large regional cardiology clinic were entered into a database (n = 8,413, mean age 63 ± 13 years). Takotsubo cardiomyopathy was diagnosed in 42 patients (0.5?%). Years of evaluation were defined as ‘early years’ (January 2002 to December 2007; n = 4350) and ‘later years’ (January 2008 to December 2013). Multivariable analyses were performed to adjust for differences in demographical and clinical variables.

Results

In later years, the age of STEMI patients was slightly higher (64 ± 13 vs. 63 ± 13 years, p < 0.001), with more patients with clinical symptoms of shock (10 vs. 7?%, p < 0.001) or a history of percutaneous coronary intervention or hypertension (10 vs. 8?%, p = 0.001 and 37 vs. 34?%, p < 0.001). Smoking and a positive family history were less often observed during later years (39 vs. 46?%, p < 0.001 and 37 vs. 42?% p < 0.001). Patients with takotsubo cardiomyopathy were more often female (81 vs. 27?%, p = 0.001). Takotsubo cardiomyopathy was more often diagnosed in the later period (0.7 vs. 0.3?%, OR 2.4, 95?% CI 1.2–4.6, p = 0.009). The higher prevalence of takotsubo cardiomyopathy in recent years remained significant after adjustment for differences in patient characteristics (OR 2.1, 95?% CI 1.1–4.3).

Conclusion

Takotsubo cardiomyopathy is currently more often diagnosed in patients with STEMI compared with in earlier years. This is probably due to the increased scientific and clinical awareness among doctors, but the prevalence is still low.
  相似文献   
993.
A Gram staining negative, rod-shaped, aerobic bacterial strain J5-3T with a single polar flagellum was isolated from coking wastewater collected from Shaoguan, Guangdong, China. It was motile and capable of optimal growth at pH 6–8, 30 °C, and 0–2 % (w/v) NaCl. Its predominant fatty acids were 11-methyl C18:1 ω7c (29.2 %), C16:0 (20.6 %), C19:0 cyclo ω8c (18.2 %), C18:0 (11.0 %), and C18:1 ω7c/C18:1 ω6c (10.9 %) when grown on trypticase soy agar. The major polar lipids were diphosphatidylglycerol, phosphatidylglycerol, two unknown glycolipids (GL1, GL2), and two unknown phospholipid (PL1, PL2). The predominant ubiquinone was Q-10, and the genome DNA G+C content was 61.7 mol %. Phylogenetic analysis based on the 16S rRNA gene sequences indicated that strain J5-3T belonged to the family Hyphomicrobiaceae in Alphaproteobacteria. It shared the 16S rRNA gene sequence similarities of 93.8–96.1 % with the genus Devosia, 94.5–94.8 % with the genus Pelagibacterium, and <92.0 % with all the other type strains in family Hyphomicrobiaceae. It can be distinguished from the closest phylogenetic neighbors based on several phenotypic and genotypic features, including α-galactosidase activity, tetracycline susceptibility, major fatty acid composition, polar lipid profile, DNA gyrase B subunit (gyrB) gene sequence, and random-amplified polymorphic DNA profile. Therefore, we consider strain J5-3T to represent a novel species of a novel genus within the family Hyphomicrobiaceae, for which the name Paradevosia shaoguanensis gen. nov., sp. nov. is proposed. The type strain of Paradevosia shaoguanensis is J5-3T (=CGMCC 1.12430T =LMG 27409T).  相似文献   
994.
995.
Bacillus strains have been widely used for the production of fibrinolytic enzymes having role in the treatment of cardiovascular disorders. Purification and overproduction of such enzymes has increased their usage in medical fields including metalloproteinases with the ability to degrade extracellular matrix (ECM). Camelysin, a neutral metalloproteinase has been isolated from different species of bacteria like Bacillus cereus, Bacillus anthracis, and Bacillus thuringiensis with fibrinolytic, collagenolytic and actin degradation activity. This project successfully demonstrated the presence of 734-bp coding DNA sequence (CDS) encoding a 20.72331 kDa camelysin gene in local strain of Bacillus thuringiensis containing a signal peptide with cleavage site between residues 19 and 20. The sequence was submitted to GenBank (KT023597) and the sequence showed high homology with the camelysin protein of closely related Bacillus species. The alignment of related proteins through ClustalW displayed difference of four amino acids (“Q” replaced by “P” at position 169 and at position 182–184, “NQE” replaced by “HLK”) in the isolated protein. Comparison including structural and functional analysis of camelysin sequences isolated from different Bacillus species was carried out using different bioinformatics tools and software. The information would help in better understanding the properties of camelysin protein and its role in pathogenicity and clinical treatments.  相似文献   
996.
997.
The majority of studies in ethnoprimatology focus on areas of sympatry where humans and nonhuman primates (hereafter, primates) naturally coexist. We argue that much can be gained by extending the field’s scope to incorporate settings where humans manage most aspects of primates’ lives, such as zoos, laboratories, sanctuaries, and rehabilitation centers (hereafter, managed settings). We suggest that the mixed-methods approach of ethnoprimatology, which facilitates examination of both humans’ and primates’ responses to one another, can reveal not only how humans’ ideas about primates shape management strategies, but also how those management strategies affect primates’ lives. Furthermore, we note that a greater focus on managed settings will strengthen links between ethnoprimatology and primate rights/welfare approaches, and will introduce new questions into discussions of ethics in primatology. For example, managed settings raise questions about when it might be justifiable to restrict primates’ freedom for a “greater good,” and the desirability of making primates’ lives more “natural” even if this would decrease their well-being. Finally, we propose that because ethnoprimatology is premised on challenging false dichotomies between categories of field site—specifically, between “natural” and “unnatural” free-ranging populations—it makes sense for ethnoprimatologists to examine settings in which humans exert considerable control over primates’ lives, given that the distinction between “wild” and “captive” is similarly unclear.  相似文献   
998.
The development of F1 hybrid varieties benefits from the synergistic effect of conventional and molecular marker-assisted breeding schemes. A sequencing run was carried out in Foeniculum vulgare (2n?=?2x?=?22) to develop the first genome draft and to identify microsatellites suitable for implementing multilocus SSR marker assays. A preliminary cytometric analysis allowed us to estimate the genome size (2C?=?2.64–2.86 pg), equal to about 1.34 Mbp for 1C genome, and to calculate the sequencing coverage (53×). The genome draft assembly into 300,408 scaffolds and its bioinformatic analysis enabled the annotation of coding and non-coding regions across the genome, including 103,306 SSR elements. A total of 100 microsatellites were randomly chosen among those with dinucleotide and trinucleotide repeat motifs and with a repeat motif length?≥?25 times and were preliminarily tested. Of these, 27 SSR markers, classified as suitable for genetic diversity analyses, were efficiently organized in five PCR multiplex assays and validated using a core collection of 100 fennel individuals potentially useful for the development of inbred lines and F1 hybrids. All SSR loci were found to be polymorphic, scoring an observed number of marker alleles Na?=?207 and an average polymorphism information content PIC?=?0.69. The SSR data were used to calculate (i) the degree of homozygosity for the individual inbred lines (0.35?<?Ho?<?0.96), to eventually plan additional selfing or sibling cycles, and (ii) the degree of genetic similarity for all possible pair-wise comparisons between parental inbred lines (GS?=?0.55–0.77), to identify the most divergent combinations for the constitution of experimental F1 hybrids. The integration of genotypic and phenotypic data was useful for implementing guidelines for precision hybrid breeding schemes in fennel.  相似文献   
999.
Energetic properties of chlorophylls in photosynthetic complexes are strongly modulated by their interaction with the protein matrix and by inter-pigment coupling. This spectral tuning is especially striking in photosystem I (PSI) complexes that contain low-energy chlorophylls emitting above 700 nm. Such low-energy chlorophylls have been observed in cyanobacterial PSI, algal and plant PSI–LHCI complexes, and individual light-harvesting complex I (LHCI) proteins. However, there has been no direct evidence of their presence in algal PSI core complexes lacking LHCI. In order to determine the lowest-energy states of chlorophylls and their dynamics in algal PSI antenna systems, we performed time-resolved fluorescence measurements at 77 K for PSI core and PSI–LHCI complexes isolated from the green alga Chlamydomonas reinhardtii. The pool of low-energy chlorophylls observed in PSI cores is generally smaller and less red-shifted than that observed in PSI–LHCI complexes. Excitation energy equilibration between bulk and low-energy chlorophylls in the PSI–LHCI complexes at 77 K leads to population of excited states that are less red-shifted (by ~?12 nm) than at room temperature. On the other hand, analysis of the detection wavelength dependence of the effective trapping time of bulk excitations in the PSI core at 77 K provided evidence for an energy threshold at ~?675 nm, above which trapping slows down. Based on these observations, we postulate that excitation energy transfer from bulk to low-energy chlorophylls and from bulk to reaction center chlorophylls are thermally activated uphill processes that likely occur via higher excitonic states of energy accepting chlorophylls.  相似文献   
1000.
Aflatoxin B1 (AFB1) is the most toxic among the mycotoxins and causes detrimental health effects on human and animals. Selenium (Se) plays an important role in chemopreventive, antioxidant, anticarcinogen, and detoxification and involved in cell cycle regulation. The aim of this study was to explore the molecular mechanisms of selenium involved in inhibition of G2/M cell cycle arrest of broiler’s jejunum. A total of 240 one-day-old healthy Cobb broilers were randomly divided into four groups and fed with basal diet (control group), 0.6 mg/kg AFB1 (AFB1 group), 0.4 mg/kg Se (+Se group), and 0.6 mg/kg AFB1 + 0.4 mg/kg Se (AFB1 + Se group) for 21 days, respectively. The histological observation and morphological analysis revealed that 0.4 mg/kg Se prevented the AFB1-associated lesions of jejunum including the shedding of the apical region of villi, the decreased villus height, and villus height/crypt ratio. The cell cycle analysis by flow cytometry showed that 0.4 mg/kg Se ameliorated the AFB1-induced G2/M phase arrest in jejunal cells. Moreover, the expressions of ATM, Chk2, p53, Mdm2, p21, PCNA, Cdc25, cyclin B, and Cdc2 analyzed by immunohistochemistry and qRT-PCR demonstrated that 0.4 mg/kg Se restored these parameters to be close to those in the control group. In conclusion, Se promoted cell cycle recovery from the AFB1-induced G2/M phase arrest by the molecular regulation of ATM pathway in the jejunum of broilers. The outcomes from the present study may lead to a better understanding of the nature of selenium’s essentiality and its protective roles against AFB1.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号