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   1731篇
  免费   104篇
  1835篇
  2023年   11篇
  2022年   31篇
  2021年   42篇
  2020年   27篇
  2019年   39篇
  2018年   58篇
  2017年   48篇
  2016年   64篇
  2015年   100篇
  2014年   97篇
  2013年   154篇
  2012年   163篇
  2011年   144篇
  2010年   82篇
  2009年   82篇
  2008年   99篇
  2007年   87篇
  2006年   86篇
  2005年   51篇
  2004年   51篇
  2003年   70篇
  2002年   49篇
  2001年   13篇
  2000年   13篇
  1999年   18篇
  1998年   10篇
  1997年   16篇
  1996年   8篇
  1995年   13篇
  1994年   7篇
  1993年   8篇
  1992年   12篇
  1991年   9篇
  1990年   11篇
  1989年   9篇
  1988年   7篇
  1987年   12篇
  1986年   3篇
  1985年   5篇
  1984年   3篇
  1983年   4篇
  1982年   4篇
  1981年   3篇
  1977年   2篇
  1976年   1篇
  1975年   2篇
  1974年   1篇
  1973年   1篇
  1967年   1篇
  1965年   1篇
排序方式: 共有1835条查询结果,搜索用时 15 毫秒
961.
Mucosal epithelia constitute the first barriers to be overcome by pathogens during infection. The induction of protective IgA in this location is important for the prevention of infection and can be achieved through different mucosal immunization strategies. Lactic acid bacteria have been tested in the last few years as live vectors for the delivery of antigens at mucosal sites, with promising results. In this work, Streptococcus pneumoniae PsaA antigen was expressed in different species of lactic acid bacteria, such as Lactococcus lactis, Lactobacillus casei, Lactobacillus plantarum, and Lactobacillus helveticus. After nasal inoculation of C57Bl/6 mice, their ability to induce both systemic (IgG in serum) and mucosal (IgA in saliva, nasal and bronchial washes) anti-PsaA antibodies was determined. Immunization with L. lactis MG1363 induced very low levels of IgA and IgG, possibly by the low amount of PsaA expressed in this strain and its short persistence in the nasal mucosa. All three lactobacilli persisted in the nasal mucosa for 3 days and produced a similar amount of PsaA protein (150-250 ng per 10(9) CFU). However, L. plantarum NCDO1193 and L. helveticus ATCC15009 elicited the highest antibody response (IgA and IgG). Vaccination with recombinant lactobacilli but not with recombinant L. lactis led to a decrease in S. pneumoniae recovery from nasal mucosa upon a colonization challenge. Our results confirm that certain Lactobacillus strains have intrinsic properties that make them suitable candidates for mucosal vaccination experiments.  相似文献   
962.
The cytotoxic potential of three cucurbitacins, 2,3,16,20(R),25-pentahydroxy-11,22-dioxo-cucurbita-5-en (cucurbitacin P, 1), 2,3,16,20(R),25-pentahydroxy-22-oxocucurbita-5-en (2) and 2,3,16,20(R),25-pentahydroxy-22-oxocucurbita-5,23(E)-diene (deacetylpicracin, 3), obtained from Cayaponia racemosa was evaluated as their ability to induce brine shrimp lethality, to inhibit the development of sea urchin eggs and tumor cell proliferation, and to lysis mouse erythrocytes. Compounds 1 and 2 were highly toxic with LC50 of (29.6+/-.1) (56.8) and (38.8 +/-.0) (76.6) micro/mL (umicro), respectively, while compound 3 was not effective at the tested concentrations. All tested compounds possessed an inhibitory effect on the proliferation of tumor cell lines, compound 1 being the most active, followed by 2 and 3. Nevertheless, no hemolytic activity or inhibition of the development of sea urchin eggs was observed for these compounds.  相似文献   
963.
The aim of this study is to determine the differences in sexual life of women with cervical cancer after surgery and radiation therapy. A total of 210 patients treated for cervical cancer at the Department of Obstetrics and Gynecology, School of Medicine, University of Zagreb, Croatia between March 2001 and March 2003 were asked to fill in the questionnaire. Sexual life had worsened in 42.86% of the surgical patients, as had in 25.00% of irradiated patients (p < 0.01). The main reason of sex life impairment was fear of pain (55.55% and 42.86%, respectively (p > 0.05)). More than 80% of patients didn't notice any changes in their partner's behavior. Almost every third woman felt certain change in her "body image", similar in both groups (p > 0.05). Need for consultations regarding sex life after diagnosis were recognized by 71.43% of patients. In conclusion we can say that considerable amount of attention should be given to psychological and sexual aspects of recovery of patients, because those aspects can significantly influence patients rehabilitation and prognosis.  相似文献   
964.
The porcellanid crab Petrolisthes armatus, with a known geographic distribution covering a wide range of latitudes, was selected to conduct a comparative study on egg production and reproductive output between two populations from Pacific and Atlantic coasts. Ovigerous females were collected between September and November 2005 in Punta Morales, Pacific coast of Costa Rica (n = 137) and from March 2005 to July 2006 in Araçá region, São Paulo, Brazil (n = 46). The mean size of females was statistically different between both populations with larger females from Brazil than Costa Rica (7.9 ± 1.31 and 6.6 ± 0.95 mm CW, respectively). Likewise, egg production was significantly different in both populations and started in Pacific Costa Rica at a smaller female size than in Brazil. Specimens from Pacific Costa Rica produced roughly three times more eggs than equally sized females from Brazil, while newly extruded embryos were larger in Brazil than in Costa Rica (0.045 and 0.039 mm3, respectively). Egg volume increased during embryogenesis by 112.8 and 164.5% in Costa Rican and Brazilian populations, respectively. The egg water content increased steadily in both populations; however, eggs produced in Brazil contained always more water than those from the same developmental stage in Costa Rica. Average reproductive output (based on dry weight) was substantially lower in Brazil (0.031) than in Costa Rica (0.065). Our results confirm an impressive intraspecific plasticity of reproductive features in an intertidal decapod. The observed phenotypical variability might be related to local environmental conditions as well as to the location of the studied population concerning its geographic range of distribution.  相似文献   
965.
While the contribution of Bacteria to bioremediation of oil-contaminated shorelines is well established, the response of Archaea to spilled oil and bioremediation treatments is unknown. The relationship between archaeal community structure and oil spill bioremediation was examined in laboratory microcosms and in a bioremediation field trial. 16S rRNA gene-based PCR and denaturing gradient gel analysis revealed that the archaeal community in oil-free laboratory microcosms was stable for 26 days. In contrast, in oil-polluted microcosms a dramatic decrease in the ability to detect Archaea was observed, and it was not possible to amplify fragments of archaeal 16S rRNA genes from samples taken from microcosms treated with oil. This was the case irrespective of whether a bioremediation treatment (addition of inorganic nutrients) was applied. Since rapid oil biodegradation occurred in nutrient-treated microcosms, we concluded that Archaea are unlikely to play a role in oil degradation in beach ecosystems. A clear-cut relationship between the presence of oil and the absence of Archaea was not apparent in the field experiment. This may have been related to continuous inoculation of beach sediments in the field with Archaea from seawater or invertebrates and shows that the reestablishment of Archaea following bioremediation cannot be used as a determinant of ecosystem recovery following bioremediation. Comparative 16S rRNA sequence analysis showed that the majority of the Archaea detected (94%) belonged to a novel, distinct cluster of group II uncultured Euryarchaeota, which exhibited less than 87% identity to previously described sequences. A minor contribution of group I uncultured Crenarchaeota was observed.  相似文献   
966.
The aim of this research was to establish if a correlation exists between the choice of an elective subject, namely subjects "Depression" and "Diabetes", and levels of depressiveness in medical students. Three groups of third year medical students attending School of medicine, Rijeka University, were tested for the level of depression using Beck's self-evaluation scale. The groups consisted of 30 non-randomly selected students that had enrolled elective subject "Depression" and 29 non-randomly selected students that had enrolled elective subject "Diabetes", and the third group of 30 randomly selected third year medical students that had enrolled none of the previously mentioned elective subjects. Median age of participants in this research was 25.24. The results showed no statistically significant difference in overall level of depressiveness among the groups. By testing for the difference between group pairs, there was a statistically significant difference between depressiveness in students attending "Depression" and "Diabetes", the latter being significantly more depressed (M = 8.30 in "Depression" group; M = 11.41 in "Diabetes" group; p = 0.04). In total there were 33 males and 56 females that participated in this research. Gender difference was also tested, and there was no statistically significant difference between sexes among groups. The difference was found only within the group of students attending "Depression" elective subject, where females scored significantly higher on Beck's questionnaire (z = 2.26; p = 0.03). The analysis of difference between items of the Beck's questionnaire showed statistically significant difference in the item "Feeling of rejection", where students attending elective subjects other then "Depression" scored significantly higher; differences in the items "Urge for punishment" and "Suicidal tendencies" were also found between "Diabetes" and "other elective subjects" group, in favor of "Diabetes" group; in the item "Weight loss" students attending "Diabetes" elective subject scored significantly higher then their peers in both other groups. The results indicate the possibility of a protective role of psycho-educative component provided to the students attending elective subject on depression within medical school environment, that has repeatedly been shown to be stressful and demanding and is beneficial for the onset of depressive disorders.  相似文献   
967.
Antiproliferative and antibacterial activities of nine glutarimide derivatives (19) were reported. Cytotoxicity of compounds was tested toward three human cancer cell lines, HeLa, K562 and MDA-MB-453 by MTT assay. Compound 7 (2-benzyl-2-azaspiro[5.11]heptadecane-1,3,7-trione), containing 12-membered ketone ring, was found to be the most potent toward all tested cell lines (IC50?=?9–27?μM). Preliminary screening of antibacterial activity by a disk diffusion method showed that Gram-positive bacteria were more susceptible to the tested compounds than Gram-negative bacteria. Minimum inhibitory concentration (MIC) determined by a broth microdilution method confirmed that compounds 1, 2, 4, 68 and 9 inhibited the growth of all tested Gram-positive and some of the Gram-negative bacteria. The best antibacterial potential was achieved with compound 9 (ethyl 4-(1-benzyl-2,6-dioxopiperidin-3-yl)butanoate) against Bacillus cereus (MIC 0.625?mg/mL; 1.97?×?10?3?mol/L). Distinction between more and less active/inactive compounds was assessed from the pharmacophoric patterns obtained by molecular interaction fields.  相似文献   
968.
The cell-cycle inhibitor p21CDKN1A has been suggested to directly participate in DNA repair, thanks to the interaction with PCNA. Yet, its role has remained unclear. Among proteins interacting with both p21 and PCNA, the histone acetyltransferase (HAT) p300 has been shown to participate in DNA repair. Here we report evidence indicating that p21 protein localizes and interacts with both p300 and PCNA at UV-induced DNA damage sites. The interaction between p300 and PCNA is regulated in vivo by p21. Indeed, loss of p21, or its inability to bind PCNA, results in a prolonged binding to chromatin and an increased association of p300 with PCNA, in UV-irradiated cells. Concomitantly, HAT activity of p300 is reduced after DNA damage. In vitro experiments show that inhibition of p300 HAT activity induced by PCNA is relieved by p21, which disrupts the association between recombinant p300 and PCNA. These results indicate that p21 is required during DNA repair to regulate p300 HAT activity by disrupting its interaction with PCNA.  相似文献   
969.
970.
Studies of macroinvertebrate communities in thermal streams are highly geographically localized and mostly faunistical, making the efforts to understand in situ water thermal regime effects on those biocoenoses barely achievable. We examined the effects of geothermal water inflow on benthic macroinvertebrate community composition in a temperate stream. Environmental data analysis has shown that water temperature is a major factor determining the faunistical composition, especially downstream of the geothermal water inflow situated some 20 m upstream of locality V3. The increase in mean annual water temperature from 11.5±4.1 °C at locality V2 to 22.0±5.0 °C at locality V3 induced an enormous shift in community composition from a diverse one, composed mainly of Gammaridae, Simuliidae, Chironomidae, Trichoptera and Ephemeroptera, and to a lesser extent of Plecoptera, Coleoptera, other Diptera, Hirudinea, Odonata, Mollusca and Oligochaeta, to a uniform one strongly dominated by Chironomidae, Mollusca and Oligochaeta, comprising 98.9±0.5% of collected individuals. While the disappearance of Plecoptera and Ephemeroptera and the increase in representation of Mollusca and Oligochaeta at locality V3 might be solely explained by water temperature increase, in the case of Chironomidae the increase in water discharge and relatively high annual water temperature variation at locality V3 had additional positive effects. However, the latter factor induced disappearance of Gammaridae at locality V3. In addition to the increase in water temperature, increase of water velocity significantly determined the longitudinal dynamics of Coleoptera.  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