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   15319篇
  免费   982篇
  国内免费   3篇
  2023年   89篇
  2022年   108篇
  2021年   394篇
  2020年   306篇
  2019年   325篇
  2018年   397篇
  2017年   364篇
  2016年   542篇
  2015年   830篇
  2014年   896篇
  2013年   1056篇
  2012年   1335篇
  2011年   1179篇
  2010年   758篇
  2009年   712篇
  2008年   951篇
  2007年   892篇
  2006年   821篇
  2005年   772篇
  2004年   721篇
  2003年   641篇
  2002年   604篇
  2001年   134篇
  2000年   95篇
  1999年   126篇
  1998年   131篇
  1997年   119篇
  1996年   100篇
  1995年   89篇
  1994年   88篇
  1993年   58篇
  1992年   55篇
  1991年   62篇
  1990年   58篇
  1989年   31篇
  1988年   38篇
  1987年   26篇
  1986年   21篇
  1985年   39篇
  1984年   54篇
  1983年   21篇
  1982年   25篇
  1981年   26篇
  1980年   27篇
  1979年   17篇
  1978年   19篇
  1977年   21篇
  1976年   17篇
  1975年   18篇
  1969年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
To further describe movement patterns and distribution of East Pacific green turtles (Chelonia mydas agassizii) and to determine threat levels for this species within the Eastern Pacific. In order to do this we combined published data from existing flipper tagging and early satellite tracking studies with data from an additional 12 satellite tracked green turtles (1996-2006). Three of these were tracked from their foraging grounds in the Gulf of California along the east coast of the Baja California peninsula to their breeding grounds in Michoacán (1337-2928 km). In addition, three post-nesting females were satellite tracked from Colola beach, Michoacán to their foraging grounds in southern Mexico and Central America (941.3-3020 km). A further six turtles were tracked in the Gulf of California within their foraging grounds giving insights into the scale of ranging behaviour. Turtles undertaking long-distance migrations showed a tendency to follow the coastline. Turtles tracked within foraging grounds showed that foraging individuals typically ranged up to 691.6 km (maximum) from release site location. Additionally, we carried out threat analysis (using the cumulative global human impact in the Eastern Pacific) clustering pre-existing satellite tracking studies from Galapagos, Costa Rica, and data obtained from this study; this indicated that turtles foraging and nesting in Central American waters are subject to the highest anthropogenic impact. Considering that turtles from all three rookeries were found to migrate towards Central America, it is highly important to implement conservation plans in Central American coastal areas to ensure the survival of the remaining green turtles in the Eastern Pacific. Finally, by combining satellite tracking data from this and previous studies, and data of tag returns we created the best available distributional patterns for this particular sea turtle species, which emphasized that conservation measures in key areas may have positive consequences on a regional scale.  相似文献   
942.
Brazil ranked second only to the United States in hectares planted to genetically modified crops in 2013. Recently corn producers in the Cerrado region reported that the control of Spodoptera frugiperda with Bt corn expressing Cry1Fa has decreased, forcing them to use chemicals to reduce the damage caused by this insect pest. A colony of S. frugiperda was established from individuals collected in 2013 from Cry1Fa corn plants (SfBt) in Brazil and shown to have at least more than ten-fold higher resistance levels compared with a susceptible colony (Sflab). Laboratory assays on corn leaves showed that in contrast to SfLab population, the SfBt larvae were able to survive by feeding on Cry1Fa corn leaves. The SfBt population was maintained without selection for eight generations and shown to maintain high levels of resistance to Cry1Fa toxin. SfBt showed higher cross-resistance to Cry1Aa than to Cry1Ab or Cry1Ac toxins. As previously reported, Cry1A toxins competed the binding of Cry1Fa to brush border membrane vesicles (BBMV) from SfLab insects, explaining cross-resistance to Cry1A toxins. In contrast Cry2A toxins did not compete Cry1Fa binding to SfLab-BBMV and no cross-resistance to Cry2A was observed, although Cry2A toxins show low toxicity to S. frugiperda. Bioassays with Cry1AbMod and Cry1AcMod show that they are highly active against both the SfLab and the SfBt populations. The bioassay data reported here show that insects collected from Cry1Fa corn in the Cerrado region were resistant to Cry1Fa suggesting that resistance contributed to field failures of Cry1Fa corn to control S. frugiperda.  相似文献   
943.
Achyrocline alata, known as Jateí-ka-há, is traditionally used to treat several health problems, including inflammations and infections. This study aimed to optimize an active extract against Streptococcus mutans, the main bacteria that causes caries. The extract was developed using an accelerated solvent extraction and chemometric calculations. Factorial design and response surface methodologies were used to determine the most important variables, such as active compound selectivity. The standardized extraction recovered 99% of the four main compounds, gnaphaliin, helipyrone, obtusifolin and lepidissipyrone, which represent 44% of the extract. The optimized extract of A. alata has a MIC of 62.5 μg/mL against S. mutans and could be used in mouth care products.  相似文献   
944.
The aim of this preclinical study was to evaluate the characteristics of the monoclonal antibody Rebmab200, which is a humanized version of the ovarian-specific murine antibody MX35. This investigation contributes to the foundation for future clinical α-radioimmunotherapy of minimal residual ovarian cancer with 211At-Rebmab200. Here, the biodistribution of 211At-Rebmab200 was evaluated, as was the utility of 99mTc-Rebmab200 for bioimaging. Rebmab200 was directly compared with its murine counterpart MX35 in terms of its in-vitro capacity for binding the immobilized NaPi2B epitope and live cells; we also assessed its biodistribution in nude mice carrying subcutaneous OVCAR-3 tumors. Tumor antigen and cell binding were similar between Rebmab200 and murine MX35, as was biodistribution, including normal tissue uptake and in-vivo tumor binding. We also demonstrated that 99mTc-Rebmab200 can be used for single-photon emission computed tomography of subcutaneous ovarian carcinomas in tumor-bearing mice. Taken together, our data support the further development of Rebmab200 for radioimmunotherapy and diagnostics.  相似文献   
945.
BackgroundChagas disease, caused by the parasite Trypanosoma cruzi, is a neglected tropical disease that causes severe human health problems. To develop a new chemotherapeutic agent for the treatment of Chagas disease, we predicted a pharmacophore model for T. cruzi dihydroorotate dehydrogenase (TcDHODH) by fragment molecular orbital (FMO) calculation for orotate, oxonate, and 43 orotate derivatives.Conclusions/SignificanceFMO-based interaction energy analyses revealed a pharmacophore model for TcDHODH inhibitor. Hydrogen bond acceptor pharmacophores correspond to Lys43 and Lys214, hydrogen bond donor and acceptor pharmacophores correspond to Asn67 and Asn194, and the aromatic ring pharmacophore corresponds to FMN, which shows important characteristics of compounds that inhibit TcDHODH. In addition, the Lys214 residue is not conserved between TcDHODH and human DHODH. Our analysis suggests that these orotate derivatives should preferentially bind to TcDHODH, increasing their selectivity. Our results obtained by pharmacophore modeling provides insight into the structural requirements for the design of TcDHODH inhibitors and their development as new anti-Chagas drugs.  相似文献   
946.

Background

To evaluate the predictive value of carotid atherosclerotic disease (CAD) and intima-media thickness (IMT) on incident cardiovascular disease and mortality in hemodialysis patients.

Methods

Multicenter, observational, prospective study including 110 patients, followed-up to 6 years. Carotid doppler ultrasonographic findings were classified in 4 degrees of severity: 1) IMT <0.9 mm, 2) IMT >0.9 mm, 3) carotid plaque with stenosis <50% and 4) plaque with stenosis >50%. The associations between IMT and CAD and cardiovascular events, total and cardiovascular mortality were assessed.

Results

83% of the patients had atherosclerotic plaques (CAD degrees 3-4). During follow-up, 29.1% of patients experienced cardiovascular events, and 28.2% died, 38.7% of cardiovascular origin. The presence of plaques was associated with cardiovascular events (p = 0.03) while calcified plaques were associated with both cardiovascular events (p = 0.01), cardiovascular mortality (p = 0.03) and non-significantly with overall mortality (p = 0.08) in the survival analysis. Carotid IMT was not associated with outcomes. Cardiovascular events correlated with CAD severity (HR 2.27, 95% CI 1.13-4.54), age (HR 1.04, 1.01-1.06), previous cardiovascular disease (HR 1.75, 1.05-4.42), dyslipidemia (HR 2.25, 1.11-4.53), lipoprotein (a) (HR 1.01, 1.00-1.02), troponin I (HR 3.89, 1.07-14.18), fibrinogen levels (HR 1.38, 0.98-1.94) and antiplatelet therapy (HR 2.14, 1.04-4.4). In an age-adjusted multivariate model, cardiovascular events were independently associated with previous coronary artery disease (HR 3.29, 1.52-7.15) and lipoprotein (a) (HR 1.01, 1.00-1.02).

Conclusions

The presence of carotid plaques and, especially, calcified plaques, are predictors of new cardiovascular events and cardiovascular mortality in hemodialysis patients, while IMT was not. The prognostic value of calcified plaques should be confirmed in future studies.  相似文献   
947.

Introduction

Minor physical anomalies (MPA) are dysmorphic features that reflect deviations in early development, are morphological variants that appear during the first trimester of pregnancy and could be used as a marker of disease risk in susceptible people. The literature agrees that the number of MPA is higher in patients with schizophrenia compared with their relatives and healthy subjects. The purpose of this study is to compare the MPA, assessed using the Gourion Scale, in complete nuclear families (families with a member with schizophrenia and control families) by determining the MPA mean, concordance and heritability for the total score on the MPA Gourion Scale for each anomaly.

Method

The sample consisted of 60 families with at least one schizophrenic patient (284 members) and 61 control families (249 members). Results: The mean total score for the scale was 5.72 ± 2.3 MPA in the case of families with at least one schizophrenic patient and 1.8 ± 4.46 MPA for control families. The average for families of patients without considering the patient in the analysis was 5.59 ± 2.3 MPA; for patients, the mean was 6.14 ± 2.4 MPA. In the analysis by anomaly differences were found only in eleven anomalies found no evidence of heritability or concordance.

Conclusions

MPA occur more frequently in patients, but a pattern of low consistency between them persists. It is concluded that MPA could be a marker of neurodevelopmental problems, but it is not suitable to consider them a Gourion scale as endophenotype.  相似文献   
948.
Deforestation and fragmentation are major components of global change; both are contributing to the rapid loss of tropical forest area with important implications for ecosystem functioning and biodiversity conservation. The forests of South Ecuador are a biological ‘hotspot’ due to their high diversity and endemism levels. We examined the deforestation and fragmentation patterns in this area of high conservation value using aerial photographs and Aster satellite scenes. The registered annual deforestation rates of 0.75% (1976–1989) and 2.86% (1989–2008) for two consecutive survey periods, the decreasing mean patch size and the increasing isolation of the forest fragments show that the area is under severe threat. Approximately 46% of South Ecuador’s original forest cover had been converted by 2008 into pastures and other anthropogenic land cover types. We found that deforestation is more intense at lower elevations (premontane evergreen forest and shrubland) and that the deforestation front currently moves in upslope direction. Improved awareness of the spatial extent, dynamics and patterns of deforestation and forest fragmentation is urgently needed in biologically diverse areas like South Ecuador.  相似文献   
949.
Tuberculosis (TB) remains a significant public health problem worldwide, but the ecology of the prevalent mycobacterial strains, and their transmission, can vary depending on country and region. Chile is a country with low incidence of TB, that has a geographically isolated location in relation to the rest of South American countries due to the Andes Mountains, but recent migration from neighboring countries has changed this situation. We aimed to assess the genotypic diversity of Mycobacterium tuberculosis complex (MTBC) strains in Santiago, Chile, and compare with reports from other Latin-American countries. We analyzed MTBC isolates from pulmonary tuberculosis cases collected between years 2008 and 2013 in Central Santiago, using two genotyping methods: spoligotyping and 12-loci mycobacterial interspersed repetitive unit-variable number of tandem repeats (MIRU-VNTRs). Data obtained were analyzed and compared to the SITVIT2 database. Mean age of the patients was 47.5 years and 61% were male; 11.6% were migrants. Of 103 strains (1 isolate/patient) included, there were 56 distinct spoligotype patterns. Of these, 16 strains (15.5%) corresponded to orphan strains in the SITVIT2 database, not previously reported. Latin American and Mediterranean (LAM) (34%) and T (33%) lineages were the most prevalent strains, followed by Haarlem lineage (16.5%). Beijing family was scarcely represented with only two cases (1.9%), one of them isolated from a Peruvian migrant. The most frequent clustered spoligotypes were SIT33/LAM3 (10.7%), SIT53/T1 (8.7%), SIT50/H3 (7.8%), and SIT37/T3 (6.8%). We conclude that LAM and T genotypes are the most prevalent genotypes of MTBC in Santiago, Chile, and together correspond to almost two thirds of analyzed strains, which is similar to strain distribution reported from other countries of Latin America. Nevertheless, the high proportion of SIT37/T3, which was rarely found in other Latin American countries, may underline a specific history or demographics of Chile related to probable human migrations and evolutions.  相似文献   
950.
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