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   2842篇
  免费   160篇
  国内免费   1篇
  2023年   21篇
  2022年   35篇
  2021年   79篇
  2020年   45篇
  2019年   70篇
  2018年   99篇
  2017年   83篇
  2016年   115篇
  2015年   175篇
  2014年   156篇
  2013年   214篇
  2012年   234篇
  2011年   225篇
  2010年   145篇
  2009年   144篇
  2008年   184篇
  2007年   158篇
  2006年   141篇
  2005年   139篇
  2004年   121篇
  2003年   96篇
  2002年   85篇
  2001年   34篇
  2000年   21篇
  1999年   34篇
  1998年   21篇
  1997年   13篇
  1996年   13篇
  1995年   9篇
  1994年   11篇
  1993年   5篇
  1992年   9篇
  1991年   4篇
  1990年   3篇
  1989年   6篇
  1988年   6篇
  1987年   3篇
  1986年   4篇
  1985年   5篇
  1984年   7篇
  1983年   5篇
  1982年   6篇
  1981年   3篇
  1980年   2篇
  1979年   3篇
  1976年   3篇
  1974年   2篇
  1973年   1篇
  1970年   1篇
  1965年   1篇
排序方式: 共有3003条查询结果,搜索用时 15 毫秒
891.
Human advance on natural habitats is a major cause of biodiversity loss. This transformation process represents a profound change in wooded environments, disrupting original communities of flora and fauna. Many species are highly dependent on forests, especially parrots (Psittaciformes) with almost a third of their species threatened by extinction. Most parrot species occur in tropical and subtropical forests, and given the forest dependence of most species, this is the main reason why habitat loss has been highlighted as the main threat for the group. Such habitat loss acts in synergy with other important threats (e.g., logging and poaching), which become especially problematic in certain developing countries along tropical latitudes. In this study, we used available information on parrot distributions, species traits, IUCN assessment, habitat loss and timber extraction for different periods, and distribution of protected areas, to determine conservation hotspots for the group, and analyze potential changes in the conservation status of these species. We detected four conservation hotspots for parrots: two in the Neotropics and two in Oceania, all of them facing different degrees of threat in regard of current habitat loss and agricultural trends. Our results suggest that the future of the group is subject to policymaking in specific regions, especially in the northeastern Andes and the Atlantic Forest. In addition, we predicted that agricultural expansion will have a further negative effect on the conservation status of parrots, pushing many parrot species to the edge of extinction in the near future. Our results have conservation implications by recommending protected areas in specific parrot conservation hotspots. Our recommendations to mitigate conservation risks to this group of umbrella species would also benefit many other coexisting species as well.  相似文献   
892.
893.
894.

Background

Schistosomiasis mansoni is a debilitating and sometimes fatal disease. Accurate diagnosis plays a key role in patient management and infection control. However, currently available parasitological methods are laborious and lack sensitivity. The selection of target antigen candidates has turned out to be a promising tool for the development of more sensitive diagnostic methods. In our previous investigations, the use of crude antigens led to false-positive results. Recently, focus has been given to highly purified Schistosoma mansoni antigens, especially to circulating antigens.

Method

Thus, our main goal was to test different types of circulating cathodic antigen glycoprotein (CCA), as “crude antigen,” the protein chain of recombinant CCA and two individual peptides. These schistosome proteins/peptides were tested in a new diagnostic method employing immunomagnetic separation based on the improvement of antigen–antibody binding.

Principal Findings

Use of recombinant CCA as a diagnostic antigen allowed us to develop a diagnostic assay with high sensitivity and specificity with no false-negative results. Interestingly, the “crude antigen” worked as a good marker for control of cure after praziquantel treatment.

Conclusions/Significance

Our new diagnostic method was superior to enzyme-linked immunosorbent assay in diagnosing low endemicity patients.  相似文献   
895.
Highlights? Stab wound injury and MCAo elicit a profound stem cell response ? Noninvasive brain injury fails to elicit a stem cell response ? SHH is upregulated and required in lesion conditions with a stem cell response ? SHH transducer deletion in astrocytes reduces their proliferative response to injury  相似文献   
896.
897.
A relevant logistic issue in the organization of a fair is to determine how stands have to be placed in the exhibition space so as to satisfy all constraints on security, ease of access, services, and so on, while maximizing the revenues coming from the exhibitors. We consider in particular the problem of allocating the maximum number of stands by satisfying all the constraints required by practical implementations. We examine a number of real-world cases, and show how basic mathematical programming models can be improved to handle specific requests from the organizers. We report the solutions obtained through an original decision support system, that embeds a number of algorithms to solve the various cases by reduction to one or more linear programs.  相似文献   
898.
BRAF is the most prevalent oncogene and an important therapeutic target in melanoma. In some cancers, BRAF is activated by rearrangements that fuse its kinase domain to 5′ partner genes. We examined 848 comparative genomic hybridization profiles of melanocytic tumors and found copy number transitions within BRAF in 10 tumors, of which six could be further characterized by sequencing. In all, the BRAF kinase domain was fused in‐frame to six N‐terminal partners. No other mutations were identified in melanoma oncogenes. One of the seven melanoma cell lines without known oncogenic mutations harbored a similar BRAF fusion, which constitutively activated the MAP kinase pathway. Sorafenib, but not vemurafenib, could block MAP kinase pathway activation and proliferation of the cell line at clinically relevant concentrations, whereas BRAFV600E mutant melanoma cell lines were significantly more sensitive to vemurafenib. The patient from whom the cell line was derived showed a durable clinical response to sorafenib.  相似文献   
899.
900.

Background

Th1 cytokines are essential for the control of M. tuberculosis infection. The role of IL-10 in tuberculosis is controversial and there is an increasing body of evidence suggesting that the relationship between Th1 cytokines and IL-10 is not as antagonistic as it was first believed, and that these cytokines may complement each other in infectious diseases.

Methods

The present study evaluated the activating capacity of CD4+ and CD8+ T cell repertoire in response to antigen stimulation through the expression of CD69 using Flow Cytometry, as well as the functionality of PBMCs by determining the cytokine profile in patients with active tuberculosis and in clinically cured patients after in vitro stimulation using ELISA. Treated patients were subdivided according to time after clinical cure (<12 months or >12 months post-treatment).

Results

We observed that T cell activation was higher in TB-treated patients, especially CD8+ T cell activation in TB-Treated >1 year. Th1 cytokines were significantly higher in TB-Treated, and the levels of IFN-γ and TNF-α increased continuously after clinical cure. Moreover, IL-10 production was significantly higher in cured patients and it was also enhanced in cured patients over time after treatment. Th17, Th2 and Th22 cytokines showed no statistically significant differences between Healthy Donors, Active-TB and TB-Treated.

Conclusions

This study describes a scenario in which potentiation of CD4+ and CD8+ T cell activation and increased Th1 cytokine production are associated with the clinical cure of tuberculosis in the absence of significant changes in Th2 cytokine production and is accompanied by increased production of IL-10. In contrast to other infections with intracellular microorganisms, this response occurs later after the end of treatment.  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