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   191篇
  免费   58篇
  2021年   3篇
  2016年   3篇
  2015年   6篇
  2014年   4篇
  2013年   7篇
  2012年   7篇
  2011年   8篇
  2010年   3篇
  2009年   3篇
  2008年   4篇
  2007年   5篇
  2006年   7篇
  2005年   7篇
  2004年   9篇
  2003年   7篇
  2002年   6篇
  2001年   9篇
  2000年   9篇
  1999年   7篇
  1998年   4篇
  1997年   4篇
  1996年   3篇
  1995年   4篇
  1994年   6篇
  1993年   4篇
  1992年   4篇
  1991年   5篇
  1990年   7篇
  1989年   3篇
  1988年   3篇
  1986年   6篇
  1985年   12篇
  1984年   6篇
  1983年   3篇
  1982年   7篇
  1981年   7篇
  1980年   2篇
  1979年   4篇
  1978年   3篇
  1977年   3篇
  1975年   4篇
  1974年   2篇
  1973年   2篇
  1971年   2篇
  1967年   3篇
  1966年   3篇
  1965年   2篇
  1960年   2篇
  1930年   1篇
  1921年   1篇
排序方式: 共有249条查询结果,搜索用时 31 毫秒
21.
Carotenoid production was determined in illuminated anaerobically maintained cultures of Rhodospirillum rubrum in media with and without 4-[beta-(diethylamino)-ethoxy]-benzophenone. In treated cultures, lycopene--which normally is not produced by R. rubrum--accumulated as the predominant pigment, and total carotenoids increased five- to sixfold.  相似文献   
22.
23.
Bats are the natural reservoirs of a number of high-impact viral zoonoses. We present a quantitative analysis to address the hypothesis that bats are unique in their propensity to host zoonotic viruses based on a comparison with rodents, another important host order. We found that bats indeed host more zoonotic viruses per species than rodents, and we identified life-history and ecological factors that promote zoonotic viral richness. More zoonotic viruses are hosted by species whose distributions overlap with a greater number of other species in the same taxonomic order (sympatry). Specifically in bats, there was evidence for increased zoonotic viral richness in species with smaller litters (one young), greater longevity and more litters per year. Furthermore, our results point to a new hypothesis to explain in part why bats host more zoonotic viruses per species: the stronger effect of sympatry in bats and more viruses shared between bat species suggests that interspecific transmission is more prevalent among bats than among rodents. Although bats host more zoonotic viruses per species, the total number of zoonotic viruses identified in bats (61) was lower than in rodents (68), a result of there being approximately twice the number of rodent species as bat species. Therefore, rodents should still be a serious concern as reservoirs of emerging viruses. These findings shed light on disease emergence and perpetuation mechanisms and may help lead to a predictive framework for identifying future emerging infectious virus reservoirs.  相似文献   
24.

Background

Progress toward meeting Millennium Development Goal 5, which aims to improve maternal and reproductive health outcomes, is behind schedule. This is despite ever increasing volumes of official development aid targeting the goal, calling into question the distribution and efficacy of aid. The 2005 Paris Declaration on Aid Effectiveness represented a global commitment to reform aid practices in order to improve development outcomes, encouraging a shift toward collaborative aid arrangements which support the national plans of aid recipient countries (and discouraging unaligned donor projects).

Methods and Findings

We conducted a systematic review to summarise the evidence of the impact on MDG 5 outcomes of official development aid delivered in line with Paris aid effectiveness principles and to compare this with the impact of aid in general on MDG 5 outcomes. Searches of electronic databases identified 30 studies reporting aid-funded interventions designed to improve maternal and reproductive health outcomes. Aid interventions appear to be associated with small improvements in the MDG indicators, although it is not clear whether changes are happening because of the manner in which aid is delivered. The data do not allow for a meaningful comparison between Paris style and general aid. The review identified discernible gaps in the evidence base on aid interventions targeting MDG 5, notably on indicators MDG 5.4 (adolescent birth rate) and 5.6 (unmet need for family planning).

Discussion

This review presents the first systematic review of the impact of official development aid delivered according to the Paris principles and aid delivered outside this framework on MDG 5 outcomes. Its findings point to major gaps in the evidence base and should be used to inform new approaches and methodologies aimed at measuring the impact of official development aid.  相似文献   
25.
Soluble human complement receptor type 1 (sCR1, TP10) has been expressed in Chinese hamster ovary (CHO) DUKX-B11 cells and shown to inhibit the classical and alternative complement pathways in vitro and in vivo. A truncated version of sCR1 lacking the long homologous repeat-A domain (LHR-A) containing the C4b binding site has similarly been expressed and designated sCR1[desLHR-A]. sCR1[desLHR-A] was shown to be a selective inhibitor of the alternative complement pathway in vitro and to function in vivo. In this study, sCR1 and sCR1[desLHR-A] were expressed in CHO LEC11 cells with an active alpha(1,3)-fucosyltransferase, which makes possible the biosynthesis of the sialyl-Lewisx (sLex) tetrasaccharide (NeuNAcalpha2-3Galbeta1-4(Fucalpha1-3)GlcNAc) during post-translational glycosylation. The resulting glycoproteins, designated sCR1sLex and sCR1[desLHR-A]sLex, respectively, retained the complement regulatory activities of their DUKX B11 counterparts, which lack alpha(1-3)-fucose. Carbohydrate analysis of purified sCR1sLex and sCR1[desLHR-A]sLex indicated an average incorporation of 10 and 8 mol of sLex/mol of glycoprotein, respectively. sLex is a carbohydrate ligand for the selectin adhesion molecules. sCR1sLex was shown to specifically bind CHO cells expressing cell surface E-selectin. sCR1[desLHR-A]sLex inhibited the binding of the monocytic cell line U937 to human aortic endothelial cells, which had been activated with tumor necrosis factor-alpha to up-regulate the expression of E-selectin. sCR1sLex inhibited the binding of U937 cells to surface-adsorbed P-selectin-IgG. sCR1sLex and sCR1[desLHR-A]sLex have thus demonstrated both complement regulatory activity and the capacity to bind selectins and to inhibit selectin-mediated cell adhesion in vitro.  相似文献   
26.
27.
Normal lymphoid tissue development and function depend upon chemokine-directed cell migration. Since chemokines signal through heterotrimeric G-protein-coupled receptors, RGS proteins, which act as GTPase-activating proteins for Galpha subunits, likely fine tune the cellular responses to chemokines. Here we show that Rgs1(-/-) mice possess B cells that respond excessively and desensitize improperly to the chemokines CXCL12 and CXCL13. Many of the B-cell follicles in the spleens of Rgs1(-/-) mice have germinal centers even in the absence of immune stimulation. Furthermore, immunization of these mice leads to exaggerated germinal center formation; partial disruption of the normal architecture of the spleen and Peyer's patches; and abnormal trafficking of immunoglobulin-secreting cells. These results reveal the importance of a regulatory mechanism that limits and desensitizes chemokine receptor signaling.  相似文献   
28.
The oncoprotein Ski represses transforming growth factor (TGF)-beta signaling in an N-CoR-independent manner. However, the molecular mechanism(s) underlying this event has not been elucidated. Here, we identify an additional domain in Ski that mediates interaction with Smad3 which is important for this repression. This domain is distinct from the previously reported N-terminal Smad3 binding domain in Ski. Individual alanine substitution of several residues in the domain significantly affected Ski-Smad3 interaction. Furthermore, combined mutations within this domain, together with those in the previously identified Smad3 binding domain, can completely abolish the interaction of Ski with Smad3, while mutation in each domain alone retained partial interaction. By introducing those mutations that abolish direct interaction with Smad3 or Smad4 individually, or in combination, we show that interaction of Ski with either Smad3 or Smad4 is sufficient for Ski-mediated repression of TGF-beta signaling. Furthermore our results clearly demonstrate that Ski does not disrupt Smad3-Smad4 heteromer formation, and recruitment of Ski to the Smad3/4 complex through binding to either Smad3 or Smad4 is both necessary and sufficient for repression.  相似文献   
29.
Immunostimulatory saponin based colloidal antigen delivery systems show promise as adjuvants for subunit vaccines. For this reason, allyl oleanolate was glycosylated at the 3-position using trichloroacetimidate donors to give monodesmodic saponins following deprotection. Bisdesmodic saponins were synthesized by double glycosylation at the 3- and 28-positions of oleanolic acid. When formulated together with cholesterol and phospholipids, ring-like, helical and rod-like nanostructures were formed depending on the saponin concentrations used. As an indication of adjuvant activity, the ability of these formulations, and the saponins by themselves, to induce dendritic cell maturation was measured, but no significant activity was observed.  相似文献   
30.

Background

Vitamin D is associated with lung function in cross-sectional studies, and vitamin D inadequacy is hypothesized to play a role in the pathogenesis of chronic obstructive pulmonary disease. Further data are needed to clarify the relation between vitamin D status, genetic variation in vitamin D metabolic genes, and cross-sectional and longitudinal changes in lung function in healthy adults.

Methods

We estimated the association between serum 25-hydroxyvitamin D [25(OH)D] and cross-sectional forced expiratory volume in the first second (FEV1) in Framingham Heart Study (FHS) Offspring and Third Generation participants and the association between serum 25(OH)D and longitudinal change in FEV1 in Third Generation participants using linear mixed-effects models. Using a gene-based approach, we investigated the association between 241 SNPs in 6 select vitamin D metabolic genes in relation to longitudinal change in FEV1 in Offspring participants and pursued replication of these findings in a meta-analyzed set of 4 independent cohorts.

Results

We found a positive cross-sectional association between 25(OH)D and FEV1 in FHS Offspring and Third Generation participants (P = 0.004). There was little or no association between 25(OH)D and longitudinal change in FEV1 in Third Generation participants (P = 0.97). In Offspring participants, the CYP2R1 gene, hypothesized to influence usual serum 25(OH)D status, was associated with longitudinal change in FEV1 (gene-based P < 0.05). The most significantly associated SNP from CYP2R1 had a consistent direction of association with FEV1 in the meta-analyzed set of replication cohorts, but the association did not reach statistical significance thresholds (P = 0.09).

Conclusions

Serum 25(OH)D status was associated with cross-sectional FEV1, but not longitudinal change in FEV1. The inconsistent associations may be driven by differences in the groups studied. CYP2R1 demonstrated a gene-based association with longitudinal change in FEV1 and is a promising candidate gene for further studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12931-015-0238-y) contains supplementary material, which is available to authorized users.  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