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   1820篇
  免费   159篇
  国内免费   262篇
  2024年   2篇
  2023年   20篇
  2022年   42篇
  2021年   100篇
  2020年   78篇
  2019年   93篇
  2018年   95篇
  2017年   56篇
  2016年   78篇
  2015年   132篇
  2014年   127篇
  2013年   179篇
  2012年   189篇
  2011年   156篇
  2010年   117篇
  2009年   92篇
  2008年   100篇
  2007年   88篇
  2006年   83篇
  2005年   64篇
  2004年   61篇
  2003年   60篇
  2002年   25篇
  2001年   21篇
  2000年   16篇
  1999年   34篇
  1998年   23篇
  1997年   20篇
  1996年   9篇
  1995年   14篇
  1994年   11篇
  1993年   16篇
  1992年   9篇
  1991年   2篇
  1990年   5篇
  1989年   4篇
  1988年   5篇
  1987年   3篇
  1985年   1篇
  1983年   3篇
  1982年   1篇
  1981年   2篇
  1978年   1篇
  1977年   2篇
  1975年   1篇
  1974年   1篇
排序方式: 共有2241条查询结果,搜索用时 15 毫秒
51.
52.

Objectives

Follicular helper T (Tfh) cells exert an important role in autoimmune diseases. Whether it might be involved in type 1 diabetes (T1D) is unknown. Our aim was to investigate the role of Tfh cells in patients with T1D and the effect of anti-CD20 monoclonal antibody (rituximab) on Tfh cells from T1D patients.

Patients and Methods

Fifty-four patients with T1D and 37 healthy controls were enrolled in the current study. 20 of those patients were treated with rituximab. The frequencies of circulating CD4+CXCR5+ICOS+T cells were analyzed by flow cytometry. The serum autoantibodies were detected by radioligand assay. The levels of IL-21, IL-6 and BCL-6 were assessed using ELISA and/or real-time PCR.

Results

Increased frequencies of circulating Tfh cells together with enhanced expression of IL-21 were detected in patients. The correlation between the frequencies of circulating Tfh cells and the serum autoantibodies or C-peptide level was comfirmed. After rituximab therapy, follow-up analysis demonstrated that the frequencies of circulating Tfh cell and serum IA2A were decreased. The levels of IL-21, IL-6 and Bcl-6 mRNA were decreased after treatment. Furthermore, beta cell function in 10 of 20 patients was improved.

Conclusions

These data indicate Tfh cells may participate in the T1D-relatede immune responses and B cells might play a role in the development of Tfh responses in the disease progression.  相似文献   
53.
目的:设计并合成抗肿瘤药物帕玛度胺的3位N取代的新型类似物。方法:从3-硝基邻苯二甲酸(2)和N-(叔丁氧羰基).L-谷氨酰胺(4)出发,经过六步反应得到目标化合物。3-硝基邻苯二甲酸(2)经脱水制得3-硝基邻苯二甲酸酐(3)。用N-(叔丁氧羰基)-L-谷氨酰胺(4)经闭环、脱保护制得3-氨基-2,6-哌啶二酮三氟乙酸盐(6)。4与6经缩合、钯碳催化氢化制得免疫调节剂Pomalidomide(8),(8)经过酰化得到3-乙酰氨基-N-(2,6-二氧代-3-哌啶基)邻苯二甲酰亚胺(1)。以(4)计总收率约34.9%。结果:得到3-乙酰氨基-N-(2,6-二氧代-3-哌啶基)-邻苯二甲酰亚胺(1),应用于细胞活性测试。结论:改进了帕玛度胺的合成工艺,得到了3位N乙酰化的新型帕玛度胺类似物(1),初步研究显示(1)的生物活性与帕玛度胺接近。  相似文献   
54.
Understanding the electron and phonon transport characteristics is crucial for designing and developing high performance thermoelectric materials. Weak scattering effects on charge carriers, characterized by deformation potential and alloy scattering potential, are favorable for thermoelectric solid solutions to enable high carrier mobility and thereby promising thermoelectric performance. Mg2(Si,Sn) solid solutions have attracted much attention due to their low cost and environmental compatibility. Usually, their high thermoelectric performance with ZT ~ 1 is ascribed to the band convergence and reduced lattice thermal conductivity caused by alloying. In this work, both a low deformation potential Ξ = 13 eV and a low alloy scattering potential U = 0.7 eV are found for the thermoelectric alloys by characterizing and modeling of thermoelectric transport properties. The band convergence is also verified by the increased density‐of‐states effective mass. It is proposed that, in addition to band convergence and reduced lattice thermal conductivity, the low deformation potential and alloy scattering potential are additional intrinsic features that contribute to the high thermoelectric performance of the solid solutions.  相似文献   
55.
以亚硫酸盐甘蔗渣浆酶解液作为原料,利用C. shehatae发酵制取燃料乙醇。结果表明:还原糖最适初始质量浓度为葡萄糖140 g/L、木糖60 g/L、酶解液总糖80 g/L。利用初始葡萄糖55.06 g/L、木糖11.18 g/L、纤维二糖4.51 g/L的亚硫酸盐甘蔗渣浆酶解液发酵,经18 h获得乙醇22.98 g/L。乙醇得率为67.23%,葡萄糖利用率为99.27%,木糖利用率为32.96%,C. shehatae适合作为蔗渣为原料的乙醇发酵菌株。  相似文献   
56.
Klotho was originally characterized as an aging suppressor gene that predisposed Klotho-deficient mice to premature aging-like syndrome. Although Klotho was recently reported to exhibit tumor suppressive properties during various malignant transformations, the functional role and molecular mechanism of Klotho in hepatocarcinogenesis remains poorly understood. In our present study, immunohistochemical Klotho staining levels in a clinical follow-up of 52 hepatoma patients were significantly associated with liver cirrhosis, tumor multiplicity and venous invasion. The overall survival rate of hepatoma patients with high Klotho expression was significantly lower than those patients with low Klotho expression. Moreover, Klotho overexpression increased cellular migration, anchorage-independent growth, and anoikis resistance in hepatoma cells. Klotho overexpression elevated p21-activated kinase 1 (PAK1) expression and shRNA-mediated PAK1 knockdown and kinase activity inhibition with kinase dead mutant PAK1 K299R coexpression or allosteric inhibitor IPA3 treatment reversed anoikis resistance in Klotho-overexpressed hepatoma cells. More importantly, the pivotal significance of upregulated VEGFR2 protein levels mediated by Klotho expression was confirmed by VEGFR2 inhibitor Axitinib and blocking antibody treatment in hepatoma cells. Axitinib treatment sensitized anoikis was reversed by constitutive active mutant PAK1 T423E coexpression in Klotho-overexpressed hepatoma cells. Conversely, knockdown of Klotho reduced VEGFR2/PAK1 dependent anoikis resistance, which could be reversed by PAK1 T423E. These results revealed a novel oncogenic function of Klotho in promoting anoikis resistance via activating VEGFR2/PAK1 signaling, thus facilitating tumor migration and invasion during hepatoma progression, which could provide a putative molecular mechanism for tumor metastasis.  相似文献   
57.

Background

The single-row and double-row fixation techniques have been widely used for rotator cuff tears. However, whether the double-row technique produces superior clinical or anatomic outcomes is still considered controversial. This study aims to use meta-analysis to compare the clinical and anatomical outcomes between the two techniques.

Methods

The Pubmed, Embase, and Cochrane library databases were searched for relevant studies published before November 1, 2012. Studies clearly reporting a comparison of the single-row and double-row techniques were selected. The Constant, ASES, and UCLA scale systems and the rotator cuff integrity rate were evaluated. The weighted mean differences and relative risks were calculated using a fixed-effects or random-effects model.

Results

Eight studies were included in this meta-analysis. The weighted mean differences of the ASES (−0.84; P = 0.04; I2 = 0%) and UCLA (−0.75; P = 0.007; I2 = 0%) scales were significantly low in the single-row group for full-thickness rotator cuff tears. For tear sizes smaller than 3 cm, no significant difference was found between the groups no matter in Constant (P = 0.95; I2 = 0%), ASES (P = 0.77; I2 = 0%), or UCLA (P = 0.24; I2 = 13%) scales. For tear sizes larger than 3 cm, the ASES (−1.95; P = 0.001; I2 = 49%) and UCLA (−1.17; P = 0.006; I2 = 0%) scales were markedly lower in the single-row group. The integrity of the rotator cuff (0.81; P = 0.0004; I2 = 10%) was greater and the partial thickness retear rate (1.93; P = 0.007; I2 = 10%) was less in the double-row group. Full-thickness retears showed no difference between the groups (P = 0.15; I2 = 0%).

Conclusion

The meta-analysis suggests that the double-row fixation technique increases post-operative rotator cuff integrity and improves the clinical outcomes, especially for full-thickness rotator cuff tears larger than 3 cm. For tear sizes smaller than 3 cm, there was no difference in the clinical outcomes between the two techniques.

Level of Evidence

Level I.  相似文献   
58.

Introduction

C-reactive protein (CRP) levels are associated with cardiovascular disease and systemic inflammation. We assessed whether CRP-associated loci were associated with serum CRP and retinal markers of microvascular disease, in Asian populations.

Methods

Genome-wide association analysis (GWAS) for serum CRP was performed in East-Asian Chinese (N = 2,434) and Malays (N = 2,542) and South-Asian Indians (N = 2,538) from Singapore. Leveraging on GWAS data, we assessed, in silico, association levels among the Singaporean datasets for 22 recently identified CRP-associated loci. At loci where directional inconsistencies were observed, quantification of inter-ethnic linkage disequilibrium (LD) difference was determined. Next, we assessed association for a variant at CRP and retinal vessel traits [central retinal artery equivalent (CRAE) and central retinal vein equivalent (CRVE)] in a total of 24,132 subjects of East-Asian, South-Asian and European ancestry.

Results

Serum CRP was associated with SNPs in/near APOE, CRP, HNF1A and LEPR (p-values ≤4.7×10−8) after meta-analysis of Singaporean populations. Using a candidate-SNP approach, we further replicated SNPs at 4 additional loci that had been recently identified to be associated with serum CRP (IL6R, GCKR, IL6 and IL1F10) (p-values ≤0.009), in the Singaporean datasets. SNPs from these 8 loci explained 4.05% of variance in serum CRP. Two SNPs (rs2847281 and rs6901250) were detected to be significant (p-value ≤0.036) but with opposite effect directions in the Singaporean populations as compared to original European studies. At these loci we did not detect significant inter-population LD differences. We further did not observe a significant association between CRP variant and CRVE or CRAE levels after meta-analysis of all Singaporean and European datasets (p-value >0.058).

Conclusions

Common variants associated with serum CRP, first detected in primarily European studies, are also associated with CRP levels in East-Asian and South-Asian populations. We did not find a causal link between CRP and retinal measures of microvascular disease.  相似文献   
59.
60.
【背景】色氨酸脱羧酶在自然界有高度特异性,行使催化色氨酸为色胺的功能。一个色氨酸脱羧酶BaTDC参与南海海绵共生菌Bacillus atrophaeus C89次级代谢产物bacillamides的生物合成过程。【目的】探究BaTDC酶学特征和底物谱,建立体外合成色胺衍生物的方法。【方法】通过构建系统发育树揭示BaTDC在进化中的地位。在温度梯度和pH梯度下进行酶反应,利用不同的色氨酸衍生物为底物,通过HPLC和UPLC-MS检测酶反应过程,表征BaTDC活性。【结果】系统发育分析显示BaTDC与肠道菌Ruminococcus gnavus亲缘关系相近。纯化重组BaTDC的最适温度为40?45 °C,最适pH值为8.0。BaTDC可以催化羟代色氨酸和卤代色氨酸包括4-氟色氨酸和5,6,7-氯色氨酸及4-溴色氨酸,得到相应的卤代色胺衍生物和血清素。【结论】本研究分析了BaTDC的特性,发现BaTDC表现出宽泛的底物耐受性,可为前体喂养或定向生物合成新型药用色胺衍生物和下游复杂天然产物奠定基础。  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