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   1280篇
  免费   114篇
  国内免费   55篇
  2023年   7篇
  2022年   8篇
  2021年   28篇
  2020年   19篇
  2019年   29篇
  2018年   47篇
  2017年   49篇
  2016年   57篇
  2015年   36篇
  2014年   60篇
  2013年   72篇
  2012年   94篇
  2011年   47篇
  2010年   59篇
  2009年   56篇
  2008年   36篇
  2007年   37篇
  2006年   37篇
  2005年   25篇
  2004年   38篇
  2003年   47篇
  2002年   57篇
  2001年   62篇
  2000年   52篇
  1999年   19篇
  1998年   15篇
  1997年   15篇
  1996年   34篇
  1995年   20篇
  1994年   12篇
  1993年   10篇
  1992年   11篇
  1991年   14篇
  1990年   10篇
  1989年   10篇
  1988年   7篇
  1987年   18篇
  1986年   7篇
  1985年   13篇
  1984年   9篇
  1983年   6篇
  1982年   10篇
  1981年   14篇
  1979年   8篇
  1977年   6篇
  1974年   5篇
  1973年   14篇
  1957年   5篇
  1935年   5篇
  1918年   8篇
排序方式: 共有1449条查询结果,搜索用时 15 毫秒
131.
目的:回顾性分析成人不典型肺结核的CT表现,探讨与其他同影性疾病的鉴别方法,做到早诊断早治疗,减低病人的治疗成本和思想负担。方法:选择性收集病变不典型。CT影像检壹曾误诊或诊断困难的病例30例做回顾性分析。所有病例均有最后治疗结果或术后病理结果证实。结果:节段性阴影12例,局灶性斑片及斑点状影8例,散在少数小结节样影4例,多发结节并空洞影3例,肺内弥漫分布的粟粒结节影3例,肺内阴影伴有肺门淋巴结及纵隔淋巴结肿大者9例以青壮年病人吸收较快,病程较短,以粟粒影和大片影表现者吸收最快。讨论:对于不典型的成人肺结核,不能片面的强调或突出某一方面征象,年龄因素、临床症状、实验室检查如结核茵素试验(PPD)、痰结核茵培养、抗结核抗体阳性在鉴别诊断中有着重要意义。进一步正确认识及探究病变的影像特点,对于正确的诊断提供了强有力的理论根据。  相似文献   
132.
The effect of glycyrrhetinic acid (GA) and GA-derivatives towards 11β-hydroxysteroid dehydrogenase (11β-HSD) was investigated. Novel compounds with modifications at positions C-3, C-11 and C-29 of the GA skeleton were prepared. Single crystal X-ray diffraction data of selected substances are reported and discussed.  相似文献   
133.
Series of novel 5-arylidene-2-arylaminothiazol-4(5H)-ones and 2-aryl(benzyl)amino-1H-imidazol-4(5H)-ones were synthesized from appropriate 2-alkylthioazol-4-ones using nucleophilic substitution in position 2 by various anilines and benzylamines and Knoevenagel reaction. X-ray structural studies of 22 revealed the structure to be intermediate between amino and imino tautomeric forms. All the target compounds were evaluated for the anticancer activity in vitro in standard National Cancer Institute 60 cancer cell lines assay. Majority of compounds showed significant antitumor cytotoxicity effect at micromolar and submicromolar level (Mean Log GI50 ranges ?5.77 to ?4.35). Some of the most potent compounds, namely 10 and 13, possessed selectively high effect on all leukemia cell lines at submicromolar level (Mean Log GI50 [leukemia lines], respectively, ?6.41 and ?6.29), which are probably associated with immunosuppressive activity. Individual cancer cell lines sensitivity to synthesized compounds and SAR studies are discussed. COMPARE analysis allowed to disclose probable modes of anticancer action for synthesized compounds, in particular showed number of high correlations with activity patterns of alkylating agents (PCC  0.606–0.731).  相似文献   
134.
The host plant expansion of a diamondback moth, Plutella xylostella (L.) (DBM) strain to snowpea (Pisum sativum L.) raised the question whether a specialist parasitoid Diadegma semiclausum (DS) could be conditioned to locate and parasitize its host on the new host plant. In a specialist parasitoid a behavioural change towards a plant outside the normal host plant range of its host due to developmental experience is not expected. The responsive behaviour, parasitism rates and fitness of three subsequent DS generations were investigated on the snowpea-strain of DBM. After three generations of DS on the pea 62.5% of females chose an DBM-infested pea plant over DBM infested cabbage. Only 16.4% of cabbage-reared DS was attracted to infested pea. Rearing of the parasitoid in host larvae on peas significantly increased the number of larvae parasitized on this host plant in the first generation; however, there was no further increase in generations 2 and 3. Larval mortality was similar for all parasitoid/DBM combinations on both host plants, but significantly higher mortality occurred in parasitoid pupae from peas. Development time of the parasitoid was slightly prolonged on the pea strain of DBM. The number of females produced by parasitoids reared on the pea strain of DBM was significantly reduced as compared to D. semiclausum reared on the cabbage strain on both host strains. Results show that DS has the potential to change its responsive behaviour in order to locate its host on a new host plant. According to the current view, a specialist parasitoid is not expected to change its reaction to a plant outside the normal host plant range of its host. Within 3 generations, responsive behaviour towards snowpea could be increased. However, fitness trade-offs, especially an extreme shift in sex ratio to males reduced reproductive success.  相似文献   
135.
Characterization of microbial communities present in a surface petroleum seep in which hydrocarbons have been biodegraded for thousands of years in order to improve the understanding on natural petroleum biodegradation. DNA was extracted from a natural, surface petroleum seep and subjected to culture independent analysis (rRNA gene-based denaturing gradient gel electrophoresis and phylogenetic analysis of clone libraries). Molecular analysis suggested dominance by acidophilic bacteria, especially Alphaproteobacteria (mainly bacteria related to Acidiphilium and Acidocella). Archaea were not detected, but fungi were present. pH of the samples was around 3.5. Acidophilic microbial communities are associated with an acidic petroleum seep. Microbial community structure analysis gives information on the environmental conditions under which petroleum biodegradation occurs. This knowledge could be applied to define conditions for specific cultivation or activity measurements. The activity of acidophilic micro-organisms deserves more attention with respect to their involvement in natural petroleum degradation. This knowledge will contribute to the design of oil bioremediation strategies for polluted acidic settings.  相似文献   
136.
The transmembrane adaptor molecule TRIM is strongly expressed within thymus and in peripheral CD4(+) T cells. Previous studies suggested that TRIM is an integral component of the T-cell receptor (TCR)/CD3 complex and might be involved in regulating TCR cycling. To elucidate the in vivo function of TRIM, we generated TRIM-deficient mice by homologous recombination. TRIM(-/-) mice develop normally and are healthy and fertile. However, the animals show a mild reduction in body weight that appears to be due to a decrease in the size and/or cellularity of many organs. The morphology and anatomy of nonlymphoid as well as primary and secondary lymphoid organs is normal. The frequency of thymocyte and peripheral T-cell subsets does not differ from control littermates. In addition, a detailed analysis of lymphocyte development revealed that TRIM is not required for either positive or negative selection. Although TRIM(-/-) CD4(+) T cells showed an augmented phosphorylation of the serine/threonine kinase Akt, the in vitro characterization of peripheral T cells indicated that proliferation, survival, activation-induced cell death, migration, adhesion, TCR internalization and recycling, TCR-mediated calcium fluxes, tyrosine phosphorylation, and mitogen-activated protein family kinase activation are not affected in the absence of TRIM. Similarly, the in vivo immune response to T-dependent and T-independent antigens as well as the clinical course of experimental autoimmune encephalomyelitis, a complex Th1-mediated autoimmune model, is comparable to that of wild-type animals. Collectively, these results demonstrate that TRIM is dispensable for T-cell development and peripheral immune functions. The lack of an evident phenotype could indicate that TRIM shares redundant functions with other transmembrane adaptors involved in regulating the immune response.  相似文献   
137.
Interleukin-17A (IL-17) promotes inflammatory renal tissue damage in mouse models of crescentic glomerulonephritis, including murine experimental autoimmune anti-myeloperoxidase glomerulonephritis, which most likely depends on IL-17-producing Th17 cells. In human anti-neutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis, however, the cellular sources of IL-17 remain to be elucidated. Therefore, we analyzed human kidney biopsies of active necrotizing and crescentic ANCA-associated glomerulonephritis by immunohistochemistry using an IL-17-specific antibody and by immunofluorescent colocalization with cell type markers. We detected numerous IL-17-expressing (IL-17(+)) cells in the glomeruli and in the tubulointerstitium. Unexpectedly, most of these IL-17(+) cells were polymorphonuclear neutrophilic granulocytes, while IL-17(+) T cells and IL-17(+) mast cells were present at significantly lower frequencies. IL-17 was not detected in other infiltrating or resident kidney cells. In those patients who had not received immunosuppressive treatment before biopsy, serum creatinine levels were positively correlated with tubulointerstitial IL-17(+) neutrophils as well as IL-17(+) T cells. Furthermore, we could demonstrate that purified human blood neutrophils expressed IL-17 protein and released it upon stimulation in vitro. In conclusion, these results support a pathogenic role for IL-17 in human ANCA-associated glomerulonephritis. Our data suggest that in the acute stage of the disease neutrophils may act as an important immediate-early innate source of IL-17 and may thereby initiate and promote ongoing renal inflammation. IL-17 may thus be a target for treating acute ANCA-associated glomerulonephritis.  相似文献   
138.
Chain binomial models and binomial autoregressive processes   总被引:2,自引:0,他引:2  
Weiß CH  Pollett PK 《Biometrics》2012,68(3):815-824
Summary We establish a connection between a class of chain‐binomial models of use in ecology and epidemiology and binomial autoregressive (AR) processes. New results are obtained for the latter, including expressions for the lag‐ conditional distribution and related quantities. We focus on two types of chain‐binomial model, extinction–colonization and colonization–extinction models, and present two approaches to parameter estimation. The asymptotic distributions of the resulting estimators are studied, as well as their finite‐sample performance, and we give an application to real data. A connection is made with standard AR models, which also has implications for parameter estimation.  相似文献   
139.
'Locked nucleic acids' (LNAs) are sugar modified nucleic acids containing the 2'-O-4'C-methylene-β-D-ribofuranoses. The substitution of RNAs with LNAs leads to an enhanced thermostability. Aptamers are nucleic acids, which are selected for specific target binding from a large library pool by the 'SELEX' method. Introduction of modified nucleic acids into aptamers can improve their stability. The stem region of a ricin A chain RNA aptamer was substituted by locked nucleic acids. Different constructs of the LNA-substituted aptamers were examined for their thermostability, binding activity, folding and RNase sensitivity as compared to the natural RNA counterpart. The LNA-modified aptamers were active in target binding, while the loop regions and the adjacent stem nucleotides remained unsubstituted. The thermostability and RNase resistance of LNA substituted aptamers were enhanced as compared to the native RNA aptamer. This study supports the approach to substitute the aptamer stem region by LNAs and to leave the loop region unmodified, which is responsible for ligand binding. Thus, LNAs possess an encouraging potential for the development of new stabilized nucleic acids and will promote future diagnostic and therapeutic applications.  相似文献   
140.
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