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   93046篇
  免费   265篇
  国内免费   887篇
  94198篇
  2021年   3篇
  2019年   1篇
  2018年   11840篇
  2017年   10670篇
  2016年   7449篇
  2015年   581篇
  2014年   276篇
  2013年   288篇
  2012年   4197篇
  2011年   12807篇
  2010年   11993篇
  2009年   8219篇
  2008年   9770篇
  2007年   11352篇
  2006年   249篇
  2005年   503篇
  2004年   963篇
  2003年   1013篇
  2002年   772篇
  2001年   255篇
  2000年   158篇
  1999年   23篇
  1998年   9篇
  1997年   23篇
  1996年   10篇
  1994年   7篇
  1993年   29篇
  1992年   21篇
  1991年   37篇
  1990年   7篇
  1989年   8篇
  1988年   17篇
  1987年   13篇
  1984年   7篇
  1983年   18篇
  1982年   2篇
  1980年   1篇
  1979年   1篇
  1978年   2篇
  1975年   7篇
  1974年   2篇
  1973年   2篇
  1972年   247篇
  1971年   274篇
  1970年   5篇
  1965年   13篇
  1962年   24篇
  1956年   5篇
  1944年   12篇
  1940年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
861.
862.

Background

Video-assisted thoracic surgery (VATS) plays an important role in thoracic surgery because it is less invasive. However, the existence of severe pleural adhesions may make VATS difficult and complicated. The aim of this study was to assess the utility of inspiration and expiration computed tomography (respiratory dynamic CT (RD-CT)) in evaluation of pleural adhesions preoperatively.

Methods

RD-CT was performed on 107 patients undergoing thoracotomies (both VATS and open). We assessed synchronous motion during respiration on RD-CT. Comparing the results of RD-CT and intraoperative findings, we assessed the utility of preoperative evaluation.

Results

A negative correlation between sliding score and adhesion grade was revealed. Sliding score in adhesion negative patients was significantly higher than that in adhesion positive patients (P?<?0.0001). The sensitivity of RD-CT was 63.6%, specificity was 74.1%, and accuracy was 72%. Among 62 patients with a CT-Respiration Ratio of less than 0.65, the sensitivity of RD-CT was 77.8%, specificity was 86.8%, and accuracy was 85.5%.

Conclusions

RD-CT may be clinically useful for detecting the presence of pleural adhesions. It can be adopted as one of the criteria for deciding the surgical approach.
  相似文献   
863.

Background

Spontaneous isolated dissection of the superior mesenteric artery (SMA) can lead to bowel ischemia, aneurysm rupture, or even death. Studies have suggested that mechanical or hemodynamic stress on the vascular wall of the SMA may be a contributor, but its pathogenesis is unclear.

Case presentation

A 57-year-old Japanese man with a history of untreated hypertension and hyperuricemia was admitted to our hospital with the sudden onset of severe epigastric pain. Laboratory findings showed elevated white blood cell count and C-reactive protein, and contrast-enhanced computed tomography (CT) of the abdomen demonstrated arterial dissection with luminal stenosis and aneurysm formation at the distal portion of the SMA after the branching of the jejunal artery, and intravenous nicardipine was administered. The patient’s epigastric pain resolved spontaneously but recurred on day 6 of his hospital stay. Contrast-enhanced abdominal CT revealed an enlarged aneurysm with wall thinning. Because of the risk of aneurysm rupture, the decision was made to perform aneurysmectomy and bowel resection on day 6. Histologic examinations revealed two separate dissecting lesions: one latent and the other resulting in aneurysm formation. Both lesions showed characteristics of segmental arterial mediolysis (SAM) with lack of arterial media, absence of internal and external elastic laminae and intimal proliferation.

Conclusions

Histologic findings in the present case suggest that mechanical or hemodynamic stress on the vascular wall and SAM-related vascular vulnerability may concomitantly contribute to the onset of isolated SMA dissection.
  相似文献   
864.

Background

EPH (erythropoietin-producing hepatocellular) receptors are clinically relevant targets in several malignancies. This report describes the effects of GLPG1790, a new potent pan-EPH inhibitor, in human embryonal rhabdomyosarcoma (ERMS) cell lines.

Methods

EPH-A2 and Ephrin-A1 mRNA expression was quantified by real-time PCR in 14 ERMS tumour samples and in normal skeletal muscle (NSM). GLPG1790 effects were tested in RD and TE671 cell lines, two in vitro models of ERMS, by performing flow cytometry analysis, Western blotting and immunofluorescence experiments. RNA interfering experiments were performed to assess the role of specific EPH receptors. Radiations were delivered using an x-6 MV photon linear accelerator. GLPG1790 (30 mg/kg) in vivo activity alone or in combination with irradiation (2 Gy) was determined in murine xenografts.

Results

Our study showed, for the first time, a significant upregulation of EPH-A2 receptor and Ephrin-A1 ligand in ERMS primary biopsies in comparison to NSM. GLPG1790 in vitro induced G1-growth arrest as demonstrated by Rb, Cyclin A and Cyclin B1 decrease, as well as by p21 and p27 increment. GLPG1790 reduced migratory capacity and clonogenic potential of ERMS cells, prevented rhabdosphere formation and downregulated CD133, CXCR4 and Nanog stem cell markers. Drug treatment committed ERMS cells towards skeletal muscle differentiation by inducing a myogenic-like phenotype and increasing MYOD1, Myogenin and MyHC levels. Furthermore, GLPG1790 significantly radiosensitized ERMS cells by impairing the DNA double-strand break repair pathway. Silencing of both EPH-A2 and EPH-B2, two receptors preferentially targeted by GLPG1790, closely matched the effects of the EPH pharmacological inhibition. GLPG1790 and radiation combined treatments reduced tumour mass by 83% in mouse TE671 xenografts.

Conclusions

Taken together, our data suggest that altered EPH signalling plays a key role in ERMS development and that its pharmacological inhibition might represent a potential therapeutic strategy to impair stemness and to rescue myogenic program in ERMS cells.
  相似文献   
865.
Cryptolaemus montrouzieri Mulsant (Coleoptera: Coccinellidae) is a ladybird native to Australia, preying on mealybugs and soft scales, and has been utilized worldwide as a biological control agent. It has long been recognized that C. montrouzieri that was introduced into the main island of Japan had failed to become established. The present study monitored yearly and seasonal occurrence of C. montrouzieri adults in citrus groves at Shizuoka Prefectural Fruit Tree Research Center in Shizuoka City, central Japan in 2008–2012 by using sticky traps and beating citrus trees. Adults of C. montrouzieri were continuously captured for 5 and 4 years in a pesticide-free citrus grove and a neighboring reduced-pesticide grove, respectively. Larvae of C. montrouzieri were observed consuming a cottony scale, Pulvinaria aurantii Cockerell, on citrus trees. These results provide unequivocal evidence for the ladybird’s establishment in central Japan. The number of trapped ladybird adults exhibited four peaks a year: in mid-April, early to late June, mid-August, and late September to early October. Adult numbers in each grove varied largely across years, showing a great increase followed by a rapid decline during a period of 4 years. Factors affecting the seasonal/yearly occurrence of C. montrouzieri adults in citrus groves are discussed.  相似文献   
866.
Dragonflies are colorful insects, and recent RNA sequencing studies have identified a number of candidate genes potentially involved in their color pattern formation and color vision. However, functional aspects of such genes have not been assessed due to the lack of molecular genetic tools applicable to dragonflies. We established an electroporation-mediated RNA interference (RNAi) procedure using the tiny dragonfly Nannophya pygmaea Rambur, 1842 (Odonata: Libellulidae) that targets the multicopper oxidase 2 gene (MCO2; also known as laccase2 gene) responsible for cuticular pigmentation in many insects. RNA sequencing of N. pygmaea and genomic survey of the dragonfly Ladona fulva identified four multicopper oxidase family genes: MCO1, MCO2, MCO3 and multicopper oxidase-related protein gene (MCORP). In N. pygmaea, MCO2 was specifically expressed around the cuticular pigmentation period, whereas MCO1 was constantly expressed. MCORP was expressed at adult stages, and MCO3 was scarcely expressed. When we applied in vivo electroporation, final instar larvae injected with MCO2 small interfering RNA became adults with patchy unpigmented regions. RNAi without in vivo electroporation did not affect cuticular pigmentation, suggesting that dragonflies do not show a systemic RNAi response. These results indicate that MCO2 is required for cuticular pigmentation across diverse insects, and highlight the usefulness of the electroporation-mediated RNAi method in dragonflies.  相似文献   
867.
Extraction of DNA from non-invasive samples (feces) has been used increasingly in genetic research on wildlife. For effective and reliable genetic analyses, knowledge about which samples should be selected in the field is essential. For this reason, we examined the process of DNA degradation in feces of deer. We collected fresh fecal pellets from three wild deer living in a warm temperate forest. We then assessed the effects of time (3, 5, and 10 days) under three environmental conditions (on the forest floor, on exposed ground, and inside the laboratory) on the rates of correct genotyping (CG), amplification failure (NA), genotyping error among positive amplification (ER), false alleles (FA), and allelic dropout (AD) of 15 microsatellite loci. The rate of CG significantly decreased, and those of NA and FA increased with increasing lapse of time. Rates of CG tended to be highest and those of NA, ER, FA, and AD to be lowest in feces kept inside, followed by those on the forest floor. Suitability of samples for DNA extraction was lowest in fecal pellets left on exposed ground, and we suspect that rain may hasten DNA degradation. NA rate could serve as a reliable indicator of the quality of fecal pellets because it was significantly positively correlated with ER rate. For efficient genetic analyses using deer feces in warm temperate zones, we recommend collecting fecal pellets within 3 days of defecation, during periods without rainfall and from under the cover of trees.  相似文献   
868.
Picrorhiza genus is emerging as an important paradigm for herbal drug formulations due to its versatile iridoid glycosides exhibition and robustness in the treatment of diverse infections including hepatic amoebiasis, cancer, malaria, ulcerative colitis and cerebral ischemia reperfusion injury. Owing to the superiority of these bioactivities, iridoid glycosides from Picrorhiza have become a hot research area over the years. A metabolic pathway for the formation of iridoid glycosides has been proposed. However, some enzymes and genes of this route are still unidentified and demand the enumeration of facilitating pathways contributing to the biosynthesis of iridoid glycosides. This review summarizes the current knowledge of all naturally occurring iridoid glycosides from Picrorhiza, their biosynthesis and pharmacological capabilities which could provide the insight into metabolic regulation and the basis for the development of new drugs.  相似文献   
869.
In this study, we evaluated the damage caused in the field by Tibraca limbativentris Stål adults at different levels of infestation (0, 1, 2, and 4 stink bugs) per three rice plants during three growth stages (V8, V13, and R4 stages) of upland rice cultivated in southwestern of Pará State, Amazon Rainforest region, Brazil. Heading time (panicle exertion) was affected by T. limbativentris infestations mainly in the vegetative stage and the whiteheads percentage in treatments ranged from 18.2 to 38%. The dead hearts percentages varied between 0 and 21.5%, and the mean number of primary branches (ramifications) ranged from 5.9?±?0.4 to 12.3?±?0.2. The number of empty spikelets was only affected in infestations with four insects/three plants, while the quantity of filled grains per panicle was affected only when infestations occurred during the vegetative stage. The total number of spikelets (filled?+?empty) per panicle decreased significantly in all phenological stages, and the percentage of damage ranged from 17 to 44% among treatments. Based on the proportion of damage observed, we suggest doubling the number of insects presently used as action threshold to 2 and 4 stink bugs per 15 stalks sampled for the vegetative stage, and of 1 or 2 stink bugs per 15 stalks sampled at the beginning of reproductive stage (R3/R4). Also, the field should be monitored during the entire vegetative stage, since most damage was observed in this phenological stage.  相似文献   
870.
Non-aquatic reproductive modes have evolved among frogs possibly favored by some advantages such as the avoidance of aquatic predators. These reproductive modes, however, make the egg clutches susceptible to terrestrial predators, among which Diptera larvae are some of the most harmful. The present work reports the predation by phorid flies of 22 egg clutches of Phyllomedusa iheringii Boulenger in the South of Brazil. Phorid specimens were identified as Megaselia bruchiana (Borgmeier & Schmitz) and Megaselia necrophaga (Enderlein), species that were reported previously to be associated with ants and dead beetles, respectively. Frog-feeding in these species is hypothesized to be use of an alternative seasonal food source. We amend the diagnoses of both Megaselia species and provide new illustrations to facilitate their identification. We also describe the male of M. bruchiana for the first time and associate males with females of both species.  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