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   1103篇
  免费   92篇
  2022年   5篇
  2021年   26篇
  2020年   14篇
  2019年   20篇
  2018年   22篇
  2017年   18篇
  2016年   26篇
  2015年   53篇
  2014年   54篇
  2013年   60篇
  2012年   81篇
  2011年   85篇
  2010年   42篇
  2009年   32篇
  2008年   56篇
  2007年   70篇
  2006年   55篇
  2005年   45篇
  2004年   65篇
  2003年   31篇
  2002年   38篇
  2001年   20篇
  2000年   18篇
  1999年   13篇
  1998年   18篇
  1997年   11篇
  1996年   7篇
  1994年   6篇
  1993年   8篇
  1992年   16篇
  1991年   14篇
  1990年   10篇
  1989年   10篇
  1988年   6篇
  1987年   5篇
  1986年   8篇
  1985年   7篇
  1984年   5篇
  1983年   11篇
  1981年   6篇
  1980年   5篇
  1979年   7篇
  1978年   5篇
  1977年   4篇
  1976年   4篇
  1972年   7篇
  1971年   4篇
  1970年   4篇
  1969年   8篇
  1968年   4篇
排序方式: 共有1195条查询结果,搜索用时 15 毫秒
91.
In recent years, an increasing number of surveys have definitively confirmed the seasonal presence of fin whales (Balaenoptera physalus) in highly productive regions of the Mediterranean Sea. Despite this, very little is yet known about the routes that the species seasonally follows within the Mediterranean basin and, particularly, in the Ionian area. The present study assesses for the first time fin whale acoustic presence offshore Eastern Sicily (Ionian Sea), throughout the processing of about 10 months of continuous acoustic monitoring. The recording of fin whale vocalizations was made possible by the cabled deep-sea multidisciplinary observatory, “NEMO-SN1”, deployed 25 km off the Catania harbor at a depth of about 2,100 meters. NEMO-SN1 is an operational node of the European Multidisciplinary Seafloor and water-column Observatory (EMSO) Research Infrastructure. The observatory was equipped with a low-frequency hydrophone (bandwidth: 0.05 Hz–1 kHz, sampling rate: 2 kHz) which continuously acquired data from July 2012 to May 2013. About 7,200 hours of acoustic data were analyzed by means of spectrogram display. Calls with the typical structure and patterns associated to the Mediterranean fin whale population were identified and monitored in the area for the first time. Furthermore, a background noise analysis within the fin whale communication frequency band (17.9–22.5 Hz) was conducted to investigate possible detection-masking effects. The study confirms the hypothesis that fin whales are present in the Ionian Sea throughout all seasons, with peaks in call detection rate during spring and summer months. The analysis also demonstrates that calls were more frequently detected in low background noise conditions. Further analysis will be performed to understand whether observed levels of noise limit the acoustic detection of the fin whales vocalizations, or whether the animals vocalize less in the presence of high background noise.  相似文献   
92.
93.
94.
Mutations in the sulfate transporter gene, SCL26A2, lead to cartilage proteoglycan undersulfation resulting in chondrodysplasia in humans; the phenotype is mirrored in the diastrophic dysplasia (dtd) mouse. It remains unclear whether bone shortening and deformities are caused solely by changes in the cartilage matrix, or whether chondroitin sulfate proteoglycan undersulfation affects also signalling pathways involved in cell proliferation and differentiation. Therefore we studied macromolecular sulfation in the different zones of the dtd mouse growth plate and these data were related to growth plate histomorphometry and proliferation analysis.A 2-fold increase of non-sulfated disaccharide in dtd animals compared to wild-type littermates in the resting, proliferative and hypertrophic zones was detected indicating proteoglycan undersulfation; among the three zones the highest level of undersulfation was in the resting zone. The relative height of the hypertrophic zone and the average number of cells per column in the proliferative and hypertrophic zones were significantly reduced compared to wild-types; however the total height of the growth plate was within normal values. The chondrocyte proliferation rate, measured by bromodeoxyuridine labelling, was also significantly reduced in mutant mice. Immunohistochemistry combined with expression data of the dtd growth plate demonstrated that the sulfation defect alters the distribution pattern, but not expression, of Indian hedgehog, a long range morphogen required for chondrocyte proliferation and differentiation.These data suggest that in dtd mice proteoglycan undersulfation causes reduced chondrocyte proliferation in the proliferative zone via the Indian hedgehog pathway, therefore contributing to reduced long bone growth.  相似文献   
95.
In a continuing study of hybrid compounds containing the α-bromoacryloyl moiety as potential anticancer drugs, we synthesized a novel series of hybrids 4ah, in which this moiety was linked to a 1,5-diaryl-1,4-pentadien-3-one system. Many of the conjugates prepared (4b, 4c, 4e and 4g) demonstrated pronounced, submicromolar antiproliferative activity against four cancer cell lines. Moreover, compound 4b induced apoptosis through the mitochondrial pathway and activated caspase-3 in a concentration-dependent manner.  相似文献   
96.
We propose a computationally efficient, bio-mechanically relevant soft-tissue simulation method for cranio-maxillofacial (CMF) surgery. Special emphasis is given to comply with the current clinical workflow. A template-based facial muscle prediction was introduced to avoid laborious segmentation from medical images. In addition, transversely isotropic mass-tensor model (MTM) was applied to realize the directional behavior of facial muscles in short computation time. Finally, sliding contact was incorporated to mimic realistic boundary condition in error-sensitive regions. Mechanical simulation result was compared with commercial finite element software. And retrospective validation study with post-operative scan of four CMF cases was performed.  相似文献   
97.
Selected biological effects of 1,4-naphthoquinone, menadione (2-methyl-1,4-naphthoquinone) and structurally related quinones from natural sources - the 5-hydroxy-naphthoquinones juglone, plumbagin and the 2-hydroxy-naphthoquinones lawsone and lapachol - were studied in human keratinocytes (HaCaT). 1,4-naphthoquinone and menadione as well as juglone and plumbagin were highly cytotoxic, strongly induced reactive oxygen species (ROS) formation and depleted cellular glutathione. Moreover, they induced oxidative DNA base damage and accumulation of DNA strand breaks, as demonstrated in an alkaline DNA unwinding assay. Neither lawsone nor lapachol (up to 100 μM) were active in any of these assays. Cytotoxic and oxidative action was paralleled by stimulation of stress signaling: all tested quinones except lawsone and lapachol strongly induced phosphorylation of the epidermal growth factor receptor (EGFR) and the related ErbB2 receptor tyrosine kinase. EGFR activation by plumbagin, juglone and menadione was attenuated by a superoxide dismutase mimetic, indicating that ROS-related mechanisms contribute to EGFR activation by these naphthoquinones.  相似文献   
98.
The endosomal pathway in neuronal dendrites is essential for membrane receptor trafficking and proper synaptic function and plasticity. However, the molecular mechanisms that organize specific endocytic trafficking routes are poorly understood. Here, we identify GRIP-associated protein-1 (GRASP-1) as a neuron-specific effector of Rab4 and key component of the molecular machinery that coordinates recycling endosome maturation in dendrites. We show that GRASP-1 is necessary for AMPA receptor recycling, maintenance of spine morphology, and synaptic plasticity. At the molecular level, GRASP-1 segregates Rab4 from EEA1/Neep21/Rab5-positive early endosomal membranes and coordinates the coupling to Rab11-labelled recycling endosomes by interacting with the endosomal SNARE syntaxin 13. We propose that GRASP-1 connects early and late recycling endosomal compartments by forming a molecular bridge between Rab-specific membrane domains and the endosomal SNARE machinery. The data uncover a new mechanism to achieve specificity and directionality in neuronal membrane receptor trafficking.  相似文献   
99.
To gain greater insight into the mechanism of dormancy release in the potato tuber, an investigation into physiological and biochemical changes in tuber and bud tissues during the transition from bud dormancy (immediately after harvest) to active bud growth was undertaken. Within the tuber, a rapid shift from storage metabolism (starch synthesis) to reserve mobilization within days of detachment from the mother plant suggested transition from sink to source. Over the same period, a shift in the pattern of [U-(14)C]sucrose uptake by tuber discs from diffuse to punctate accumulation was consistent with a transition from phloem unloading to phloem loading within the tuber parenchyma. There were no gross differences in metabolic capacity between resting and actively growing tuber buds as determined by [U-(14)C]glucose labelling. However, marked differences in metabolite pools were observed with large increases in starch and sucrose, and the accumulation of several organic acids in growing buds. Carboxyfluorescein labelling of tubers clearly demonstrated strong symplastic connection in actively growing buds and symplastic isolation in resting buds. It is proposed that potato tubers rapidly undergo metabolic transitions consistent with bud outgrowth; however, growth is initially prevented by substrate limitation mediated via symplastic isolation.  相似文献   
100.
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