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   1495篇
  免费   92篇
  2021年   7篇
  2020年   9篇
  2019年   10篇
  2018年   14篇
  2017年   13篇
  2016年   28篇
  2015年   34篇
  2014年   51篇
  2013年   96篇
  2012年   110篇
  2011年   97篇
  2010年   65篇
  2009年   81篇
  2008年   97篇
  2007年   88篇
  2006年   89篇
  2005年   89篇
  2004年   87篇
  2003年   83篇
  2002年   91篇
  2001年   9篇
  2000年   13篇
  1999年   20篇
  1998年   25篇
  1997年   16篇
  1996年   19篇
  1995年   15篇
  1994年   15篇
  1993年   13篇
  1992年   19篇
  1990年   11篇
  1989年   6篇
  1988年   7篇
  1987年   10篇
  1985年   8篇
  1984年   13篇
  1983年   14篇
  1982年   10篇
  1981年   7篇
  1980年   11篇
  1978年   6篇
  1977年   7篇
  1976年   5篇
  1975年   8篇
  1974年   5篇
  1972年   5篇
  1971年   4篇
  1968年   4篇
  1963年   5篇
  1962年   4篇
排序方式: 共有1587条查询结果,搜索用时 31 毫秒
51.
The hitherto unknown larva of Rhyacophila ferox Graf, 2006, is described and discussed in the context of contemporary Rhyacophilidae keys. In addition, zoogeographical and ecological notes are included.  相似文献   
52.
53.
Annitella apfelbecki is one of three Annitella species with distribution ranges restricted to the Balkan Peninsula. In this paper, we describe the hitherto unknown female of A. apfelbecki and give the most important morphological features to enable its identification and separation from the other Annitella females. Additionally, we provide new data on distribution and discuss zoogeography, life cycle and ecology of this species.  相似文献   
54.
En el curso de las investigaciones sobre la fauna de las aguas subterráneas límnicas de la Argentina, se encontraron en pocas muestras una sorprendente cantidad de especies de crustáceos.

Una de las especies (Leptobathynella richerti) fué descubierta poco antes en Paraguay y 3 especies (Parastygocaris goerssi, P. andina e Ingolfiella uspallatae) ya han sido publicadas por el autor. Aqui se describen, se ilustran y se discuten 2 especies del género Parabathynella, 1 Nitocrella y 12 Parastenocaris como nuevas para la ciencia. Se completa la descripción de Leptobathynella richerti. Por falta de material, otras 5 especies aún no se han podido determinar.

Por consiguiente, la fauna de las aguas subterráneas neotropicales prueba ser abun‐dante y representada por mudias especies también en la Argentina. Un análisis biogeo‐gráfico del material aparecerá en otra oportunidad con relaciones más ámplias.

Las investigaciones serán continuadas.  相似文献   
55.
As the Lyme disease spirochaete Borrelia burgdorferi shuttles back and forth between arthropod vector and vertebrate host, it encounters vastly different and hostile environments. Major mechanisms contributing to the success of this pathogen throughout this complex transmission cycle are phase and antigenic variation of abundant and serotype‐defining surface lipoproteins. These peripherally membrane‐anchored virulence factors mediate niche‐specific interactions with vector/host factors and protect the spirochaete from the perils of the mammalian immune response. In this issue of Molecular Microbiology, Tilly, Bestor and Rosa redefine the roles of two lipoproteins, OspC and VlsE, during mammalian infection. Using a variety of promoter fusions in combination with a sensitive in vivo ‘use it or lose it’ gene complementation assay, the authors demonstrate that proper sequential expression of OspC followed by VlsE indeed matters. A previously suggested general functional redundancy between these and other lipoproteins is shown to be limited and dependent on an immunodeficient experimental setting that is arguably of diminished ecological relevance. These data reinforce the notion that OspC plays a unique role during initial infection while the antigenically variant VlsE proteins allow for persistence in the mammalian host.  相似文献   
56.
Methionine, folic acid, betaine and choline interact in the one-carbon metabolism which provides methyl groups for methylation reactions. An optimal intake of these nutrients during pregnancy is required for successful completion of fetal development and evidence is growing that they could be involved in metabolic long-term programming. However, the biological pathways involved in the action of these nutrients are still poorly known. This study investigated the interaction between methyl donors and protein content in maternal diet during the preconceptual, pregnancy and lactation periods and the consequences on the rat offspring in the short and long term. Methyl donor supplementation reduced leptin secretion in offspring, whereas insulin levels were mostly affected by protein restriction. The joint effect of protein restriction and methyl donor excess strongly impaired postnatal growth in both gender and long term weight gain in male offspring only, without affecting food intake. In addition, rats born from protein restricted and methyl donor supplemented dams gained less weight when fed a hypercaloric diet. Methylation of the leptin gene promoter in adipose tissue was increased in methyl donor supplemented groups but not affected by protein restriction only. These results suggest that maternal methyl donor supplementation may influence energy homeostasis in a gender-dependent manner, without affecting food intake. Moreover, we showed that macronutrients and micronutrients in maternal diet interact to influence the programming of the offspring.  相似文献   
57.
Several markers identify cancer stem cell-like populations, but little is known about the functional roles of stem cell surface receptors in tumor progression. Here, we show that the endothelial protein C receptor (EPCR), a stem cell marker in hematopoietic, neuronal and epithelial cells, is crucial for breast cancer growth in the orthotopic microenvironment of the mammary gland. Mice with a hypomorphic allele of EPCR show reduced tumor growth in the PyMT-model of spontaneous breast cancer development and deletion of EPCR in established PyMT tumor cells significantly attenuates transplanted tumor take and growth. We find expansion of EPCR+ cancer stem cell-like populations in aggressive, mammary fat pad-enhanced human triple negative breast cancer cells. In this model, EPCR-expressing cells have markedly increased mammosphere- and tumor-cell initiating activity compared to another stable progenitor-like subpopulation present at comparable frequency. We show that receptor blocking antibodies to EPCR specifically attenuate in vivo tumor growth initiated by either EPCR+ cells or the heterogenous mixture of EPCR+ and EPCR- cells. Furthermore, we have identified tumor associated macrophages as a major source for recognized ligands of EPCR, suggesting a novel mechanism by which cancer stem cell-like populations are regulated by innate immune cells in the tumor microenvironment.  相似文献   
58.
59.
Each year businesses, governments, and homeowners in the United States invest around one fifth of gross domestic product into the creation of capital assets such as buildings, machinery, and software to enable production and consumption. Use of capital is typically included to some extent in environmental life cycle assessments of goods and services but is not incorporated into most environmentally extended input‐output (EEIO) models, including the US Environmental Protection Agency's USEEIO. Capital assets are typically created in years prior to their use, so a challenge lies in distributing the impacts of their creation over time. In this work, a highly detailed capital flow matrix approach is followed to distribute the use of fixed capital assets to consuming industries. Data from the US Bureau of Economic Analysis's Fixed Asset Accounts is merged with its Industry Accounts data by the creation of concordance tables. Public highways and streets are partially reallocated to industries operating vehicles. The resulting capital use matrix is later combined into a modified USEEIO. “Housing” is found to be the largest consumer of fixed assets, followed by general government, fossil fuel extraction, and financial industries involved in leasing. Construction, vehicles, and machinery are mostly used by industries in the form of fixed assets. The types of fixed assets used by industries are consistent with expectations: housing is dominated by structures, transport by equipment, and information industries by intellectual property products.  相似文献   
60.
Why is it hard to divide attention between dissimilar activities, such as reading and listening to a conversation? We used functional magnetic resonance imaging (fMRI) to study interference between simple auditory and visual decisions, independently of motor competition. Overlapping activity for auditory and visual tasks performed in isolation was found in lateral prefrontal regions, middle temporal cortex and parietal cortex. When the visual stimulus occurred during the processing of the tone, its activation in prefrontal and middle temporal cortex was suppressed. Additionally, reduced activity was seen in modality-specific visual cortex. These results paralleled impaired awareness of the visual event. Even without competing motor responses, a simple auditory decision interferes with visual processing on different neural levels, including prefrontal cortex, middle temporal cortex and visual regions.  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