全文获取类型
收费全文 | 3201篇 |
免费 | 231篇 |
国内免费 | 2篇 |
专业分类
3434篇 |
出版年
2023年 | 11篇 |
2022年 | 27篇 |
2021年 | 58篇 |
2020年 | 35篇 |
2019年 | 51篇 |
2018年 | 62篇 |
2017年 | 56篇 |
2016年 | 95篇 |
2015年 | 138篇 |
2014年 | 164篇 |
2013年 | 232篇 |
2012年 | 258篇 |
2011年 | 243篇 |
2010年 | 163篇 |
2009年 | 156篇 |
2008年 | 161篇 |
2007年 | 206篇 |
2006年 | 190篇 |
2005年 | 177篇 |
2004年 | 152篇 |
2003年 | 137篇 |
2002年 | 138篇 |
2001年 | 31篇 |
2000年 | 28篇 |
1999年 | 30篇 |
1998年 | 37篇 |
1997年 | 30篇 |
1996年 | 28篇 |
1995年 | 17篇 |
1994年 | 23篇 |
1993年 | 18篇 |
1992年 | 19篇 |
1991年 | 23篇 |
1990年 | 28篇 |
1989年 | 15篇 |
1988年 | 16篇 |
1987年 | 12篇 |
1986年 | 17篇 |
1985年 | 17篇 |
1984年 | 17篇 |
1983年 | 8篇 |
1982年 | 15篇 |
1981年 | 14篇 |
1980年 | 15篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1972年 | 6篇 |
1969年 | 5篇 |
排序方式: 共有3434条查询结果,搜索用时 15 毫秒
61.
Giner RM Mancini L Kamal AM Perretti M 《Biochemical and biophysical research communications》2007,354(2):414-419
We tested whether glucocorticoids modulated osteoblast expression of the annexin 1 system, including the ligand and two G-coupled receptors termed formyl-peptide receptor (FPR) and FPR-like-1 (FPRL-1). In Saos-2 cells, rapid up-regulation of FPR mRNA upon cell incubation with dexamethasone (0.01-1 microM) was observed, with significant changes as early as 2h and a more marked response at 24h; annexin 1 and FPRL-1 mRNA changes were more subtle. At the protein level, dexamethasone provoked a rapid externalization of annexin 1 (maximal at 2h) followed by delayed time-dependent changes in the cell cytosol. Saos-2 cell surface expression of FPR or FPRL-1 could not be detected, even when dexamethasone was added with the bone modelling cytokines interleukin-6 or interleukin-1. The uneven modulation of the annexin 1 system (mediator and its putative receptors) in osteoblasts might lead to a better understanding of how these complex biochemical pathways become operative in bone. 相似文献
62.
Anaerobic Bioremediation of Groundwater Containing a Mixture of 1,1,2,2-Tetrachloroethane and Chloroethenes 总被引:2,自引:0,他引:2
This study investigated the biotransformation pathways of 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA) in the presence of chloroethenes
(i.e. tetrachloroethene, PCE; trichloroethene, TCE) in anaerobic microcosms constructed with subsurface soil and groundwater
from a contaminated site. When amended with yeast extract, lactate, butyrate, or H2 and acetate, 1,1,2,2-TeCA was initially dechlorinated via both hydrogenolysis to 1,1,2-trichloroethane (1,1,2-TCA) (major
pathway) and dichloroelimination to dichloroethenes (DCEs) (minor pathway), with both reactions occurring under sulfidogenic
conditions. In the presence of only H2, the hydrogenolysis of 1,1,2,2-TeCA to 1,1,2-TCA apparently required the presence of acetate to occur. Once formed, 1,1,2-TCA
was degraded predominantly via dichloroelimination to vinyl chloride (VC). Ultimately, chloroethanes were converted to chloroethenes
(mainly VC and DCEs) which persisted in the microcosms for very long periods along with PCE and TCE originally present in
the groundwater. Hydrogenolysis of chloroethenes occurred only after highly reducing methanogenic conditions were established.
However, substantial conversion to ethene (ETH) was observed only in microcosms amended with yeast extract (200 mg/l), suggesting
that groundwater lacked some nutritional factors which were likely provided to dechlorinating microorganisms by this complex
organic substrate. Bioaugmentation with an H2-utilizing PCE-dechlorinating Dehalococcoides spp. -containing culture resulted in the conversion of 1,1,2,2-TeCA, PCE and TCE to ETH and VC. No chloroethanes accumulated
during degradation suggesting that 1,1,2,2-TeCA was degraded through initial dichloroelimination into DCEs and then typical
hydrogenolysis into ETH and VC. 相似文献
63.
AIDS Vaccination Studies Using an Ex Vivo Feline Immunodeficiency Virus Model: Detailed Analysis of the Humoral Immune Response to a Protective Vaccine 总被引:1,自引:0,他引:1 下载免费PDF全文
64.
Ann H. Klopp Lara Lacerda Anshul Gupta Bisrat G. Debeb Travis Solley Li Li Erika Spaeth Wei Xu Xiaomei Zhang Michael T. Lewis James M. Reuben Savitri Krishnamurthy Mauro Ferrari Rogério Gaspar Thomas A. Buchholz Massimo Cristofanilli Frank Marini Michael Andreeff Wendy A. Woodward 《PloS one》2010,5(8)
Introduction
Normal and malignant breast tissue contains a rare population of multi-potent cells with the capacity to self-renew, referred to as stem cells, or tumor initiating cells (TIC). These cells can be enriched by growth as “mammospheres” in three-dimensional cultures.Objective
We tested the hypothesis that human bone-marrow derived mesenchymal stem cells (MSC), which are known to support tumor growth and metastasis, increase mammosphere formation.Results
We found that MSC increased human mammary epithelial cell (HMEC) mammosphere formation in a dose-dependent manner. A similar increase in sphere formation was seen in human inflammatory (SUM149) and non-inflammatory breast cancer cell lines (MCF-7) but not in primary inflammatory breast cancer cells (MDA-IBC-3). We determined that increased mammosphere formation can be mediated by secreted factors as MSC conditioned media from MSC spheroids significantly increased HMEC, MCF-7 and SUM149 mammosphere formation by 6.4 to 21-fold. Mammospheres grown in MSC conditioned media had lower levels of the cell adhesion protein, E-cadherin, and increased expression of N-cadherin in SUM149 and HMEC cells, characteristic of a pro-invasive mesenchymal phenotype. Co-injection with MSC in vivo resulted in a reduced latency time to develop detectable MCF-7 and MDA-IBC-3 tumors and increased the growth of MDA-IBC-3 tumors. Furthermore, E-cadherin expression was decreased in MDA-IBC-3 xenografts with co-injection of MSC.Conclusions
MSC increase the efficiency of primary mammosphere formation in normal and malignant breast cells and decrease E-cadherin expression, a biologic event associated with breast cancer progression and resistance to therapy. 相似文献65.
66.
Ariel A. Muñoz Jonathan Barichivich Duncan A. Christie Wouter Dorigo David Sauchyn Álvaro González‐Reyes Ricardo Villalba Antonio Lara Natalia Riquelme Mauro E. González 《Austral ecology》2014,39(2):158-169
Araucaria araucana (Araucaria) is a long‐lived conifer growing along a sharp west–east biophysical gradient in the Patagonian Andes. The patterns and climate drivers of Araucaria growth have typically been documented on the driest part of the gradient relying on correlations with meteorological records, but the lack of in situ soil moisture observations has precluded an assessment of the growth responses to soil moisture variability. Here, we use a network of 21 tree‐ring width chronologies to investigate the spatiotemporal patterns of tree growth through the entire gradient and evaluate their linkages with regional climate and satellite‐observed surface soil moisture variability. We found that temporal variations in tree growth are remarkably similar throughout the gradient and largely driven by soil moisture variability. The regional spatiotemporal pattern of tree growth was positively correlated with precipitation (r = 0.35 for January 1920–1974; P < 0.01) and predominantly negatively correlated with temperature (r = ?0.38 for January–March 1920–1974; P < 0.01) during the previous growing season. These correlations suggest a temporally lagged growth response to summer moisture that could be associated with known physiological carry‐over processes in conifers and to a response to moisture variability at deeper layers of the rooting zone. Notably, satellite observations revealed a previously unobserved response of Araucaria growth to summer surface soil moisture during the current rather than the previous growing season (r = 0.65 for 1979–2000; P < 0.05). This new response has a large spatial footprint across the mid‐latitudes of the South American continent (35°–45°S) and highlights the potential of Araucaria tree rings for palaeoclimatic applications. The strong moisture constraint on tree growth revealed by satellite observations suggests that projected summer drying during the coming decades may result in regional growth declines in Araucaria forests and other water‐limited ecosystems in the Patagonian Andes. 相似文献
67.
Zinc nutritional status and its relationships with hyperinsulinemia in obese children and adolescents 总被引:4,自引:0,他引:4
A perturbation of zinc metabolism has been noted in subjects with obesity. The present work intends to investigate whether
the zinc nutritional status is associated with hyperinsulinemia in obesity. A study was carried out in a group of obese children
and adolescents (n=23) and compared to a control group (n=21), both between 7 and 14 yr of age. Software analyzed diet information from 3-d food records. Body composition was evaluated
by body mass index (BMI), bioelectrical impedance, and skinfold measurements. Zinc nutritional status was evaluated by Zn
determination in plasma, erythrocyte, and 24-h urine, by atomic absorption spectrophotometry (λ=213.9 nm). Insulin was measured
by radioimmunoassay (Linco Res). Diets consumed by both groups had marginal concentrations of zinc. Zinc concentrations in
plasma and erythrocytes were significantly lower in the obese group. Urinary zinc excretion and serum insulin were significantly
higher in the same group, although the insulinemia and the parameters of zinc nutritional status were not significantly correlated.
As a result, considering that zinc is part of the synthesis and secretion of this hormone, an assessment is necessary of the
possible participation of the oligoelement in the mechanisms of insulin resistance, commonly present in obese patients. 相似文献
68.
Iron deficiency can have nonhematological manifestations, some of which may affect the gastrointestinal tract. The aim of
this study was to determine if iron-deficiency anemia in growing rats affected small-bowel permeability as assessed by the
urinary ratio of lactulose and mannitol. Thirty-seven male Harlan Sprague-Dawley rats (21 d of age) were randomly divided
into two groups and fed either an iron-deficient (n=19) or an iron-sufficient diet (n=18) that contained either 13.5 or 43.8 mg of iron/kg diet, respectively. Animals were evaluated between 25 and 38 d of dietary
treatment. Intestinal permeability was assessed by measuring the lactulose/mannitol urinary ratio following administration
of a solution that contained the two sugars. At the end of the study, the mean body weight of rats fed the low-iron diet was
approx 95% that of the controls. The mean hemoglobin (g/dL) was significantly lower in the low-iron diet group (11.2±1.4)
than in the control group (16.9±0.8) (p=0.001). The liver iron concentration (μg/g) of the anemic group (41.4±4.7) was also statistically (p=0.001) lower than in the control group (116.6±18.2). The lactulose/mannitol ratio was lower in the anemic rats (2.0±0.7)
than in the control group (2.6±0.7) (p=0.008), a finding that is not suggestive of intestinal mucosal atrophy, previously described in anemic children. 相似文献
69.
70.
Persistence of the effect of frugivore identity on post‐dispersal seed fate: consequences for the assessment of functional redundancy 下载免费PDF全文
Ana Paula Lugon Marion Boutefeu Emilie Bovy Fernando Z. Vaz‐de‐Mello Marie‐Claude Huynen Mauro Galetti Laurence Culot 《Biotropica》2017,49(3):293-302
Large frugivores play an important role as seed dispersers and their extinction may affect plant regeneration. The consequences of such extinctions depend on the likelihood of other species being functionally redundant and on how post‐dispersal events are affected. We assess the functional redundancy of two seed dispersers of the Atlantic Forest, the muriqui (Brachyteles arachnoides) and the tapir (Tapirus terrestris) through the comparison of their seed dispersal quality, taking into account post‐dispersal events. We compare tapirs and muriquis for: (1) the dung beetle community associated with their feces; (2) the seed burial probability and burial depth by dung beetles; and (3) the seed mortality due to predators or other causes according to burial depth. We determine how seed burial affects seed dispersal effectiveness (SDE) and compare the dispersal quality of four plant species dispersed by these frugivores. Muriqui feces attract 16‐fold more dung beetles per gram of fecal matter and seeds experience 10.5‐fold more burial than seeds in tapir feces. In both feces types, seed mortality due to predation decreases with burial depth but seed mortality due to other causes increases. Total seed mortality differ within plant species according to the primary disperser. Therefore, the effect of seed burial on SDE varies according to the plant species, burial depth, and primary disperser. As tapirs and muriquis differently affect the seed fate, they are not functionally redundant. Since the effect of the primary disperser persists into the post‐dispersal events, we should consider the cascading effects of these processes when assessing functional redundancy. 相似文献