全文获取类型
收费全文 | 324篇 |
免费 | 19篇 |
专业分类
343篇 |
出版年
2023年 | 3篇 |
2019年 | 3篇 |
2017年 | 4篇 |
2016年 | 8篇 |
2015年 | 14篇 |
2014年 | 10篇 |
2013年 | 10篇 |
2012年 | 9篇 |
2011年 | 14篇 |
2010年 | 8篇 |
2009年 | 8篇 |
2008年 | 30篇 |
2007年 | 12篇 |
2006年 | 12篇 |
2005年 | 12篇 |
2004年 | 11篇 |
2003年 | 6篇 |
2002年 | 3篇 |
2001年 | 6篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1998年 | 7篇 |
1995年 | 4篇 |
1992年 | 5篇 |
1990年 | 4篇 |
1988年 | 3篇 |
1986年 | 4篇 |
1984年 | 2篇 |
1981年 | 3篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1976年 | 3篇 |
1973年 | 3篇 |
1971年 | 7篇 |
1970年 | 3篇 |
1969年 | 3篇 |
1968年 | 4篇 |
1967年 | 4篇 |
1965年 | 2篇 |
1958年 | 3篇 |
1957年 | 2篇 |
1956年 | 2篇 |
1955年 | 2篇 |
1923年 | 2篇 |
1916年 | 5篇 |
1904年 | 2篇 |
1903年 | 2篇 |
1902年 | 2篇 |
1900年 | 2篇 |
排序方式: 共有343条查询结果,搜索用时 15 毫秒
81.
82.
83.
84.
Many autotrophs vary their allocation to nutrient uptake in response to environmental cues, yet the dynamics of this plasticity
are largely unknown. Plasticity dynamics affect the extent of single versus multiple nutrient limitation and thus have implications
for plant ecology and biogeochemical cycling. Here we use a model of two essential nutrients cycling through autotrophs and
the environment to determine conditions under which different plastic or fixed nutrient uptake strategies are adaptive. Our
model includes environment-independent costs of being plastic, environment-dependent costs proportional to the rate of plastic
change, and costs of being mismatched to the environment, the last of which is experienced by both fixed and plastic types.
In equilibrium environments, environment-independent costs of being plastic select for tortoise strategies—fixed or less plastic
types—provided that they are sufficiently close to co-limitation. At intermediate levels of environmental fluctuation forced
by periodic nutrient inputs, more hare-like plastic strategies prevail because they remain near co-limitation. However, the
fastest is not necessarily the best. The most adaptive strategy is an intermediate level of plasticity that keeps pace with
environmental fluctuations, but is not faster. At high levels of environmental fluctuation, the environment-dependent cost
of changing rapidly to keep pace with the environment becomes prohibitive and tortoise strategies again dominate. The existence
and location of these thresholds depend on plasticity costs and rate, which are largely unknown empirically. These results
suggest that the expectations for single nutrient limitation versus co-limitation and therefore biogeochemical cycling and
autotroph community dynamics depend on environmental heterogeneity and plasticity costs. 相似文献
85.
86.
Zavitsanou K Nguyen V Greguric I Chapman J Ballantyne P Katsifis A 《Journal of molecular histology》2007,38(4):313-319
In the present study we aimed to establish an animal model of dexamethasone (DEX)-induced apoptosis in the thymus of rats.
The degree of apoptosis was determined in the same animals at 6 and 11 h after a single administration of DEX (5 mg/kg, ip)
by (a) in vivo biodistribution of the uptake of [123I]Annexin V, a biomarker of the early stages of apoptosis; (b) in vitro evaluation of the apoptotic index (percentage of number
of apoptotic cells versus total number of cells) in the form of DNA fragmentation, on tissue sections using in situ oligo
ligation (ISOL). ISOL demonstrated a 62- and 90-fold increase of apoptotic index at 6 and 11 h after DEX administration respectively,
in the outer part of the thymic lobule (cortex) and a 25- and 54-fold increases in the inner part of the thymic lobule (medulla)
in the corresponding treatment groups. In the biodistribution study, [123I]Annexin V uptake was significantly increased in the thymus of rats 11 h after DEX administration (by 1.3- to 1.4-fold) and
significantly decreased at the 6-h time point. We conclude that the specificity of the apoptotic signal provided by isotopic
methods in vivo would always require confirmation by complementary in vitro techniques that verify the assessment of ongoing
apoptosis accurately. 相似文献
87.
In forensic casework, Y chromosome short tandem repeat markers (Y-STRs) are often used to identify a male donor DNA profile in the presence of excess quantities of female DNA, such as is found in many sexual assault investigations. Commercially available Y-STR multiplexes incorporating 12-17 loci are currently used in forensic casework (Promega's PowerPlex Y and Applied Biosystems' AmpFlSTR Yfiler). Despite the robustness of these commercial multiplex Y-STR systems and the ability to discriminate two male individuals in most cases, the coincidence match probabilities between unrelated males are modest compared with the standard set of autosomal STR markers. Hence there is still a need to develop new multiplex systems to supplement these for those cases where additional discriminatory power is desired or where there is a coincidental Y-STR match between potential male participants. Over 400 Y-STR loci have been identified on the Y chromosome. While these have the potential to increase the discrimination potential afforded by the commercially available kits, many have not been well characterized. In the present work, 91 loci were tested for their relative ability to increase the discrimination potential of the commonly used 'core' Y-STR loci. The result of this extensive evaluation was the development of an ultra high discrimination (UHD) multiplex DNA typing system that allows for the robust co-amplification of 14 non-core Y-STR loci. Population studies with a mixed African American and American Caucasian sample set (n = 572) indicated that the overall discriminatory potential of the UHD multiplex was superior to all commercial kits tested. The combined use of the UHD multiplex and the Applied Biosystems' AmpFlSTR Yfiler kit resulted in 100% discrimination of all individuals within the sample set, which presages its potential to maximally augment currently available forensic casework markers. It could also find applications in human evolutionary genetics and genetic genealogy. 相似文献
88.
The LFA-1 adhesion molecule is required for protective immunity during pulmonary Mycobacterium tuberculosis infection 总被引:2,自引:0,他引:2
Ghosh S Chackerian AA Parker CM Ballantyne CM Behar SM 《Journal of immunology (Baltimore, Md. : 1950)》2006,176(8):4914-4922
Host immunity to Mycobacterium tuberculosis is mediated by T cells that recognize and activate infected macrophages to control intracellular bacterial replication. The early appearance of T cells in the lungs of infected mice correlates with greater resistance to infection. However, it is unknown whether the trafficking of T cells to the lung following infection is dependent upon the expression of certain adhesion molecules. To address this question, we infected knockout (KO) mice that have defective expression of CD11a, CD11b, CD18, CD62, CD103, or beta7. We found that the integrins CD11a and CD18 are absolutely required for host resistance following infection with aerosolized M. tuberculosis. Although Ag-specific T cells are generated following infection of CD11a KO mice, T cell priming is delayed, T cell trafficking to the lung is impaired, and fewer ESAT6-specific CD4+ T cells are found in the lungs of CD11a KO mice compared with control mice. Thus, LFA-1 (CD11a/CD18) plays an essential role in immunity to M. tuberculosis infection. 相似文献
89.
Evidence exists that increased levels of physical activity decrease the population burden of cardiovascular disease (CVD). Although risk factors for CVD, including plasma lipids and lipoproteins, have been associated with physical activity, studies including a sizeable number of minority participants are lacking. Our purpose was to interrogate the longitudinal effect of physical activity on plasma lipids and lipoproteins in the African American and white participants of the Atherosclerosis Risk in Communities (ARIC) Study. Nine years of follow-up data on 8,764 individuals aged 45–64 years at baseline were used in linear mixed-effects models to estimate the association between increases in baseline physical activity on mean change in HDL, LDL, total cholesterol, and triglyceride levels. Increases in the level of activity were associated with increases in HDL in all strata and decreases in triglycerides among white participants. Physical activity was associated with LDL in all women, while the association with total cholesterol was limited to African American women. This study is one of the few to investigate the effect of physical activity on lipids and lipoproteins in a race- and sex-specific manner. Overall our results highlight the importance of physical activity on plasma lipid profiles and provide evidence for novel differential associations. 相似文献
90.