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   100篇
  免费   6篇
  2020年   2篇
  2019年   2篇
  2018年   2篇
  2017年   1篇
  2016年   4篇
  2015年   15篇
  2014年   9篇
  2013年   15篇
  2012年   13篇
  2011年   2篇
  2010年   16篇
  2009年   9篇
  2008年   1篇
  2007年   2篇
  2006年   1篇
  2002年   1篇
  2001年   1篇
  1999年   1篇
  1997年   2篇
  1992年   1篇
  1989年   1篇
  1971年   1篇
  1970年   1篇
  1969年   1篇
  1968年   1篇
  1967年   1篇
排序方式: 共有106条查询结果,搜索用时 18 毫秒
71.

Background

Categories of imperilment like the global IUCN Red List have been transformed to probabilities of extinction and used to rank species by the amount of imperiled evolutionary history they represent (e.g. by the Edge of Existence programme). We investigate the stability of such lists when ranks are converted to probabilities of extinction under different scenarios.

Methodology and Principal Findings

Using a simple example and computer simulation, we show that preserving the categories when converting such list designations to probabilities of extinction does not guarantee the stability of the resulting lists.

Significance

Care must be taken when choosing a suitable transformation, especially if conservation dollars are allocated to species in a ranked fashion. We advocate routine sensitivity analyses.  相似文献   
72.
Enterotoxigenic Escherichia coli (ETEC) infection is the most common type of porcine postweaning colibacillosis (PWC). Among fimbriae of porcine ETEC strains the best studied family of fimbriae are the members of F4 adhesins, existing in at least three variants: ab, ac, ad. Active immunization against porcine PWC is difficult due to: i) ETEC strains are only one of the essential predisposing factors, ii) the success of vaccinal antigen uptake depends on the presence of enterocyte receptors for F4 adhesins, iii) the intestinal immune system may react with tolerance or hypersensitivity to the same antigens depending on the dose and form of the vaccinal immunogen, and iv) kinetics of the specific immune responses may be different in the case of F4 (earlier) and the other ETEC adhesins, particularly F18 (later). The aim of this study was to test the effectiveness of a live attenuated F4ac+ non-ETEC vaccine against porcine PWC by analyzing quantitative differences in the small intestinal lymphoid and myeloid cell subsets of immunized (with or without levamisole given as an adjuvant) vs control non-immunized pigs. Four week-old pigs were intragastrically immunized with a vaccine candidate F4ac + non-ETEC strain 2407 at day 0, challenged 7 days later with a virulent F4ac+ strain ETEC 11-800/1/94, euthanatized at day 13 and sampled for immunohistology. Non-immunized pigs received saline at day 0 and were processed as the principals. Immunophenotypes of lymphoid and myeloid cell subsets were demonstrated within jejunal and ileal mucosa by immunohistochemical avidinbiotin complex method and corresponding morphometric data were analyzed using software program Lucia G for digital image analyses. Monoclonal antibodies reactive with surface molecules on porcine immune cells such as CD3, CD45RA, CD45RC, CD21 and SWC3 enabled clear insight into distribution patterns and amount of these cells within the gut-associated lymphoid tissues (GALT) examined. The numbers of jejunal and ileal cell subsets tested were significantly increased (at P<0.5 or lower) in both principal groups (vaccinated or levamisole primed-vaccinated) of pigs, compared to those recorded in the control non-vaccinated pigs. Based on the histomorphometric quantification of porcine intestinal immune cells from the GALT compartments tested, it is possible to differentiate the responses of pigs immunized by an experimental mucosal vaccine from those of non-immunized pigs.Key words: adjuvanted/nonadjuvanted E. coli vaccine, intestinal immune cells, pigs.  相似文献   
73.
The effects of salicylic acid (SA) and salinity on the activity of apoplastic antioxidant enzymes were studied in the leaves of two wheat (Triticum aestivam L.) cultivars: salt-tolerant (Gerek-79) and salt-sensitive (Bezostaya). The leaves of 10-d-old seedlings grown at nutrient solution with 0 (control), 250 or 500 mM NaCl were sprayed with 0.01 or 0.1 mM SA. Then, the activities of catalase (CAT), peroxidase (POX) and superoxide dismutase (SOD) were determined in the fresh leaves obtained from 15-d-old seedlings. The NaCl applications increased CAT and SOD activities in both cultivars, compared to those of untreated control plants. In addition, the NaCl increased POX activity in the salt-tolerant while decreased in the salt-sensitive cultivar. In control plants of the both cultivars, 0.1 mM SA increased CAT activity, while 0.01 mM SA slightly decreased it. SA treatments also stimulated SOD and POX activity in the salt-tolerant cultivar but significantly decreased POX activity and had no effect on SOD activity in the saltsensitive cultivar. Under salinity, the SA treatments significantly inhibited CAT activity, whereas increased POX activity. The increases in POX activity caused by SA were more pronounced in the salt-tolerant than in the salt-sensitive cultivar. SOD activity was increased by 0.01 mM SA in the salt-tolerant while increased by 0.1 mM SA treatment in the salt-sensitive cultivar.  相似文献   
74.
Eggs, hatchlings, and adult loggerhead turtles, and incubation durations of clutches, were measured on three Turkish beaches (Dalyan, Fethiye and G?ksu Delta), and some physical features of nests were compared. These features were not statistically different among the beaches, except for nest depth and distance to the high water mark. There was a positive relationship between hatchling mass and egg size. The carapace length of hatchlings was correlated with both egg diameter and incubation duration. The duration of asynchronous emergence of hatchlings on Fethiye beach was slightly longer than on the other two beaches, and the size of hatchlings decreased as asynchronous emergence proceeded. Of the hatchlings that emerged first, those that died were significantly smaller in SCL and mass than those that lived. These results suggest that smaller hatchlings may not be vigorous enough to emerge earlier from nests, and that they may be less fit.  相似文献   
75.
Enabling the paradigm of quality by design requires the ability to quantitatively correlate material properties and process variables to measureable product performance attributes. In this study, we show how heterogeneities in compacted ribbon densities quantitatively correlate to tablet mechanical properties. These density variations, which have been purposely modulated by internal and external lubrications, are characterized longitudinally and transversally by nondestructive ultrasonic and X-ray micro-computed tomography measurements. Subsequently, different transversal regions of the compacted ribbon are milled under the same conditions, and granules with nominally the same particle size distribution are utilized to manufacture cylindrical tablets, whose mechanical properties are further analyzed by ultrasonic measurements. We consider three different ribbon conditions: no lubrication (case 1); lubricated powder (case 2); and lubricated tooling (hopper, side sealing plates, feed screws, and rolls) (case 3). This study quantitatively reveals that variation in local densities in ribbons (for case 1) and process conditions (i.e., internal case 2 and external lubrication case 3) during roller compaction significantly affect the mechanical properties of tablets even for granules with the same particle size distribution. For case 1, the mechanical properties of tablets depend on the spatial location where granules are produced. For cases 2 and 3, the ribbon density homogeneity was improved by the use of a lubricant. It is demonstrated that the mechanical performances of tablets are decreased due to applied lubricant and work-hardening phenomenon. Moreover, we extended our study to correlate the speed of sound to the tensile strength of the tablet. It is found that the speed of sound increases with the tensile strength for the tested tablets.  相似文献   
76.
Local movements of receptors in the plasma membrane have been extensively studied, as it is generally believed that the dynamics of membrane distribution of receptors regulate their functions. However, the properties of large-scale (> 5 μm) receptor movements in the membrane are relatively obscure. In the present study, we addressed the question as to whether the large-scale movement of receptor in the plasma membrane at the whole cell level can be explained quantitatively by its local diffusive properties. We used HEK 293 cells transfected with human β2-adrenoceptor fused to photoconvertible fluorescent protein dendra2 as a model system; and found that 1) functional integrity of the dendra2-tagged receptor remains apparently intact; 2) in a mesoscopic scale (~ 4 μm), ~ 90% of the receptors are mobile on average, and receptor influx to, and out-flux from a membrane area can be symmetrically explained by a diffusion-like process with an effective diffusion coefficient of ~ 0.1 μm2/s; 3) these mobility parameters are not affected by the activity state of the receptor (assessed by using constitutively active receptor mutants); 4) in the macroscopic scale (4-40 μm), although a slowly diffusing fraction of receptors (with D < 0.01 μm2/s) is identifiable in some cases, the movement of the predominant fraction is perfectly explained by the same effective diffusion process observed in the mesoscopic scale, suggesting that the large scale structure of the cell membrane as felt by the receptor is apparently homogeneous in terms of its mesoscopic properties. We also showed that intracellular compartments and plasma membrane are kinetically connected even at steady-state.  相似文献   
77.
The distribution of EGF receptors (EGF-R) was examined in normal, hyaline membrane diseased and pneumonic newborn lung tissues by immunohistochemical methods under the light microscope. The PAP technique with polyclonal antibodies was performed to demonstrate the EGF receptor localisation in these tissues. Strong EGF-R reactivity was observed on bronchiolar epithelium and type I and type II alveolar cells in normal newborn lung tissues; whereas, poor reactivity was observed in alveolar macrophages. On the other hand, strong immunoreactivity was detected in type I alveolar cells and alveolar macrophages in hyaline membrane disease, but no reactivity was present in type II alveolar cells. The strongest immunoreactivity was observed in alveolar macrophages of newborn pneumonic lung tissues. In conclusion, the most meaningful form of reactivity was observed in normal newborn lung tissues of airway track and respiration area. This result is related with the maturation of the lungs after birth.  相似文献   
78.
Data are presented concerning the antifungal activity of 11 natural isothiocyanates and 27 synthetized analogues in Aspergillus niger, Penicillium cyclopium, and Rhizopus oryzae, as well as in 13 additional saprophytic and parasitic fungi. A remarkable antifungal activity was observed in some analogues of benzylisothiocyanate and β-phenylethylisothiocyanate. The latter-mentioned compounds have not been described previously. In the group of benzylisothiocyanates, a correlation, which was inversely proportional, was detected between ed100 values for A. niger and R. oryzae and the corresponding molar solubilities of compounds in water. In contradistinction, no relationship was observed between antifungal activity and chemical reactivity of investigated derivatives.  相似文献   
79.
Brain’s alpha activity and alpha responses belong to major electrical signals that are related to sensory/cognitive signal processing. The present study aims to analyze the spontaneous alpha activity and visual evoked alpha response in drug free euthymic bipolar patients. Eighteen DSM-IV euthymic bipolar patients (bipolar I n = 15, bipolar II n = 3) and 18 healthy controls were enrolled in the study. Patients needed to be euthymic at least for 4 weeks and psychotrop free for at least 2 weeks. Spontaneous EEG (4 min eyes closed, 4 min eyes open) and evoked alpha response upon application of simple visual stimuli were analyzed. EEG was recorded at 30 positions. The digital FFT-based power spectrum analysis was performed for spontaneous eyes closed and eyes open conditions and the response power spectrum was also analyzed for simple visual stimuli. In the analysis of spontaneous EEG, the ANOVA on alpha responses revealed significant results for groups (F(1,34) = 8.703; P < 0.007). Post-hoc comparisons showed that spontaneous EEG alpha power of healthy subjects was significantly higher than the spontaneous EEG alpha power of euthymic patients. Furthermore, visual evoked alpha power of healthy subjects was significantly higher than visual evoked alpha power of euthymic patients (F(1,34) = 4.981; P < 0.04). Decreased alpha activity in spontaneous EEG is an important pathological EEG finding in euthymic bipolar patients. Together with an evident decrease in evoked alpha responses, the findings may lead to a new pathway in search of biological correlates of cognitive impairment in bipolar disorder.  相似文献   
80.

Background

Telomeres are potential markers of mitotic cellular age and are associated with physical ageing process. Long-term endurance training and higher aerobic exercise capacity (VO2max) are associated with improved survival, and dynamic effects of exercise are evident with ageing. However, the association of telomere length with exercise training and VO2max has so far been inconsistent. Our aim was to assess whether muscle telomere length is associated with endurance exercise training and VO2max in younger and older people.

Methods

Twenty men; 10 young (22–27 years) and 10 old (66–77 years), were studied in this cross-sectional study. Five out of 10 young adults and 5 out of 10 older were endurance athletes, while other halves were exercising at a medium level of activity. Mean telomere length was measured as telomere/single copy gene-ratio (T/S-ratio) using quantitative real time polymerase chain reaction. VO2max was measured directly running on a treadmill.

Results

Older endurance trained athletes had longer telomere length compared with older people with medium activity levels (T/S ratio 1.12±0.1 vs. 0.92±0.2, p = 0.04). Telomere length of young endurance trained athletes was not different than young non-athletes (1.47±0.2 vs. 1.33±0.1, p = 0.12). Overall, there was a positive association between T/S ratio and VO2max (r = 0.70, p = 0.001). Among endurance trained athletes, we found a strong correlation between VO2max and T/S ratio (r = 0.78, p = 0.02). However, corresponding association among non-athlete participants was relatively weak (r = 0.58, p = 0.09).

Conclusion

Our data suggest that VO2max is positively associated with telomere length, and we found that long-term endurance exercise training may provide a protective effect on muscle telomere length in older people.  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