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   610篇
  免费   46篇
  656篇
  2024年   2篇
  2023年   6篇
  2022年   7篇
  2021年   14篇
  2020年   7篇
  2019年   25篇
  2018年   15篇
  2017年   17篇
  2016年   20篇
  2015年   34篇
  2014年   29篇
  2013年   42篇
  2012年   69篇
  2011年   51篇
  2010年   27篇
  2009年   33篇
  2008年   39篇
  2007年   24篇
  2006年   26篇
  2005年   22篇
  2004年   20篇
  2003年   21篇
  2002年   13篇
  2001年   7篇
  2000年   6篇
  1999年   8篇
  1998年   12篇
  1997年   7篇
  1996年   5篇
  1995年   5篇
  1994年   10篇
  1993年   6篇
  1992年   2篇
  1991年   2篇
  1990年   4篇
  1988年   3篇
  1987年   2篇
  1986年   2篇
  1985年   3篇
  1984年   1篇
  1982年   1篇
  1981年   1篇
  1980年   1篇
  1978年   1篇
  1977年   1篇
  1975年   2篇
  1974年   1篇
排序方式: 共有656条查询结果,搜索用时 15 毫秒
181.
Loss of genetic diversity is thought to lead to increased risk of extinction in endangered populations due to decreasing fitness of homozygous individuals. Here, we evaluated the presence of inbreeding depression in a long‐lived seabird, the European shag (Phalacrocorax aristotelis), after a severe decline in population size by nearly 70%. During three reproductive seasons, 85 breeders were captured and genotyped at seven microsatellite loci. Nest sites were monitored during the breeding season to estimate reproductive success as the number of chicks surviving to full‐size‐grown per nest. Captured birds were tagged with a ring with an individual code, and resighting data were collected during 7‐year period. We found a strong effect of multilocus heterozygosity on female reproductive performance, and a significant, although weaker, effect on breeder survival. However, our matrix population model suggests that this relatively small effect of genetic diversity on breeder survival may have a profound effect on fitness. This highlights the importance of integrating life history consequences in HFC studies. Importantly, heterozygosity was correlated across loci, suggesting that genomewide effects, rather than single loci, are responsible for the observed HFCs. Overall, the HFCs are a worrying symptom of genetic erosion in this declining population. Many long‐lived species are prone to extinction, and future studies should evaluate the magnitude of fitness impact of genetic deterioration on key population parameters, such as survival of breeders.  相似文献   
182.
Thrombin-activatable fibrinolysis inhibitor (TAFI) plays a role in the regulation of coagulation and inflammation. In addition to inhibiting the fibrinolytic system, TAFI may also regulate the bradykinin and complement systems. We hypothesized that TAFI also plays a role in defense mechanisms of the gastric mucosa during Helicobacter pylori infection. This study comprised 65 patients with gastroduodenal disorders: 41 patients with H. pylori infection, 13 without, and 11 patients with cured H. pylori infection. The gastric intramucosal concentrations of TAFI were measured by enzyme immunoassay. The gastric levels of TAFI and plasminogen activator inhibitor-1 were significantly increased in patients with H. pylori compared to those without infection or cured H. pylori . The presence of TAFI was detected in gastric mucosal epithelial cells. The concentration of TAFI was correlated with the degree of gastric mucosal atrophy, inflammation, and disease activity. These results show that TAFI is present in the gastric mucosa and that it may play a role in the pathogenesis of H. pylori infection-associated gastroduodenal disorders.  相似文献   
183.
Behavioral training in neurofeedback has proven to be an essential complement to generalize the effects of pharmacological support in subjects who have attention deficit with hyperactivity disorder (ADHD). Therefore, this investigation attempts to analyze the efficacy of neurofeedback compared with pharmacological support and the combination of both. Participants were 131 students, classified into four groups: control (did not receive neurofeedback or pharmacological support), neurofeedback group, pharmacological support group, and combined group (neurofeedback + pharmacological support). Participants’ executive control and cortical activation were assessed before and after treatment. Results indicate that the combined group obtained more benefits and that the neurofeedback group improved to a greater extent in executive control than the pharmacological support group. It is concluded that this kind of training may be an alternative to stimulate activation in subjects with ADHD.  相似文献   
184.
The genotype frequencies of three metabolic polymorphisms were determined in a sample of a typical community in central Mexico. CYP1A1*3, GSTM1, and GSTT1 polymorphisms were studied in 150 donors born in Mexico and with Mexican ascendants; with respect to ethnicity the subjects can be considered Mestizos. PCR reactions were used to amplify specific fragments of the selected genes from genomic DNA. An unexpected 56.7% frequency of the CYP1A1*3 allele (which depends on the presence of a Val residue in the 462 position of the enzyme, instead of Ile) was found, the highest described for open populations of different ethnic origins (i.e., Caucasian, Asian, African, or African American). The GSTM1 null genotype was found with a frequency of 42.6%, which is not different from other ethnicities, whereas the GSTT1 null genotype had a frequency of 9.3%, one of the lowest described for any ethnic group but comparable to the frequency found in India (9.7%). The frequency of the combined genotype CYP1A1*3/*3 and the GSTM1 null allele is one of the highest observed to date (or perhaps the highest): 13.7% among all the ethnicities studied, including Caucasians and Asians, whereas the combination of CYP1A1*3/*3 with the GSTT1 null allele reached only 2.8%. The GSTM1 null allele combined with the GSTT1 null allele, on the other hand, has one of the lowest frequencies described, 4.24%, comparable to the frequencies found in African Americans and Indians. Finally, the combined CYP1A1*3/*3, GSTM1 null allele, and GSTT1 null allele genotype could not be found in the sample studied; it is assumed that the frequency of carriers of these combined genotypes is less than 1%. CYP1A1*3 and CYP1A1*2 polymorphisms were also evaluated in 50 residents in a community of northern Mexico; the CYP1A1*3 frequency was 54%, similar to that found in the other community studied, and the CYP1A1*2 frequency was 40%, which is high compared to Caucasians and Asians but comparable to the frequency found in Japanese and lower than the frequency found in Mapuche Indians. Haplotype frequencies for these CYP1A1 polymorphisms were estimated, and a linkage disequilibrium value (D) of 0.137 was calculated.  相似文献   
185.
186.

Objectives

To study the prevalence of delirium in the residential environment and to analyse the associated clinical, functional and mental factors.

Material and methods

A cross-sectional epidemic study was conducted on a population of elderly persons institutionalised in 2011 in 6 nursing homes in Asturias. Socio-demographic, clinical, functional (Barthel Index [BI]) and mental (Mini-Mental State Examination [MMSE]) variables were collected. Delirium was defined by the Confusion Assessment Method.

Results

A total of 505 elderly were included in the study (age 83.30 ± 7.33 years, with 67.70% women), and scores on the MMSE of 17.19 ± 10.35 and a BI score of 55.11 ± 35.82. The prevalence of delirium was 11.70%. On examining the risk of delirium among the studied variables, there was statistical significance when considering: BI, MMSE, dementia, pressure ulcers, or urinary catheter, and the prescribing of clomethiazole, ACTH-I or trazodone. In the analysis of the variables in the logistic regression with BI, diagnosis of dementia, the prescribing of clomethiazole or trazodone, in the equation, there was a statistical significance associated with delirium.

Conclusions

The prevalence of delirium in a residential environment in our study population was 11.7%. The results show that BI, diagnosis of dementia, and prescribing of clomethiazole or trazodone were associated with risk of delirium in institutionalised patients.  相似文献   
187.
This study evaluated the protective effect of protein, as dependent on osmolarity, and the quality of water sources used to prepare embryo culture media. In Experiment 1, two concentrations of NaCl were used to obtain culture media with normal (280 mOSM) and low (245 mOSM) osmolarity, each supplemented with either bovine serum albumin (BSA) or polyvinyl alcohol (PVA). Low osmolarity improved blastocyst rates in the presence of BSA (P < 0.01) and tended to do it in medium containing PVA (P < 0.07). Furthermore, low osmolarity allowed PVA to increase inner cell mass (ICM) numbers and ICM/total cell rate (P < 0.05), while trophectoderm (TE) and total cell counts tended to decrease (P < 0.08). In Experiment 2, culture media were prepared with two water sources (Milli-Q and Sigma-W3500-) in combination with BSA or PVA. Both water sources yielded similar embryo development rates, but in the presence of BSA, Milli-Q water produced embryos with increased ICM/total cells rates (P < 0.05). On the contrary, Sigma water tended to increase trophectoderm cell counts (P < 0.08). In conclusion, the present study showed that low osmolarity is beneficial to embryo development and combinations of macromolecule and osmolarity influence trophectoderm differentiation. Both Milli-Q and Sigma supported embryo development at comparable rates, although in the presence of BSA, blastocysts obtained in the medium prepared with Milli-Q water had superior quality in terms of ICM/total cells rates.  相似文献   
188.
Controlling the cross-presentation of exogenous Ags to CD8+ T cells represents a major step for designing new vaccination strategies. Whereas several recombinant pseudo-viral particles have been used as delivery systems for triggering potent CTL responses to heterologous exogenous Ags, the adjuvant properties of virus-like particles (VLPs) themselves were little questioned. Here, we analyzed the contribution of the porcine parvovirus (PPV)-VLPs to the induction of protective cellular responses to exogenous Ags carried by an independent delivery system. Microspheres, which are known to transfer exogenous Ags into the MHC class I pathway, were chosen for delivering the immunodominant OVA(257-264) CD8+ T cell epitope (B-OVAp). This delivery system fulfills the requirements in terms of cross-presentation, but fails to induce cross-priming of specific CD8+ T cells. Coinjection of PPV-VLPs with B-OVAp results in the priming of potent CTL responses and type 1-biased immunity in a CD4- and CD40-independent manner, as efficiently as the recombinant PPV-VLPs carrying the same epitope (PPV-OVAp). Furthermore, vaccination with PPV-VLPs and B-OVAp was fully efficient to protect mice against the development of OVA-bearing melanoma. These findings indicate that PPV-VLPs act not only as a delivery system but also as a strong adjuvant when independently provided with exogenous Ag. Thus, dissociation between delivery system and adjuvant would provide a more flexible and reliable system to induce potent and protective CTL.  相似文献   
189.
190.
Although it is generally accepted that the tribe Genisteae has rather homogeneous pollen, we have found considerable variation in pollen size and morphology in Cytisus and related genera (Argyrocytisus and Chamaecytisus), which match taxonomic groups defined on morphological grounds. The results of Principal Component Analysis of pollen characteristics show the following: Cytisus Sect. Alburnoides is well-delimited, with the smallest pollen grains and the simplest pattern in the tectum, fossulate-perforate to perforate; No separation of C. villosus (Sect. Cytisus) from Sect. Alburnoides is observed; Cytisus Sect. Spartopsis, with the largest pollen grains and reticulate to reticulate-fossulate ornamentation, forms an isolated group; C. fontanesii (Sect. Heterocytisus) stands apart from other Cytisus species by its homogeneous ornamentation and the perforate instead of psilate-punctate margo. Thus, the pollen data supports its transfer to the separated genus Chronanthos; Pollen ornamentation and morphology also support the separation of Argyrocytisus battandieri and Chamaecytisus mollis from Cytisus.  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