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   21871篇
  免费   1499篇
  国内免费   9篇
  2023年   112篇
  2022年   265篇
  2021年   580篇
  2020年   351篇
  2019年   470篇
  2018年   595篇
  2017年   479篇
  2016年   809篇
  2015年   1215篇
  2014年   1345篇
  2013年   1660篇
  2012年   1951篇
  2011年   1892篇
  2010年   1176篇
  2009年   1016篇
  2008年   1376篇
  2007年   1287篇
  2006年   1179篇
  2005年   1082篇
  2004年   981篇
  2003年   927篇
  2002年   751篇
  2001年   121篇
  2000年   108篇
  1999年   135篇
  1998年   125篇
  1997年   115篇
  1996年   89篇
  1995年   86篇
  1994年   91篇
  1993年   88篇
  1992年   53篇
  1991年   57篇
  1990年   62篇
  1989年   46篇
  1988年   50篇
  1987年   39篇
  1986年   42篇
  1985年   51篇
  1984年   39篇
  1983年   44篇
  1982年   32篇
  1981年   42篇
  1980年   26篇
  1979年   34篇
  1978年   32篇
  1977年   29篇
  1976年   25篇
  1975年   22篇
  1973年   17篇
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
431.
The objectives of this study were to identify tissue-specific differentially methylated regions (T-DMR’s) in the folate transport genes in placental tissue compared with leukocytes, and from placental tissues obtained from normal infants or with neural tube defects (NTDs). Using pyrosequencing, we developed methylation assays for the CpG islands (CGIs) and the CGI shore regions of the folate receptor α (FOLR1), proton-coupled folate transporter (PCFT) and reduced folate carrier 1 (RFC1) genes. The T-DMRs differed in location for each gene and the difference in methylation ranged between 2 and 54%. A higher T-DMR methylated fraction was associated with a lower mRNA level of the FOLR1 and RFC1 genes. Methylation fractions differed according to RFC1 80G > A genotype in the NTD cases and in leukocytes from subjects with high total plasma homocysteine (tHcy). There were no differences in methylated fraction of folate transporter genes between NTD cases and controls. We suggest that T-DMRs participate in the regulation of expression of the FOLR1 and RFC1 genes, that the RFC1 80G > A polymorphism exerts a gene-nutrition interaction on DNA methylation in the RFC1 gene, and that this interaction appears to be most prominent in NTD-affected births and in subjects with high tHcy concentrations.  相似文献   
432.
433.
Plant interactions with environmental factors cause changes in the metabolism and regulation of biochemical and physiological processes. Plant defense against pathogenic microorganisms depends on an innate immunity system that is activated as a result of infection. There are two mechanisms of triggering this system: basal immunity activated as a result of a perception of microbe-associated molecular patterns through pattern recognition receptors situated on the cell surface and effector-triggered immunity (ETI). An induced biosynthesis of bioactive secondary metabolites, in particular phytoalexins, is one of the mechanisms of plant defense to fungal infection. Results of the study on narrow leaf lupin (Lupinus angustifolius L.) plants infected with the anthracnose fungus Colletotrichum lupini and treated with fungal phytotoxic metabolites are described in the paper. The C. lupini phytotoxins were isolated from liquid cultures, purified and partially characterized with physicochemical methods. Accumulation of secondary metabolites on leaf surface and within the tissues of plants either infected, treated with the fungal phytotoxin or submitted to both treatments was studied using GC-MS and LC-MS, respectively. Substantial differences in isoflavone aglycones and glycoconjugate profiles occurred in response to different ways of plant treatment.  相似文献   
434.
Various radioligands have been used to characterize and quantify the platelet P2Y12 receptor, which share several weaknesses: (a) they are metabolically unstable and substrates for ectoenzymes, (b) they are agonists, and (c) they do not discriminate between P2Y1 and P2Y12. We used the [3H]PSB-0413 selective P2Y12 receptor antagonist radioligand to reevaluate the number of P2Y12 receptors in intact platelets and in membrane preparations. Studies in humans showed that: (1) [3H]PSB-0413 bound to 425 ± 50 sites/platelet (KD = 3.3 ± 0.6 nM), (2) 0.5 ± 0.2 pmol [3H]PSB-0413 bound to 1 mg protein of platelet membranes (KD = 6.5 ± 3.6 nM), and (3) competition studies confirmed the known features of P2Y12, with the expected rank order of potency: AR-C69931MX > 2MeSADP ≫ ADPβS > ADP, while the P2Y1 ligand MRS2179 and the P2X1 ligand α,β-Met-ATP did not displace [3H]PSB-0413 binding. Patients with severe P2Y12 deficiency displayed virtually no binding of [3H]PSB-0413 to intact platelets, while a patient with a dysfunctional P2Y12 receptor had normal binding. Studies in mice showed that: (1) [3H]PSB-0413 bound to 634 ± 87 sites/platelet (KD = 14 ± 4.5 nM) and (2) 0.7 pmol ± 0.3 [3H]PSB-0413 bound to 1 mg protein of platelet membranes (KD = 9.1 ± 5.3 nM). Clopidogrel and other thiol reagents like pCMBS or DTT abolished the binding both to intact platelets and membrane preparations. Therefore, [3H]PSB-0413 is an accurate and selective tool for radioligand binding studies aimed at quantifying P2Y12 receptors, to identify patients with P2Y12 deficiencies or quantify the effect of P2Y12 targeting drugs.  相似文献   
435.
Anna Aragno 《Biosemiotics》2013,6(3):473-488
In my continuing efforts to build a bridge between psychoanalytic findings and biosemiotics here, as in previous works, ‘biosemiotic’ refers to the hierarchy of meaning-forms (from biological to semiotic-organizations) underlying an updated psychoanalytic model of mind. Within this framework I present a broad range of bio-semiotic phenomena, processes, dynamics, defenses, and universal and unique internalized interpersonal patterns, that in psychoanalysis all commonly fall under the broad heading of the “Unconscious.” Reconceptualized as interpretive data within the purview of a psychoanalytic discourse-semantic this biosemiotic framework posits an epigenetic continuum of human meaning-organizations originating at basic organic levels, moving upward through biological, psycho-somatic and affective expression, proto-semiotic transmissions, represented forms, and finally to explicit linguistic signs and complex symbol systems. In addition to assuming an uninterrupted epigenetic continuum crystallizing in hierarchic organization, this framework accentuates the multilayered and increasingly condensed quality of higher more elaborate organizations of meaning in human communication, drawing attention to persisting biological undercurrents in implied sense, intent, and motivation, all of which impact on repressive/defensive mechanisms. Drawing from previous works (Aragno 1997, 2005, 2008a, b, Psychoanalytic Inquiry 29(1):30–47, 2009, Biosemiotics 3:57–77, 2010, 2011a, Signs 5:71–74, 2011b, Journal of the American Psychoanalytic Association, Centennial Paper, Special Centennial Issue 59(2):239–288, 2011b, Signs 5:29–70, 2011c) in which I labored to update and revise Freud’s first topographical theory of mind, this paper presents the phenomenology of unconscious ‘data’ for the purpose of introducing a diverse range of non-linguistic signifying forms from which psychoanalysts infer mental processes and ‘interpret’ meanings. An important underlying premise regarding psychoanalytic data and its relation to the basic biosemiotic ‘agenda’ is that until grounded in an updated developmental theory of mind inclusive of pre- and proto-semiotic-forms, that is evolutionarily plausible, epistemologically based, and correlates with contemporary neuroscience, the term “sign” is merely an abstract linguistic ‘label’ rather than a mental act with antecedent developmental stages manifesting meanings through different forms and modes of expression. Drawn from the yields of the psychoanalytic method and semantic this revised metatheoretical approach provides insights into the sensory-emotive, bodily origins of unconscious layers of non-linguistic signification thereby expanding our understanding of the formative stages of the ‘semiotic function’ in human evolution. This being the third in a series of papers integrating the yields of psychoanalytic methodology with the underlying premises of ‘Biosemiotics,’ some familiarity with the background knowledge provided in the previous two is strongly recommended.  相似文献   
436.
Recombinant adeno-associated viral (AAV) vectors are known to safely and efficiently transduce the retina. Among the various AAV serotypes available, AAV2/5 and 2/8 are the most effective for gene transfer to photoreceptors (PR), which are the most relevant targets for gene therapy of inherited retinal degenerations. However, the search for novel AAV serotypes with improved PR transduction is ongoing. In this work we tested vectors derived from five AAV serotypes isolated from porcine tissues (referred to as porcine AAVs, four of which are newly identified) for their ability to transduce both the murine and the cone-enriched pig retina. Porcine AAV vectors expressing EGFP under the control of the CMV promoter were injected subretinally either in C57BL/6 mice or Large White pigs. The resulting retinal tropism was analyzed one month later on histological sections, while levels of PR transduction were assessed by Western blot. Our results show that all porcine AAV transduce murine and porcine retinal pigment epithelium and PR upon subretinal administration. AAV2/po1 and 2/po5 are the most efficient porcine AAVs for murine PR transduction and exhibit the strongest tropism for pig cone PR. The levels of PR transduction obtained with AAV2/po1 and 2/po5 are similar, albeit not superior, to those obtained with AAV2/5 and AAV2/8, which evinces AAV2/po1 and 2/po5 to be promising vectors for retinal gene therapy.  相似文献   
437.

Background

Effective promotion of exercise could result in substantial savings in healthcare cost expenses in terms of direct medical costs, such as the number of medical appointments. However, this is hampered by our limited knowledge of how to achieve sustained increases in physical activity.

Objectives

To assess the effectiveness of a Primary Health Care (PHC) based physical activity program in reducing the total number of visits to the healthcare center among inactive patients, over a 15-month period.

Research Design

Randomized controlled trial.

Subjects

Three hundred and sixty-two (n = 362) inactive patients suffering from at least one chronic condition were included. One hundred and eighty-three patients (n = 183; mean (SD); 68.3 (8.8) years; 118 women) were randomly allocated to the physical activity program (IG). One hundred and seventy-nine patients (n = 179; 67.2 (9.1) years; 106 women) were allocated to the control group (CG). The IG went through a three-month standardized physical activity program led by physical activity specialists and linked to community resources.

Measures

The total number of medical appointments to the PHC, during twelve months before and after the program, was registered. Self-reported health status (SF-12 version 2) was assessed at baseline (month 0), at the end of the intervention (month 3), and at 12 months follow-up after the end of the intervention (month 15).

Results

The IG had a significantly reduced number of visits during the 12 months after the intervention: 14.8 (8.5). The CG remained about the same: 18.2 (11.1) (P = .002).

Conclusions

Our findings indicate that a 3-month physical activity program linked to community resources is a short-duration, effective and sustainable intervention in inactive patients to decrease rates of PHC visits.

Trial Registration

ClinicalTrials.gov NCT00714831  相似文献   
438.
Nanobubbles and microbubbles are non-invasive ultrasound imaging contrast agents that may potentially enhance diagnosis of tumors. However, to date, both nanobubbles and microbubbles display poor in vivo tumor-selectivity over non-targeted organs such as liver. We report here cyanine 5.5 conjugated nanobubbles (cy5.5-nanobubbles) of a biocompatible chitosan–vitamin C lipid system as a dual ultrasound-fluorescence contrast agent that achieved tumor-selective imaging in a mouse tumor model. Cy5.5-nanobubble suspension contained single bubble spheres and clusters of bubble spheres with the size ranging between 400–800 nm. In the in vivo mouse study, enhancement of ultrasound signals at tumor site was found to persist over 2 h while tumor-selective fluorescence emission was persistently observed over 24 h with intravenous injection of cy5.5-nanobubbles. In vitro cell study indicated that cy5.5-flurescence dye was able to accumulate in cancer cells due to the unique conjugated nanobubble structure. Further in vivo fluorescence study suggested that cy5.5-nanobubbles were mainly located at tumor site and in the bladder of mice. Subsequent analysis confirmed that accumulation of high fluorescence was present at the intact subcutaneous tumor site and in isolated tumor tissue but not in liver tissue post intravenous injection of cy5.5-nanobubbles. All these results led to the conclusion that cy5.5-nanobubbles with unique crosslinked chitosan–vitamin C lipid system have achieved tumor-selective imaging in vivo.  相似文献   
439.
In the case of melanoma, advances in therapies are slow, which raises the need to evaluate new therapeutic strategies and natural products with potential cancer cell inhibiting effect. Pseudofactin II (PFII), a novel cyclic lipopeptide biosurfactant has been isolated from the Arctic strain of Pseudomonas fluorescens BD5. The aim of this study was to investigate the effect of PFII on A375 melanoma cells compared with the effect of PFII on Normal Human Dermis Fibroblast (NHDF) cells and elucidate the underlying mechanism of PFII cytotoxic activity. Melanoma A375 cells and NHDF cells were exposed to PFII or staurosporine and apoptotic death was assessed by monitoring caspase 3-like activity and DNA fragmentation. From time-dependent monitoring of lactate dehydrogenase (LDH) release, Ca2+ influx, and a correlation between Critical Micelle Concentration (CMC) we concluded that cell death is the consequence of plasma membrane permeabilisation by micelles. This finding suggests that pro-apoptotic mechanism of PFII is different from previously described cyclic lipopeptides. The mechanism of PFII specificity towards malignant cells remains to be discovered. The results of this study show that PFII could be a new promising anti-melanoma agent.  相似文献   
440.
Cattle pastoralism is an important trait of African cultures. Ethnographic studies describe the central role played by domestic cattle within many societies, highlighting its social and ideological value well beyond its mere function as ‘walking larder’. Historical depth of this African legacy has been repeatedly assessed in an archaeological perspective, mostly emphasizing a continental vision. Nevertheless, in-depth site-specific studies, with a few exceptions, are lacking. Despite the long tradition of a multi-disciplinary approach to the analysis of pastoral systems in Africa, rarely do early and middle Holocene archaeological contexts feature in the same area the combination of settlement, ceremonial and rock art features so as to be multi-dimensionally explored: the Messak plateau in the Libyan central Sahara represents an outstanding exception. Known for its rich Pleistocene occupation and abundant Holocene rock art, the region, through our research, has also shown to preserve the material evidence of a complex ritual dated to the Middle Pastoral (6080–5120 BP or 5200–3800 BC). This was centred on the frequent deposition in stone monuments of disarticulated animal remains, mostly cattle. Animal burials are known also from other African contexts, but regional extent of the phenomenon, state of preservation of monuments, and associated rock art make the Messak case unique. GIS analysis, excavation data, radiocarbon dating, zooarchaeological and isotopic (Sr, C, O) analyses of animal remains, and botanical information are used to explore this highly formalized ritual and the lifeways of a pastoral community in the Holocene Sahara.  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