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   123665篇
  免费   7884篇
  国内免费   22篇
  131571篇
  2023年   750篇
  2022年   1002篇
  2021年   2009篇
  2020年   1607篇
  2019年   1856篇
  2018年   3265篇
  2017年   2880篇
  2016年   3845篇
  2015年   4964篇
  2014年   5373篇
  2013年   7105篇
  2012年   8140篇
  2011年   7610篇
  2010年   5033篇
  2009年   4156篇
  2008年   6063篇
  2007年   5849篇
  2006年   5586篇
  2005年   4648篇
  2004年   4686篇
  2003年   4159篇
  2002年   3883篇
  2001年   3154篇
  2000年   3012篇
  1999年   2404篇
  1998年   1199篇
  1997年   919篇
  1996年   910篇
  1995年   834篇
  1994年   763篇
  1993年   692篇
  1992年   1373篇
  1991年   1278篇
  1990年   1194篇
  1989年   1251篇
  1988年   1052篇
  1987年   1069篇
  1986年   967篇
  1985年   1015篇
  1984年   840篇
  1983年   753篇
  1982年   645篇
  1979年   838篇
  1978年   641篇
  1975年   727篇
  1974年   779篇
  1973年   737篇
  1972年   674篇
  1969年   702篇
  1968年   660篇
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
951.
BACKGROUND : Genetic and environmental factors likely influence susceptibility to nonsyndromic cryptorchidism, a common disease presenting at birth or in later childhood. We compared cases and controls to define differential risk factors for congenital versus acquired cryptorchidism. METHODS : We compared questionnaire and clinical data from cases of congenital cryptorchidism (n = 230), acquired cryptorchidism (n = 182) and hernia/hydrocele (n = 104) with a group of healthy male controls (n = 358). Potential predictor variables (p < 0.2 in univariable analysis) were included in stepwise multivariable logistic regression models. RESULTS : Temporary (odds ratio [OR], 0.5; 95% confidence interval [CI], 0.4–0.8) or exclusive (OR, 0.6; 95% CI, 0.4–0.9) breastfeeding was reduced and soy formula feeding increased (OR, 1.8; 95% CI, 1.2–2.9) in acquired but not congenital or hernia/hydrocele groups. The highest risk estimates were observed for primary soy formula feeding with limited or no breastfeeding (OR 2.5; 95% CI, 1.4–4.3; adjusted OR, 2.7; 95% CI, 1.4–5.4) in the acquired group. Primary feeding risk estimates were equivalent or strengthened when multivariable models were limited to age greater than 2 years, full‐term or not small for gestational age, or Caucasian subjects. Pregnancy complications and increased maternal exposure to cosmetic or household chemicals were not consistently associated with either form of cryptorchidism in these models. CONCLUSIONS : Our data support reduced breastfeeding and soy formula feeding as potential risk factors for acquired cryptorchidism. Although additional studies are needed, hormonally active components of breast milk and soy formula could influence the establishment of normal testis position in the first months of life, leading to apparent ascent of testes in childhood. Birth Defects Research (Part A), 2012. © 2012 Wiley Periodicals, Inc.  相似文献   
952.
Epigenetic modifications, such as aberrant DNA promoter methylation, are frequently observed in cervical cancer. Identification of hypermethylated regions allowing discrimination between normal cervical epithelium and high-grade cervical intraepithelial neoplasia (CIN2/3), or worse, may improve current cervical cancer population-based screening programs. In this study, the DNA methylome of high-grade CIN lesions was studied using genome-wide DNA methylation screening to identify potential biomarkers for early diagnosis of cervical neoplasia. Methylated DNA Immunoprecipitation (MeDIP) combined with DNA microarray was used to compare DNA methylation profiles of epithelial cells derived from high-grade CIN lesions with normal cervical epithelium. Hypermethylated differentially methylated regions (DMRs) were identified. Validation of nine selected DMRs using BSP and MSP in cervical tissue revealed methylation in 63.2–94.7% high-grade CIN and in 59.3–100% cervical carcinomas. QMSP for the two most significant high-grade CIN-specific methylation markers was conducted exploring test performance in a large series of cervical scrapings. Frequency and relative level of methylation were significantly different between normal and cancer samples. Clinical validation of both markers in cervical scrapings from patients with an abnormal cervical smear confirmed that frequency and relative level of methylation were related with increasing severity of the underlying CIN lesion and that ROC analysis was discriminative. These markers represent the COL25A1 and KATNAL2 and their observed increased methylation upon progression could intimate the regulatory role in carcinogenesis. In conclusion, our newly identified hypermethylated DMRs represent specific DNA methylation patterns in high-grade CIN lesions and are candidate biomarkers for early detection.  相似文献   
953.
The community composition of arbuscular mycorrhizal fungi (AMF) was analyzed in roots of Gypsophila struthium growing in gypsum soils under semiarid conditions. In order to investigate the effect of plant community degradation on the AMF biodiversity at the single species level, on the basis of the plant community complexity level, we selected four areas affected by degradation and shrub species spatial heterogeneity. The AM fungal community colonizing G. struthium was investigated from the morphological and molecular points of view. All plants were well colonized and showed a high level of infective AM propagules. Roots were analyzed by polymerase chain reaction, restriction fragment length polymorphism screening, and sequence analyses of the ribosomal DNA small subunit region. Four AM fungal types were identified and clustered into the AM fungal family: Glomeraceae, Glomus being the only taxon present. One fungal type was present in all the selected areas. Two fungal types are distinct from any previously published sequences and could be specific to gypsum soils. The chemical–physical properties of the soil were not correlated with the AMF diversity in roots. Our data show vegetation cover complexity-dependent differences in the AM fungal community composition.  相似文献   
954.
Glutamic acid decarboxylase (GAD) activity was measured in homogenates of conidia and both submerged and aerial mycelia of Trichoderma viride. The GAD activity in conidia had a temperature optimum at 30 degrees C and a pH optimum at pH 4. GAD was stimulated by EDTA (2 mM) and was insensitive to treatment with calmodulin antagonists calmidazolium (10 microM) or phenothiazine neuroleptics (60 microM). Cyclosporin A (up to 300 microM) partially inhibited GAD in the homogenate, but not in the supernatant obtained after centrifuging the homogenate. Attempts to release GAD activity from the homogenate using high ionic strength, detergents, or urea failed. Freezing-thawing led to the partial increase of activity in the conidial homogenate. These results indicate that GAD is a membrane-bound enzyme. The highest specific activity of GAD was present in the mitochondrial/vacuolar organellar fraction. Germination of conidia in the submerged culture led to a temporary decrease in GAD activity. After prolonged cultivation, the activity displayed quasi-oscillatory changes. The stationary state was characterized by a high GAD activity. The presence of gamma-aminobutyric acid in the submerged mycelia was demonstrated. In surface culture in the dark, GAD activity increased in a monophasic manner until conidia formation. The illumination of dark-cultivated mycelia by a white-light pulse caused a dramatic increase in GAD activity. Light-induced changes were not observed in mutants with delayed onset of conidiation. In the dark or upon illumination by light pulse, the increase of GAD activity preceded the appearance of conidia. Thus, GAD activity in T. viride is closely associated with its developmental status and may represent a link between differentiation events and energy metabolism.  相似文献   
955.
Iodine deficiency is a factor that may compromise child development, but is not the only one. Other health determinants, some of them outside the healthcare system, are able to influence development. Fighting iodine deficiency may be a pragmatic and useful strategy if it is found to be not maleficent, beneficial to health, and cost-effective, and does not make us lose the notion that child development goes beyond psychomotor or cognitive performance. This article analyzes such constraints from a critical point of view.  相似文献   
956.
A new hypothesis is offered regarding the pathomechanism of generalized epilepsy with spike-wave paroxysms (GESw) based on the pertaining literature and personal investigations. The first section is devoted to a critical overview of the development of theories regarding GESw. The centrencephalic theory, the debate on subcortical versus cortical origin, the "corticoreticular" hypothesis of Gloor and, finally, the "dyshormic" concept of Niedermeyer are outlined. In the next section it is shown that there is a particular optimum zone between sleep and wakefulness and between REM and slow wave sleep which highly favours the occurrence of spike-wave paroxysms. According to our investigations into the dynamics within this critical zone, the spike-wave paroxysms always appear with characteristic fluctuations of the level of consciousness where the changes towards awakening are always followed by rebounds towards sleep. Hence, the dynamic properties of this unstable border zone become especially interesting in the genesis of spike-wave paroxysms. It has been shown that even without epilepsy, a dynamics can be observed in the micro-oscillations in the depth of sleep which could be interpreted according to the reciprocal induction regulation model. In our concept the process of falling asleep emerges from rebounds of the sleep promoting system in response to sensory inputs streaming in from the external environment. According to this model, arousal influences in sleep have a sleep promoting effect. We interpret in this way all synchronized EEG reactions elicited by sensory stimuli and we consider K-complex type synchronization reactions as a "building stone" of the process of falling asleep which contains the whole process in concentrated form. The manifold similarities between the K-complex and the spike-wave pattern are demonstrated. On this basis spike-wave paroxysms can be regarded as an epileptic "caricature" of the sleep induction momentum reflected in the K-complex phenomenon. Hence, the GESw is the epileptic disorder of the sleep promotion function. This hypothesis resolves and explains many contradictory features of our knowledge about this mechanism and gives a new biologically oriented framework for further research. In the light of the hypothesis it has been attempted to interpret some of the characteristic features of the GESw: the genetic determination, the age dependency, the link with the sleep-waking cycle as well as the functional-anatomical characteristics and the symptoms of the seizures.  相似文献   
957.
The influence of several factors on the chemiluminescence (CL) activity of haemocytes from the Mediterranean mussel (Mytilus galloprovincialis) was studied. Haemocytes were stimulated in vitro with different concentrations of zymosan, phorbol 12-myristate 13-acetate (PMA) and lipopolysaccharide (LPS) (adding superoxide dismutase, SOD, to the zymosan-stimulated haemocytes in order to test the specificity of the reaction). The in vitro effect of the clam pathogens Vibrio tapetis (bacteria) and a Perkinsus atlanticus-like protozoan tentatively named Pseudoperkinsus taapetis on the mussel haemocytes CL response was also assessed. To study the in vivo stimulation of haemocytes, mussels were inoculated with zymosan and the CL response of their haemocytes was subsequently measured. Zymosan added in vitro produced the highest CL response, although PMA also enhanced the CL emission and, in addition, increased the zymosan-stimulated CL. LPS and V. tapetis did not activate haemocytes. SOD significantly decreased the CL emission in zymosan-stimulated haemocytes. P. tapetis cells, as well as their extracellular products, inhibited the CL response to zymosan. Haemocytes from mussels injected with zymosan showed lower levels of stimulation than in vitro treated cells, and CL increased with time after injection.  相似文献   
958.
A major challenge in combating the human immunodeficiency virus (HIV) epidemic is the development of vaccines capable of inducing potent, persistent cellular immunity and broadly reactive neutralizing antibody responses to HIV type 1 (HIV-1). We report here the results of a preclinical trial using the chimpanzee model to investigate a combination vaccine strategy involving sequential priming immunizations with different serotypes of adenovirus (Ad)/HIV-1(MN)env/rev recombinants and boosting with an HIV envelope subunit protein, oligomeric HIV(SF162) gp140deltaV2. The immunogenicities of replicating and nonreplicating Ad/HIV-1(MN)env/rev recombinants were compared. Replicating Ad/HIV recombinants were better at eliciting HIV-specific cellular immune responses and better at priming humoral immunity against HIV than nonreplicating Ad-HIV recombinants carrying the same gene insert. Enhanced cellular immunity was manifested by a greater frequency of HIV envelope-specific gamma interferon-secreting peripheral blood lymphocytes and better priming of T-cell proliferative responses. Enhanced humoral immunity was seen in higher anti-envelope binding and neutralizing antibody titers and better induction of antibody-dependent cellular cytotoxicity. More animals primed with replicating Ad recombinants mounted neutralizing antibodies against heterologous R5 viruses after one or two booster immunizations with the mismatched oligomeric HIV-1(SF162) gp140deltaV2 protein. These results support continued development of the replicating Ad-HIV recombinant vaccine approach and suggest that the use of replicating vectors for other vaccines may prove fruitful.  相似文献   
959.
Seed micromorphology of 24 taxa of the genera Anacamptis and Orchis was examined under light and scanning electron microscopy. Seed qualitative characters appear very useful at the supraspecific level in the subtribe Orchidinae. Based on our observations, the sculpturing of the periclinal walls of the medial testa cells, the seed shape and several features of the anticlinal walls of the apical cells showed variability between the studied taxa, with special relevance of the first character. According to this factor, we found one type of seed for Anacamptis and two for Orchis. The taxonomic value of seed coat characters is compared with the last taxonomic proposals for the genera Anacamptis and Orchis.  相似文献   
960.
Auxin modulates a range of plant developmental processes including embryogenesis, organogenesis, and shoot and root development. Recent studies have shown that plant hormones also strongly influence metabolic networks, which results in altered growth phenotypes. Modulating auxin signalling pathways may therefore provide an opportunity to alter crop performance. Here, we performed a detailed physiological and metabolic characterization of tomato (Solanum lycopersicum) mutants with either increased (entire) or reduced (diageotropicadgt) auxin signalling to investigate the consequences of altered auxin signalling on photosynthesis, water use, and primary metabolism. We show that reduced auxin sensitivity in dgt led to anatomical and physiological modifications, including altered stomatal distribution along the leaf blade and reduced stomatal conductance, resulting in clear reductions in both photosynthesis and water loss in detached leaves. By contrast, plants with higher auxin sensitivity (entire) increased the photosynthetic capacity, as deduced by higher Vcmax and Jmax coupled with reduced stomatal limitation. Remarkably, our results demonstrate that auxin‐sensitive mutants (dgt) are characterized by impairments in the usage of starch that led to lower growth, most likely associated with decreased respiration. Collectively, our findings suggest that mutations in different components of the auxin signalling pathway specifically modulate photosynthetic and respiratory processes.  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