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   22088篇
  免费   1885篇
  国内免费   1442篇
  25415篇
  2024年   51篇
  2023年   233篇
  2022年   579篇
  2021年   861篇
  2020年   625篇
  2019年   788篇
  2018年   838篇
  2017年   628篇
  2016年   935篇
  2015年   1415篇
  2014年   1641篇
  2013年   1729篇
  2012年   2052篇
  2011年   1946篇
  2010年   1165篇
  2009年   1119篇
  2008年   1281篇
  2007年   1175篇
  2006年   1076篇
  2005年   903篇
  2004年   873篇
  2003年   701篇
  2002年   563篇
  2001年   367篇
  2000年   294篇
  1999年   259篇
  1998年   195篇
  1997年   153篇
  1996年   150篇
  1995年   119篇
  1994年   109篇
  1993年   67篇
  1992年   89篇
  1991年   72篇
  1990年   81篇
  1989年   62篇
  1988年   44篇
  1987年   45篇
  1986年   28篇
  1985年   27篇
  1984年   33篇
  1983年   13篇
  1982年   10篇
  1981年   7篇
  1980年   2篇
  1978年   2篇
  1967年   2篇
  1964年   1篇
  1962年   1篇
  1950年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Fifty normal noninfarct patients and 12 cases with infarcts of the cerebrum were examined with routine magnetic resonance imaging and echo-planar diffusion-weighted imaging. The diffusion-weighted three-dimensional images were reconstructed with volume-rendering processing on workstation. Precentral gyrus, post-central gyrus, superior parietal lobule, superior frontal gyrus, precentral sulcus, central sulcus, postcentral sulcus, intraparietal sulcus and superior frontal sulcus were best shown of all structures with an arbitrary score of 2.61–2.77. Supramarginal gyrus, middle frontal gyrus, inferior frontal gyrus and lateral sulcus were clearly shown in the majority of the cerebra with average scores of 2.0–2.49; angular gyrus, inferior frontal sulcus and superior temporal gyrus were not demonstrated satisfactorily and their average scores were 1.67–1.89. Middle temporal gyrus, inferior temporal gyrus, superior temporal sulcus and inferior temporal sulcus were difficult to identify, and thus had average scores of 0.87–1.26. Brain surface structures were better displayed in the older group of individuals than in the younger group. The structures in the 12 cases with acute or chronic cerebrum infarcts were also satisfactorily demonstrated with this new technique.  相似文献   
102.
Hyporesponsiveness to growth factors is one of the fundamental characteristics of senescent cells. We previously reported that the up-regulation of caveolin attenuates the growth factor response and the subsequent downstream signal cascades in senescent human diploid fibroblasts. Therefore, in the present experiment, we investigated the modulation of caveolin status in senescent cells to determine the effect of caveolin on mitogenic signaling efficiency and cell cycling. We reduced the level of caveolin-1 in senescent human diploid fibroblasts using its antisense oligonucleotides and small interfering RNA, and this resulted in the restoration of normal growth factor responses such as the increased phosphorylation of Erk, the nuclear translocation of p-Erk, and the subsequent activation of p-Elk upon epidermal growth factor stimulation. Moreover, DNA synthesis and the re-entry of senescent cells into cell cycle were resumed upon epidermal growth factor stimulation concomitantly with decreases in p53 and p21. Taken together, we conclude that the loss of mitogenic signaling in senescent cells is strongly related to their elevated levels of caveolin-1 and that the functional recovery of senescent cells at least in the terms of growth factor responsiveness and cell cycle entry might be achieved simply by lowering the caveolin level.  相似文献   
103.
104.
Hong MY  Lee EM  Jo YH  Park HC  Kim SR  Hwang JS  Jin BR  Kang PD  Kim KG  Han YS  Kim I 《Gene》2008,413(1-2):49-57
The 15,360-bp long complete mitogenome of Caligula boisduvalii possesses a gene arrangement and content identical to other completely sequenced lepidopteran mitogenomes, but different from the common arrangement found in most insect order, as the result of the movement of tRNA(Met) to a position 5'-upstream of tRNA Ile. The 330-bp A+T-rich region is apparently capable of forming a stem-and-loop structure, which harbors the conserved flanking sequences at both ends. Dissimilar to what has been seen in other sequenced lepidopteran insects, the initiation codon for C. boisduvalii COI appears to be TTG, which is a rare, but apparently possible initiation codon. The ATP8, ATP6, ND4L, and ND6 genes, which neighbor another PCG at their 3' end, all harbored potential sequences for the formation of a hairpin structure. This is suggestive of the importance of such structures for the precise cleavage of the mRNA of mature PCGs. Phylogenetic analyses of available sequenced species of Bombycoidea, Pyraloidea, and Tortricidea supported the morphology-based current hypothesis that Bombycoidea and Pyraloidea are monophyletic (Obtectomera). As previously suggested, Bombycidae (Bombyx mori and B. mandarina) and Saturniidae (Antheraea pernyi and C. boisduvalii) formed a reciprocal monophyletic group.  相似文献   
105.

Background

The Supreme? laryngeal mask airway (SLMA) is a single-use LMA with double lumen design that allows separation of the respiratory and the alimentary tract, hence potentially reducing the gastric volume and risk of aspiration. The purpose of this prospective cohort study is to evaluate the the role of the SLMA as an airway technique for women undergoing category 2 and 3 Cesarean delivery under general anesthesia.

Methods

We recruited 584 parturients who underwent category 2 or 3 Cesarean delivery under general anesthesia, in which 193 parturients underwent category 2 and 391 parturients underwent category 3 Cesarean delivery. The primary outcome was insertion success rate at 1st attempt in SLMA insertion. The secondary outcomes included anaesthetic, obstetric outcomes and maternal side effects associated with airway device.

Results

The 1st attempt insertion success rate was 98.3%, while the overall insertion success rate was 100%. The mean (Standard deviation) time to effective ventilation was 15.6 (4.4) seconds. Orogastric tube insertion was successful at the 1st attempt in all parturients. There was no clinical evidence of aspiration or regurgitation. No episodes of hypoxemia, laryngospasm or bronchospasm were observed intra-operatively. The incidence of complications was low and with good maternal satisfaction reported.

Conclusions

The SLMA could be an alternative effective airway in category 2 and 3 parturients emergency Cesarean Delivery under general anesthesia in a carefully-selected obstetric population.

Trial registration

Clinical Trials Registration: Clinicaltrials.gov Registration NCT02026882. Registered on December 31, 2013.
  相似文献   
106.
Fourteen title compounds, 1-substituted-5-substitutedphenylthio-4-pyrazolaldoxime ester derivatives 4a-4n, were synthesized from the starting material 1-substitutedphenyl-3-methyl-5-substitutedphenylthio-4-pyrazolaldoximes 3 by treatment with acyl chloride. The synthesized compounds were characterized by physical constants, and the structures of the title compounds were further confirmed by IR, 1H NMR, 13C NMR and elemental analysis. The bioassay results showed that title compounds possessed weak to good anti-TMV bioactivity with 4l showing significant enhancement of disease resistance in tobacco leaves with high affinity for TMV CP.  相似文献   
107.
Li JM  Jin ZX 《Genetica》2008,133(3):227-234
Inter-simple sequence repeat (ISSR) markers were used to determine the genetic variation and genetic differentiation of nine populations of Emmenopterys henryi Oliv., an endangered plant endemic to China. Relatively low genetic diversity was detected at population level (the percentage of polymorphic loci P=22.56%, the number of alleles per locus A=1.183+/-0.045, the effective number of alleles per locus A(E)=1.007+/-0.345, Nei's gene diversity h=0.071+/-0.017, Shannon information index I=0.104+/-0.025). However, the genetic diversity at species level was relatively high (P=56.05%; A=1.561+/-0.498, A(E)=1.325+/-0.371, h=0.191+/-0.199, I=0.287+/-0.284). Analysis of molecular variance showed that most of the ISSR variation (68.03%) in E. henryi occurred among populations. The estimated Nm from F (ST )was 0.235. It indicated that the fragmentation and isolation of populations might result from specific evolutionary history and anthropogenic activity. Consequently, genetic drift might play an important role in determining the genetic structure of E. henryi. Conservation strategies for this endangered species are proposed based on the genetic data.  相似文献   
108.
Wong KY  So CC  Loong F  Chung LP  Lam WW  Liang R  Li GK  Jin DY  Chim CS 《PloS one》2011,6(4):e19027
miR-124-1 is a tumour suppressor microRNA (miR). Epigenetic deregulation of miRs is implicated in carcinogenesis. Promoter DNA methylation and histone modification of miR-124-1 was studied in 5 normal marrow controls, 4 lymphoma, 8 multiple myeloma (MM) cell lines, 230 diagnostic primary samples of acute myeloid leukaemia (AML), acute lymphoblastic leukaemia (ALL), chronic myeloid leukaemia (CML), chronic lymphocytic leukaemia (CLL), MM, and non-Hodgkin's lymphoma (NHL), and 53 MM samples at stable disease or relapse. Promoter of miR-124-1 was unmethylated in normal controls but homozygously methylated in 4 of 4 lymphoma and 4 of 8 myeloma cell lines. Treatment of 5-Aza-2'-deoxycytidine led to miR-124-1 demethylation and re-expression of mature miR-124, which also associated with emergence of euchromatic trimethyl H3K4 and consequent downregulation of CDK6 in myeloma cells harboring homozygous miR-124-1 methylation. In primary samples at diagnosis, miR-124-1 methylation was absent in CML but detected in 2% each of MM at diagnosis and relapse/progression, 5% ALL, 15% AML, 14% CLL and 58.1% of NHL (p<0.001). Amongst lymphoid malignancies, miR-124-1 was preferentially methylated in NHL than MM, CLL or ALL. In primary lymphoma samples, miR-124-1 was preferentially hypermethylated in B- or NK/T-cell lymphomas and associated with reduced miR-124 expression. In conclusion, miR-124-1 was hypermethylated in a tumour-specific manner, with a heterochromatic histone configuration. Hypomethylation led to partial restoration of euchromatic histone code and miR re-expression. Infrequent miR-124-1 methylation detected in diagnostic and relapse MM samples showed an unimportant role in MM pathogenesis, despite frequent methylation found in cell lines. Amongst haematological cancers, miR-124-1 was more frequently hypermethylated in NHL, and hence warrants further study.  相似文献   
109.
110.
PIWI‐interacting RNAs (piRNAs) silence transposons in germ cells to maintain genome stability and animal fertility. Rhino, a rapidly evolving heterochromatin protein 1 (HP1) family protein, binds Deadlock in a species‐specific manner and so defines the piRNA‐producing loci in the Drosophila genome. Here, we determine the crystal structures of Rhino‐Deadlock complex in Drosophila melanogaster and simulans. In both species, one Rhino binds the N‐terminal helix–hairpin–helix motif of one Deadlock protein through a novel interface formed by the beta‐sheet in the Rhino chromoshadow domain. Disrupting the interface leads to infertility and transposon hyperactivation in flies. Our structural and functional experiments indicate that electrostatic repulsion at the interaction interface causes cross‐species incompatibility between the sibling species. By determining the molecular architecture of this piRNA‐producing machinery, we discover a novel HP1‐partner interacting mode that is crucial to piRNA biogenesis and transposon silencing. We thus explain the cross‐species incompatibility of two sibling species at the molecular level.  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