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   1860篇
  免费   86篇
  国内免费   1篇
  2022年   11篇
  2021年   39篇
  2020年   30篇
  2019年   24篇
  2018年   44篇
  2017年   33篇
  2016年   50篇
  2015年   70篇
  2014年   95篇
  2013年   136篇
  2012年   131篇
  2011年   143篇
  2010年   85篇
  2009年   77篇
  2008年   104篇
  2007年   95篇
  2006年   107篇
  2005年   76篇
  2004年   61篇
  2003年   62篇
  2002年   46篇
  2001年   36篇
  2000年   26篇
  1999年   27篇
  1998年   12篇
  1997年   9篇
  1996年   13篇
  1995年   18篇
  1994年   14篇
  1993年   9篇
  1992年   18篇
  1991年   7篇
  1989年   8篇
  1988年   6篇
  1985年   9篇
  1984年   8篇
  1982年   10篇
  1981年   7篇
  1979年   6篇
  1978年   6篇
  1976年   6篇
  1975年   7篇
  1973年   7篇
  1972年   11篇
  1971年   8篇
  1967年   8篇
  1961年   7篇
  1957年   8篇
  1956年   7篇
  1927年   8篇
排序方式: 共有1947条查询结果,搜索用时 671 毫秒
111.
Radical paring of the cleft edge during a primary cleft operation or repeated secondary surgeries can result in tightness of the upper lip. The degree of the resulting side-to-side tension can vary, from mild cases for which improvement is sought through realignment of the misplaced oral sphincter muscle in secondary revision, to severe cases for which the possibility of a lip switch flap must be considered. When the lip tightness accompanies more than three-quarters loss of the Cupid's bow, an Abbé flap is an alternative. However, the lip switch flap is far from ideal, in both artistic and functional perspectives, and should be avoided if at all possible in mild to moderate degrees of lip tightness. This study presents a method of correcting horizontal cleft upper lip tightness, especially of the vermilion. The method involves local transfer of an inferiorly based rectangular flap from the relatively redundant upper two-thirds to the lower one-third of the upper lip and vermilion. Primary indications for the technique include vermilion tightness with half to three-quarters loss of Cupid's bow. The method has the advantage of supplementing the horizontal lip dimension on the cleft side and restoring a natural Cupid's bow, thereby repositioning the shifted philtral column and adding fullness to the lower one-third of the upper lip. Incorporation of the upper lip scar in the rectangular flap removes ugly scars and spares the lower lip from surgical violation. The orbicularis sphincter function, as seen in facial animation, was well regained. Twenty unilateral and three bilateral cases with a maximal follow-up period of 4.5 years are presented.  相似文献   
112.
The effect of media composition on microspore culture was investigated in one tetraploid and two diploid potatoes. The viability of microspores isolated from 4.5 to 5 mm buds was in the range of 33 to 52%. In media for anther culture, microspores showed no further development and lost viability within 2 days. In M1 medium containing mineral components, sucrose, uridine, cytidine, myo-inositol, glutamine and lactalbumin hydrolysate, 18 to 37% of microspores underwent mitosis within 14 days. Up to 95% of the divisions were symmetric and produced equal nuclei. Some symmetrically divided microspores eventually produced structures with 3 to 10 nuclei. The proportion of the total microspore population producing multinuclear structures reached 9% in diploid clones responsive to anther culture and 1 to 2% in recalcitrant cv. Borka. Symmetric mitoses in M1 medium were induced in the presence of glutamine and lactalbumin hydrolysate. Nucleosides and myo-inositol had no effect on microspore division. In the absence of all organic components except sucrose, most mitoses were asymmetric, formation of multinuclear structures was reduced and most pollen accumulated starch indicative of gametophytic fate. In complete M1 medium, starch accumulation was suppressed. Suppression also occurred in asymmetrically divided microspores, indicating a direct inhibition of pollen development independent of the mode of microspore division. This inhibitory effect of M1 medium might present a stress which triggers the induction of symmetric microspore division and subsequent formation of multinuclear structures. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
113.
Carbohydrate-peptide esters which mimic the reactivity of sugar 6-phosphates in nonenzymatic glycations were used as model compounds for the study of the Maillard reaction in vitro. We found that intramolecular cyclization of the monosaccharide ester in which the sugar moiety (D-glucose or D-galactose) is linked, through the C-6 hydroxy group, to the C-terminal carboxy group of the endogenous opioid pentapeptide leucine-enkephalin, in methanol as the solvent, resulted in the formation of imidazolidinone diastereoisomers having cis or trans relative geometry of the substituents at the imidazolidinone ring moiety. The diastereoisomeric imidazolidinones were separated and each transformed by hydrolysis into the corresponding D-gluco- and D-galacto-related imidazolidinone products of leucine-enkephalin. Along with the previous evidence that, from the same sugar-peptide esters by changing the reaction conditions Amadori rearrangement products could be obtained [Horvat et al. (1998) J Chem Soc Perkin Trans 1:99–13], the presented results point to the possibility that similar carbohydrate-related imidazolidinones may also be generated in the early stage of the Maillard reaction in vivo.  相似文献   
114.
Bacillus thuringiensis 656-3, isolated from a soil sample collected at mushroom houses, showed high toxicity to mushroom flies, Lycoriella mali and Coboldia fuscipes. B. thuringiensis 656-3 produced bipyramidal inclusions and reacted with the H antiserum of B. thuringiensis subsp. morrisoni (H8a8b). The plasmid and protein profiles of B. thuringiensis 656-3 were similar to those of its reference strain, subsp. morrisoni PG-14. However, PCR analysis using cry gene primers showed that B. thuringiensis 656-3, unlike its reference strain, had cry4A, cry4B, cry10A, cry11A, and cry1Ac genes, suggesting that B. thuringiensis 656-3 was a unique strain with respect to gene type. In addition, B. thuringiensis 656-3 showed a high level of toxicity against mushroom flies, L. mali and C. fuscipes.  相似文献   
115.
Neural stem cells (NSCs) are tissue-specific stem cells with self-renewal potential that can give rise to neurons and glia in vivo and in vitro. The aim of this study was to transplant NSCs as whole neurospheres into intact brain and assess the fate and phenotype of their progeny generated in vivo. We isolated NSCs from E14 foetal rat forebrains and cultured them in basic fibroblast and epidermal growth factor-supplemented serum-free medium in the form of neurospheres in vitro. Neurospheres were transplanted into the intact brains of 2 Wistar rats and after a period of 3 weeks, grafted brains were examined immunohistochemically. Neurospheres formed solid grafts that were found in the lateral ventricle and in the velum interpositum under the hippocampus. The majority of cells in the transplanted tissue were identified as beta-III-tubulin(+), NeuN(+), PanNF(+) and synaptophysin(+) neurons and were accumulated throughout the graft centre. GFAP(+) astrocytes were scattered throughout the entire graft and astrocyte processes delimited the outer and perivascular surfaces. A great number of NG2(+) oligodendrocyte precursors was detected. Nestin(+) endothelial cells were found to line capillaries growing in the transplant. These data indicate that nestin(+) NSCs prevailing in neurospheres differentiate following transplantation into nestin(-) neuronal and glial cells which confirms the multipotency of NSCs. Three weeks posttransplantation neuronal and astrocyte cells reached terminal differentiation (formation of synaptic vesicles and superficial and perivascular limiting membranes) while elements of oligodendroglial cell lineage remained immature. Grafting stem cells as non-dissociated neurospheres provide cells with favourable conditions which facilitate cell survival, proliferation and differentiation. However, in the intact brain, grafted neurosphere cells were not found to integrate with the brain parenchyma and formed a compact structure demarcated from its surroundings.  相似文献   
116.
A double-blind, placebo-controlled clinical trial was performed to investigate the effectiveness of a herbal-based dentifrice in the control of gingivitis. Forty volunteers completed the 84-day study. All subjects were balanced for parameters measured - plaque index (PI), community periodontal index of treatment needs (CPITN) and papillary bleeding index (PBI). The dentifrice was effective in reducing symptoms of gingivitis as evaluated by the CPITN and PBI indexes.  相似文献   
117.
Nanoseconds long molecular dynamics (MD) trajectories of differently active complexes of human cyclin-dependent kinase 2 (inactive CDK2/ATP, semiactive CDK2/Cyclin A/ATP, fully active pT160-CDK2/Cyclin A/ATP, inhibited pT14-; pY15-; and pT14,pY15,pT160-CDK2/Cyclin A/ATP) were compared. The MD simulations results of CDK2 inhibition by phosphorylation at T14 and/or Y15 sites provide insight into the structural aspects of CDK2 deactivation. The inhibitory sites are localized in the glycine-rich loop (G-loop) positioned opposite the activation T-loop. Phosphorylation of T14 and both inhibitory sites T14 and Y15 together causes ATP misalignment for phosphorylation and G-loop conformational change. This conformational change leads to the opening of the CDK2 substrate binding box. The phosphorylated Y15 residue negatively affects substrate binding or its correct alignment for ATP terminal phospho-group transfer to the CDK2 substrate. The MD simulations of the CDK2 activation process provide results in agreement with previous X-ray data.  相似文献   
118.
We report full-term development of nuclear transfer embryos following nuclear exchange at the 2-cell stage. Nuclei from 2-cell rat embryos were transferred into enucleated 2-cell embryos and developed to term after transfer to recipients (NT2). Pronuclear exchange in zygotes was used for comparison (NT1). Zygotes and 2-cell embryos were harvested from 4-week-old female Sprague-Dawley rats. Nuclear transfer was performed by transferring the pronuclei or karyoplasts into the perivitelline space of recipient embryos followed by electrofusion to reconstruct embryos. Fused couplets were cultured for 4 or 24 h before being transferred into day 1 pseudopregnant recipients (Hooded Wistar) at the 1- or 2-cell stage. In vitro culture was also carried out to check the developmental competence of the embryos. In vitro development to the blastocyst stage was not significantly different between the two groups (NT1, 34.3%; NT2, 45.0%). Two of three recipients from NT1 and two of five recipients from NT2 became pregnant. Six pups (3 from NT1, 3 from NT2) were delivered from the four foster mothers. Three female pups survived; 2 from NT1 and 1 from NT2. At 2 months of age these pups appeared healthy, and were mated with Sprague-Dawley males. One rat derived from NT1 delivered 15 pups (5 males, 10 females) as did the rat from NT2 (7 males, 8 females). Our results show that by using karyoplasts from 2-cell stage embryos as nuclear donors and reconstructing them with enucleated 2-cell embryos, healthy rats can be produced.  相似文献   
119.
Dai Q  Zajicek J  Castellino FJ  Prorok M 《Biochemistry》2003,42(43):12511-12521
The association of a ligand with its cognate cell surface receptor can be facilitated by interactions between the ligand and the lipid phase of the cell membrane. With respect to the N-methyl-D-aspartate receptor (NMDAR), we have previously established a low affinity, nonreceptor-mediated interaction of the peptidic conantokins with synaptic membranes in conjunction with a high affinity binding to the NMDARs present therein [Klein, R. C., Prorok, M., and Castellino, F. J. (2003) J. Pept. Res. 61, 307-317]. In the current study, several techniques including size-exclusion chromatography, circular dichroism, fluorescence, and NMR spectroscopies were used to investigate the binding, conformation, and orientation of conantokins and their variants to a variety of phospholipid (PL) vesicles and multilayers. We have found that conantokins bind to PLs and that the effectors Ca(2+) and spermine slightly increase this binding ability. The conantokins preserve a high degree of helical conformation when bound to vesicles in the presence of Ca(2+). In the absence of Ca(2+), only conantokin-G (con-G) manifests an increase in conantokin helicity with increasing vesicle concentration. In solution, the conantokins appear to be localized at the headgroup of vesicles and do not insert into the hydrophobic core of the bilayer. On aligned PL films, the helical axis of the conantokins can either reside normal to the membrane surface or partition in a parallel orientation, depending on the nature of the conantokins and the PLs used. These orientation preferences may be conjoined with the biological activities of the conantokins.  相似文献   
120.
We calculated occurrences of all dinucleotide and trinucleotide microsatellites in the human, mouse, and yeast genomes. The microsatellites were considered separately not only according to the repeated dinucleotide or trinucleotide and the microsatellite length but also according to the starting/terminal nucleotide. The analysis showed that dramatically non-equal amounts occurred in the human genome of microsatellites that differed only by the terminal nucleotides. For example, the 23-mer (TTG)(7)TT occurs 635 times in the human genome whereas (GTT)(7)GT is present only three times in the human genome though the two 23-mers share a 22 nucleotide sequence. The dramatically non-equal occurrences of microsatellites differing only by the terminal nucleotides are observed for most dinucleotide and trinucleotide microsatellites and in all analyzed genomes. We suppose that the strikingly non-equal genomic occurrences of these closely related microsatellites originate from conformational properties of DNA.  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