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   4522篇
  免费   345篇
  国内免费   2篇
  4869篇
  2023年   24篇
  2022年   46篇
  2021年   106篇
  2020年   82篇
  2019年   81篇
  2018年   122篇
  2017年   67篇
  2016年   150篇
  2015年   234篇
  2014年   267篇
  2013年   318篇
  2012年   430篇
  2011年   333篇
  2010年   226篇
  2009年   232篇
  2008年   255篇
  2007年   230篇
  2006年   251篇
  2005年   218篇
  2004年   218篇
  2003年   197篇
  2002年   193篇
  2001年   64篇
  2000年   47篇
  1999年   40篇
  1998年   66篇
  1997年   54篇
  1996年   46篇
  1995年   38篇
  1994年   26篇
  1993年   31篇
  1992年   26篇
  1991年   26篇
  1990年   12篇
  1989年   8篇
  1988年   11篇
  1987年   13篇
  1986年   7篇
  1985年   4篇
  1984年   8篇
  1982年   9篇
  1981年   9篇
  1980年   4篇
  1979年   4篇
  1978年   8篇
  1977年   4篇
  1975年   3篇
  1972年   4篇
  1971年   4篇
  1970年   2篇
排序方式: 共有4869条查询结果,搜索用时 15 毫秒
991.
An ultrastructural study of the manchette was carried out on 21 species belonging to the culicid genera Aedes, Anopheles, Culex and Toxorhynchites . Each genus has a characteristic morphological type of manchette. Toxorhynchites seems to have the type found in many other Diptera, while the other genera have unusual types.  相似文献   
992.
Bigard, Xavier A., Chantal Janmot, Danièle Merino,Françoise Lienhard, Yannick C. Guezennec, and Anne D'Albis.Endurance training affects myosin heavy chain phenotype inregenerating fast-twitch muscle. J. Appl.Physiol. 81(6): 2658-2665, 1996.The aim of thisstudy was to analyze the effects of treadmill training (2 h/day, 5 days/wk, 30 m/min, 7% grade for 5 wk) on the expression of myosinheavy chain (MHC) isoforms during and after regeneration of afast-twitch white muscle [extensor digitorum longus (EDL)]. Male Wistar rats were randomly assigned to a sedentary(n = 10) or an endurance-trained (ET;n = 10) group. EDL muscle degeneration and regeneration were induced by two subcutaneous injections of a snaketoxin. Five days after induction of muscle injury, animals were trainedover a 5-wk period. It was verified that ~40 days after venomtreatment, central nuclei were present in the treated EDL muscles fromsedentary and ET rats. The changes in the expression of MHCs in EDLmuscles were detected by using a combination of biochemical andimmunocytochemical approaches. Compared with contralateral nondegenerated muscles, relative concentrations of types I, IIa, andIIx MHC isoforms in ET rats were greater in regenerated EDL muscles(146%, P < 0.05; 76%,P < 0.01; 87%,P < 0.01, respectively). Their elevation corresponded to a decreasein the relative concentration of type IIb MHC (36%,P < 0.01). Although type I accountedfor only 3.2% of total myosin in regenerated muscles from the ETgroup, the cytochemical analysis showed that the proportion of positive staining with the slow MHC antibody was markedly greater in regenerated muscles than in contralateral ones. Collectively, these results demonstrate that the regenerated EDL muscle is sensitive to endurance training and suggest that the training-induced shift in MHC isoforms observed in these muscles resulted from an additive effect of regeneration and repeated exercise.

  相似文献   
993.
Summary A proinsulin analog constructed by site-directed mutagenesis was expressed as a fusion protein that formed inclusion bodies inside the cells. It was purified from the isolated inclusion bodies and proinsulin was obtained by trifluoro-acetic acid, dimethyl sulfoxide and hydrochloric acid cleavage. The released proinsulin analog was confirmed by its molecular weight as determined by SDS-PAGE.  相似文献   
994.

Background

During the last decade, the use of 5-aminolevulinic acid (5-ALA) has been steadily increasing in neurosurgery. The study''s main objectives were to prospectively evaluate the effectiveness and safety of 5-ALA when used in clinical practice setting on high-grade gliomas’ patients.

Methods

National, multicenter and prospective observational study. Inclusion criteria: authorized conditions of use of 5-ALA. Exclusion criteria: contraindication to 5-ALA, inoperable or partial resected tumors, pregnancy and children. Epidemiological, clinical, laboratory, radiological, and safety data were collected. Effectiveness was assessed using complete resection of the tumor, and progression-free and overall survival probabilities.

Results

Between May 2010 and September 2014, 85 patients treated with 5-ALA were included, and 77 were suitable for the effectiveness analysis. Complete resection was achieved in 41 patients (54%). Surgeons considered suboptimal the fluorescence of 5-ALA in 40% of the patients assessed. The median duration of follow-up was 12.3 months. The progression-free survival probability at 6 months was 58%. The median duration overall survival was 14.2 months. Progression tumor risk factors were grade of glioma, age and resection degree; and death risk factors were grade of glioma and gender. No severe adverse effects were reported. At one month after surgery, new or increased neurological morbidity was 6.5%. Hepatic enzymes were frequently increased within the first month after surgery; however, they subsequently normalized, and this was found to have no clinical significance.

Conclusion

In clinical practice, the 5-ALA showed a good safety profile, but the benefits related to 5-ALA have not been yet clearly shown. The improved differentiation expected by fluorescence between normal and tumor cerebral tissue was suboptimal in a relevant number of patients; in addition, the expected higher degree of resection was lower than in clinical trials as well as incomplete resection was not identified as a prognostic factor risk for death. Because optimal fluorescence was correlated to higher complete resection rate, further research is needed to identify patients (or tumors) with more surgery benefits when using the 5-ALA.  相似文献   
995.
Cystic fibrosis in Finland: a molecular and genealogical study   总被引:4,自引:4,他引:0  
Summary The incidence of cystic fibrosis (CF) in Finland is one tenth that in other Caucasian populations. To study the genetics of CF in Finland, we used a combined molecular and genealogical approach. Out of the 20 Finnish families with a living CF patient, 19 were typed for eight closely linked restriction fragment length polymorphisms (RFLP) at the MET, D7S8, and D7S23 loci. The birthplaces of the parents and grandparents were traced using population registries. Allele and haplotype frequencies in Finland are similar to those of other European and North American populations, but are modified by sampling: two regional CF gene clusters, evidently the results of a founder effect, were identified. Generally, the gene was evenly distributed over the population, carrier frequency being estimated at approximately 1.3%. We conclude that CF in Finland is caused by the common Caucasian mutation(s), and that the low frequency of the gene can be explained by a negative sampling effect and genetic drift.  相似文献   
996.
Summary We studied the wintering of European Starlings by means of ringing recoveries since 1960 and the distribution of roosting sites in the last fifty years. An overall decrease was found in the number of roosts as well as in wintering birds. However, this is due to a a considerable decrease in the south and east, whereas in the north and northeast the number of wintering birds seems to be stable ore even lightly increasing. This geographical difference can be correlated with the decline of breeding populations in North and Central Europa and/ore with the growing populations in France and Belgium respectively. In the last decade, the number of urban roosts in Spain increased.  相似文献   
997.
Tanymastix stagnalis is known from seven locations in Ireland. These range from small to large temporary water bodies. It was earlier shown that the best hatching and survival of eggs and young stages occurred in the presence of soil and at temperatures between about 10° and 15°. Some eggs hatched after one drying period, others after up to at least 4 drying and wetting periods. It was found from field observations that the habitat appeared after a minimum of 200 mm of rain in a 2-month period. This occurred on average more than once per year, mostly between June and February. A computer model was constructed using field data, and meteorological data (rainfall and temperature) covering 100 years, to decide how secure the organism was in two contrasting habitats. This showed that in both habitats there was little likelihood of extinction from metorological causes. There was, however, the possibility of destruction of the smaller habitats either through drainage or conversion to permanent ponds through compaction of the soil by heavy farm animals. The distribution and the possibility of harvesting the organisms is briefly discussed.  相似文献   
998.
Summary The probe pCS. 7, isolated from an HpaII tiny fragments (HTF) island and tightly linked to the mutation causing cystic fibrosis (CF), detects a polymorphism with the rare-cutter restriction enzymeBssHII. In a single digest, the resulting restriction fragment length polymorphism (RFLP) cannot be detected by either conventional or pulse-field gel electrophoresis. A double digest withBssHII in conjunction with a six-cutter restriction enzyme that does not recognize a site containing a CpG dinucleotide enables the probe to be used routinely for prenatal diagnosis and carrier exclusion. This strategy can be used to identify polymorphisms in HTF islands.  相似文献   
999.
The incidence of cystic fibrosis (CF) in Finland, 125 000 newborn, is one of the lowest in Caucasian populations. The F508 mutation accounts for 18/40 (45%) of CF chromosomes in Finland. Other mutations were therefore sought among the remaining 55%. Twelve out of 40 chromosomes (30%) were found to carry 394delTT, whereas G542X and 3732delA were each detected in one chromosome. Eight mutations remained unidentified using a testing panel for 26 mutations. Mutation 394delTT was associated exclusively with haplotype 23-36-13. Five unknown mutations were associated with different haplotypes for microsatellite markers, whereas three shared the same haplotype. Most F508 mutations and all unidentified mutations originated from regions of old and dense settlement in the coastal regions, whereas 394delTT was geographically clustered and enriched in a rural location, consistent with a local founder effect. The remote location of Finland and her population history give a plausible explanation for the rarity of CF in Finland.  相似文献   
1000.
Analysis of expressed sequence tags from oil palm (Elaeis guineensis)   总被引:3,自引:0,他引:3  
This is the first report of a systematic study of genes expressed by means of expressed sequence tag (EST) analysis in oil palm, a species of the Arecales order, a phylogenetically key clade of monocotyledons that is not widely represented in the sequence databases. Five different cDNA libraries were generated from male and female inflorescences, shoot apices and zygotic embryos and unidirectional systematic sequencing was performed. A total of 2411 valid EST sequences were thus obtained. Cluster analysis enabled the identification of 209 groups of related sequences and 1874 singletons. Putative functions were assigned to 1252 of the set of 2083 non-redundant ESTs obtained. The EST database described here is a first step towards gene discovery and cDNA array-based expression analysis in oil palm.  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