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   552篇
  免费   40篇
  国内免费   1篇
  2023年   4篇
  2022年   1篇
  2021年   7篇
  2020年   18篇
  2019年   41篇
  2018年   30篇
  2017年   11篇
  2016年   13篇
  2015年   23篇
  2014年   15篇
  2013年   30篇
  2012年   13篇
  2011年   14篇
  2010年   23篇
  2009年   12篇
  2008年   25篇
  2007年   20篇
  2006年   25篇
  2005年   17篇
  2004年   12篇
  2003年   9篇
  2002年   16篇
  2001年   23篇
  2000年   16篇
  1999年   17篇
  1998年   18篇
  1997年   9篇
  1996年   20篇
  1995年   17篇
  1994年   5篇
  1993年   12篇
  1992年   11篇
  1991年   4篇
  1990年   6篇
  1989年   13篇
  1988年   5篇
  1987年   7篇
  1986年   3篇
  1985年   3篇
  1984年   5篇
  1983年   3篇
  1982年   3篇
  1981年   2篇
  1980年   4篇
  1979年   1篇
  1978年   3篇
  1976年   2篇
  1974年   1篇
  1973年   1篇
排序方式: 共有593条查询结果,搜索用时 15 毫秒
541.
In this study, the cell-associated and extracellular peptidases of Trypanosoma cruzi grown in modified Roitman’s complex (MRC) medium were analyzed by measuring peptidase activity in gelatin-containing zymograms. Our results showed that the cell-associated peptidases as well as peptidases extracellularly released by T. cruzi displayed two distinct proteolytic classes: cysteine and metallopeptidase activities. The major cysteine peptidase, cruzipain, synthesized by T. cruzi cells was detected in cellular parasite content, as a 50 kDa reactive polypeptide, after probing with anti-cruzipain antibody. In addition, metallo-type peptidases belonging to the matrix metallopeptidase-9 (MMP-9) family were revealed, after Western blotting, as a 97 kDa protein band in cellular extract and an 85 kDa polypeptide in both cellular and secreted parasite extracts. The MMP-9-like activity present in cells and spent culture medium was immunoprecipitated by an anti-MMP-9 polyclonal antibody. The surface location of MMP-9-like proteins in T. cruzi was also evidenced by means of flow cytometry analysis. Furthermore, doxycycline that has direct MMP-9 inhibiting properties in vitro, inhibited MMP-9-like activities in gel zymography, immunoprecipitation and flow cytometry analyses. This is the first report of the presence of MMP-9-like molecules in T. cruzi. The presence of a matrix extracellular-degrading enzyme may play a role in the T. cruzi-host cell interaction, making this enzyme a potential target for future drug development against this pathogenic trypanosomatid.  相似文献   
542.
Incubation of T. cruzi epimastigotes with the lectin Cramoll 1,4 in Ca2+ containing medium led to agglutination and inhibition of cell proliferation. The lectin (50 μg/ml) induced plasma membrane permeabilization followed by Ca2+ influx and mitochondrial Ca2+ accumulation, a result that resembles the classical effect of digitonin. Cramoll 1,4 stimulated (five-fold) mitochondrial reactive oxygen species (ROS) production, significantly decreased the electrical mitochondrial membrane potential (ΔΨm) and impaired ADP phosphorylation. The rate of uncoupled respiration in epimastigotes was not affected by Cramoll 1,4 plus Ca2+ treatment, but oligomycin-induced resting respiration was 65% higher in treated cells than in controls. Experiments using T. cruzi mitochondrial fractions showed that, in contrast to digitonin, the lectin significantly decreased ΔΨm by a mechanism sensitive to EGTA. In agreement with the results showing plasma membrane permeabilization and impairment of oxidative phosphorylation by the lectin, fluorescence microscopy experiments using propidium iodide revealed that Cramoll 1,4 induced epimastigotes death by necrosis.  相似文献   
543.
In a recent study, we reported that the combined average mutation rate of 10 di-, 6 tri-, and 8 tetranucleotide repeats in Drosophila melanogaster was 6.3 x 10(-6) mutations per locus per generation, a rate substantially below that of microsatellite repeat units in mammals studied to date (range = 10(-2)-10(-5) per locus per generation). To obtain a more precise estimate of mutation rate for dinucleotide repeat motifs alone, we assayed 39 new dinucleotide repeat microsatellite loci in the mutation accumulation lines from our earlier study. Our estimate of mutation rate for a total of 49 dinucleotide repeats is 9.3 x 10(-6) per locus per generation, only slightly higher than the estimate from our earlier study. We also estimated the relative difference in microsatellite mutation rate among di-, tri-, and tetranucleotide repeats in the genome of D. melanogaster using a method based on population variation, and we found that tri- and tetranucleotide repeats mutate at rates 6.4 and 8.4 times slower than that of dinucleotide repeats, respectively. The slower mutation rates of tri- and tetranucleotide repeats appear to be associated with a relatively short repeat unit length of these repeat motifs in the genome of D. melanogaster. A positive correlation between repeat unit length and allelic variation suggests that mutation rate increases as the repeat unit lengths of microsatellites increase.   相似文献   
544.
Although cloning of mammals has been achieved successfully, the percentage of live offspring is very low because of reduced fetal size and fewer implantation sites. Recent studies have attributed such pathological conditions to abnormal reprogramming of the donor cell used for cloning. The inability of the oocyte to fully restore the differentiated status of a somatic cell to its pluripotent and undifferentiated state is normally evidenced by aberrant DNA methylation patterns established throughout the genome during development to blastocyst. These aberrant methylation patterns are associated with abnormal expression of imprinted genes, which among other genes are essential for normal embryo development and gestation. We hypothesized that embryo loss and low implantation rates in cattle derived by somatic cell nuclear transfer (SCNT) are caused by abnormal epigenetic reprogramming of imprinted genes. To verify our hypothesis, we analyzed the parental expression and the differentially methylated domain (DMD) methylation status of the H19 gene. Using a parental-specific analysis, we confirmed for the first time that H19 biallelic expression is tightly associated with a severe demethylation of the paternal H19 DMD in SCNT embryos, suggesting that these epigenetic anomalies to the H19 locus could be directly responsible for the reduced size and low implantation rates of cloned embryos in cattle.  相似文献   
545.
Preface     
"Although few people die of pain,many people die in pain and even more live in pain"——IASP On October11,2004,International Association for the Study of Pain(IASP)  相似文献   
546.
Summary The morphology of the uterine microvasculature during early placentation was investigated by light microscopy, scanning electron microscopy of microvascular corrosion casts and transmission electron microscopy in rats 26 and 50 h after initiation of implantation. Increased vascular permeability at implantation sites was observed as a positive blue-dye test, spacing of vessels, and as localized extravasations of resin from postcapillary venules in the center of the endometrium. The subepithelial capillary plexus in the primary decidual zone adjacent to the blastocyst was shut down 50 h after initiation of implantation, most probably due to swelling of the metabolically activated endothelium and volume expansion of the decidual cells. This phenomenon coincided with the mesometrial orientation of the inner cell mass of the blastocyst; it may be a uterine mechanism to direct the ectoplacental cone toward the patent vessels in the mesometrial portion of the uterus. The remaining vessels at implantation sites were generally fewer, larger in diameter, more irregular in caliber, and more uniformly oriented along the implantation axis than their counterparts at inter-implantation sites. No vascular sprouts were observed during the interval studied.  相似文献   
547.
Leishmania are protozoa that invade mononuclear phagocytes with the involvement of different ligand-receptor systems, including mannose receptors. Until now, scant data are available concerning the mechanisms that govern the infection of Leishmania in other host cell types such as fibroblasts. Our aim was to analyze the expression of mannose receptors in primary cultures of skin fibroblasts (SF) further characterizing their role during the invasion of promastigotes of Leishmania (L.) amazonensis. Both fluorescent, light, and electron microscopy assays revealed that SF have mannose receptors since they bound and internalized mannosylated ligands in addition to being positively labeled by fuc-BSA-FITC probes. d-mannose competition assays revealed the participation of mannose receptors during the parasite association with SF presenting upregulated receptor expression during the initial steps of the infection. After longer periods of Leishmania:fibroblasts contact, the modulation noted in the host mannose receptors was reverted concomitantly to the infection control, suggesting that the parasites were required for the alteration maintenance and providing evidences that the SF may display microbicidal mechanisms to control the Leishmania infection.  相似文献   
548.
We investigated the effect of a subminimal concentration of penicillin on the ultrastructure and protein profile of Group G streptococci. In cells treated with penicillin (1/3 MIC), the protein content increased by 50%, and several protein bands with a molecular mass of 14–70 kDa were detected. In the hydrophilic phase, carbohydrate-containing proteins were detected by PAS staining, and in the hydrophobic phase, a group of proteins that reacted strongly with homologous antisera were observed. In terms of cell structure, Triton X-114 extraction was found to induce alterations in the cross wall of untreated cells. In bacteria treated with penicillin but not extracted with Triton X-114, the cell wall was observed to detach itself, and regions with reduced amounts of cellular material appeared in the cytoplasm. After Triton-X114 extraction, these penicillin-treated cells exhibited profound morphological changes, leading in some cases to lysis.  相似文献   
549.
550.
Background  ERCP-directed brush cytology is used to sample lesions of the pancreatic and biliary ducts and the ampulla of Vater. With conventional preparations, the sensitivity and specificity range from 44% to 63% and 80% to 98%, respectively, and increased N : C ratio, nuclear molding and loss of honeycombing are reliable features of malignancy. The performance and morphology of specimens prepared by ThinPrep, a liquid-based cytology technique is mostly unknown.
Methods  The laboratory information system was searched for all cases prepared by ThinPrep. Patient disease classification of benign or malignant was determined by linkage with the provincial cancer registry and was the gold standard against which sensitivity, specificity, positive (PPV) and negative predictive values (NPV) were calculated. True positives and negatives were reviewed to identify predictive cytomorphologic features.
Results  Between 1996 and 2001, there were 149 ThinPrep specimens; 55 (37%) were reported as positive for malignancy and 94 (63%) as negative. Disease was classified as malignant in 86 (58%) patients and benign in 63 (42%). There were 42 false negative, 11 false positive, 52 true negative, and 44 true positive cytology results. Sensitivity was 51.2% (CI; 40.2 : 62.0), specificity 82.5% (CI; 70.5 : 90.6), and PPV and NPV 80.0% (CI; 66.6 : 89) and 55.3% (CI; 44.7 : 65.5), respectively. Cell groups with crowded, enlarged, irregular nuclei and nuclear features of vesicular chromatin and large, multiple irregular nucleoli correlated with malignant disease, while monolayered sheets of uniform columnar cells, regular nuclei and a finely granular chromatin correlated with benign disease.
Conclusions  The performance of ThinPrep brushings from this anatomic site equals conventional preparations. Cytomorphologic features of malignancy are more frequent and pronounced with ThinPrep.  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