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   2334篇
  免费   165篇
  国内免费   1篇
  2023年   8篇
  2021年   21篇
  2020年   12篇
  2019年   23篇
  2018年   33篇
  2017年   45篇
  2016年   60篇
  2015年   63篇
  2014年   92篇
  2013年   128篇
  2012年   135篇
  2011年   144篇
  2010年   95篇
  2009年   98篇
  2008年   132篇
  2007年   141篇
  2006年   156篇
  2005年   142篇
  2004年   156篇
  2003年   162篇
  2002年   166篇
  2001年   37篇
  2000年   35篇
  1999年   40篇
  1998年   38篇
  1997年   34篇
  1996年   25篇
  1995年   22篇
  1994年   23篇
  1993年   23篇
  1992年   17篇
  1991年   15篇
  1990年   12篇
  1989年   13篇
  1988年   14篇
  1987年   11篇
  1986年   9篇
  1985年   11篇
  1984年   21篇
  1983年   13篇
  1982年   9篇
  1981年   9篇
  1980年   7篇
  1979年   6篇
  1978年   6篇
  1977年   4篇
  1975年   5篇
  1974年   4篇
  1973年   3篇
  1972年   7篇
排序方式: 共有2500条查询结果,搜索用时 15 毫秒
1.
In order to evaluate the importance of estrogen production in tumor and surrounding tissues, we measured mRNA expression levels of 5 enzymes participating to estrogen synthesis in situ and 4 breast cancer-related proteins in 27 pairs of tumor and non-malignant tissues. Steroid sulfatase (STS) mRNA was more frequently detected in tumor tissues rather than in their non-malignant counterparts. Estrogen sulfotransferase (EST) was constantly expressed with high level not only in tumor tissues but also in their surrounding non-malignant counterparts. In contrast, mRNA expression levels of aromatase, and 17β-hydroxysteroid dehydrogenase type I and II were relatively low and detected only in small proportion of the patients. We also measured the mRNA expression levels of the same nine genes in tumor tissues of 197 breast cancer patients, and analyzed relationship between the mRNA expression level and the clinicopathological parameters. The mRNA expression levels of STS, aromatase and erbB2 in tumor tissues increased as breast cancer progressed. The tumoral mRNA expression levels of STS, estrogen receptor β, and erbB2 in patients with recurrence were higher than those in patients without recurrence. Upregulation of STS expression plays an important role in tumor progression of human breast cancer and is considered to be responsible for estrogen production in tumor and surrounding tissues.  相似文献   
2.
Accumulation of hyperphosphorylated tau in the entorhinal cortex (EC) is one of the earliest pathological hallmarks in patients with Alzheimer’s disease (AD). It can occur before significant Aβ deposition and appears to “spread” into anatomically connected brain regions. To determine whether this early-stage pathology is sufficient to cause disease progression and cognitive decline in experimental models, we overexpressed mutant human tau (hTauP301L) predominantly in layer II/III neurons of the mouse EC. Cognitive functions remained normal in mice at 4, 8, 12 and 16 months of age, despite early and extensive tau accumulation in the EC. Perforant path (PP) axon terminals within the dentate gyrus (DG) contained abnormal conformations of tau even in young EC-hTau mice, and phosphorylated tau increased with age in both the EC and PP. In old mice, ultrastructural alterations in presynaptic terminals were observed at PP-to-granule cell synapses. Phosphorylated tau was more abundant in presynaptic than postsynaptic elements. Human and pathological tau was also detected within hippocampal neurons of this mouse model. Thus, hTauP301L accumulation predominantly in the EC and related presynaptic pathology in hippocampal circuits was not sufficient to cause robust cognitive deficits within the age range analyzed here.  相似文献   
3.
Summary Streptomyces peucetius cells were immobilized by entrapment in calcium alginate and a photosensitive synthetic polymer, and used for the production of daunorubicin (daunomycin), which is known to be an antitumour reagent. The use of cultivation media removed insoluble components in a natural medium prevented rapid decrease in daunorubicin titer after maximum production. These entrapped cells could be used at least five times for repeated daunorubicin production; the total cultivation period was 45 days.  相似文献   
4.
5.
Summary The mutagen-sensitive mutant mus(1)104 D1 of Drosophila melanogaster maps to a position on the X chromosome very close to the meiotic mutant mei-41 D5 . Both mutants have been characterized as mutagen-sensitive and defective in post-replication repair. In the present report we show by complementation studies that mus(1)104 and mus(1)103 are allelic with mei-41. In addition, two reported alleles of mus(1)104 lie between the mei-41 alleles A10 and D5. The size of the mei-41 locus is estimated to be about 0.1 centimorgans (cM). Because several alleles of mei-41 have been shown to reduce recombination and increase meiotic chromosome loss and nondisjunction, mus(1)104 D1 females were examined for defects in meiosis. Although there was no evidence for reduced recombination on the second chromosome in homozygous mus(1)104 D1 females, heterozygous mus(1)104 D1 /mei-41 >D5 and mus(1)104 D1 /deficiency females showed reduced levels of recombination. However, there was no evidence of an increase in nondijunction in these females.We dedicate this article to the memory of Larry Sandler, who passed away suddenly on February 7, 1987  相似文献   
6.
Summary An attempt at cytochemical demonstration of acidification proton-translocating ATPase (H+-ATPase) of Golgi complex in rat pancreatic acinar cells has been made by using p-nitrophenylphosphatase (NPPase) cytochemistry which is used for detecting of Na+-K+-ATPase (Mayahara et al. 1980) and gastric H+-K+-ATPase (Fujimoto et al. 1986). K+-independent NPPase activity was observed on the membrane of the trans cisternae of Golgi complex, but not inside of cisternae. The localization of NPPase activity is different from that of acid phosphatase activity where reaction products were seen on the inside of the trans Golgi cisternae. Since this activity was insensitive to vanadate, ouabain and independent of potassium ions, it was distinct from plasma membranous ATPases such as Na+-K+-ATPase and Ca2+-ATPase. The K+-independent NPPase activity was diminished by the inhibitors of H+-ATPase such as N-ethylmaleimide (NEM) and 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS). The NPPase reaction products were also seen on the membranes of other acidic organelles, i.e., lysosomes, endosomes, autophagosomes and coated vesicles. These results suggest that NPPase activity on the membrane of the Golgi complex and other acidic organelles corresponds with H+-ATPase which plays a role in acidification.  相似文献   
7.
Summary Biosynthesis and processing of the protective protein for -galactosidase in normal and galactosialidosis fibroblasts were investigated using specific antiserum preparations. A 45-kd precursor was processed to a mature 30-kd protein in normal fibroblasts. The mature protective protein was not detected in any of the twelve galactosialidosis fibroblast strains examined in this study. The precursor was not detected in two cases and in the others was of heterogeneous molecular weight, i.e., normal, abnormally low, or abnormally high. These molecular abnormalities were not correlated with clinical manifestations of the patients.  相似文献   
8.
We investigated the effects of interruption of the impulse flow in the habenulopeduncular pathways by local infusion of tetrodotoxin on the acetylcholine and choline content in selected dopamine rich regions in the forebrain and midbrain in rats. The tetrodotoxin infusion caused a marked increase in acetylcholine content in the medial frontal cortex, striatum and ventral tegmental area+interpeduncular nucleus, but not in the limbic area or the substantia nigra, whereas choline content was reduced only in both the striatum and ventral tegmental area+interpeduncular nucleus. There was an increase in 3,4-dihydroxyphenylacetic acid content in the striatum after the manipulation. These findings suggest that the dorsal diencephalic conduction system may be involved in the integration of the activity of cholinergic neurons in the forebrain and midbrain regions and striatal dopanine neurons may play a role in the modulation of cholinergic neurons.  相似文献   
9.
S Itoh  A Takashima  Y Maeda 《Peptides》1992,13(5):1007-1012
Previous studies have demonstrated that NMDA receptor antagonists and protein kinase C inhibitors induced marked memory impairment in rats, but that peripherally administered cerulein (CER) prevented these effects. In the present study, the effect of subcutaneously administered CER on amnesia induced by protein synthesis inhibitors was examined in passive and active avoidance responses and in the Morris water maze test. Intraperitoneal injection of the inhibitors produced marked memory impairment, but the effect was abolished by combined administration with CER. The effective dose of subcutaneously injected CER was, on a molar basis, three thousand- and six thousandfold less than the dose of anisomycin, and two hundred eighty- and three thousandfold less than the dose of puromycin in the passive and active avoidance response experiments, respectively. Similarly, in the Morris water maze test, behavioral disturbances produced by the protein synthesis inhibitors were abolished by CER. These results indicate the effectiveness of CER in preventing memory impairment induced by protein synthesis inhibitors.  相似文献   
10.
Purified bovine brain myosin contained approximately 1 and 3 mol of protein-bound phosphate/mol myosin in the light chains and heavy chains, respectively. Large portions of this light chain- and heavy chain-bound phosphate (about 0.8 and 2.4 mol, respectively) were removed by incubation with a brain phosphoprotein phosphatase and potato acid phosphatase, respectively. Upon phosphorylation of the dephosphorylated brain myosin with myosin light chain kinase and casein kinase II, about 1.6 and 3.0 mol of phosphate was incorporated into the light chains and heavy chains, respectively, while much lower levels of phosphate were incorporated into the non-dephosphorylated brain myosin under the same conditions. The actin-activated Mg2+-ATPase activity of brain myosin rephosphorylated with myosin light chain kinase was about twice as high as that of dephosphorylated brain myosin (about 30 and 15 nmol phosphate/mg/min, respectively). On the other hand, whereas the rephosphorylated brain myosin superprecipitated rapidly with F-actin, the rate of superprecipitation of the dephosphorylated brain myosin was extremely low. Under appropriate conditions, a loose network of tiny superprecipitates, which formed initially throughout the solution, contracted to form eventually a large and dense particle. These results indicate that phosphorylation of the light chains of brain myosin is a prerequisite for the contraction of brain actomyosin. The role of phosphorylation of the heavy chains by casein kinase II remains to be elucidated.  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