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   4420篇
  免费   271篇
  2023年   10篇
  2022年   20篇
  2021年   46篇
  2020年   22篇
  2019年   38篇
  2018年   47篇
  2017年   46篇
  2016年   84篇
  2015年   141篇
  2014年   150篇
  2013年   291篇
  2012年   282篇
  2011年   282篇
  2010年   189篇
  2009年   165篇
  2008年   275篇
  2007年   257篇
  2006年   285篇
  2005年   264篇
  2004年   253篇
  2003年   256篇
  2002年   231篇
  2001年   84篇
  2000年   75篇
  1999年   75篇
  1998年   53篇
  1997年   54篇
  1996年   53篇
  1995年   50篇
  1994年   34篇
  1993年   40篇
  1992年   56篇
  1991年   67篇
  1990年   40篇
  1989年   44篇
  1988年   29篇
  1987年   30篇
  1986年   32篇
  1985年   23篇
  1984年   30篇
  1983年   25篇
  1982年   21篇
  1981年   30篇
  1980年   9篇
  1979年   6篇
  1978年   11篇
  1977年   16篇
  1976年   11篇
  1975年   16篇
  1974年   9篇
排序方式: 共有4691条查询结果,搜索用时 46 毫秒
41.
42.
The mode of antitumor action of rHu-TNF was elucidated in BALB/c mice bearing Meth A fibrosarcoma 7 days after transplantation with respect to time course, dose-response relationships and selectivity of the effects. The maximal cytotoxic effect on tumor cells revealed by inhibition of DNA synthesis and maximal lesional effect on tumor vasculature revealed by change in blood pool-size in the tissue were detected at 30 min and I h after administration of rHu-TNF, respectively. The dose-response relationship between cytotoxic and tumoricidal effects of rHu-TNF was irrespective of administration route. ED50s of these antitumor effects afteri.v. administration of rHu-TNF were about 50 times as high as ED50s afteri.t. administration. ED50 ofi.t. given rHu-TNF for vascular effect was about 20 times as high as that for cytotoxicity while ED50 ofi.v. rHu-TNF for vascular effect was only 2–3 times as high as that for cytotoxicity. The whole body autoradiographies with [125I] HSA giveni.v. to see the blood influx into tumor tissue and [14C]thymidine given i.v. to see DNA synthesis in the whole body after administration of rHu-TNF revealed that the distribution of radioactivity was markedly changed in the tumor alone without any detectable change in other whole body tissues.In conclusion, thein vivo antitumor effect of rHu-TNF giveni.t. ori.v., appears to be exerted through the direct action on Meth A sarcoma rather than indirectly on tumor vasculature. Under present conditions, the effect of rHu-TNF in the whole body tissues seems rather selective on cells and vasculature of the tumor.  相似文献   
43.
Summary During the course of studies to characterize mutations of the CYP17 gene that cause the 17-hydroxylase-deficient form of congenital adrenal hyperplasia we have discovered two ostensibly unrelated Mennonite families in which affected individuals are homozygous for the same mutation. The defect is a four-base duplication in exon 8 of the CYP17 gene, which alters the reading frame encoding the C-terminal 26 animo acids of cytochrome P45017.  相似文献   
44.
Extracellular matrix of meningiomas was studied by light and electron microscopy with the aid of immunohistochemical techniques. Special attention was paid to the distribution of type I, III, IV, V collagens and laminin with a comparison between meningothelial and fibroblastic types. Connective tissue fibers and basement membrane were not found among the tumor cells in the meningothelial type, but were found in the fibroblastic type. The immunolocalizations were consistently demonstrated extracellularly, but were not within the cytoplasm. Type I, III and V collagens were usually demonstrated in the fibrous septum in the meningothelial type, while they were localized among the tumor cells in the fibroblastic type. Furthermore, type IV collagen and laminin were demonstrated within the vascular walls or around the syncytium in the meningothelial type, while they were localized among the tumor cells in the fibroblastic type. In both types the expression of type IV collagen and laminin was closely related to the distribution of basement membrane. Although meningothelial and fibroblastic meningiomas showed quite different distribution of extracellular matrices, the profile of collagen types expressed by these two basic types was essentially the same. The cellular derivation of meningiomas was discussed with particular attention to the structure of human arachnoid villi and meninges.  相似文献   
45.
Molecular phylogenetic analyses using mitochondrial NADH dehydrogenase subunit 5 (ND5) gene sequences representing all 15 species and the majority of subspecies or races of theOhomopterus ground beetles from all over the Japanese archipelago have uncovered a remarkable evolutionary history. Clustering of the species in the molecular phylogenetic tree is linked to their geographic distribution and does not correlate with morphological characters. Taxonomically the same species or the members belonging to the same species-group fall out in more than two different places on the ND5 tree. Evidence has been presented against a possible participation of ancestral polymorphism and random lineage sorting or of hybrid individuals for the observed distribution of mitochondrial DNA haplotypes. The most plausible explanation of our results is that parallel evolution took place in different lineages. Most notably,O. dehaanii, O. yaconinus, andO. japonicus in a lineage reveal almost identical morphology with those of the same species (or subspecies) but belonging to the phylogenetically remote lineages.The nucleotide sequence data reported in this paper will appear in the DDBJ, EMBL, and GenBank nucleotide sequence databases with accession numbers D50711-DD-50733 and D87131-D87186.  相似文献   
46.
Persistent infections with mumps virus were established in several human lymphoid cells of T-cell origin (Molt-4, TALL-1, and CCRF-CEM) and human monocyte cells (U937 and THP-1). 2′,5′-Oligoadenylate synthetase (2–5AS) activity was demonstrated to be only slightly induced by interferon (IFN) or TPA (12-O-tetradecanoyl-phorbol-13-acetate) treatment in these cells. Treatment of the persistently infected cells with IFN or TPA did not stimulate an increase in the amount of synthetase mRNA. Induction of cell differentiation and augmentation of IFN production by TPA were demonstrated in U937 cells persistently infected with mumps virus (U937-MP). Similar results for IFN production were obtained from differentiated U937 cells. It is suggested that cell differentiation of U937 cells might be associated with the development of IFN inducibility.  相似文献   
47.
48.
Novel mono-O-acetylated GM3s, one containing 9-O-acetylN-glycolyl neuraminic acid and another containing 6-O-acetyl galactose, were isolated as a mixture from equine erythrocytes, and the structures were characterized by one- and two-dimensional proton nuclear magnetic resonance (NMR) and fast atom bombardment-mass spectrometry (FAB-MS). The position of theO-acetyl residue was identified by the downfield shift of the methylene protons at C-9 ofN-glycolyl neuraminic acid (9-O-Ac GM3) and C-6 of galactose (6-O-Ac GM3) in the NMR spectrum, in comparison to the respective non-acetylated counterparts. To confirm the presence of 6-O-Ac GM3, theO-acetylated GM3 mixture was desialylated withArthrobacter neuraminidase, giving 6-O-acetyl galactosyl glucosylceramide, the structure of which was estimated by NMR and FAB-MS, together with non-acetylated lactosylceramide with a ratio of 1:1. Abbreviations: Ac, acetyl; Gc, glycolyl; NeuGc,N-Gc neuraminic acid; GM3 (Gc), GM3 containing NeuGc (II3NeuGc-LacCer); 4-O-Ac GM3 (Gc), GM3 containing 4-O-Ac NeuGc; 9-O-Ac GM3 (Gc), GM3 containing 9-O-Ac NeuGc; 6-O-Ac GM3 (Gc), GM3 containing 6-O-Ac Gal; 1D-NMR, one-dimensional nuclear magnetic resonance spectrometry; 2D-COSY, two-dimensional chemical shift-correlated spectrometry; FAB-MS, fast atom bombardment-mass spectrometry; GLC, gas-layer chromatography; GC-MS, gas chromatography-mass spectrometry; TLC, thin-layer chromatography; Ggl, ganglioside; Cer, ceramide; CMH, monohexosylceramide; LacCer, lactosylceramide; 6-O-Ac LacCer, LacCer containing 6-O-Ac Gal; Me2SO-d6,2H6-dimethylsufloxide; CMW, chloroform-methanol-water; Nomenclature and abbreviations of glycosphingolipids follow the system of Svennerholm (J Neurochem [1963]10: 613–23) and those recommended by the IUPAC-IUB Nomenclature Commission (Lipids [1977]12: 455–68).  相似文献   
49.
A novel mono-sulfated glycosphingolipid based on the gangliotriaose core structure was isolated from rat kidney. The isolation procedure involved extraction of lipids with chloroform/methanol, mild alkaline methanolysis, column chromatographies with anion exchangers and silica beads. The structure was characterized by compositional analysis, FTIR spectroscopy, methylation analysis,1H-NMR spectroscopy and negative-ion liquid secondary ion mass spectrometry (LSIMS) using the intact glycolipid and its desulfation product. The two dimensional chemical shift correlated spectroscopy provided information on the sugar sequence as well as anomeric configurations, and indicated the presence of a 3-O-sulfatedN-acetylgalactosamine within the molecule. Negative-ion LSIMS with high- and low-energy collision-induced dissociation defined the sugar sequence and ceramide composition, confirming the presence of a sulfatedN-acetylgalactosamine at the non-reducing terminus. From these results, the complete structure was proposed to be HSO3-3GalNAc1-4Gal1-4Glc1-1Cer (Gg3Cer III3-sulfate, SM2b). Abbreviations: Abbreviations for sulfated glycolipids [17] follow the modifications of the nomenclature system of Svennerholm for gangliosides [37], and the designation of the other glycosphingolipids follows the IUPAC-IUB recommendations [38]. Cer, ceramide; LacCer, lactosylceramide, Gal1-4Glc1-1Cer; Gg3Cer, gangliotriaosylceramide, GalNAc1-4Gal1-4Glc1-1Cer; Gg4Cer, gangliotetraosylceramide, Gal1-3GalNAc1-4Gal1-4Glc1-1Cer; iGb4Cer, isoglobotetraosylceramide, GalNAc1-3Gal1-3Gal1-4Glc1-1Cer; Gb4Cer, globotetraosylceramide, GalNAc1-3Gal1-4Gal1-4Glc1-1Cer; SM4s, galactosylceramide sulfate, GalCer I3-sulfate; SM3, lactosylceramide sulfate, LacCer II3-sulfate; SM2a, Gg3Cer II3-sulfate; SM2b, Gg3Cer III3-sulfate; SB2, Gg3Cer II3,III3-bis-sulfate; SM1a, Gg4Cer II3-sulfate; SM1b, Gg4Cer IV3-sulfate; SB1a, Gg4Cer II3,IV3-bissulfate; GLC, gas-liquid chromatography; GC-MS, gas chromatography-mass spectrometry; DQF, double quantum filtered; COSY, chemical-shift-correlated spectroscopy; LSIMS, liquid secondary ion mass spectrometry; CID, collision-induced dissociation; MS/MS, tandem mass spectrometry.  相似文献   
50.
Maitani T  Kubota H  Sato K  Yamada T 《Plant physiology》1996,110(4):1145-1150
The induction of phytochelatins (PCs) and their desglycyl peptides (both are referred to as class III metallothionein [CIIIMT]) by exposure to various metals (Ag+, As3+, As5+, Cd2+, Cu2+, Ga3+, Hg2+, In3+, Ni2+, Pb2+, Pd2+, Se4+, and Zn2+) and the metal composition in the CIIIMTs were investigated in root cultures of Rubia tinctorum L. All of these metal species induced PCs to various degrees when analyzed by the postcolumn derivatization high-performance liquid chromatography method. The desglycyl peptides of PCs often were also present. However, only Ag, Cd, and Cu were bound to the CIIIMTs that they induced when analyzed by the high-performance liquid chromatography-inductively coupled plasma-atomic emission spectrometry method. Cu was also bound to the CIIIMTs induced by Ag+, As3+, and Cd2+. After Ag+ exposure, an Fe peak that may be of Fe-CIIIMT was also observed. However, most of the metal species studied were not bound to the CIIIMTs that they induced.  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