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   7748篇
  免费   560篇
  国内免费   1篇
  2021年   85篇
  2019年   63篇
  2018年   75篇
  2017年   82篇
  2016年   118篇
  2015年   191篇
  2014年   226篇
  2013年   551篇
  2012年   320篇
  2011年   368篇
  2010年   255篇
  2009年   256篇
  2008年   332篇
  2007年   364篇
  2006年   333篇
  2005年   345篇
  2004年   377篇
  2003年   337篇
  2002年   288篇
  2001年   306篇
  2000年   296篇
  1999年   265篇
  1998年   100篇
  1997年   75篇
  1996年   63篇
  1995年   67篇
  1994年   53篇
  1993年   64篇
  1992年   137篇
  1991年   146篇
  1990年   141篇
  1989年   130篇
  1988年   116篇
  1987年   128篇
  1986年   112篇
  1985年   94篇
  1984年   76篇
  1983年   53篇
  1982年   61篇
  1981年   55篇
  1979年   65篇
  1978年   58篇
  1977年   62篇
  1975年   47篇
  1974年   59篇
  1973年   62篇
  1972年   40篇
  1971年   40篇
  1969年   49篇
  1968年   49篇
排序方式: 共有8309条查询结果,搜索用时 15 毫秒
51.
Lymphokine activated killer (LAK) cells are generated by culture of lymphocytes with interleukin 2 (IL-2) in short term culture (3 to 5 days) and are used for adoptive immunotherapy for advanced cancer patients. The culture condition hitherto reported are essentially based on the rotating culture system, in which the maximum cell density was at 2 X 10(6) cell/ml and the cell recovery was usually less than 100%. The inability to induce LAK cells efficiently in vitro made the culturing of cells for therapy rather difficult and costly work because the mean infusion dose of LAK cells of one patient requires more than 1 X 10(10)/ml. We have therefore attempted to culture lymphocytes in 10 times higher concentration comparing with conventional methods. By using a new dialyzing culture system under continuous regulation of the amount of infused IL-2, nutrition medium, and pO2 and pCO2, we could culture cells at 2 X 10(7)/ml for more than 21 days and the resulted LAK cells showed a 100 times increase of activity on a per cell basis. By limiting dilution procedure, these killer cells mostly express T cell markers such as CD3 and CD8 but dose not express CD16.  相似文献   
52.
An exotoxin (HS-6) produced by Nocardia otitidiscaviarum isolated from certain lesions of cutaneous nocardiosis of a male 82-year-old patient induced severe injuries in the pancreas, liver, stomach, small intestine, heart, thymus and kidney of male ICR mice. Mice given Nocardia-free preparation of HS-6 at a dose of 1 mg/kg of body weight developed several autophagic vacuoles in the pancreas and liver within 20 min after the i.p. injection. Thereafter, the autophagic vacuoles increased in number and size with time. About 24 hr after the administration of HS-6, the liver showed marked accumulation of fat droplets in the cytoplasm of the hepatocytes. Although they contained abundant autophagic vacuoles in the regions of RER, there were no lipomatoses in the acinar cells of the pancreas, those of the chief cells and smooth muscle cells of the stomach, Paneth cells, goblet cells, smooth muscle cells of the small intestine, and plasma cells in the digestive tract. Biochemical examinations revealed that HS-6 had no significant effect on the protein synthesis of reticulocytes. Inoculation of the Nocardia into the mouse peritoneal cavities caused marked granulomatoses in the pancreas, liver and regional lymph nodes, but did not develop autophagic vacuoles in RER regions of these organs.  相似文献   
53.
The expression of class II major histocompatibility complex (MHC) antigens on alveolar epithelial cells and macrophages was investigated immunocytochemically in paraquat-induced alveolitis in the rat lung. Until 2 days after paraquat injection, class II MHC antigens were expressed on the type II alveolar epithelium without any inflammatory cellular infiltration. From the 4th to the 7th day after paraquat injection, class II MHC antigen-positive macrophages increased in the alveolar spaces, whereas the expression on the type II alveolar epithelium became obscure. Over 10 days after the injection, interstitial fibrosis progressed and the intra-alveolar inflammatory infiltrates decreased. Epithelial cells lining the thickened fibrous septa no longer expressed class II MHC antigens. These results suggest that chemical stimuli can induce class II MHC antigen expression on the type II alveolar epithelium in the early stage of cellular injury, followed by inflammatory infiltration and interstitial fibrosis.  相似文献   
54.
Partial nucleotide sequences of the large subunit of ribulose-1,5-bisphosphate carboxylase (rubisco) gene (1333 base pairs: about 90% of the gene) from several seed plants were determined. Phylogenetic trees based on amino acid sequences were inferred by using the neighbor joining and maximum likelihood methods. The results indicate (1) monophyly of gnetum group (Ephedra, Gnetum, Welwitschia), (2) monophyly of extant gymnosperms containing gnetum group, which contradicts the results of morphological data.  相似文献   
55.
Placental transmission of Pneumocystis carinii in mice was examined in 39 animals obtained by caesarean section from 17 pregnant SCID females experimentally infected with P. carinii. When examined with toluidine blue O, DAPI and immunofluorescent antibody stains, P. carinii was detected in the lungs of infected mothers but not in the lungs of caesarean section-derived neonates even after the neonates were treated with dexamethasone for 8 weeks. However, 13 neonates born to five infected females developed P. carinii pneumonia. These results indicate that P. carinii cannot be transmitted transplacentally in mice.  相似文献   
56.
K Shichijo  M Ito  I Sekine 《Life sciences》1991,49(26):2023-2029
The mechanism of low susceptibility to stress in gastric lesion formation in spontaneously hypertensive rats (SHR) was investigated focusing on the sympathetic and parasympathetic nervous systems. In the gastric tissues of SHR, norepinephrine (NE) and dopamine (DA) contents were higher, while acetylcholine content and choline acetyltransferase activity were lower than those of Wistar-Kyoto rats (WKY). Water-immersion restraint induced gastric lesions frequently in WKY (ulcer indices : 52 +/- 7mm2) but less frequently in SHR (ulcer indices : 3 +/- 1mm2). Although NE content decreased in both SHR and WKY as a result of water-immersion restraint, it remained higher in SHR than in WKY. ACh content decreased by the procedure in WKY but not in SHR. DA content was increased by the procedure in all gastric regions of SHR. The gastric lesions induced in SHR were aggravated by pretreatment with 6-hydroxydopamine, an agent for chemical sympathectomy, following decreases of NE and DA contents. These results indicate that the relative sympathetic hyperfunction, parasympathetic hypofunction and dopaminergic mechanism in the stomach contribute to the prevention of gastric lesion formation in SHR.  相似文献   
57.
The pea phytochrome I (PI) cDNA clone, pPP1001, was expressedin E. coli. The plasmid pPP1001 contains pea PI cDNA which coversthe entire coding region with the Shine-Dalgarno consensus sequencejoined upstream of the cDNA in an expression vector pNUT6. ThepPP1001 transformants formed typical inclusion bodies when culturedat 32?C. However, when cultured at 37?C or in the presence ofisopropyl-ß-D-thiogalactopyranoside (IPTG) at 32?C,the bacteria lysed before inclusion body formation. Immuno-stainingwith anti-PI monoclonal antibody, mAP5, of transformants fixedby cold methanol showed that stainable materials were distributedin whole cytoplasmic region. When the inclusion bodies wereobserved clearly, the regions corresponding to the inclusionbodies became difficult to stain. Western blot analysis, however,showed that a ca. 100 kDa PI polypeptide was detected in thefraction from inclusion bodies and a ca. 90 kDa PI polypeptidefrom the soluble fraction. The amino acid sequence analysisof purified 100 kDa PI sample indicated that its amino terminusis blocked. However, minor signals in one experiment yieldeda sequence corresponding to the expected amino terminus of peaPI except for the initiation methionine. One of the anti-peaPI monoclonal antibodies, mAP9, that recognizes the near N-terminusof pea phytochrome was reactive to the 100 kDa polypeptide. (Received June 22, 1990; Accepted November 18, 1990)  相似文献   
58.
M Kojima  H Kimura  M Ohnishi  Y Fujino  S Ito 《Phytochemistry》1991,30(4):1165-1168
The novel diglycosyldiacylglycerol (DGDG), galactosyl(beta 1----6)-galactosyl(beta 1----3')-diacylglycerol (B isomer), present in Adzuki beans was found to be distributed together with the well-known galactosyl(alpha 1----6)-galactosyl(beta 1----3')-diacylglycerol (A isomer), in all (10) of the higher plants examined. The highest levels were found in leguminous seeds were the amounts were always less than 33% of the total DGDG of mature seeds. The highest proportion of the B isomer was found in Adzuki bean seed DGDG (26-33%), with the lowest in pea seed DGDG (2%). The amounts of the B isomer in DGDG of Adzuki and kidney beans cotyledons were almost equal to those in mature seeds. Immature seeds and hypocotyls of three kinds of beans also contained the B isomer in small amounts compared with the mature beans, while only trace amounts of the isomer was found in other organs such as leaves, stems, pods, roots and generative organs of plants, except for root from kidney beans. The molecular species composition of the principal diacylglycerol moieties in the A and B isomers of DGDG were found to be significantly different among several plant seeds, although the component diacylglycerol species were qualitatively similar to each other.  相似文献   
59.
Augmentation rhinoplasty: observations on 1200 cases   总被引:1,自引:0,他引:1  
Over the past 14 years, from January of 1975 to December of 1988, we have done 1263 aesthetic rhinoplasties using ear cartilage. In the field of augmentation rhinoplasty, many kinds of materials, such as bone, septal cartilage, ear cartilage, and prostheses, were used. In this paper, we limit discussion to our experience with the technique for the augmentation of the nasal dorsum using the ear cartilage and compare this with other materials. Patient ages ranged from 15 to 72 years, with an average of 24 years. Some 95 percent of patients (1199) were female, and only 5 percent (64) were male. Patients were followed for a minimum of 6 months and a maximum of 20 months, with average follow-up only 8 months. Of course, we know that this is a very short follow-up period, but we could not follow patients longer because if they had no complaint about the results at the 6-month visit, they never returned, despite our efforts. Five-hundred and ten of the 1263 patients (40 percent) had been augmented elsewhere, and the silicone prosthesis was already in place. However, 753 patients (60 percent) had no previous operation. For the 510 patients (secondary rhinoplasty patients), too-high or too-large a prosthesis was the largest complaint in number, totaling 378 cases (74 percent), and psychological dissatisfaction, such as pain or an uncomfortable sensation, was the second largest in number, totaling 104 cases (20 percent). For the 753 patients (primary rhinoplasty patients), the main complaint was too-short or too-flat a nose (100 percent).(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
60.
Messenger RNAs (mRNAs) specific for NGK1 and NGK2 potassium channels were synthesized from complementary DNAs (cDNAs) that had been cloned from mouse neuroblastoma x rat glioma hybrid NG108-15 cells. Outward pottasium currents were evoked by 5 s depolarizing voltage commands in Xenopus oocytes injected with NGK1- or NGK2-specific mRNAs. The NGK1 or NGK2 currents showed different activation and inactivation kinetics, and different pharmacological sensitivities. The threshold potential for activation of the NGK2 current (-14 mV) was more positive than that for the NGK1 (-36 mV). The NGK2 current showed faster inactivation during a 5 s depolarizing pulse than did the NGK1 current. Inactivation was best fit by time constants of 0.37, 1.5 and 19 s for the NGK2 current and 4.4 and 19 s for NGK1. Extracellularly applied tetraethylammonium chloride (TEA) was 1000 times more potent on the NGK2 current than the NGK1 current. Furthermore we examined outward current following co-injection of an equal amount of mRNAs for NGK1 and NGK2. The timecourse of inactivation differed from either alone or from a simple sum of the two individual currents. TEA sensitivity could not be explained by summation of the two homomultimeric channels. These findings suggest that both NGK1 and NGK2 proteins assemble to form heteromultimeric K+ channels in addition to homomultimeric K+ channels. NGK2 channels and the heteromultimeric channels may be responsible for the native transient outward current with slow inactivation in NG108-15 hybrid cells.  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