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   1828篇
  免费   130篇
  国内免费   4篇
  2024年   5篇
  2023年   25篇
  2022年   61篇
  2021年   104篇
  2020年   60篇
  2019年   97篇
  2018年   117篇
  2017年   68篇
  2016年   87篇
  2015年   78篇
  2014年   90篇
  2013年   163篇
  2012年   122篇
  2011年   126篇
  2010年   77篇
  2009年   62篇
  2008年   73篇
  2007年   72篇
  2006年   54篇
  2005年   54篇
  2004年   58篇
  2003年   49篇
  2002年   39篇
  2001年   14篇
  2000年   13篇
  1999年   11篇
  1998年   10篇
  1996年   13篇
  1994年   5篇
  1992年   7篇
  1990年   12篇
  1989年   4篇
  1988年   7篇
  1987年   8篇
  1986年   7篇
  1985年   9篇
  1984年   9篇
  1983年   4篇
  1982年   5篇
  1981年   5篇
  1980年   8篇
  1979年   6篇
  1978年   7篇
  1977年   8篇
  1976年   4篇
  1974年   5篇
  1973年   5篇
  1971年   3篇
  1970年   3篇
  1969年   4篇
排序方式: 共有1962条查询结果,搜索用时 31 毫秒
991.
Abstract Bidirectional signaling between the urogenital sinus epithelium and mesenchyme is an essential element of prostate development that regulates ductal morphogenesis, growth, and differentiation. Comparable interactions between the epithelium and stroma in the adult prostate appear to regulate normal growth homeostasis. Alterations in the stromal–epithelial dialogue that recapitulate features of the mesenchymal–epithelial interactions of development may play a critical role in the development of benign prostatic hyperplasia and in the progression of prostate cancer. For this reason, the mesenchymal–epithelial interactions of development are of considerable interest. In this review, we provide an overview of the mesenchymal contribution to rodent prostate development with an emphasis on the stage just before ductal budding (embryonic day 16; E16) and describe the isolation, characterization and utility of a newly established E16 urogenital sinus mesenchymal cell line.  相似文献   
992.
In this study, morphological, ultrastructural and physiological modifications of faba bean (Vicia faba cv Giza 461) leaves in response to bean yellow mosaic virus (BYMV) infection and salicylic acid (SA) treatments were examined. Under BYMV stress, leaves showed symptoms including severe mosaic, mottling, crinkling, size reduction and deformations. Three weeks after virus inoculation, photosynthetic rate, pigment contents and transpiration rate were significantly reduced in response to BYMV infection.

Ultrastructural investigations of BYMV-infected leaves demonstrated that most chloroplasts with increased stromal area became spherical in shape and some lost their envelopes, either partially or totally. The internal structures of chloroplast, grana and thylakoids were dilated. Two kinds of inclusions were detected in BYMV-infected leaves: straight or slightly curved bands sometimes coiled or looped at the end, and electron opaque crystals with varied shapes. BYMV-infected cells showed lower chloroplast number in comparison to the control.

Spraying of SA on faba bean leaves helped to reduce or prevent the harmful effects produced after virus infection. Application of 100 μM SA three days before inoculation restored the metabolism of infected leaves to the levels of healthy controls. SA treatment improved plant health by increasing the photosynthesis rates, pigment contents and levels of other parameters studied similar to control values.

Moreover, SA treatment increased plant resistance against BYMV. This was observed through induction of chloroplast number, reduction in percentage of infected plants, decrease in disease severity and virus concentration of plants treated with SA prior to BYMV inoculation. Cells of SA-treated samples showed well-developed chloroplasts with many starch grains and well-organized cell organelles.

The present results provide an overview of the negative effects on faba bean leaves due to BYMV infection from physiological and subcellular perspectives. Also, a role of SA involved in induction of resistance against BYMV infection in bean plants is discussed.  相似文献   

993.
Aneuploidy of germ cells contributes to reduced fertility, foetal wastage and genetic defects. The possible risk of aneuploidy induction by the cancer chemotherapeutic drugs amsacrine (AMSA) and nocodazole (NOC) was investigated in male mice. Two molecular cytogenetic approaches were used: (1) the BrdU-incorporation assay to test the altered duration of meiotic divisions and (2) the sperm-FISH assay to determine aneuploidy induction during meiosis by observing hyperhaploid and diploid sperm. Sperm were sampled from the Caudae epididymes of treated and solvent control males. Single intraperitoneal injections with NOC (35 mg/kg) and AMSA (15 mg/kg) caused a meiotic delay of 24h. The timing of sperm sampling for the sperm-FISH assay was adjusted accordingly, i.e. 23 days after treatment. Mice were treated with 18, 35 and 50 mg/kg of NOC, or 5, 10, 15 and 20 mg/kg of AMSA. Significant dose-dependent increases above the concurrent controls in the frequencies of hyperhaploid sperm were found with both agents. Significant increases in the frequencies of diploid sperm were found only with AMSA. These results provide a basis for genetic counselling of patients under AMSA or NOC chemotherapy. During a period of 3-4 months after the end of chemotherapy, they may stand a higher risk of siring chromosomally abnormal offspring.  相似文献   
994.
Flagellin (fliC) genes of 12 Shigella boydii and five Shigella dysenteriae strains were characterized. Though these strains are nonmotile, the cryptic fliCSB gene, cloned from S. boydii strain C3, is functional for expression of flagellin. It consists of 1,704 bp, and encodes 568 amino acid residues (57,918 Da). The fliCSD gene from S. dysenteriae strain 16 consists of 1,650 bp encoding 549 amino acid residues (57,591 Da) and contains an IS1 element inserted in its 3' end. The two genes are composed of the 5'-constant, central variable and 3'-constant sequences, like other known fliC genes. The two genes share high homology in nucleotide and amino acid sequences with each other and also with the Escherichia coli fliCE gene, indicating that both genes are closely related to the fliCE gene. Comparison of the central variable sequences of six different fliC genes showed that the fliCSB and fliCSD genes share low homology in amino acid sequence with the other fliC genes, suggesting that they encode antigenic determinants intrinsic to respective subgroups. However, Southern blotting using as probes the central variable sequences of several fliC genes showed that four of 12 S. boydii strains have a fliC gene similar to that of Shigella flexneri, and that among five fliC genes from S. dysenteriae strains, one is similar to that of S. flexneri, two are similar to that of S. boydii, and only one is unique to S. dysenteriae. Some of these variant alleles were verified by immunoblotting with flagellins produced from cloned fliC genes. The presence of variant fliC alleles in S. boydii and S. dysenteriae indicates that subdivision into subgroups does not reflect the ancestral flagella H antigenic relationships. These data will be useful in considering the evolutionary divergence of the Shigella spp..  相似文献   
995.
Seven flavonoids have been isolated from the seeds of Lonchocarpus costaricensis of which four appear to be novel. The new compounds have been identified as the β-hydroxychalcone demethylpraecansone-B and the flavanones 7-(3,3-dimethylallyloxy)-8-(3-hydroxy-3-methyl-trans-but-1-enyl)flavanone, 7-(3,3-dimethylallyloxy)-8-(3,3-dimethylallyl)-5-methoxyflavanone and 8-(3,3-dimethylallyl)-5,7-dimethoxyflavanone.  相似文献   
996.
The ability of dromedary skim milk to form an acid curd during a lactic acid starter fermentation was investigated. The activity of the starter in dromedary milk was characterized by a longer lag phase (∼5 vs. ∼1 h) and by an earlier decline phase. This suggests the presence of inhibiting factors. The maximum buffering capacity of dromedary milk as well as its minimum apparent viscosity were obtained at lower pH values. Similarly, its elastic modulus appeared later (pH 5.7 vs. 6.3). Because these rheological and biochemical events took place at lower pH values, dromedary skim milk seems to present a higher physical stability toward the increase of acidity. Determination of the rheological and microscopic characteristics of the dromedary milk coagulum (pH 4.4) did not reveal curd formation but indicated a fragile and heterogeneous structure. This coagulum, which is very different from that of cows' milk, seems to be made up of dispersed casein flakes. Journal of Industrial Microbiology & Biotechnology (2001) 26, 263–270. Received 01 May 2000/ Accepted in revised form 26 January 2001  相似文献   
997.
Two antisera, denoted R41 and R42, were raised against a synthetic peptide from the murine Clara cell-specific protein CC10, and one antiserum, denoted R40, was raised against human recombinant uteroglobin, the human homolog of murine CC10. Purified antigen-specific antisera, denoted R40AP, R41AP, and R42AP were prepared using peptide columns. The purified antisera were characterized by dot blots, immunohistochemistry, and immunoblots. Immunohistochemistry of mouse lung showed specific labeling of Clara cells in distal bronchioles by all three antisera. In human lung, the antiuteroglobin antiserum specifically labeled Clara cells, while the anti-mouse peptide antisera had weak crossreactivity and higher background staining. Electron microscopy revealed immunogold labeling of CC10 granules in Clara cells of mouse lung with all antisera. All antisera also labeled a 5-kDa protein on immunoblots of mouse lung homogenates. The surface epithelium of the alveolar air spaces around the distal bronchioles were CC10 positive suggesting a functional activity for CC10 in the lung parenchyma distal to Clara cells. R40AP immunohistochemical staining of sections of normal human lungs and lungs from patients with surfactant protein B deficiency, bronchopneumonia, and idiopathic alveolar proteinosis illustrate the utility of the anti-human CC10 antibody for diagnostic pathology.  相似文献   
998.
The effect of five different concentrations (2, 4, 6, 8 and 10 mg ml-1) of an aqueous extracts of Lupinus albus, Ammi visnaga and Xanthium pungens were tested on growth and aflatoxin production by Aspergillus flavus in a chemically defined medium. All the plants inhibited mycelial growth and aflatoxin formation. The inhibitory effect was proportional with the applied concentration. Growth and aflatoxin production appeared to be correlated processes. The nature of the plant extract also affected the ratio of B1 to B2, and there was no correlation between the inhibition of aflatoxins or growth of the fungus and the resultant B1: B2 ratio.  相似文献   
999.
The aim of this study was to introduce a simple, reproducible, and less expensive method for isolation of alpha-lactalbumin, beta-lactoglobulin, and bovine serum albumin from cow's milk while retaining their antigenicity. Whey (lactoserum) was obtained by isolating casein from defatted milk using hydrochloric acid. Globulins were then precipitated from whey by half-saturated ammonium sulfate and beta-lactoglobulin was purified further using Sephadex G-50 gel filtration. The proteins in the supernatant were also fractionated using diethylaminoethyl cellulose chromatography in which beta-lactoglobulin was separated from alpha-lactalbumin and bovine serum albumin. The latter two proteins that co-eluted in anion-exchange chromatography were then gently isolated from each other by Sephadex G-50 gel filtration. Pure beta-lactoglobulin was also obtained by anion-exchange chromatography of the ammonium sulfate-precipitated globulins. Using enzyme-linked immunosorbent assay (ELISA), Western blotting, and ELISA inhibition assay, antigenicity of the purified proteins was evaluated. Our results showed high purity and well-preserved antigenicity of alpha-lactalbumin, beta-lactoglobulin, and bovine serum albumin thus purified.  相似文献   
1000.
We used two, 3-min field stimulation cycles 30 min apart (S1, S2) in 3H-norepinephrine-loaded, superfused rat nucleus tractus solitarii-dorsal motor vagal nucleus (NTS-DVN) slices. The stimulation-induced release was expressed as the area above the baseline. Drugs were introduced 12 min before S2 and drug actions were characterized in terms of alterations of S2/S1 ratios. The S2/S1 ratio was 1.047 (0.946-1.159, n = 4, geometric mean and 95% confidence interval) in controls and 0.336 (0.230-0.490, n = 3), 0.726 (0.590-0.892, n = 4), 0.613 (0.594-0.683, n = 4) and 0.665 (0.500-0.886, n = 4) in the presence of 10(-6) M clonidine, D-Ala(2),MePhe(4),Gly(5)-ol-enkephalin (DAMGO), endomorphin-1 (Tyr-Pro-Trp-Phe-NH(2), EM-1) and -2 (Tyr-Pro-Phe-Phe-NH(2), EM-2) [the latter two in the presence of 10(-4) M diprotin A, an inhibitor of dipeptidyl-aminopeptidase IV (DAP-IV) enzyme]. The effect of DAMGO at 10(-5) M was significantly higher than at 10(-6) M, whereas the effect of endomorphins did not differ at the two concentration levels. Diprotin A potentiated only very modestly the action of endomorphins. These data (a) confirm the presence of functional mu-opioid receptors in the vagal complex, (b) render it likely that the enzymic degradation of endomorphins is not a highly effective process in brain slices and (c) may suggest that the apparent ceiling in the effect of endomorphins might be related to their partial agonist property.  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