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   4277篇
  免费   308篇
  国内免费   6篇
  4591篇
  2023年   21篇
  2022年   49篇
  2021年   76篇
  2020年   41篇
  2019年   51篇
  2018年   108篇
  2017年   82篇
  2016年   131篇
  2015年   220篇
  2014年   238篇
  2013年   288篇
  2012年   315篇
  2011年   323篇
  2010年   188篇
  2009年   176篇
  2008年   228篇
  2007年   191篇
  2006年   221篇
  2005年   181篇
  2004年   175篇
  2003年   171篇
  2002年   152篇
  2001年   95篇
  2000年   98篇
  1999年   74篇
  1998年   41篇
  1997年   30篇
  1996年   28篇
  1995年   22篇
  1994年   30篇
  1993年   24篇
  1992年   47篇
  1991年   33篇
  1990年   31篇
  1989年   28篇
  1988年   22篇
  1987年   25篇
  1986年   17篇
  1985年   17篇
  1984年   17篇
  1983年   20篇
  1979年   20篇
  1977年   16篇
  1975年   19篇
  1974年   20篇
  1973年   17篇
  1972年   20篇
  1970年   13篇
  1969年   13篇
  1967年   13篇
排序方式: 共有4591条查询结果,搜索用时 15 毫秒
31.
Retinal neovascularization in retinopathy of prematurity (ROP) is the most common cause of blindness for children. Despite evidence that hypoxia inducible factor (HIF)‐1α ‐VEGF axis is associated with the pathogenesis of ROP, the inhibitors of HIF‐1α have not been established as a therapeutic target in the control of ROP pathophysiology. We investigated the hypothesis that degradation of HIF‐1α as a master regulator of angiogenesis in hypoxic condition, using β‐lapachone, would confer protection against hypoxia‐induced retinopathy without affecting physiological vascular development in mice with oxygen‐induced retinopathy (OIR), an animal model of ROP. The effects of β‐lapachone were examined after intraocular injection in mice with OIR. Intraocular administration of β‐lapachone resulted in significant reduction in hypoxia‐induced retinal neovascularization without retinal toxicity or perturbation of developmental retinal angiogenesis. Our results demonstrate that HIF‐1α–mediated VEGF expression in OIR is associated with pathological neovascularization, not physiological angiogenesis. Thus, strategies blocking HIF‐1α in the developing eye in the pathological hypoxia could serve as a novel therapeutic target for ROP.  相似文献   
32.
33.
Here, we show that Elovl3 (elongation of very long-chain fatty acids 3) was involved in the regulation of the progression of adipogenesis through activation of peroxisome proliferator-activated receptor (PPAR)γ in mouse adipocytic 3T3-L1 cells. The expression of the Elovl3 gene increased during adipogenesis, the expression pattern of which was similar to that of the PPARγ gene. Troglitazone, a PPARγ agonist, enhanced Elovl3 expression in adipocytes, as it did that of other PPARγ target genes. Promoter-reporter analysis demonstrated that three PPAR-responsive elements in the Elovl3 gene promoter had the potential to activate its expression in 3T3-L1 cells. Moreover, a chromatin immunoprecipitation assay revealed that PPARγ bound these PPAR-responsive elements of the Elovl3 promoter. When the Elovl3 mRNA level was suppressed by its siRNAs, the level of intracellular triglycerides was significantly decreased, and the expression levels of adipogenic, lipolytic, and lipogenic genes were also repressed. In a mammalian two-hybrid assay, C18:1 and C20:1 very long-chain fatty acids (VLCFAs), which are the products of Elovl3 and activated PPARγ function. In addition, these same VLCFAs could prevent the Elovl3 siRNA-mediated suppression of adipogenesis by enhancing the expression of adipogenic, lipolytic, and lipogenic genes in adipocytes. Moreover, this VLCFAs-mediated activation was repressed by a PPARγ antagonist. These results indicate that the expression of the Elovl3 gene was activated by PPARγ during adipogenesis. Elovl3-produced C18:1 and C20:1 VLCFAs acted as agonists of PPARγ in 3T3-L1 cells. Thus, the Elovl3-PPARγ cascade is a novel regulatory circuit for the regulation of adipogenesis through improvement of PPARγ function in adipocytes.  相似文献   
34.
Androgen deprivation therapy, also known as chemical castration, has been used as an adjunct to psychotherapy for sex offenders. Goserelin and bicalutamide are drugs used for chemical castration. Serotonin (5-hydroxytryptamine, 5-HT) is a key neurotransmitter involved in mood changes, such as depression. We investigated the effects of surgical and chemical castration on depressive symptoms in rats. Surgical castration was performed through a bilateral orchiectomy. Bicalutamide was administrated orally once a day for 84 consecutive days. Goserelin acetate was implanted subcutaneously into the anterior abdominal wall, and this implantation was repeated 3 times at 28-day intervals. Testosterone levels were detected by enzyme-linked immunosorbent assays. Sexual behaviors were analyzed by measuring mount latency, mount frequency, intromission latency, and intromission frequency. The forced swimming test was performed to evaluate rats’ depression status. To detect 5-HT and tryptophan hydroxylase (TPH)-positive cells in the dorsal raphe, immunohistochemistry for 5-HT and TPH and western blotting for 5-HT1A receptors and TPH were performed. Surgical castration and goserelin decreased testosterone levels and suppressed sexual behaviors. However, bicalutamide did not inhibit sexual behaviors, although it reduced testosterone levels to a limited extent. Both surgical and chemical castration induced depression in rats. The expression of 5-HT, TPH, and 5-HT1A receptors in the dorsal raphe was significantly decreased by both surgical castration and chemical castration via bicalutamide and goserelin. The present results showed that surgical and chemical castration for 12 weeks induced a depressive state in rats by inhibiting serotonergic function through 5-HT1A receptors.  相似文献   
35.
The condition and feeding behaviour of burbot, a widespread potamodromous species in riverine and lacustrine environments, were compared in order to evaluate the importance of both in three artificial systems. Subadult burbot were sampled in three temperate reservoirs in spring, and one of them also in summer and autumn. Standardised abundance and sizes of burbot were comparable between the reservoirs, but the conditions were significantly different. The Clark’s condition coefficient and index of fullness were independent of burbot size and individuals. Feeding behaviour in terms of abundance and composition of consumed food was environment-dependent. The most common prey category was aquatic insect larvae, dominating in the riverine environment. Permanent water invertebrates, microcrustaceans, beetles and crayfish were less common food and eaten more often in the lacustrine environment. Prey-fish were usually the most common species of suitable size. The food analyses demonstrated opportunistic feeding behaviour with selection of prey associated with benthic habitat and suitable size. Burbot is therefore flexible not only in environments utilisation, but also feeding strategy.  相似文献   
36.
Poly(A)-binding proteins are highly conserved among eukaryotes and regulate stability of mRNA and translation. Among C. elegans homologues, pab-1 mutants showed defects in germline mitotic proliferation. Unlike pab-1 mutants, pab-1 RNAi at every larval stage caused arrest of germline development at the following stage, indicating that pab-1 is required for the entire postembryonic germline development. This idea is supported by the observations that the mRNA level of pab-1 increased throughout postembryonic development and its protein expression was germline-enriched. PAB-1 localized to P granules and the cytoplasm in the germline. PAB-1 colocalized with CGH-1 and CAR-1 and affected their localization, suggesting that PAB-1 is a component of processing (P)-bodies that interacts with them. The mRNA and protein levels of representative germline genes, rec-8, GLP-1, rme-2, and msp-152, were decreased after pab-1 RNAi. Although the mRNA level of msp-152 was increased in cgh-1 mutant, it was also significantly reduced by pab-1 RNAi. Our results suggest that PAB-1 positively regulates the mRNA levels of germline genes, which is likely facilitated by the interaction of PAB-1 with other P-body components, CGH-1 and CAR-1.  相似文献   
37.
The ability of the lignin-degrading microorganism Phanerochaete chrysosporium to attack polyethylene and polypropylene was investigated using a series of polymer blends containing 10, 20 and 30% lignin obtained from the waste product of pulp and paper industry. In the cultivation medium, lignin peroxidase and Mn(II)peroxidase activities were detected. Degradation was verified by quantitative u.v. spectrophotometric analysis of the cultivation medium and by liberation of CO2 from the blends. Measurement of the tensile strength after 30-days cultivation showed that the mechanical properties of the polymer blends were decreased during the biodegradation process. The isolation of oligomer fractions by tetrahydrofuran (THF) extraction of biodegraded polymers and their characterization by gel permeation chromatography (GPC), u.v. and Fourier transmission infrared (FTIR) spectroscopy indicates that biotransformation of the lignin component during the cultivation process initiates partial biodegradation of the synthetic polymer matrix.  相似文献   
38.
Phytophthora spp. are serious pathogens that threaten numerous cultivated crops, trees, and natural vegetation worldwide. The soybean pathogen P. sojae has been developed as a model oomycete. Here, we report a bacterial artificial chromosome (BAC)-based, integrated physical map of the P. sojae genome. We constructed two BAC libraries, digested 8,681 BACs with seven restriction enzymes, end labeled the digested fragments with four dyes, and analyzed them with capillary electrophoresis. Fifteen data sets were constructed from the fingerprints, using individual dyes and all possible combinations, and were evaluated for contig assembly. In all, 257 contigs were assembled from the XhoI data set, collectively spanning approximately 132 Mb in physical length. The BAC contigs were integrated with the draft genome sequence of P. sojae by end sequencing a total of 1,440 BACs that formed a minimal tiling path. This enabled the 257 contigs of the BAC map to be merged with 207 sequence scaffolds to form an integrated map consisting of 79 superscaffolds. The map represents the first genome-wide physical map of a Phytophthora sp. and provides a valuable resource for genomics and molecular biology research in P. sojae and other Phytophthora spp. In one illustration of this value, we have placed the 350 members of a superfamily of putative pathogenicity effector genes onto the map, revealing extensive clustering of these genes.  相似文献   
39.
Mustafa Akyol  Kamil Koç 《Biologia》2006,61(5):487-495
Two new species of Neophyllobius (Acari, Camerobiidae) viz. N. populus sp. n., N. karabagiensis sp. n., and a new species of Tycherobius (Acari, Camerobiidae), Tycherobius dazkiriensis sp. n., are described from Turkey and illustrated based on females and a protonymph. A key to the known species of Tycherobius is presented.  相似文献   
40.
Open‐circuit voltage (VOC) losses in organic photovoltaics (OPVs) inhibit devices from reaching VOC values comparable to the bandgap of the donor–acceptor blend. Specifically, nonradiative recombination losses (?Vnr) are much greater in OPVs than in silicon or perovskite solar cells, yet the origins of this are not fully understood. To understand what makes a system have high or low loss, an investigation of the nonradiative recombination losses in a total of nine blend systems is carried out. An apparent relationship is observed between the relative domain purity of six blends and the degree of nonradiative recombination loss, where films exhibiting relatively less pure domains show lower ?Vnr than films with higher domain purity. Additionally, it is shown that when paired with a fullerene acceptor, polymer donors which have bulky backbone units to inhibit close π–π stacking exhibit lower nonradiative recombination losses than in blends where the polymer can pack more closely. This work reports a strategy that ensures ?Vnr can be measured accurately and reports key observations on the relationship between ?Vnr and properties of the donor/acceptor interface.  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