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   6697篇
  免费   580篇
  国内免费   6篇
  2023年   19篇
  2022年   102篇
  2021年   165篇
  2020年   85篇
  2019年   95篇
  2018年   160篇
  2017年   125篇
  2016年   216篇
  2015年   360篇
  2014年   408篇
  2013年   466篇
  2012年   587篇
  2011年   526篇
  2010年   318篇
  2009年   249篇
  2008年   402篇
  2007年   385篇
  2006年   315篇
  2005年   313篇
  2004年   304篇
  2003年   211篇
  2002年   222篇
  2001年   208篇
  2000年   144篇
  1999年   113篇
  1998年   52篇
  1997年   35篇
  1996年   45篇
  1995年   33篇
  1994年   30篇
  1993年   23篇
  1992年   56篇
  1991年   36篇
  1990年   39篇
  1989年   34篇
  1988年   31篇
  1987年   37篇
  1986年   24篇
  1985年   24篇
  1984年   19篇
  1983年   17篇
  1982年   20篇
  1981年   22篇
  1980年   16篇
  1979年   21篇
  1978年   20篇
  1977年   22篇
  1976年   20篇
  1974年   17篇
  1969年   13篇
排序方式: 共有7283条查询结果,搜索用时 31 毫秒
951.
There is a difference in the susceptibility to inflammation between the umbilical vein (UV) and the umbilical arteries (UAs). This led us to hypothesize that there is an intrinsic difference in the pro-inflammatory response between UA and UV. Real-time quantitative RT-PCR and microarray analysis revealed higher expression of interleukin (IL)-1β and IL-8 mRNA in the UV and differential expression of 567 genes between the UA and UV associated with distinct biological processes, including the immune response. Differential expression of human leukocyte antigen (HLA)-DRA mRNA between the UA and UV was due to unexpected HLA-DR+ cells migrating via the umbilical vessels into Wharton's jelly, more frequently in the UV. A significant proportion of these cells co-expressed CD45 and type I pro-collagen, and acquired CD163 or α-smooth muscle actin immunoreactivity in Wharton's jelly. Migrating cells were also found in the chorionic and stem villous vessels. Furthermore, the extent of migration increased with progression of gestation, but diminished in intrauterine growth restriction (IUGR). The observations herein strongly suggest that circulating foetal fibrocytes, routing via umbilical and placental vessels, are a reservoir for key cellular subsets in the placenta. This study reports fibrocytes in the human umbilical cord and placenta for the first time, and a novel role for both circulating foetal cells and the umbilical vessels in placental development, which is deranged in IUGR.  相似文献   
952.
A simulation program that runs on a geographic information system (GIS) was developed to predict the multi-species size-structure dynamics of forest stands. Because important characteristics of a forest stand, including woody biomass accumulation, carbon storage, commercial value of timber, and functions for environmental conservation, can be inferred from the size structures of the component populations, management plans can be made from the predictions of the size-structure dynamics. For example, the simulation can incorporate various forms of thinning; forest managers can then try several thinning plans in simulated forest stands and choose the appropriate plan that achieves the best results. Using GIS to predict the size-structure dynamics of forest stands is of practical importance, because GIS has been used widely in forest management and can easily handle spatial distributions of environmental information (e.g., climate, geology, soils) that may influence tree performance. To predict size-structure dynamics, the program numerically solves a continuum equation that describes size-structure dynamics based on growth and mortality rates of individual trees. When predicting size-structure dynamics of a forest stand, the program obtains the environmental information of the stand from a database stored in the GIS and calculates environmental factors such as warmth index and potential evapotranspiration/precipitation ratio that influence growth and mortality rates. The simulation program calculates growth and mortality rates using published growth and mortality models that incorporate the effects of size of the individual, competition between trees, and abiotic environmental factors. To demonstrate the effects of abiotic environmental factors on the multi-species size-structure dynamics, sensitivity analyses were conducted. The size-structure dynamics varied in a way that was predictable from the responses of the growth and mortality rates to variations in the abiotic environmental factors. To demonstrate the size-structure dynamics in different locations, five test runs of the simulation program were also performed using the same initial size-structure and five different sets of abiotic environmental conditions from five locations. At the end of the simulation, the predicted size structures differed because the growth and mortality rates differed among the five locations. Finally, the response of the size structure to thinning was clarified. The result showed how the size structure of a component species in a forest stand is dependent on the presence of other species.  相似文献   
953.
Optimal feed rate strategy is studied for fed-batch culture of recombinant cells with plasmid instability and with different death rates for the plasmid-free cells (PFC) and plasmid-bearing cells (PBC). Most of the fed-batch fermentation is known to have first-order singularity and therefore a single singular arc. However, this study shows that a singular arc with second-order singularity and therefore two distinct singular arcs are possible for a recombinant cell process if PFC and PBC are subjected to death, and their specific growth rates are proportional to each other. Two types of singular arcs are elucidated and analyzed. The optimal policies over the singular arcs are theoretically explored as these findings reveal qualitative information on the singular arc, which is critically important in providing the optimal initial conditions in numerical computation of optimal feed rate profile.  相似文献   
954.
Lim KT  Miyazaki K  Kimura N  Izawa M  Kannagi R 《Proteomics》2008,8(16):3263-3273
We provide here an example of clinical application of functional glycoproteomics for cancer diagnosis. Sialyl Lewis a and sialyl Lewis x glycotopes, which are the specific ligands for selectins, and variant forms of CD44, which are the adhesion molecules recognizing hyaluronate, are both implicated in cancer metastasis. The CD44 variants modified by the sialyl Lewis a and sialyl Lewis x glycotopes are expected to have dual functions, serving as ligands for vascular selectins, and simultaneously having binding activity to vascular bed hyaluronate, and are expected to figure heavily in cancer metastasis. We developed a heterogeneous sandwich assay system to detect soluble CD44v specifically modified by the cancer-associated sialyl Lewis a/x glycotopes, using the extracellular domain of CD44v cleaved by the metalloproteinase ADAM10 as standard molecules. We also developed the assay system for CD44v modified by normal epithelial glycotopes including disialyl Lewis a and sialyl 6-sulfo Lewis x. The results indicated that serum levels of soluble CD44v modified by cancer-associated glycotopes were frequently increased in patients with cancers, while those of CD44v modified by the nonmalignant glycotopes tended to be elevated in patients with benign disorders.  相似文献   
955.
956.
Background/aims Several studies have reported varying results of the influence of ACE gene on ACEI/ARB therapy. The efficacy of high dose ARB and its influence on ACE gene have not been explored. This is a 6 year randomised trial in IgA nephritis comparing high dose ARB (Losartan 200 mg/day) with normal dose ARB (Losartan 100 mg/day), normal dose ACEI (20 mg/day) and low dose ACEI (10 mg/day). Results Patients on high dose ARB had significantly lower proteinuria, 1.0 ± 0.8 gm/day compared to 1.7 ± 1.0 g/day in the other groups (P = 0.0005). The loss in eGFR was 0.7 ml min?1year?1 for high dose ARB compared to 3.2–3.5 ml min?1year?1 for the other three groups (P = 0.0005). There were more patients on high dose ARB with improvement in eGFR compared to other three groups (P < 0.001). Comparing patients with the three ACE genotypes DD, ID and II, all three groups responded well to therapy with decrease in proteinuria (P < 0.002). Only those on low dose ACEI (10 mg/day) with the I allele had increased in ESRF (P = 0.037). Conclusion High dose ARB is more efficacious in reducing proteinuria and preserving renal function when compared with normal dose ARB and ACEI, and also obviates the genomic influence of ACE gene polymorphism on renal survival.  相似文献   
957.
958.
This study reports that Aurora-A (Aur-A) phosphorylates Fas-associated factor-1 (FAF1) at Ser-289 and Ser-291. Forced expression of a FAF1 mutant mimicking phosphorylation at Ser-289 and Ser-291 (FAF1 DD), but not phosphorylation-deficient FAF1 (FAF1 AA), reduced Aur-A expression. However, transfection of FAF1 DD failed to reduce Aur-A expression in the presence of MG132 and MG115, indicating that this decrease is proteasome-mediated. Additionally, transfection of FAF1 DD suppressed the expression of Aur-A in ts20-BALB cells lacking E1 ubiquitin (Ub) activating enzyme activity at restrictive temperatures and also reduced the expression of Aur-A S51D, a mutant resistant to Ub-dependent degradation. Our data indicate that phosphorylated FAF1 mediates the ubiquitin-independent, proteasome-dependent degradation of Aur-A. Overexpression of FAF1 DD blocked Aur-A-induced centrosome amplification and accumulated cells in G(2)/M phase, representing cellular phenotypes consistent with the anticipated loss of Aur-A. Collectively, our findings support the negative feedback regulation of Aur-A via phosphorylation of the death-promoting protein, FAF1, and disclose the presence of molecular cross-talk between constituents of the cell cycle and cell death machinery.  相似文献   
959.
Plants produce p-aminobenzoate (pABA) in chloroplasts and use it for folate synthesis in mitochondria. In plant tissues, however, pABA is known to occur predominantly as its glucose ester (pABA-Glc), and the role of this metabolite in folate synthesis has not been defined. In this study, the UDP-glucose:pABA acyl-glucosyltransferase (pAGT) activity in Arabidopsis extracts was found to reside principally (95%) in one isoform with an apparent K(m) for pABA of 0.12 mm. Screening of recombinant Arabidopsis UDP-glycosyltransferases identified only three that recognized pABA. One of these (UGT75B1) exhibited a far higher k(cat)/K(m) value than the others and a far lower apparent K(m) for pABA (0.12 mm), suggesting its identity with the principal enzyme in vivo. Supporting this possibility, ablation of UGT75B1 reduced extractable pAGT activity by 95%, in vivo [(14)C]pABA glucosylation by 77%, and the endogenous pABA-Glc/pABA ratio by 9-fold. The K(eq) for the pABA esterification reaction was found to be 3 x 10(-3). Taken with literature data on the cytosolic location of pAGT activity and on cytosolic UDP-glucose/UDP ratios, this K(eq) value allowed estimation that only 4% of cytosolic pABA is esterified. That pABA-Glc predominates in planta therefore implies that it is sequestered away from the cytosol and, consistent with this possibility, vacuoles isolated from [(14)C]pABA-fed pea leaves were estimated to contain> or =88% of the [(14)C]pABA-Glc formed. In total, these data and the fact that isolated mitochondria did not take up [(3)H]pABA-Glc, suggest that the glucose ester represents a storage form of pABA that does not contribute directly to folate synthesis.  相似文献   
960.
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