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   752篇
  免费   72篇
  国内免费   1篇
  2023年   5篇
  2022年   9篇
  2021年   11篇
  2020年   6篇
  2019年   18篇
  2018年   19篇
  2017年   22篇
  2016年   23篇
  2015年   39篇
  2014年   49篇
  2013年   48篇
  2012年   72篇
  2011年   51篇
  2010年   41篇
  2009年   30篇
  2008年   44篇
  2007年   33篇
  2006年   52篇
  2005年   30篇
  2004年   32篇
  2003年   40篇
  2002年   35篇
  2001年   14篇
  2000年   8篇
  1999年   10篇
  1998年   4篇
  1997年   9篇
  1996年   4篇
  1995年   5篇
  1994年   2篇
  1993年   3篇
  1992年   6篇
  1991年   2篇
  1990年   2篇
  1989年   2篇
  1988年   2篇
  1987年   4篇
  1986年   3篇
  1985年   2篇
  1984年   4篇
  1983年   4篇
  1982年   3篇
  1980年   5篇
  1979年   3篇
  1977年   2篇
  1971年   3篇
  1966年   1篇
  1965年   1篇
  1961年   2篇
  1960年   1篇
排序方式: 共有825条查询结果,搜索用时 15 毫秒
41.
Intra-amniotic lipopolysaccharide (LPS) and cytokines may decrease respiratory distress syndrome (RDS) and increase chronic lung disease in the newborn. The aim was to identify the primary inflammatory mediators regulating the expression of surfactant proteins (SP) in explants from immature (22-day-old fetus) and mature (30-day term fetus and 2-day-old newborn) rabbits. In immature lung, interleukin (IL)-1alpha and IL-1beta upregulated the expression of SP-A and SP-B. These effects of IL-1 were diminished, and SP-C mRNA was suppressed additively in the presence of tumor necrosis factor (TNF)-alpha and either LPS or interferon (IFN)-gamma. LPS, TNF-alpha, or IFN-gamma had no effect alone. In explants from the term fetus and the newborn, LPS, IL-1alpha, and TNF-alpha additively suppressed the SPs. LPS acutely induced IL-1alpha in alveolar macrophages in mature lung but not in the immature lung. IFN-gamma that generally has low expression in intrauterine infection decreased the age dependence of the other agonists' effects on SPs. The present study serves to explain the variation of the pulmonary outcome after an inflammatory insult. We propose that IL-1 from extrapulmonary sources induces the SPs in premature lung and is responsible for the decreased risk of RDS in intra-amniotic infection.  相似文献   
42.
Hydrostatic pressure has a profound effect on cartilage tissue and chondrocyte metabolism. Depending on the type and magnitude of pressure various responses can occur in the cells. The mechanisms of mechanotransduction at cellular level and the events leading to specific changes in gene expression are still poorly understood. We have previously shown that induction of stress response in immortalized chondrocytes exposed to high static hydrostatic pressure increases the stability of heat shock protein 70 mRNA. In this study, our aim was to examine the effect of high pressure on gene expression profile and to study whether stabilization of mRNA molecules is a general phenomenon under this condition. For this purpose a cDNA array analysis was used to compare mRNA expression profile in pressurized vs. non-pressurized human chondrosarcoma cells (HCS 2/8). mRNA stability was analyzed using actinomycin-treated and nontreated samples collected after pressure treatment. A number of immediate-early genes, and genes regulating cell cycle and growth were up-regulated due to high pressure. Decrease in osteonectin, fibronectin, and collagen types VI and XVI mRNAs was observed. Also bikunin, cdc37 homologue and Tiam1, genes linked with hyaluronan metabolism, were down-regulated. In general, stability of down-regulated mRNA species appeared to increase. However, no increase in mRNA above control level due to stabilization was noticed in the genes available in the array. On the other hand, mRNAs of certain immediate-early genes, like c-jun, jun-B and c-myc, became destabilized under pressure treatment. Increased accumulation of mRNA on account of stabilization under high pressure conditions seems to be a tightly regulated, specific phenomenon.  相似文献   
43.
44.
Novel morphology in Enteromorpha (Ulvophyceae) forming green tides   总被引:1,自引:0,他引:1  
"Green tides" are vast accumulations of unattached green macroalgae associated with eutrophicated marine environments. They have major ecological and economic impacts globally, so an understanding of their origin and persistence is required in order to make management decisions. Blooms predominantly consist of two common fouling genera of the Ulvales, Ulva (distromatic sheets) and Enteromorpha (monostromatic tubes). In the Baltic Sea and elsewhere green tides have increased over the last few decades. On the west coast of Finland, summer blooms consist of monostromatic sheets resembling Monostroma (Codiolales). We identified these as Enteromorpha intestinalis by comparative analyses of rDNA internal transcribed spacer 1 (ITS1), 5.8S, and ITS2 sequences, the first time monostromatic sheets have been found in the genus Enteromorpha. Ordinary attached E. intestinalis sporulated freely in culture, but the sheets reproduced only by cell regeneration into typical tubular thalli. The ITS sequences were identical to those of attached E. intestinalis populations in southwestern Finland, but differed by two substitutions from other Baltic sequences. We infer that this bloom originated from local attached populations and now reproduces clonally by fragmentation. This study provides further evidence of radical changes in gross morphology of green algae under eutrophicated conditions and the need for molecular identification.  相似文献   
45.
46.
C-reactive protein (CRP) is a sensitive marker of inflammation induced by both IL-6 and IL-1. Thus, genetic variation in these genes could be associated with the variety in C-reactive protein levels, and therefore with the severity of the entire inflammatory response. Even a subtle elevation in baseline CRP levels in healthy individuals has been found to significantly increase the risk for cardiovascular diseases. Therefore, to find out the possible role of pro-inflammatory cytokines in CRP baseline regulation we conducted a study of 338 healthy blood donors whose CRP levels were determined and whose single nucleotide polymorphisms of IL1A(C/T)-889, IL1B(C/T)-511, IL1B(C/T) + 3954, IL6(G/C)-174 and ILRN (a VNTR) both genotyped and haplotyped. The data revealed an association between CRP levels and the IL1B + 3954 genotype. Also, the bilocus haplotype IL1B-511*1/IL1B + 3954*2 was more frequent in subjects with below median CRP levels (< 0.72 mg/l), and composite genotype analysis of IL1B-511/IL1B + 3954 supported this finding. Our findings suggest that in healthy people, basal CRP levels are regulated by IL1B but not by IL6 genetics.  相似文献   
47.
A single accidental event such as the fallout released from the Chernobyl reactor in 1986 can expose millions of people to non-natural environmental radiation. Ionizing radiation increases the frequency of germline mutations in experimental studies, but the genetic effects of radiation in humans remain largely undefined. To evaluate the hereditary effects of low radiation doses, we compared the minisatellite mutation rates of 155 children born to Estonian Chernobyl cleanup workers after the accident with those of their siblings born prior to it. All together, 94 de novo paternal minisatellite mutations were found at eight tested loci (52 and 42 mutants among children born after and before the accident, respectively). The minisatellite mutation rate was nonsignificantly increased among children born after the accident (0.042 compared to 0.036, OR 1.33, 95% CI 0.80-2.20). Furthermore, there was some indication of an increased mutation rate among offspring born after the accident to workers who had received doses of 20 cSv or above compared with their siblings born before the accident (OR 3.0, 95% CI 0.97-9.30). The mutation rate was not associated with the father's age (OR 1.04, 95% CI 0.94-1.15) or the sex of the child (OR 0.95, 95% CI 0.50-1.79). Our results are consistent with both no effect of radiation on minisatellite mutations and a slight increase at dose levels exceeding 20 cSv.  相似文献   
48.
Species living in highly fragmented landscapes typically occur as metapopulations with frequent turnover of local populations. The turnover rate depends on population sizes and connectivities, but it may also depend on the phenotypic and genotypic composition of populations. The Glanville fritillary butterfly (Melitaea cinxia) in Finland uses two host plant species, which show variation in their relative abundances at two spatial scales: locally among individual habitat patches and regionally among networks of patches. Female butterflies in turn exhibit spatial variation in genetically determined host plant preference within and among patch networks. Emigration, immigration and establishment of new populations have all been shown to be strongly influenced by the match between the host plant composition of otherwise suitable habitat patches and the host plant preference of migrating butterflies. The evolutionary consequences of such biased migration and colonization with respect to butterfly phenotypes might differ depending on spatial configuration and plant species composition of the patches in heterogeneous patch networks. Using a spatially realistic individual-based model we show that the model-predicted evolution of host plant preference due to biased migration explains a significant amount of the observed variation in host plant use among metapopulations living in dissimilar networks. This example illustrates how the ecological extinction-colonization dynamics may be linked with the evolutionary dynamics of life history traits in metapopulations.  相似文献   
49.
Surgical transfer of in vivo produced conventionally frozen-thawed embryos of farmed European polecat (Mustela putorius) was investigated as a part of an ex-situ preservation program which has the long-term aim of developing a genome resource bank for the endangered European mink (Mustela lutreola). Eighteen oestrous yearling European polecat donors were mated once daily on two consecutive days using 13 fertile males. The donors were surgically flushed for embryos 8-9 days after the first mating. The embryo recovery rate was 60% (116 embryos/193 corpora lutea). The embryos were cryopreserved with 1.5 M ethylene glycol in a programmable freezer using a conventional slow freezing protocol. The thawed embryos were surgically transferred either after dilution with 0.5 M sucrose or directly without removal of ethylene glycol. To induce ovulation, eight recipient females were mated once daily on two consecutive days with vasectomized males starting 7 or 8 days before embryo transfer. The recipients received 7-11 embryos each and three recipients delivered a total of nine pups after a gestation length of 44-46 days. The embryo survival rate was 10% (9 pups/93 frozen embryos). This report describes the first successful cryopreservation of embryos in the Mustelidae family resulting in viable offspring. The low embryo survival rate, however, indicates that the freezing-thawing protocol needs to be improved.  相似文献   
50.
Special training methods in wrestling have been assumed to improve the stability and tolerance of the neck. The aim of this study was to measure the neck strength levels reached in an extremely physically demanding sport. A neck strength measurement system was used to measure various parameters of maximal isometric neck strength in Finnish senior wrestlers competing at the international level. The results were compared with those achieved by junior wrestlers and a control group. The means (SD) of the maximal isometric neck strength for cervical rotation were 0.4 (0.1) Nm.kg(-1) for the senior wrestlers, 0.3 (0.1) Nm.kg(-1) for the junior wrestlers, and 0.2 (0.1) Nm.kg(-1) for the nonsportsmen. The respective results for cervical flexion were 4.4 (1.4), 3.8 (0.7), and 2.3 (0.8) Nm.kg(-1); for extension, 6.0 (1.1), 5.9 (0.7), and 4.0 (0.9) Nm.kg(-1). Neck strength in flexion seems to improve more than in extension as the result of wrestling. The greatest difference was found in rotation, which in the senior wrestlers was almost 3 times that in the nonsportsmen. There was great individual variation within all groups, and the results revealed weaknesses in all directions. Maximal neck strength measurements provide information useful in planning training programs to correct possible muscle deficiency and imbalance.  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