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   11170篇
  免费   944篇
  国内免费   6篇
  2023年   53篇
  2022年   84篇
  2021年   166篇
  2020年   147篇
  2019年   128篇
  2018年   250篇
  2017年   250篇
  2016年   349篇
  2015年   527篇
  2014年   580篇
  2013年   702篇
  2012年   800篇
  2011年   767篇
  2010年   471篇
  2009年   409篇
  2008年   640篇
  2007年   581篇
  2006年   578篇
  2005年   535篇
  2004年   503篇
  2003年   411篇
  2002年   402篇
  2001年   190篇
  2000年   158篇
  1999年   143篇
  1998年   84篇
  1997年   87篇
  1996年   89篇
  1995年   81篇
  1994年   61篇
  1993年   79篇
  1992年   103篇
  1991年   97篇
  1990年   107篇
  1989年   78篇
  1988年   68篇
  1987年   96篇
  1986年   67篇
  1985年   84篇
  1984年   78篇
  1983年   91篇
  1982年   68篇
  1981年   59篇
  1980年   44篇
  1979年   67篇
  1978年   63篇
  1977年   54篇
  1976年   50篇
  1975年   46篇
  1974年   59篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Acute megakaryoblastic leukaemia (AMkL) is a rare subtype of acute myeloid leukaemia (AML) representing 5% of all reported cases, and frequently diagnosed in children with Down syndrome. Patients diagnosed with AMkL have low overall survival and have poor outcome to treatment, thus novel therapies such as CAR T cell therapy could represent an alternative in treating AMkL. We investigated the effect of a new CAR T cell which targets CD41, a specific surface antigen for M7-AMkL, against an in vitro model for AMkL, DAMI Luc2 cell line. The performed flow cytometry evaluation highlighted a percentage of 93.8% CAR T cells eGFP-positive and a limited acute effect on lowering the target cell population. However, the interaction between effector and target (E:T) cells, at a low ratio, lowered the cell membrane integrity, and reduced the M7-AMkL cell population after 24 h of co-culture, while the cytotoxic effect was not significant in groups with higher E:T ratio. Our findings suggest that the anti-CD41 CAR T cells are efficient for a limited time spawn and the cytotoxic effect is visible in all experimental groups with low E:T ratio.  相似文献   
142.
To clarify whether cigarette smoking during pregnancy causes an organic alteration in placental estrogen producing ability, we determined the catalytic activity of aromatase by the tritiated water assay, and tissue level of aromatase cytochrome P-450 (P-450arom) by the specific enzyme-linked immunosorbent assay, in placental samples from nonsmokers and smokers. As pregnancy progressed, both aromatase activity and P-450arom concentration increased in placentas from nonsmokers and smokers. However, the gradient of the increase was significantly less in heavy smokers (20 cigarettes a day) than in normal and moderate smokers (<20 cigarettes a day). At term, the mean aromatase activity and P-450arom concentration in placentas from heavy smokers were significantly lower than in nonsmokers and moderate smokers, while aromatase activity per P-450arom (turnover rate) and the mean placental weight were comparable among the three groups. In contrast, the ratio of aryl hydrocarbon hydroxylase activity to aromatase activity was higher in placentas from heavy smokers. Immunohistochemical studies showed that P-450arom was localized in the cytoplasm of syncytiotrophoblasts of chorionic villi in placentas from both nonsmokers and smokers. These results suggest that the induction of placental P-450arom during gestation is suppressed by maternal smoking, resulting in a reduction in estrogen producing ability, while placental xenobiotic P-450 is induced.  相似文献   
143.
Summary Explants of tomato (Lycopersicon esculentum Mill cv. Ailsa Craig) were co-cultivated with Agrobacterium tumefaciens C58C1Rifr::pGSFR1161 in the presence of 20 (M acetosyringone). Transformed root clones were selected on kanamycin medium and the presence of the nptII gene in the plant DNA confirmed by the polymerase chain reaction. Root clones derived from acetosyringone treatment grew more vigorously in the presence of kanamycin and synthesized a greater amount of NPT-II enzyme. The conclusion is that acetosyringone treatment enhances the transformation process, possibly by stimulating multiple insertions of the T-DNA into the host genome.Abbreviations AS acetosyringone - CTAB hexadecyltrimethylammonium bromide - EDTA ethylenediaminetetraacetic acid - ELISA enzyme-linked immunosorbant assay - MS Murashige and Skoog (1962) medium - PCR polymerase chain reaction  相似文献   
144.
Gulls, as largely flexible opportunistic individuals, have been increasingly breeding in many cities around the world, but it is still unclear whether urban habitats are of equal or higher quality than traditional natural habitats or represent an ecological trap with immediate reproductive benefits but longer-term detrimental consequences to health. Here we present a study of breeding parameters (nest density, egg dimensions, clutch size, hatching success and adult body condition) and physiological parameters (erythrocyte sedimentation rate, heterophil/lymphocyte ratio, haemoglobin concentration and measurements of oxidative stress) as indicators of the general health condition of Yellow-legged Gull Larus michahellis adults and chicks from natural and urban colonies. Yellow-legged Gulls in the largest urban area (Porto) laid smaller eggs and clutches, showed a significantly lower occurrence of inflammatory processes in chicks, and showed a slower early chick growth than in the natural colony of Deserta. This suggests that urban gulls might be facing important trade-offs between the advantages of breeding in lower density urban colonies, with fewer intraspecific interactions and a lower disease transmission probability, and the disadvantages of having an anthropogenic diet usually lower in nutritional value.  相似文献   
145.
Successful protected area networks must represent biodiversity across taxonomic groups. However, too often plant species are overlooked in conservation planning, and the resulting protected areas may, as a result, fail to encompass the most important sites for plant diversity. The Mozambique Tropical Important Plant Areas project sought to promote the conservation of Mozambique's flora through the identification of Important Plant Areas (IPAs). Here, we use the Weighted Endemism including Global Endangerment (WEGE) index to identify the richest areas for rare and endemic plants in Mozambique and subsequently evaluate how well represented these hotspots are within the current protected area and IPA networks. We also examine the congruence between IPA and protected areas to identify opportunities for strengthening the conservation of plants in Mozambique. We found that high WEGE scores, representing areas rich in endemic/near-endemic and threatened species, predict the presence of IPAs in Mozambique, but do not predict the presence of protected areas. We also find that there is limited overlap between IPAs and protected areas in Mozambique. We demonstrate how IPAs could be an important tool for ensuring priority sites for plant diversity are included within protected area network expansions, particularly following the adoption of the “30 by 30” target agreed within the post-2020 Convention on Biological Diversity framework, with great potential for this method to be replicated elsewhere in the global tropics.  相似文献   
146.
147.
At low concentrations ethanol is metabolized largely by alcohol dehydrogenase to acetaldehyde, while at higher concentrations a microsomal ethanol oxidising system (MEOS) is involved, namely cytochrome P450 IIE1, which also probably generates free radical species. In hyperthyroidism hepatic glutathione stores are depleted and net superoxide anion production occurs. In contrast, in hypothyroidism hepatic glutathione may be increased and thus renders the liver less sensitive to alcohol generated free radical production. Steroid hormones inhibit lipid peroxidation. Sixty male Wistar rats either underwent thyroidectomy, adrenalectomy, or sham procedures. Twenty control animals were pair fed with thyroidectomized animals, whilst another twenty fed ad libitum. An intraperitoneal injection of alcohol (75 mmol/kg) was given 2.5 h prior to sacrifice to half the animals in each group, the remainder receiving saline. The total hepatic glutathione contents of the pair fed and the ad libitum groups were not different, but were significantly increased by thyroidectomy (p = <0.001). This effect was significantly reduced by alcohol (p < 0.01). The sham procedures and dietary restrictions had no effect. The ethanol alone reduced total hepatic glutathione, but this only reached statistical significance in the thyroidectomized and sham-adrenalectomized groups. Hepatic malonaldehyde (MDA) levels were significantly reduced in the thyroidectomy group but alcohol had no effect on them. We conclude that hypothyroidism increased hepatic glutathione status, presumably by reducing radical production by enzyme systems, which would otherwise consume this important scavenger. Long term exposure to ethanol with induction of MEOS is probably required for it to generate toxic levels of free radical species.  相似文献   
148.
Pancreatic cancer is a highly aggressive form of cancer with a poor prognosis, partly due to ‘austerity’, a phenomenon of tolerance to nutrient deprivation and survival in its hypovascular tumor microenvironment. Anti-austerity agents which preferentially diminish the survival of cancer cells under nutrition starvation is regarded as new generation anti-cancer agents. This study investigated the potential of Piper longum constituents as anti-austerity agents. The ethanolic extract of Piper longum was found to have preferential cytotoxicity towards PANC-1 human pancreatic cancer cells in a nutrient-deprived medium (NDM). Further investigation led to the identification of pipernonaline ( 3 ) as the lead compound with the strongest anti-austerity activity, inducing cell death and inhibiting migration in a normal nutrient medium, as well as strongly inhibiting the Akt/mTOR/autophagy pathway. Therefore, pipernonaline ( 3 ) holds promise as a novel antiausterity agent for the treatment of pancreatic cancer.  相似文献   
149.
Curcumin has a plethora of biological properties, making this compound potentially effective in the treatment of several diseases, including cancer. However, curcumin clinical use is compromised by its poor pharmacokinetics, being crucial to find novel analogs with better pharmacokinetic and pharmacological properties. Here, we aimed to evaluate the stability, bioavailability and pharmacokinetic profiles of monocarbonyl analogs of curcumin. A small library of monocarbonyl analogs of curcumin 1a–q was synthesized. Lipophilicity and stability in physiological conditions were both assessed by HPLC-UV, while two different methods assessed the electrophilic character of each compound monitored by NMR and by UV-spectroscopy. The potential therapeutic effect of the analogs 1a–q was evaluated in human colon carcinoma cells and toxicity in immortalized hepatocytes. Our results showed that the curcumin analog 1e is a promising agent against colorectal cancer, with improved stability and efficacy/safety profile.  相似文献   
150.
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