全文获取类型
收费全文 | 35584篇 |
免费 | 3234篇 |
国内免费 | 5260篇 |
专业分类
44078篇 |
出版年
2024年 | 122篇 |
2023年 | 505篇 |
2022年 | 1175篇 |
2021年 | 1881篇 |
2020年 | 1366篇 |
2019年 | 1744篇 |
2018年 | 1590篇 |
2017年 | 1232篇 |
2016年 | 1665篇 |
2015年 | 2421篇 |
2014年 | 2912篇 |
2013年 | 3009篇 |
2012年 | 3629篇 |
2011年 | 3290篇 |
2010年 | 2111篇 |
2009年 | 1877篇 |
2008年 | 2103篇 |
2007年 | 1893篇 |
2006年 | 1653篇 |
2005年 | 1351篇 |
2004年 | 1105篇 |
2003年 | 1038篇 |
2002年 | 871篇 |
2001年 | 547篇 |
2000年 | 474篇 |
1999年 | 435篇 |
1998年 | 284篇 |
1997年 | 259篇 |
1996年 | 230篇 |
1995年 | 183篇 |
1994年 | 178篇 |
1993年 | 124篇 |
1992年 | 132篇 |
1991年 | 104篇 |
1990年 | 79篇 |
1989年 | 76篇 |
1988年 | 61篇 |
1987年 | 40篇 |
1986年 | 51篇 |
1985年 | 60篇 |
1984年 | 23篇 |
1983年 | 29篇 |
1982年 | 29篇 |
1981年 | 23篇 |
1980年 | 10篇 |
1979年 | 20篇 |
1978年 | 9篇 |
1974年 | 8篇 |
1973年 | 11篇 |
1971年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
间断低氧对大鼠下丘脑超微结构及前增食欲素水平的影响 总被引:1,自引:0,他引:1
目的探讨睡眠中间断低氧对大鼠下丘脑前增食欲素及受体水平的影响以及下丘脑超微结构的变化。方法大鼠分成对照组、间断低氧组和持续低氧组,分别给予吸入空气,持续低氧和间断低氧气体,并在实验开始后1d、3d、1w和4w应用RT-PCR方法测定大鼠下丘脑前增食欲素及受体水平,分析其间的变化关系,电镜观察下丘脑的超微结构变化。结果与对照组和持续低氧组比较,间断低氧4w后大鼠下丘脑前增食欲素mRNA水平明显降低,受体水平升高,但在持续低氧和对照组之间无明显差异。在低氧后1d、3d、7d后大鼠下丘脑前增食欲素mRNA降低,受体水平升高,在4w后,持续低氧组则接近正常。急性持续低氧大鼠超微结构变化更严重,而慢性间断低氧变化更持久。结论慢性间断低氧可以引起下丘脑前增食欲素下降及受体水平升高,急性持续低氧也可引起上述变化,而慢性持续低氧未引起增食欲素改变;慢性间断低氧大鼠下丘脑超微结构表现为严重而持久的变化。 相似文献
32.
Five new guaiane sesquiterpenes, 1 – 5 , were isolated from the culture broth of the endophytic fungus Xylaria sp. YM 311647, isolated from Azadirachta indica A. Juss . The structures of these compounds were elucidated on the basis of spectroscopic analyses, and their inhibitory activities against five pathogenic fungi were evaluated. All guaiane sesquiterpenes showed moderate or weak antifungal activities in a broth microdilution assay. 相似文献
33.
In super-resolution microscopy methods based on single-molecule switching, the rate of accumulating single-molecule activation events often limits the time resolution. Here we developed a sparse-signal recovery technique using compressed sensing to analyze images with highly overlapping fluorescent spots. This method allows an activated fluorophore density an order of magnitude higher than what conventional single-molecule fitting methods can handle. Using this method, we demonstrated imaging microtubule dynamics in living cells with a time resolution of 3 s. 相似文献
34.
35.
Involvement of regulatory volume decrease in the migration of nasopharyngeal carcinoma cells 总被引:7,自引:0,他引:7
Mao JW Wang LW Jacob T Sun XR Li H Zhu LY Li P Zhong P Nie SH Chen LX 《Cell research》2005,15(5):371-378
The transwell chamber migration assay and CCD digital camera imaging techniques were used to investigate the relationship between regulatory volume decrease (RVD) and cell migration in nasopharyngeal carcinoma cells (CNE-2Z cells). Both migrated and non-migrated CNE-2Z cells, when swollen by 47% hypotonic solution, exhibited RVD which was inhibited by extracellular application of chloride channel blockers adenosine 5‘-triphosphate (ATP), 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and tamoxifen. However, RVD rate in migrated CNE-2Z cells was bigger than that of non-migrated cells and the sensitivity of migrated cells to NPPB and tamoxifen was higher than that of nonmigrated cells. ATP, NPPB and tamoxifen also inhibited migration of CNE-2Z cells. The inhibition of migration was positively correlated to the blockage of RVD, with a correlation coefficient (r) = 0.99, suggesting a functional relationship between RVD and cell migration. We conclude that RVD is involved in cell migration and RVD may play an important role in migratory process in CNE-2Z cells. 相似文献
36.
Combination of bone tissue engineering and BMP-2 gene transfection promotes bone healing in osteoporotic rats 总被引:12,自引:0,他引:12
OBJECTIVE: The aim of this study was to develop a feasible approach to promote bone healing in osteoporotic rats using autogenous bone tissue-engineering and gene transfection of human bone morphogenetic protein 2 (hBMP-2). METHODS: Bone marrow stromal cells (BMSCs) from the left tibia of osteoporotic rats were transfected with the hBMP-2 gene in vitro which was confirmed by immunohistochemistry, in situ hybridization and Western blotting. Autogenous transfected or untransfected BMSCs were seeded on macroporous coral hydroxyapatite (CHA) scaffolds. Each cell-scaffold construct was implanted into a defect site which was created in the ramus of the mandible of osteoporotic rats. Four or eight weeks after implantation in situ hybridization was performed in BMSCs transfected with hBMP-2, X-ray examinations, histological and histomorphological analyses were used to evaluate the effect of tissue-engineered bone on osseous defect repair. RESULTS: Newly formed bone was observed at the margin of the defect 4 weeks after implantation with BMSCs transfected with BMP-2. Mature bone was observed 8 weeks after treatment. In the control group there was considerably less new bone and some adipose tissue was observed at the defect margins 8 weeks after implantation. CONCLUSIONS: Autogenous cells transfected with hBMP-2 promote bone formation in osteoporotic rats. BMSC-mediated BMP-2 gene therapy used in conjunction with bone tissue engineering may be used to successfully treat bone defects in osteoporotic rats. This method provides a powerful tool for bone regeneration and other tissue engineering. 相似文献
37.
目的:研究高血压病患者肠系膜动脉平滑肌细胞钙激活钾通道(KCa)的功能活动。方法:应用膜片钳制技术内面向外式单通道记录方法。结果:①人肠系膜动脉平滑肌细胞KCa开放具有电压依赖性。KCa通道电导在高血压组、正常组分别为191.4pS、197.7pS。胞内侧应用TEA可阻断通道。②增加浴液中Ca2 浓度(从0增至10-8、10-7、5×10-7、10-6mol/L),各组KCa开放概率(Po)均呈浓度依赖性增加,高血压组Po从0.016增至0.023、0.031、0.053、0.094,正常组Po从0.004增至0.023、0.041、0.072、0.184。通道平均开放时间延长,平均关闭时间缩短。③Ca2 浓度为0时,高血压组KCa开放概率明显高于正常组,在其它Ca2 浓度下高血压组KCa开放概率等于或低于正常组。结论:高血压病患者肠系膜动脉平滑肌细胞KCa的Ca2 敏感性较低,可能促进高血压的发生。 相似文献
38.
By adapting a laser scanning microscope with a titanium sapphire femtosecond pulsed laser and transmission optics, we are able to produce live cell images based on the nonlinear optical phenomenon of second harmonic generation (SHG). Second harmonic imaging (SHIM) is an ideal method for probing membranes of living cells because it offers the high resolution of nonlinear optical microscopy with the potential for near-total avoidance of photobleaching and phototoxicity. The technique has been implemented on three cell lines labeled with membrane-staining dyes that have large nonlinear optical coefficients. The images can be obtained within physiologically relevant time scales. Both achiral and chiral dyes were used to compare image formation for the case of single- and double-leaflet staining, and it was found that chirality plays a significant role in the mechanism of contrast generation. It is also shown that SHIM is highly sensitive to membrane potential, with a depolarization of 25 mV resulting in an approximately twofold loss of signal intensity. 相似文献
39.
外源基因的表达及其对细菌种群的影响对于群体感应系统和合成生物学产业的研究具有重要意义。然而,人们对于表达外源蛋白的细菌本身的行为模式仍然知之甚少。为了研究菌落生长和外源基因表达的过程究竟受到哪些因素的影响,文中测量了受Lux类受体调控的外源基因在N-酰基高丝氨酸内酯 (N-acyl homoserine lactone,N-AHL) 信号分子诱导下的表达,并模拟了其对细菌种群动态的影响。文中建立了一个假设性的数学模型,对信号分子诱导表达下细菌种群生长受影响的现象进行了分析。先前的研究通常将细菌种群生长受群体感应系统影响的现象归咎于合成群体感应信号分子的消耗与N-AHL信号分子的毒性,文中提供了对于这种生存压力的另一种可能的解释。 相似文献
40.
Xiong XH Han S Wang JH Jiang ZH Chen W Jia N Wei HL Cheng H Yang YX Zhu B You S He JY Hou W Chen MX Yu CJ Jiao YH Zhang WC 《Journal of bacteriology》2011,193(1):315-316
Ketogulonicigenium vulgare is characterized by the efficient production of 2KGA from L-sorbose. Ketogulonicigenium vulgare Y25 is known as a 2-keto-L-gulonic acid-producing strain in the vitamin C industry. Here we report the finished, annotated genome sequence of Ketogulonicigenium vulgare Y25. 相似文献