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   8270篇
  免费   712篇
  国内免费   3篇
  8985篇
  2023年   30篇
  2022年   77篇
  2021年   108篇
  2020年   69篇
  2019年   75篇
  2018年   112篇
  2017年   103篇
  2016年   181篇
  2015年   319篇
  2014年   367篇
  2013年   444篇
  2012年   565篇
  2011年   564篇
  2010年   416篇
  2009年   356篇
  2008年   500篇
  2007年   504篇
  2006年   460篇
  2005年   482篇
  2004年   506篇
  2003年   445篇
  2002年   476篇
  2001年   70篇
  2000年   56篇
  1999年   116篇
  1998年   139篇
  1997年   56篇
  1996年   79篇
  1995年   64篇
  1994年   73篇
  1993年   85篇
  1992年   79篇
  1991年   61篇
  1990年   74篇
  1989年   47篇
  1988年   61篇
  1987年   59篇
  1986年   55篇
  1985年   49篇
  1984年   69篇
  1983年   48篇
  1982年   65篇
  1981年   73篇
  1980年   46篇
  1979年   46篇
  1978年   40篇
  1977年   43篇
  1976年   35篇
  1975年   25篇
  1974年   27篇
排序方式: 共有8985条查询结果,搜索用时 12 毫秒
71.
72.
Three-dimensional quantitative ultrasound spectroscopic imaging of prostate was investigated clinically for the noninvasive detection and extent characterization of disease in cancer patients and compared to whole-mount, whole-gland histopathology of radical prostatectomy specimens. Fifteen patients with prostate cancer underwent a volumetric transrectal ultrasound scan before radical prostatectomy. Conventional-frequency (~ 5 MHz) ultrasound images and radiofrequency data were collected from patients. Normalized power spectra were used as the basis of quantitative ultrasound spectroscopy. Specifically, color-coded parametric maps of 0-MHz intercept, midband fit, and spectral slope were computed and used to characterize prostate tissue in ultrasound images. Areas of cancer were identified in whole-mount histopathology specimens, and disease extent was correlated to that estimated from quantitative ultrasound parametric images. Midband fit and 0-MHz intercept parameters were found to be best associated with the presence of disease as located on histopathology whole-mount sections. Obtained results indicated a correlation between disease extent estimated noninvasively based on midband fit parametric images and that identified histopathologically on prostatectomy specimens, with an r2 value of 0.71 (P < .0001). The 0-MHz intercept parameter demonstrated a lower level of correlation with histopathology. Spectral slope parametric maps offered no discrimination of disease. Multiple regression analysis produced a hybrid disease characterization model (r2 = 0.764, P < .05), implying that the midband fit biomarker had the greatest correlation with the histopathologic extent of disease. This work demonstrates that quantitative ultrasound spectroscopic imaging can be used for detecting prostate cancer and characterizing disease extent noninvasively, with corresponding gross three-dimensional histopathologic correlation.  相似文献   
73.
Orally bioavailable, dual inhibitors of TIE-2/VEGF-R2 were identified by elaborating the C3/N13 SAR around a fused pyrrolodihydroindazolocarbazole scaffold. Analogs bearing a C3-thiophencarbonyl group were evaluated in enzymatic and cellular biochemical assays; two orally bioavailable analogs were further profiled in functional assays and found to inhibit microvessel growth in rat aortic explant cultures and inhibit Ang-1-stimulated chemotaxis of HUVECs.  相似文献   
74.
The functions of the two proteins defective in autosomal dominant polycystic kidney disease, polycystin-1 and polycystin-2, have not been fully clarified, but it has been hypothesized that they may heterodimerize to form a "polycystin complex" involved in cell adhesion. In this paper, we demonstrate for the first time the existence of a native polycystin complex in mouse kidney tubular cells transgenic for PKD1, non-transgenic kidney cells, and normal adult human kidney. Polycystin-1 is heavily N-glycosylated, and several glycosylated forms of polycystin-1 differing in their sensitivity to endoglycosidase H (Endo H) were found; in contrast, native polycystin-2 was fully Endo H-sensitive. Using highly specific antibodies to both proteins, we show that polycystin-2 associates selectively with two species of full-length polycystin-1, one Endo H-sensitive and the other Endo H-resistant; importantly, the latter could be further enriched in plasma membrane fractions and co-immunoprecipitated with polycystin-2. Finally, a subpopulation of this complex co-localized to the lateral cell borders of PKD1 transgenic kidney cells. These results demonstrate that polycystin-1 and polycystin-2 interact in vivo to form a stable heterodimeric complex and suggest that disruption of this complex is likely to be of primary relevance to the pathogenesis of cyst formation in autosomal dominant polycystic kidney disease.  相似文献   
75.
76.

Background  

Tumour regression observed in many conditional mouse models following oncogene inactivation provides the impetus to develop, and a platform to preclinically evaluate, novel therapeutics to inactivate specific oncogenes. Inactivating single oncogenes, such as c-Myc, can reverse even advanced tumours. Intriguingly, transient c-Myc inactivation proved sufficient for sustained osteosarcoma regression; the resulting osteocyte differentiation potentially explaining loss of c-Myc's oncogenic properties. But would this apply to other tumours?  相似文献   
77.
The cultivation of exotic Penaeus vannamei in Thailand began on a very limited scale in the late 1990s, but a Thai government ban on the cultivation of P. monodon in freshwater areas in 2000 led many Thai shrimp farmers to shift to cultivation of P. vannamei. Alarmed by the possibility of Taura syndrome virus (TSV) introduction, the Thai Department of Fisheries required that imported stocks of P. vannamei be certified free of TSV by RT-PCR (Reverse Trasciption Polymerase Chain Reaction) testing. During the interval of allowed importation, over 150,000 broodstock shrimp were imported, 67% of these from China and Taiwan. Despite the safeguards, TSV outbreaks occurred and we confirmed the first outbreak by RT-PCR in early 2003. This resulted in a governmental ban on all shrimp broodstock imports from February 2003, but TSV outbreaks have continued, possibly due to original introductions or to the continued illegal importation of stocks. To determine the origin of the TSV in Thailand, the viral coat protein gene VP1 was amplified by RT-PCR from several shrimp specimens found positive for TSV by RT-PCR from January to November 2003. These included 7 samples from P. vannamei disease outbreaks in Thailand, 3 other non-diseased shrimp samples from Thailand and Burma and 6 samples including P. vannamei and P. japonicus from China. Comparison revealed that the Thai, Burmese and Chinese TSV types formed a clade distinct from a clade of TSV types from the Americas.  相似文献   
78.
79.
Polymyxin peptide conjugated to the end groups of highly branched poly(N-isopropyl acrylamide) was shown to bind to a Gram negative bacterium, Pseudomonas aeruginosa . The nonbound polymer had a lower critical solution temperature (LCST) above 60 °C. However, binding caused aggregation, which was disrupted on cooling of the bacteria and polymer mixture. The data indicate that polymer binding of bacteria occurred by interaction of the end groups with lipopolysaccharide and that the binding decreased the LCST to below 37 °C. Cooling then progressed the polymer/bacteria aggregate through a bound LCST into an open polymer coil conformation that was not adhesive to P. aeruginosa .  相似文献   
80.
The report of R. J. Gillies, M. P. Rosenberg, and D. W. Deamer (1981, J. Cell. Phys., 108, 115–122) that sea urchin fertilization acid is anaerobically produced CO2, was reinvestigated by inseminating Strongylocentrotus purpuratus eggs in HCO?3-free seawater, then bubbling the seawater with N2 to remove volatile acid. Fertilization acid production occurred in HCO?3-free seawater and with N2-bubbling, the pH rose 0.28 ± 0.08 unit, significantly less than the rise of 0.63 ± 0.14 unit during N2-bubbling of HCO?3-free seawater that had been acidified with CO2 and similar to the rise of 0.18 ± 0.07 unit when acidification was with HCl. We conclude that most, if not all, of the sea urchin fertilization acid is nonvolatile and thus is not CO2; since it is not a weak acid, it must be H+.  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