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   98886篇
  免费   842篇
  国内免费   1562篇
  101290篇
  2024年   25篇
  2023年   102篇
  2022年   245篇
  2021年   355篇
  2020年   249篇
  2019年   307篇
  2018年   12079篇
  2017年   10841篇
  2016年   7694篇
  2015年   1009篇
  2014年   748篇
  2013年   760篇
  2012年   4795篇
  2011年   13270篇
  2010年   12274篇
  2009年   8525篇
  2008年   10068篇
  2007年   11609篇
  2006年   469篇
  2005年   717篇
  2004年   1165篇
  2003年   1192篇
  2002年   922篇
  2001年   369篇
  2000年   254篇
  1999年   99篇
  1998年   67篇
  1997年   71篇
  1996年   46篇
  1995年   29篇
  1994年   40篇
  1993年   51篇
  1992年   49篇
  1991年   66篇
  1990年   18篇
  1989年   19篇
  1988年   23篇
  1987年   21篇
  1986年   10篇
  1985年   6篇
  1984年   9篇
  1983年   20篇
  1982年   7篇
  1972年   246篇
  1971年   274篇
  1965年   13篇
  1962年   24篇
  1956年   5篇
  1944年   12篇
  1940年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.

Background  

In cancer cells the three-dimensional (3D) telomere organization of interphase nuclei into a telomeric disk is heavily distorted and aggregates are found. In Hodgkin's lymphoma quantitative FISH (3D Q-FISH) reveals a major impact of nuclear telomere dynamics during the transition form mononuclear Hodgkin (H) to diagnostic multinuclear Reed-Sternberg (RS) cells. In vitro and in vivo formation of RS-cells is associated with the increase of very short telomeres including "t-stumps", telomere loss, telomeric aggregate formation and the generation of "ghost nuclei".  相似文献   
192.
Loranthus yadoriki, one of the Korean mistletoe species, has been already known for anti-viral effects, but the molecular basis that it caused apoptosis in cancer cells was not definitely revealed yet. The aim of this study was to estimate the mechanisms of apoptotic cell death of the extract from Loranthus yadoriki (named as ELY) in human cervix HeLa cells. We identified that ELY prevented the proliferation of HeLa cells between 50 and 300 μg/mL which did not affect non-cancerous HaCaT cells. In addition, ELY induced a morphological change and nucleus disruption as well as an accumulation of sub-G1 phase in HeLa cells. The mechanism study, by using western blot analysis, showed that the phosphorylation of Fas-associated death domain (FADD), Bim and Bak was up-regulated by ELY treatment. Furthermore, the expression of cytochrome c and Apaf-1 was increased by ELY treatment. In immunofluorescence staining, the increased intensity of cleaved caspase-3 and cleaved PARP was also observed under ELY treatment. Sequentially, the caspase cascade was activated by ELY from caspase-8 to caspase-3 and from caspase-9 to caspase-3, in both extrinsic and intrinsic pathways. The results of this study demonstrate that ELY has anti-cancer effects on human cervix cancer HeLa cells via caspase cascade in apoptotic signaling pathways.  相似文献   
193.
Invading pathogens elicit potent immune responses in cells through interactions between structurally conserved molecules derived from the pathogens and specialized innate immune receptors such as the Toll-like receptors (TLRs). Nucleic acid is one of the principal TLR ligands. Nucleic acid-sensing TLRs recognize an array of nucleic acids, including double-stranded RNA, single-stranded RNA, and DNAs with specific sequence motifs. Although ligand-induced dimerization is commonly observed followed by TLR activation, both the specific recognition mechanisms and the ligand–receptor interactions vary among different TLRs. In this review, we highlight our current understanding of how these receptors recognize their cognate ligands based on the recent advances in structural biology.  相似文献   
194.
Peruvian yellow-tailed woolly monkeys (Oreonax flavicauda) are considered Critically Endangered (IUCN Categories A4c). The International Primatological Society also considers them one of the world’s 25 most endangered primate species and therefore a conservation priority. However, there is little concerted conservation action, and the existing protected area network may be inadequate to protect this species from extinction. Until recently this species has been the focus of few studies and its distributional limits remain unknown. I present results of a range-wide survey of Oreonax flavicauda in northeastern Peru. I conducted 53 presence/absence field surveys at 43 sites between March 2007 and March 2010, with data collected for an additional 7 sites from other researchers. I chose sites where the species was previously reported or following suggestions from predictive GIS modeling. Oreonax flavicauda was present at 35 sites, all presence records were in Ficus spp.–dominated cloud forests between 1500 and 2650 m above sea level. I give the geographical limits of this species distribution throughout the north, east, and west of its range; the exact extent of its range to the south requires further investigation. Oreonax flavicauda continues to be threatened throughout its range. The major threats I identified at the survey locations were the continued conversion of forests to cattle pasture, opening of new access routes into virgin areas, and both commercial and subsistence hunting. My results suggest that existing conservation measures may be inadequate at protecting this species but that substantial opportunities do exist. Further surveys need to be made in the southern distribution of this species to determine more accurately extant habitat.  相似文献   
195.
Sphedamnocarpus andersonii, a new species from the Ihorombe Region of Madagascar, is described and illustrated. The twiggy, white-flowered shrubs are characterized by linear leaves, unique in the genus.  相似文献   
196.
The management of field margin strips for the enhancement of biodiversity of plant-insect communities and natural-enemy populations was studied on experimental farms near Göttingen (Germany). Young and old, sown and naturally developed field margin strips were compared and differences to large fallows established. The five types of field margin strips (around cereal fields) were: (1, 2) 1- or 6-year-old naturally developed strips, (3) strips sown with a Phacelia mixture, (4) strips sown with a mixture of 19 wild flower species, and (5) strips sown with winter wheat or oat as a control. The naturally developed vegetation of the field margin strips was dominated by aggressive weeds, presumably due to the intensive farming practices and the fertile soils. Cirsium arvense populations decreased, while Elymus repens populations increased with age of habitat. Sowings were suitable to suppress these aggressive weeds. Potted plants of mugwort (Artemisia vulgaris) and red clover (Trifolium pratense) were exposed in the field margin strips to study arthropod colonization of these experimentally standardized plant patches. Arthropod species richness did not differ between field margin types, reflecting the overall similarity in floristic diversity, but sprayed and strip-free edges of cereal fields had a reduced diversity. Dispersal of insect populations of red clover into the cereal fields decreased with increasing distance, but benefited from adjacent field margin strips. Populations of predators (mainly spiders) as well as predator-prey ratios were significantly larger in 6-year-old than in 1-year-old strips emphasizing the importance of habitat age for natural enemies and possible biological control. Predator-prey ratios were also higher on old than young fallows. Large fallows had greater predator-prey ratios than small field margin strips emphasizing the trophic-level hypothesis of island biogeography in that the relative importance of natural enemies increased with habitat area. Insect species richness was only marginally influenced by area and not by age. As species richness of predators did not increase with area and age, species diversity and the possible biological-control function did not covary.  相似文献   
197.
198.
199.
200.

Background  

Head and neck cancers (HNC) are relatively common and often very serious diseases in both dogs and humans. Neoplasms originating in the head and neck region are a heterogeneous group. HNC often has an unfavourable prognosis and the proximity of the tissue structures renders extirpation of tumours with sufficient margins almost incompatible with preservation of functionality. In humans oral malignant melanoma (OMM) is extremely rare, but represents a particular challenge since it is highly aggressive as is the canine counterpart, which thus may be of interest as a spontaneous animal model.  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