全文获取类型
收费全文 | 13486篇 |
免费 | 1055篇 |
国内免费 | 12篇 |
出版年
2024年 | 15篇 |
2023年 | 79篇 |
2022年 | 194篇 |
2021年 | 361篇 |
2020年 | 187篇 |
2019年 | 228篇 |
2018年 | 304篇 |
2017年 | 236篇 |
2016年 | 403篇 |
2015年 | 609篇 |
2014年 | 678篇 |
2013年 | 811篇 |
2012年 | 1120篇 |
2011年 | 1119篇 |
2010年 | 701篇 |
2009年 | 642篇 |
2008年 | 895篇 |
2007年 | 874篇 |
2006年 | 864篇 |
2005年 | 754篇 |
2004年 | 717篇 |
2003年 | 688篇 |
2002年 | 611篇 |
2001年 | 146篇 |
2000年 | 87篇 |
1999年 | 156篇 |
1998年 | 161篇 |
1997年 | 107篇 |
1996年 | 105篇 |
1995年 | 65篇 |
1994年 | 71篇 |
1993年 | 75篇 |
1992年 | 39篇 |
1991年 | 47篇 |
1990年 | 35篇 |
1989年 | 45篇 |
1988年 | 27篇 |
1987年 | 22篇 |
1986年 | 17篇 |
1985年 | 45篇 |
1984年 | 42篇 |
1983年 | 26篇 |
1982年 | 21篇 |
1981年 | 29篇 |
1980年 | 25篇 |
1979年 | 12篇 |
1978年 | 10篇 |
1977年 | 12篇 |
1976年 | 8篇 |
1975年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Embryonic stem cell therapy for diabetes mellitus 总被引:9,自引:0,他引:9
There is a compelling need to develop novel therapies for diabetes mellitus. Recent successes in the transplantation of islets of Langerhans are seen as a major breakthrough. However, there is huge disparity between potential recipients and the availability of donor tissue. Human embryonic stem cells induced to form pancreatic beta cells could provide a replenishable supply of tissue. Early studies on the spontaneous differentiation of mouse embryonic stem cells have laid the foundation for a more directed approach based on recapitulating the events that occur during the development of the pancreas in the mouse. A high yield of definitive endoderm has been achieved, and although beta-like cells can be generated in a step-wise manner, the efficiency is still low and the final product is not fully differentiated. Future challenges include generating fully functional islet cells under Xeno-free and chemically defined conditions and circumventing the need for immunosuppression. 相似文献
992.
993.
The quality of pharmaceutical products such as ginseng is important for ensuring consumer safety and efficacy. Ginseng is
an expensive herb, and adulteration with other cheaper products may occur. Quality assurance of ginseng is needed since many
of its commercial products now come in various formulations such as capsules, powder, softgels and tea. Thus traditional means
of authentication via smell, taste or physical appearance are hardly reliable. Herbs like ginseng tend to exhibit characteristic
infrared fingerprints due to their different chemical constituents. Here we report for the first time a rapid means of distinguishing
American and Asian ginsengs from two morphological fakes – sawdust and Platycodon grandiflorum, via pattern differences and principal component analysis of their infrared spectra. Our results show that ginseng can be
distinguished from both sawdust and Platycodon grandiflorum, hence there is a potential of using infrared spectroscopy as a novel analytical technique in the authentication of ginseng. 相似文献
994.
995.
Response to genotoxic stress may trigger the activation of distinct mechanisms that serve to promote cell death, including apoptosis and necrosis. In this study we examined the response of human fibroblasts, either proficient or deficient for the damage-activated protein kinase ataxia telangiectasia-mutated (ATM), to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Analysis of both long- and short-term viability shows that both ATM-proficient YZ-5 and ATM-deficient EBS-7 fibroblasts display a cytotoxic response to MNNG. Consistent with activation of apoptosis in response to MNNG, we observed increased caspase-3 cleavage and activity, appearance of fragmented nuclei, and increased staining with annexin V in both ATM-proficient and -deficient fibroblasts. Flow cytometry demonstrated that these cell lines also display a nonapoptotic cell death in response to MNNG. This form of cell death is associated with activation of poly-ADP ribose polymerase (PARP), and analysis of PARP activity indicated increased protein poly(ADP-ribosylation) in YZ-5 when compared to EBS-7. This PARP activity was accompanied by apoptosis-inducing factor release and translocation from the mitochondria to the nucleus. Finally, the PARP inhibitor 3,4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2H)-isoquinolinone (DPQ) or the caspase-3 inhibitor benzyloxycarbonyl-VAD-fluoromethyl ketone dramatically diminished the cytotoxic response to MNNG, reinforcing the roles for apoptotic and nonapoptotic cell death in human fibroblasts treated with MNNG. From these findings, we conclude that MNNG induces a heterogeneous death response in human fibroblasts. 相似文献
996.
Kuzdzal-Fick Jennie J.; Foster Kevin R.; Queller David C.; Strassmann Joan E. 《Behavioral ecology》2007,18(2):433-437
Understanding the ecological benefits of social actions is centralto explaining the evolution of social behavior. The social amoebaDictyostelium discoideum has been well studied and is a modelfor social evolution and development, but surprisingly littleis known about its ecology. When starving, thousands of thenormally solitary amoebae aggregate to form a differentiatedmulticellular organism known as a slug. The slug migrates towardthe soil surface where it metamorphoses into a fruiting bodyof hardy spores held up by a dead stalk comprising about one-fifthof the cells. Multicellularity in D. discoideum is thought tohave evolved to lift the spores above the hazards of the soilwhere spores can be picked up for long-distance dispersal. Here,we show that multicellularity has another advantage: local dispersalto new food sources. We find that cells shed by D. discoideumslugs during migration consume and remove bacteria in the pathof the slug, although slugs themselves do not breakup. We alsoshow that slugs are adept at local dispersal by comparing migrationof slugs with migration of individual cells of the mutant, CAP2,which cannot aggregate and so rely only on cellular movement.In particular, the solitary cells of the aggregation mutantare unable to cross a soil barrier, easily crossed by slugs.We propose that the exploitation of local food patches is animportant selective benefit favoring multicellular cooperationin D. discoideum. 相似文献
997.
Much of the pioneering work on the genetics of cell cycle regulation was accomplished using budding and fission yeast. The relative simplicity of these single-celled organisms allowed investigators to readily identify and assign roles to individual genes. While the molecular mechanisms worked out in yeast are more or less identical to those operating in higher organisms, additional layers of control must exist in multicellular organisms to coordinate the timing of developmental events occurring in different cells and tissues. Here we discuss experimental approaches for studying cell cycle processes in the nematode Caenorhabditis elegans. 相似文献
998.
Keith Lombardo Jeffrey S. Fehmi Kevin J. Rice Emilio A. Laca 《Restoration Ecology》2007,15(2):177-178
We found no significant effect (p > 0.38) of clipping surrounding non‐native annuals on the performance or survival of plantings of the native Nassella pulchra. However, the preplanting productivity of the three sites had a significant effect (p < 0.001) on the demography and water relations of Nassella. Survival was 85% on the lowest productivity site and it decreased by 6% for each additional 100 kg in average aboveground productivity between 2,400 and 3,600 kg/ha. Plants experienced greater water stress in higher productivity sites. Together, these results suggest that the original habitat of N. pulchra may have been in more marginal ecological sites rather than the more fertile soils of the Central Valley. 相似文献
999.
de Queiroz K 《Systematic biology》2007,56(6):956-974
Although the proposition that higher taxa should correspond to clades is widely accepted, current nomenclature does not distinguish clearly between different clades in nested series. In particular, the same name is often applied to a total clade, its crown clade, and clades originating with various nodes, branches, and apomorphies in between. An integrated system of clade names is described based on categories of clades defined with respect to lineages that have survived to the present time. In this system, the most widely known names are applied to crown clades, the names of total clades are formed by adding a standard prefix to the names of the corresponding crowns, and the names of apomorphy clades describe the specific apomorphies with which they originated. Relative to traditional approaches, this integrated approach to naming clades is both more precise concerning the associations of names with particular clades and more efficient with regard to the cognitive effort required to recognize the names of corresponding crown and total clades. It also seems preferable to five alternatives that could be used to make the same distinctions. The integrated system of clade names has several advantages, including the facilitation of communication among biologists who study distantly related clades, promoting a broader conceptualization of the origins of distinctive clades of extant organisms and emphasizing the continuous nature of evolution. 相似文献
1000.
Old World arenavirus infection interferes with the expression of functional alpha-dystroglycan in the host cell 下载免费PDF全文
alpha-Dystroglycan (alpha-DG) is an important cellular receptor for extracellular matrix (ECM) proteins as well as the Old World arenaviruses lymphocytic choriomeningitis virus (LCMV) and the human pathogenic Lassa fever virus (LFV). Specific O-glycosylation of alpha-DG is critical for its function as receptor for ECM proteins and arenaviruses. Here, we investigated the impact of arenavirus infection on alpha-DG expression. Infection with an immunosuppressive LCMV isolate caused a marked reduction in expression of functional alpha-DG without affecting biosynthesis of DG core protein or global cell surface glycoprotein expression. The effect was caused by the viral glycoprotein (GP), and it critically depended on alpha-DG binding affinity and GP maturation. An equivalent effect was observed with LFVGP. Viral GP was found to associate with a complex between DG and the glycosyltransferase LARGE in the Golgi. Overexpression of LARGE restored functional alpha-DG expression in infected cells. We provide evidence that virus-induced down-modulation of functional alpha-DG perturbs DG-mediated assembly of laminin at the cell surface, affecting normal cell-matrix interactions. 相似文献