全文获取类型
收费全文 | 193863篇 |
免费 | 9997篇 |
国内免费 | 62篇 |
专业分类
203922篇 |
出版年
2021年 | 1135篇 |
2020年 | 1049篇 |
2019年 | 1060篇 |
2018年 | 3537篇 |
2017年 | 3386篇 |
2016年 | 5857篇 |
2015年 | 10246篇 |
2014年 | 10017篇 |
2013年 | 11627篇 |
2012年 | 12632篇 |
2011年 | 9915篇 |
2010年 | 6903篇 |
2009年 | 5559篇 |
2008年 | 7230篇 |
2007年 | 7142篇 |
2006年 | 6645篇 |
2005年 | 11952篇 |
2004年 | 10376篇 |
2003年 | 8310篇 |
2002年 | 5619篇 |
2001年 | 3509篇 |
2000年 | 2745篇 |
1999年 | 3523篇 |
1998年 | 1227篇 |
1997年 | 885篇 |
1992年 | 3291篇 |
1991年 | 3262篇 |
1990年 | 3195篇 |
1989年 | 3005篇 |
1988年 | 2920篇 |
1987年 | 2620篇 |
1986年 | 2389篇 |
1985年 | 2477篇 |
1984年 | 1813篇 |
1983年 | 1456篇 |
1982年 | 944篇 |
1981年 | 930篇 |
1979年 | 1710篇 |
1978年 | 1258篇 |
1977年 | 1053篇 |
1976年 | 1076篇 |
1975年 | 1344篇 |
1974年 | 1490篇 |
1973年 | 1487篇 |
1972年 | 1302篇 |
1971年 | 1316篇 |
1970年 | 1154篇 |
1969年 | 1131篇 |
1968年 | 1028篇 |
1967年 | 1051篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
92.
P V Sergeev A S Dukhanin A V Seme?kin 《Biulleten' eksperimental'no? biologii i meditsiny》1987,104(12):678-681
In has been shown that cortisol immobilized on polyvinylpyrrolidone (PVP-GC) affects cyclic AMP production stimulated by adenosine and isoproterenol in rat thymocytes. This effect of PVP-GC is specific for cortisol: antiglucocorticoid progesterone (at a concentration of 10(-5) M) inhibited completely the action of PVP-GC on the intracellular cAMP level. It is suggested that cortisol effect on cAMP production is one of the mechanisms of glucocorticoid hormone action in target cells. 相似文献
93.
94.
95.
96.
Natural killer T (NKT) cells are a component of innate and adaptive immune systems implicated in immune, autoimmune responses and in the control of obesity and cancer. NKT cells develop from common CD4+ CD8+ double positive (DP) thymocyte precursors after the rearrangement and expression of T cell receptor (TCR) Vα14-Jα18 gene. Temporal regulation and late appearance of Vα14-Jα18 rearrangement in immature DP thymocytes has been demonstrated. However, the precise control of lifetime of DP thymocytes in vivo that enables distal rearrangements remains incompletely defined. Here we demonstrate that T cell factor (TCF)-1, encoded by the Tcf7 gene, is critical for the extended lifetime of DP thymocytes. TCF-1-deficient DP thymocytes fail to undergo TCR Vα14-Jα18 rearrangement and produce significantly fewer NKT cells. Ectopic expression of Bcl-xL permits Vα14-Jα18 rearrangement and rescues NKT cell development. We report that TCF-1 regulates expression of RORγt, which regulates DP thymocyte survival by controlling expression of Bcl-xL. We posit that TCF-1 along with its cofactors controls the lifetime of DP thymocytes in vivo. 相似文献
97.
Mélody Porlier Marc Bélisle Dany Garant 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2009,364(1523):1543-1554
Improving our knowledge of the links between ecology and evolution is especially critical in the actual context of global rapid environmental changes. A critical step in that direction is to quantify how variation in ecological factors linked to habitat modifications might shape observed levels of genetic variability in wild populations. Still, little is known on the factors affecting levels and distribution of genetic diversity at the individual level, despite its vital underlying role in evolutionary processes. In this study, we assessed the effects of habitat quality on population structure and individual genetic diversity of tree swallows (Tachycineta bicolor) breeding along a gradient of agricultural intensification in southern Québec, Canada. Using a landscape genetics approach, we found that individual genetic diversity was greater in poorer quality habitats. This counter-intuitive result was partly explained by the settlement patterns of tree swallows across the landscape. Individuals of higher genetic diversity arrived earlier on their breeding grounds and settled in the first available habitats, which correspond to intensive cultures. Our results highlight the importance of investigating the effects of environmental variability on individual genetic diversity, and of integrating information on landscape structure when conducting such studies. 相似文献
98.
99.
S Kafert S Luther I B?ll K Wagner A Ganser M Eder 《The Journal of biological chemistry》1999,274(46):33064-33071
To analyze the function of each subunit of the receptor for granulocyte-macrophage colony-stimulating factor (GM-CSF), GMR, we previously generated a single-chain chimeric receptor by fusion of the extracellular and transmembrane domain from the alpha-subunit (alpha-GMR) to the intracellular part of the beta-subunit (beta-GMR) introducing an additional glutamate residue at the fusion site (alpha/beta-GMR). We demonstrated the capacity of alpha/beta-GMR to bind GM-CSF with low affinity and to induce GM-CSF-dependent activation of tyrosine kinase activity and proliferation in transfected Ba/F3 cells. To further compare the functions of wild type and chimeric receptors, we now report that this alpha/beta-GMR is sufficient to mediate morphological changes, expression of alpha(4)- and beta(1)-integrin receptor subunits, and serine-phosphorylation of Akt kinase. To analyze the function of the glutamate residue at the fusion region of alpha/beta-GMR various point mutants changing this amino acid and its position were expressed in Ba/F3 cells. None of these mutants was capable of supporting GM-CSF-dependent proliferation; however, when beta-GMR was coexpressed, GM-CSF mediated short and long term proliferation. Interestingly, some mutants but not alpha/beta-GMR can induce proliferation in the presence of an anti-alpha-GMR antibody. These data demonstrate the significance of a glutamate residue in the transmembrane region of alpha/beta-GMR for ligand-induced receptor activation. 相似文献
100.