全文获取类型
收费全文 | 34241篇 |
免费 | 3157篇 |
国内免费 | 390篇 |
专业分类
37788篇 |
出版年
2023年 | 197篇 |
2022年 | 418篇 |
2021年 | 714篇 |
2020年 | 513篇 |
2019年 | 665篇 |
2018年 | 767篇 |
2017年 | 694篇 |
2016年 | 1135篇 |
2015年 | 1772篇 |
2014年 | 1861篇 |
2013年 | 2275篇 |
2012年 | 2671篇 |
2011年 | 2428篇 |
2010年 | 1655篇 |
2009年 | 1447篇 |
2008年 | 1882篇 |
2007年 | 1860篇 |
2006年 | 1700篇 |
2005年 | 1673篇 |
2004年 | 1546篇 |
2003年 | 1529篇 |
2002年 | 1384篇 |
2001年 | 525篇 |
2000年 | 445篇 |
1999年 | 503篇 |
1998年 | 372篇 |
1997年 | 286篇 |
1996年 | 249篇 |
1995年 | 230篇 |
1994年 | 244篇 |
1993年 | 216篇 |
1992年 | 308篇 |
1991年 | 260篇 |
1990年 | 252篇 |
1989年 | 256篇 |
1988年 | 222篇 |
1987年 | 193篇 |
1986年 | 205篇 |
1985年 | 186篇 |
1984年 | 186篇 |
1983年 | 146篇 |
1982年 | 158篇 |
1981年 | 122篇 |
1980年 | 126篇 |
1979年 | 118篇 |
1977年 | 130篇 |
1976年 | 94篇 |
1975年 | 90篇 |
1974年 | 101篇 |
1971年 | 80篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
Marie Salomonsson Bo Carlsson Johan Hggblad 《The Journal of steroid biochemistry and molecular biology》1994,50(5-6):313-318
It is generally accepted that the Kd for hormone binding to estrogen receptors in extracts ranges between 0.1–1 nM and that binding displays positive cooperativity due to formation of homodimers. After carefully optimizing assay procedures, to diminish ligand depletion phenomena and to fully control recoveries, we find a single class of non-interacting high affinity hormone binding sites with a Kd of approx. 10 pM. Ligand depletion was avoided by decreasing receptor concentrations to 5–8 pM. We were therefore obliged to employ radioiodinated estradiol as a probe as the specific radioactivity of tritiated estradiol was too low to maintain the accuracy of the binding assay. Human estrogen receptor extracted from the MCF7 cell line and recombinantly produced (in yeast) wild-type human receptor have identical equilibrium hormone binding characteristics. 相似文献
182.
Per Putkonen Charlotta Nilsson Kerstin Hild Reinhold Benthin Martin Cranage Anne Marie Aubertin Gunnel Biberfeld 《Journal of medical primatology》1993,22(2-3):100-103
Several groups have reported protection against experimental SIV infection in macaques immunized with a whole inactivated virus vaccine. The aim of the current study was to investigate whether five macaques vaccinated with whole inactivated SIV and previously shown to be protected against challenge with two divergent strains of SIV grown on human cells could resist challenge with a subsequent homologous SIV grown on macaque cells. We show here that this same vaccine did not protect when the challenge virus was grown on primary cells of monkey origin. 相似文献
183.
Vertical structure of seed banks and the impact of depth of burial on recruitment in two temporary marshes 总被引:3,自引:0,他引:3
The structure of the seed bank (including Chara oospores), in relation to depth within the sediment and disturbance, was studied in two Rhône delta temporary marshes for two years. The seeds of all species were concentrated in the top 2 cm of sediment with very low numbers beeing found below 4 cm. When an exclosure eliminated disturbances of the sediment by animals, the vertical repartition of seeds at site 2 was more pronounced than outside the exclosure.In experiment 1, the emergence capacity of seeds from different depths and buried under layers of sterile equivalent to those in the field was measured. Depending of the species, 22 to 98% of the seeds germinated from unburied seeds in the top 2 cm. Only 1% of the oospores of Chara (from site 2) at 2 to 4 cm depth in the sediment emerged.In experiment 2, surface seed bank samples were placed under 0, 2 or 4 cm sterile sediment depth. The samples contained numerous recent seeds and the emergence percentage reached 41% (for Ruppia maritima). Only the seeds of Zannichellia spp failed to germinate from a depth of 2 cm or more. The emergence percentage from 2 cm depth or more was always lower than at the surface. These experiments showed that both burial and ageing of seeds decrease germination capacity.The majority of the active seeds located at the surface germinate when the marsh is flooded. Seeds located between 2 and 4 cm can be brought back to the surface by disturbances and play the role of a reserve involved in maintenance of populations that go without seed production for one or some years. 相似文献
184.
185.
186.
Neil C. Talbot Vernon G. Pursel Caird E. Rexroad Jr. Thomas J. Caperna Anne M. Powell Roger T. Stone 《In vitro cellular & developmental biology. Animal》1994,30(12):851-858
Summary The secondary culture of non-transformed parenchymal hepatocytes has not been possible. STO feeder cell-dependent secondary
cultures of fetal pig hepatocytes were established by colony isolation from primary cultures of 26-d fetal livers. The liver
cells had the typical polygonal morphology of parenchymal hepatocytes. They also spontaneously differentiated to form small
biliary canaliculi between individual cells or progressed further to large multicellular duct-like structures or cells undergoing
gross lipid accumulation and secretion. The secondary hepatocyte cultures expressed alpha-fetoprotein (AFP), albumin, and
β-fibrinogen mRNA, and conditioned medium from the cells contained elevated levels of transferrin and albumin. STO feeder
cell co-culture may be useful for the sustainable culture of hepatocytes from other species. 相似文献
187.
Anne N. Murphy Edward J. Unsworth William G. Stetler-Stevenson 《Journal of cellular physiology》1993,157(2):351-358
Tissue inhibitor of metalloproteinase-2 (TIMP-2), a protease inhibitor that binds to the latent and active forms of 72 kDa type IV collagenase (gelatinase A), was found to inhibit the in vitro proliferation of human microvascular endothelial (HME) cells stimulated with bFGF and 5% serum. The maximal inhibitory effect of TIMP-2 on incorporation of 3H-thymidine was evident 24 hours after bFGF stimulation of these cells and ranged between 45 and 60%. The half-maximal effective concentration of TIMP-2 was 107 ± 12 nM (S.D.). In contrast, TIMP-1 was not found to slow the growth of HME cells. The inhibition of cell proliferation observed with TIMP-2 was not mimicked by addition to the culture medium of BB94, a general matrix metalloproteinase inhibitor, nor antibodies to the 72 kDa type IV collagenase. In addition to growth, two other cell functions associated with the angiogenic process were tested for sensitivity to TIMP-2. Cell adhesion to tissue culture platic was slightly stimulated by TIMP-2, and cell migration was inhibited with short-term exposure to TIMP-2, but neither process was affected by longer-term exposure. The ability of TIMP-2 to inhibit cultured endothelial cell proliferation independent of protease inhibitory activity suggests that TIMP-2 may have additional actions which may limit neovascularization associated with solid tumor growth and metastasis in vivo. © 1993 Wiley-Liss, Inc. 相似文献
188.
189.
Effects of Motility and Adsorption Rate Coefficient on Transport of Bacteria through Saturated Porous Media 总被引:7,自引:3,他引:4 下载免费PDF全文
Anne K. Camper Jason T. Hayes Paul J. Sturman Warren L. Jones Alfred B. Cunningham 《Applied microbiology》1993,59(10):3455-3462
Three strains of Pseudomonas fluorescens with different motility rates and adsorption rate coefficients were injected into porous-medium reactors packed with l-mm-diameter glass spheres. Cell breakthrough, time to peak concentration, tailing, and cell recovery were measured at three interstitial pore velocities (higher than, lower than, and much lower than the maximal bacterial motility rate). All experiments were done with distilled water to reduce the effects of growth and chemotaxis. Contrary to expectations, motility did not result in either early breakthrough or early time to peak concentration at flow velocities below the motility rate. Bacterial size exclusion effects were shown to affect breakthrough curve shape at the very low flow velocity, but no such effect was seen at the higher flow velocity. The tendency of bacteria to adsorb to porous-medium surfaces, as measured by adsorption rate coefficients, profoundly influenced transport characteristics. Cell recoveries were shown to be correlated with the ratio of advective to adsorptive transport in the reactors. Adsorption rate coefficients were found to be better predictors of microbial transport phenomena than individual characteristics, such as size, motility, or porous-medium hydrodynamics. 相似文献
190.
Extracellular matrix (ECM) is an important mediator of endothelial functions such as adhesion, spreading, migration, proliferation, and maintenance of differentiated functions. Attachment of cultured cells to tissue culture polystyrene (TCPS) is dependent on vitronectin which adsorbs onto the surface from the serum in the culture medium. Vitronectin (VN) will adsorb efficiently to TCPS even if the latter has been coated with another matrix molecule and blocked with albumin. This means that studies of the interactions of cells with individual coated ECM molecules will be confounded by the presence of adsorbed VN if serum is present in the culture medium. In this study, the adhesion, spreading, growth, and output of endogenous matrix molecules by bovine corneal endothelial (BCE) cells were measured on five different matrix substrates using medium which had been depleted of vitronectin to avoid such confounding effects. The same cell adhesion and spreading maxima were achieved on vitronectin, fibronectin (FN), laminin (LM), and types I and IV collagen (col I, col IV). The coating concentrations required to achieve these maxima, however, differed among the substrates, LM needing considerably higher concentrations than the other substrates for both maximal adhesion and spreading and FN needing higher concentrations for cell spreading. When cells were continuously passaged on each of the five substrates coated at concentrations optimal for cell spreading, no differences in cell proliferation rates or cell morphology were observed. Significant differences, however, were observed in the subcellular output of endogenous matrix molecules (FN, LM, col IV, and thrombospondin) between the different substrates. Col I was a poor substrate for the production of all ECM molecules tested over the 10 passages of the experiment, whereas col IV was a consistently good substrate. LM and FN substrates displayed differential effects on the output of different ECM molecules. VN was unique in that BCE cells at early passage on this substrate produced high levels of endogenous matrix molecules, whereas with continued passage on this substrate, a progressive decline in ECM secretion was observed. These results show that incorporation of individual molecules into the ECM by BCE cells in culture is significantly affected by the nature of the substratum. They further suggest that passage of endothelial cells in media containing serum (which results in coating of VN onto the substrate) may result in a progressive reduction of ECM output. 相似文献