全文获取类型
收费全文 | 3908篇 |
免费 | 332篇 |
国内免费 | 2篇 |
专业分类
4242篇 |
出版年
2023年 | 19篇 |
2022年 | 29篇 |
2021年 | 58篇 |
2020年 | 34篇 |
2019年 | 40篇 |
2018年 | 83篇 |
2017年 | 53篇 |
2016年 | 91篇 |
2015年 | 162篇 |
2014年 | 205篇 |
2013年 | 200篇 |
2012年 | 291篇 |
2011年 | 259篇 |
2010年 | 174篇 |
2009年 | 155篇 |
2008年 | 220篇 |
2007年 | 240篇 |
2006年 | 208篇 |
2005年 | 220篇 |
2004年 | 172篇 |
2003年 | 204篇 |
2002年 | 175篇 |
2001年 | 42篇 |
2000年 | 29篇 |
1999年 | 31篇 |
1998年 | 46篇 |
1997年 | 37篇 |
1996年 | 31篇 |
1995年 | 30篇 |
1994年 | 22篇 |
1993年 | 27篇 |
1992年 | 15篇 |
1991年 | 30篇 |
1990年 | 39篇 |
1989年 | 23篇 |
1988年 | 30篇 |
1987年 | 26篇 |
1986年 | 16篇 |
1985年 | 17篇 |
1984年 | 22篇 |
1983年 | 19篇 |
1982年 | 22篇 |
1981年 | 24篇 |
1980年 | 19篇 |
1978年 | 15篇 |
1977年 | 16篇 |
1974年 | 14篇 |
1969年 | 13篇 |
1968年 | 13篇 |
1967年 | 15篇 |
排序方式: 共有4242条查询结果,搜索用时 0 毫秒
31.
Various human Burkitt lymphoma and LCL lines established in vitro and their derived somatic cell hybrids were tested for their comparative EBV receptor levels in a virus binding assay. Their graded C3b and C3d complement receptor expression was estimated simultaneously by means of isotope labeled rosette marker cells. The receptor concentration of each cell line was related to Raji as the standard of comparison, K 562, P3HR-1, and YACUT were used as negative controls. In general, the charging curves for EBV and C3d receptors parallelled each other (r = 0.97) while C3b receptor charging showed no correlation (r < 0.60). In the Raji hybrids between the C3b receptor positive Raji parent and various patents that were negative for this receptor, C3b receptor expression was low or negative. In contrast, the C3d negative P3HR-1 line gave rise to hybrids, after fusion with receptor-positive cells, that were intermediate with regard to their C3d receptor expression. The host range restriction of the Epstein-Barr virus is determined at the receptor level. The close relationship between the EBV receptor and the C3d receptor, a B-lymphocyte-specific moiety, suggests that the moderate interaction with EBV with the B lymphocytes may have had a selective advantage, favoring the presence of EBV. Since EBV causes lytic infections after artificial introduction into nonnatural host cells, it may represent a B-lymphocyte-specific host range mutant, derived from an originally lytic herpesvirus with a much broader target cell range. 相似文献
32.
Regulation and Glucocorticoid-Independent Induction of Lipocortin I in Cultured Astrocytes 总被引:3,自引:0,他引:3
Peter J. Gebicke-Haerter Angelika Schobert Peter Dieter Paul Honegger† Georg Hertting 《Journal of neurochemistry》1991,57(1):175-183
Stimulation of prostaglandin (PG) release in rat astroglial cultures by various substances, including phorbol esters, melittin, or extracellular ATP, has been reported recently. It is shown here that glucocorticoids (GCs) reduced both basal and stimulated PGD2 release. Hydrocortisone, however, did not inhibit ATP-, calcium ionophore A23187-, or tetradecanoyl phorbol acetate (TPA)-stimulated arachidonic acid release, and only TPA stimulations were affected by dexamethasone. GC-mediated inhibition of PGD2 release thus appeared to exclude regulation at the phospholipase A2 (PLA2) level. Therefore, the effects of GCs on the synthesis of lipocortin I (LC I), a potent, physiological inhibitor of PLA2, were studied in more detail. Dexamethasone was not able to enhance de novo synthesis of LC I in freshly seeded cultures and failed to increase LC I synthesis in 2-3-week-old cultures. It is surprising that LC I was the major LC synthesized in those cultures, and marked amounts accumulated with culture time, reaching plateau levels at approximately day 10. In contrast, LC I was barely detectable in vivo. This tonic inhibition of PLA2 is the most likely explanation for unsuccessful attempts to evoke PG release in astrocyte cultures by various physiological stimuli. GC receptor antagonists (progesterone and RU 38486) given throughout culture time reduced LC I accumulation and simultaneously increased PGD2 release. Nonetheless, a substantial production of LC I persisted in the presence of antagonists. Therefore, LC I induction did not seem to involve GC receptor activation. This was confirmed in serum- and GC-free brain cell aggregate cultures. Here also a marked accumulation of LC I was observed.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
33.
Eric P. Krenning Roel Docter Bert Bernard Theo Visser Georg Hennemann 《FEBS letters》1982,140(2):229-233
34.
35.
Peter J. Gebicke-Haerter Andr s Seregi Angelika Schobert Georg Hertting 《Neurochemistry international》1988,13(4):475-480
The role of protein kinase C (PKC) and calcium in the stimulation of prostaglandin D2 (PGD2) synthesis was investigated in primary rat astroglial cultures using the phorbol esters phorbol 12-myristate, 13-acetate (PMA), phorbol 12,13-dibutyrate (PDB) and the calcium ionophore A23187. Both phorbol esters and the ionophore were able to stimulate PGD2 synthesis in a concentration dependent manner. The inactive stereoisomers of PMA and PDB had no significant effect. Combinations of subthreshold concentrations of phorbol esters (10 nM PMA or 10 nM PBD) potentiated PG formation induced by 100 nM A23187. An even more pronounced effect was observed when phorbol ester concentrations were increased to 100nM. The contribution of extra- and intracellular calcium in phorbol ester or A23187 stimulated PGD2 synthesis was evaluated by carrying out experiments with calcium-free media plus EGTA or with the intracellular calcium-chelating agent TMB-8. Ionophore stimulated PGD2 release was shut down to basal values upon removal of extracellular calcium, whereas phorbol ester stimulated PGD2 formation persisted at a reduced level. It was unabated also upon further addition of EGTA. In the presence of TMB-8, however, phorbol ester stimulated PGD2 synthesis was completely suppressed. These data strongly suggest that PKC has an additional effect on the activation of phospholipase A2 and subsequent prostanoid synthesis, which is independent from extracellular calcium and, thus, support the concept of more than one metabolic pathway in astrocytes that synergistically regulate phospholipase A2 activity. 相似文献
36.
Barbara M Seiffert Juliane Vier Georg H?cker 《Biochemical and biophysical research communications》2002,290(1):359-365
Caspase family cell death proteases are activated during apoptosis through the oligomerization of caspase-binding "adapter" proteins. In the nematode Caenorhabditis elegans one adapter protein, CED-4, exists. Here we report an analysis of CED-4 protein expressed in insect Sf9 cells by infection with recombinant baculovirus. During expression, CED-4 assumed a perinuclear spherical or reticular localization where it was partly resistant to extraction with nonionic detergents. Both purified FLAG-CED-4 and GST-FLAG-CED-4 proteins were present in solution as large complexes. FLAG-CED-4 complexes were estimated by gel filtration to have a molecular weight of approximately 500 kDa to >1.2 MDa, while GST-FLAG-CED-4 complexes appeared somewhat smaller. Unlike its mammalian homologue Apaf-1, CED-4 exhibited a marked preference for ATP over dATP in filter binding studies and in competition experiments. ATP hydrolysis was required neither for complex stability nor for binding of CED-3. These features are likely to be relevant for CED-4's function as a caspase adapter. 相似文献
37.
Haberer G Young S Bharti AK Gundlach H Raymond C Fuks G Butler E Wing RA Rounsley S Birren B Nusbaum C Mayer KF Messing J 《Plant physiology》2005,139(4):1612-1624
Maize (Zea mays or corn) plays many varied and important roles in society. It is not only an important experimental model plant, but also a major livestock feed crop and a significant source of industrial products such as sweeteners and ethanol. In this study we report the systematic analysis of contiguous sequences of the maize genome. We selected 100 random regions averaging 144 kb in size, representing about 0.6% of the genome, and generated a high-quality dataset for sequence analysis. This sampling contains 330 annotated genes, 91% of which are supported by expressed sequence tag data from maize and other cereal species. Genes averaged 4 kb in size with five exons, although the largest was over 59 kb with 31 exons. Gene density varied over a wide range from 0.5 to 10.7 genes per 100 kb and genes did not appear to cluster significantly. The total repetitive element content we observed (66%) was slightly higher than previous whole-genome estimates (58%-63%) and consisted almost exclusively of retroelements. The vast majority of genes can be aligned to at least one sequence read derived from gene-enrichment procedures, but only about 30% are fully covered. Our results indicate that much of the increase in genome size of maize relative to rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana) is attributable to an increase in number of both repetitive elements and genes. 相似文献
38.
Böcker W Rossmann O Docheva D Malterer G Mutschler W Schieker M 《The journal of gene medicine》2007,9(7):585-595
BACKGROUND: Human mesenchymal stem cells (hMSCs) are a promising target for ex vivo gene therapy and lentiviruses are excellent gene transfer vehicles in hMSCs since they achieve high transduction rates with long-term gene expression. Nevertheless, senescence of hMSCs may limit therapeutic applications due to time-consuming cell selection and viral titration. Here, we describe a fast and reliable method to determine functional lentiviral titer by quantitative polymerase chain reaction (qPCR) after highly efficient ex vivo gene transfer in hMSCs. METHODS: Lentivirus production was tested with different types of packaging systems. Using p24 ELISA remaining viral particles were detected in the cell culture supernatant. The lentiviral gene transfer efficiency was quantified by FACS analysis. Lentiviral titers were determined by qPCR of expressed transgenes. RESULTS: Third-generation self-inactivating vectors showed highly efficient gene transfer in hMSCs. No viral antigen was detected in the cell culture supernatant after four media changes, suggesting the absence of infectious particles after 4 days. We observed a linear correlation between virus dilution and level of transgene expression by qPCR analysis, therefore allowing viral titering by quantification of transgene expression. Finally, we demonstrated that transduced hMSCs retained their stem cell character by differentiation towards adipogenic, osteogenic and chondrogenic lineages. CONCLUSIONS: Quantification of transgene copy numbers by qPCR is a fast and reliable method to determine functional lentiviral titer after ex vivo gene transfer in hMSCs. 相似文献
39.
Developmental changes of nerve growth factor and its mRNA in the rat hippocampus: Comparison with choline acetyltransferase 总被引:4,自引:0,他引:4
Georg Auburger Rolf Heumann Rainer Hellweg Sigrun Korsching Hans Thoenen 《Developmental biology》1987,120(2):322-328
Previous experiments have demonstrated that in the septo-hippocampal system choline acetyltransferase (ChAT) is induced by nerve growth factor (NGF) (Gnahn et al. (1983) Dev. Brain Res. 9, 45-52) and that hippocampal NGF and mRNANGF levels are correlated with the density of cholinergic innervation (Korsching et al. (1985) EMBO J. 4, 1389-1393). In the present investigation we have compared the developmental changes of ChAT, NGF, and mRNANGF levels in this system. During the postnatal development of the hippocampus the time courses of NGF and ChAT were well correlated including the most rapid increase between P12 and P14. This increase in hippocampal NGF was preceded by a corresponding increase in mRNANGF. The developmental changes in hippocampal NGF levels were also closely reflected by corresponding changes in the septum. This, together with previous observations (Korsching et al., 1985) that the adult septum, in spite of relatively high NGF levels, does not contain measurable quantities of mRNANGF, suggests that the NGF levels in the septum are determined by the quantity of NGF transported retrogradely from the field of innervation rather than by local synthesis. During the prenatal period hippocampal NGF levels were relatively high, whereas the mRNANGF was below the level of detection. Since the ingrowth of septal fibers, and with that also the removal of NGF by retrograde transport, begins around birth, the relatively high prenatal NGF levels probably result from an accumulation produced by a small copy number of mRNANGF prior to the removal of NGF by retrograde axonal transport. It is concluded that the correlation of the developmental changes in NGF and mRNANGF with the ChAT activity in the hippocampus further supports the concept of a physiological role of NGF in the central nervous system. 相似文献
40.
During the peri-implantation period, molecular signaling between embryo and endometrium (layer of tissue lining the uterus lumen) is supposed to be crucial for the maintenance of pregnancy. To investigate embryo-induced alterations in the proteome of bovine endometrium in the preattachment period (day 18), we used monozygotic cattle twins (generated by embryo splitting) as a model eliminating genetic variability as a source for proteome differences. One of the twins was pregnant after the transfer of two in vitro produced blastocysts, while the corresponding twin received a sham-transfer and served as a nonpregnant control. The two-dimensional fluorescence difference gel electrophoresis (2-D DIGE) analysis of the endometrium samples of three twin pairs (pregnant/nonpregnant) revealed four proteins with significantly higher abundance (p < 10(-9)) in each sample derived from the pregnant animals: Rho GDP dissociation inhibitor beta; 20 alpha-hydroxysteroid dehydrogenase (20 alpha-HSD); soluble NADP(+)-dependent isocitrate dehydrogenase 1; and acyl-CoA-binding protein. To verify the accuracy of the 2-D DIGE quantification, the abundances of 20 alpha-HSD were quantified by a targeted cleavable isotope-coded affinity tag (ICAT) approach. The mass spectrometry-based ICAT quantification matched perfectly the results obtained by 2-D DIGE quantification, demonstrating the accuracy of our data. These results demonstrate that our model (monozygotic twins) in combination with the appropriate analytical tools is particularly suitable for the detection of the proteins involved in the embryo-maternal interactions. 相似文献