全文获取类型
收费全文 | 2788篇 |
免费 | 179篇 |
国内免费 | 3篇 |
专业分类
2970篇 |
出版年
2023年 | 10篇 |
2022年 | 26篇 |
2021年 | 39篇 |
2020年 | 19篇 |
2019年 | 31篇 |
2018年 | 33篇 |
2017年 | 28篇 |
2016年 | 55篇 |
2015年 | 104篇 |
2014年 | 93篇 |
2013年 | 151篇 |
2012年 | 219篇 |
2011年 | 208篇 |
2010年 | 128篇 |
2009年 | 107篇 |
2008年 | 155篇 |
2007年 | 197篇 |
2006年 | 162篇 |
2005年 | 143篇 |
2004年 | 158篇 |
2003年 | 172篇 |
2002年 | 146篇 |
2001年 | 22篇 |
2000年 | 12篇 |
1999年 | 30篇 |
1998年 | 32篇 |
1997年 | 32篇 |
1996年 | 25篇 |
1995年 | 28篇 |
1994年 | 25篇 |
1993年 | 22篇 |
1992年 | 27篇 |
1991年 | 21篇 |
1990年 | 19篇 |
1989年 | 18篇 |
1988年 | 11篇 |
1987年 | 14篇 |
1986年 | 17篇 |
1985年 | 22篇 |
1984年 | 20篇 |
1983年 | 15篇 |
1982年 | 19篇 |
1981年 | 19篇 |
1980年 | 15篇 |
1979年 | 10篇 |
1977年 | 13篇 |
1975年 | 13篇 |
1974年 | 10篇 |
1969年 | 8篇 |
1968年 | 8篇 |
排序方式: 共有2970条查询结果,搜索用时 15 毫秒
991.
Tiffany Cheng Christina Yang Norbert WeberHubert T. Kim Alfred C. Kuo 《Biochemical and biophysical research communications》2012,426(4):544-550
Treatment of mesenchymal stem cells (MSCs) with fibroblast growth factor 2 (FGF-2) during monolayer expansion leads to increased expression of cartilage-related molecules during subsequent pellet chondrogenesis. This may be due to faster differentiation and/or a durable change in phenotype. In order to evaluate changes over time, we assessed chondrogenesis of human MSCs at early and late time points during pellet culture using real-time PCR, measurement of glycosaminoglycan accumulation, and histology. Marked enhancement of chondrogenesis was seen early compared to controls. However, the differences from controls in gene expression dramatically diminished over time. Depending on conditions, increases in glycosaminoglycan accumulation were maintained. These results suggest that FGF-2 can enhance the kinetics of MSC chondrogenesis, leading to early differentiation, possibly by a priming mechanism. 相似文献
992.
Norbert Bleier Róbert Lehoczki Dóra újváry László Szemethy Sándor Csányi 《Acta theriologica》2012,57(4):351-359
Game damage to agriculture represents one of the most important and most frequent human?Cwildlife conflicts worldwide. In Hungary and in the other European countries, damage caused by wild boar (Sus scrofa) and red deer (Cervus elaphus) is the most significant. The volume of damage is correlated with the population density of the game species, however, not exclusively. Since the growing expansion of wild ungulates is a general tendency in Europe, increasing conflicts can be envisaged. The objective of our study was to assess the possible relationships between the amount of agricultural game damage and big game population density considering some other factors (e.g. habitat structure, sown area of cultivated plants) by analysing wildlife management and land-use statistics for the 19 counties of Hungary from 1997 to 2008. According to the results, crop damage positively correlated with the population density of red deer and wild boar, with the length of forest edge, and the proportion of the sown area of maize. According to the regression model, these factors could be accountable for 74.2?% of the total agricultural game damage. 相似文献
993.
Pan J Heberle FA Tristram-Nagle S Szymanski M Koepfinger M Katsaras J Kučerka N 《Biochimica et biophysica acta》2012,1818(9):2135-2148
We have determined the molecular structures of commonly used phosphatidylglycerols (PGs) in the commonly accepted biologically relevant fluid phase. This was done by simultaneously analyzing small angle neutron and X-ray scattering data, with the constraint of measured lipid volumes. We report the temperature dependence of bilayer parameters obtained using the one-dimensional scattering density profile model - which was derived from molecular dynamics simulations - including the area per lipid, the overall bilayer thickness, as well as other intrabilayer parameters (e.g., hydrocarbon thickness). Lipid areas are found to be larger than their phosphatidylcholine (PC) counterparts, a result likely due to repulsive electrostatic interactions taking place between the charged PG headgroups even in the presence of sodium counterions. In general, PG and PC bilayers show a similar response to changes in temperature and chain length, but differ in their response to chain unsaturation. For example, compared to PC bilayers, the inclusion of a first double bond in PG lipids results in a smaller incremental change to the area per lipid and bilayer thickness. However, the extrapolated lipid area of saturated PG lipids to infinite chain length is found to be similar to that of PCs, an indication of the glycerol-carbonyl backbone's pivotal role in influencing the lipid-water interface. 相似文献
994.
Schmal H Pilz IH Mehlhorn AT Dovi-Akue D Kirchhoff C Südkamp NP Gerlach U Niemeyer P 《Cytotherapy》2012,14(7):868-876
Background aimsBone morphogenetic protein-2 (BMP-2) and its receptor type 1A (BMPR-1A) play significant roles in cartilage metabolism. The aim of this study was to evaluate a possible correlation between intra-articular expression of these proteins and the degree of osteoarthritis (OA) in human knees.MethodsBiopsies of synovia and debrided cartilage were taken in 15 patients undergoing autologous chondrocyte implantation. Expression of BMP-2 and BMPR-1A was evaluated semi-quantitatively by immunohistologic staining. These data were complemented by grading of cartilage lesions according to International Cartilage Repair Society (ICRS), defect size, duration of complaints, knee osteoarthritis scoring system (KOSS) and Henderson and Kellgren–Lawrence scores. General histologic stainings were used to determine Mankin, Pritzker and Krenn scores.ResultsThe expression of BMPR-1A but not of BMP-2 was significantly higher in cartilage biopsies taken in type 3 lesions with intact subchondral layer compared with type 4 defects (P < 0.05). In cartilage areas of bordering sectors, the intensity of immunohistologic staining of BMPR-1A was statistically significantly higher in mature cartilage compared with repair zones (P < 0.05). Expression of BMP-2 and its receptor 1A correlated in the cartilage biopsies (P < 0.02) but not in the synovia. The degree of OA was scored in all biopsies according to Mankin and Pritzker, and these scores correlated statistically significantly with BMPR-1A expression in the synovia (P < 0.05). In patients with an osteochondritis dissecans, the degree of OA was higher compared with other causes of chondromalacia, as evaluated by defect size, ICRS score, duration of complaints, Pritzker score and expression of BMPR-1A in cartilage (P < 0.05).ConclusionsThese data support the role of BMPR-1A as an indicator of OA progression in human knees with circumscribed cartilage lesions. 相似文献
995.
Background aimsMesenchymal stromal cells (MSC) are heterogeneous and only a subset possesses multipotent differentiation potential. It has been proven that long-term culture has functional implications for MSC. However, little is known how the composition of subpopulation changes during culture expansion.MethodsWe addressed the heterogeneity of MSC using limiting-dilution assays at subsequent passages. In addition, we used a cellular automaton model to simulate population dynamics under the assumption of mixed numbers of remaining cell divisions until replicative senescence. The composition of cells with adipogenic or osteogenic differentiation potential during expansion was also determined at subsequent passages.ResultsNot every cell was capable of colony formation upon passaging. Notably, the number of fibroblastoid colony-forming units (CFU-f) decreased continuously, with a rapid decay within early passages. Therefore the CFU-f frequency might be used as an indicator of the population doublings remaining before entering the senescent state. Predictions of the cellular automaton model suited the experimental data best if most cells were already close to their replicative limit by the time of culture initiation. Analysis of differentiated clones revealed that subsets with very high levels of adipogenic or osteogenic differentiation capacity were only observed at early passages.ConclusionsThese data support the notion of heterogeneity in MSC, and also with regard to replicative senescence. The composition of subpopulations changes during culture expansion and clonogenic subsets, especially those with the highest differentiation capacity, decrease already at early passages. 相似文献
996.
Timothy John Tranbarger Wanwisa Kluabmongkol Duangjai Sangsrakru Fabienne Morcillo James W Tregear Somvong Tragoonrung Norbert Billotte 《BMC plant biology》2012,12(1):1
Background
The oil palm (Elaeis guineensis Jacq.) is a perennial monocotyledonous tropical crop species that is now the world's number one source of edible vegetable oil, and the richest dietary source of provitamin A. While new elite genotypes from traditional breeding programs provide steady yield increases, the long selection cycle (10-12 years) and the large areas required to cultivate oil palm make genetic improvement slow and labor intensive. Molecular breeding programs have the potential to make significant impacts on the rate of genetic improvement but the limited molecular resources, in particular the lack of molecular markers for agronomic traits of interest, restrict the application of molecular breeding schemes for oil palm. 相似文献997.
Lichtenstern T Mücke N Aebi U Mauermann M Herrmann H 《Journal of structural biology》2012,177(1):54-62
We have generated human recombinant keratins K8 and K18 and describe conditions to quantitatively follow their assembly into filaments. When renatured individually from 8M urea into a low ionic strength/high pH-buffer, K8 was present in a dimeric to tetrameric form as revealed by analytical ultracentrifugation. In contrast, K18 sedimented as a monomer. When mixed in 8 M urea and renatured together, K8 and K18 exhibited s-value profiles compatible with homogeneous tetrameric complexes. This finding was confirmed by sedimentation equilibrium centrifugation. Subsequently, these tetrameric starter units were subjected to assembly experiments at various protein concentrations. At low values such as 0.0025 g/l, unit-length filaments were abundantly present after 2s of assembly. During the following 5 min, filaments grew rapidly and by measuring the length of individual filaments we were able to generate time-dependent length profiles. These data revealed that keratins K8/K18 assemble several times faster than vimentin and desmin. In addition, we determined the persistence length l(p) of K8/K18 filaments to be in the range of 300 nm. Addition of 1 mM MgCl(2) increases l(p) to 480 nm indicating that magnesium ions affect the interaction of keratin subunits within the filament during assembly to some extent. 相似文献
998.
The behaviour of fossil organisms can typically be inferred only indirectly, but rare fossil finds can provide surprising insights. Here, we report from the Eocene Messel Pit Fossil Site between Darmstadt and Frankfurt, Germany numerous pairs of the fossil carettochelyid turtle Allaeochelys crassesculpta that represent for the first time among fossil vertebrates couples that perished during copulation. Females of this taxon can be distinguished from males by their relatively shorter tails and development of plastral kinesis. The preservation of mating pairs has important taphonomic implications for the Messel Pit Fossil Site, as it is unlikely that the turtles would mate in poisonous surface waters. Instead, the turtles initiated copulation in habitable surface waters, but perished when their skin absorbed poisons while sinking into toxic layers. The mating pairs from Messel are therefore more consistent with a stratified, volcanic maar lake with inhabitable surface waters and a deadly abyss. 相似文献
999.
Kustiariyah Tarman Gottfried J. Palm Andrea Porzel Kurt Merzweiler Norbert Arnold Ludger A. Wessjohann Martin Unterseher Ulrike Lindequist 《Phytochemistry letters》2012,5(1):83-86
From an endophytic Daldinia eschscholzii strain isolated from the agar-producing red alga Gracilaria sp. SGR-1, collected from the coast of South Sulawesi, Indonesia, a new lactone helicascolide C (1) was obtained as colourless crystals from the ethyl acetate extract together with the related structurally known compound helicascolide A (2). The structure of the new compound 1 reveals a carbonyl group replacing an alcohol group of compound 2. The structure of 1 was elucidated by X-ray diffraction and spectral analyses. Compound 1 showed fungistatic activity against the phytopathogenic fungus Cladosporium cucumerinum. 相似文献
1000.
Stoppe C Fries M Rossaint R Grieb G Coburn M Simons D Brücken D Bernhagen J Pallua N Rex S 《PloS one》2012,7(4):e33512