全文获取类型
收费全文 | 96514篇 |
免费 | 611篇 |
国内免费 | 884篇 |
专业分类
98009篇 |
出版年
2022年 | 22篇 |
2021年 | 51篇 |
2020年 | 33篇 |
2019年 | 36篇 |
2018年 | 11881篇 |
2017年 | 10704篇 |
2016年 | 7527篇 |
2015年 | 710篇 |
2014年 | 442篇 |
2013年 | 469篇 |
2012年 | 4446篇 |
2011年 | 13040篇 |
2010年 | 12155篇 |
2009年 | 8384篇 |
2008年 | 9977篇 |
2007年 | 11574篇 |
2006年 | 426篇 |
2005年 | 731篇 |
2004年 | 1149篇 |
2003年 | 1215篇 |
2002年 | 976篇 |
2001年 | 286篇 |
2000年 | 195篇 |
1999年 | 72篇 |
1998年 | 84篇 |
1997年 | 69篇 |
1996年 | 52篇 |
1995年 | 40篇 |
1994年 | 53篇 |
1993年 | 65篇 |
1992年 | 44篇 |
1991年 | 62篇 |
1990年 | 40篇 |
1989年 | 40篇 |
1988年 | 41篇 |
1987年 | 39篇 |
1986年 | 18篇 |
1985年 | 17篇 |
1984年 | 27篇 |
1983年 | 43篇 |
1982年 | 25篇 |
1981年 | 21篇 |
1980年 | 26篇 |
1979年 | 14篇 |
1975年 | 16篇 |
1974年 | 14篇 |
1972年 | 250篇 |
1971年 | 278篇 |
1965年 | 15篇 |
1962年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
81.
Simon Lovestone C. Hugh Reynolds Donna Latimer Daniel R. Davis Brian H. Anderton Jean-Marc Gallo Diane Hanger Sandrine Mulot Betina Marquardt Silvia Stabel James R. Woodgett Christopher C. J. Miller 《Current biology : CB》1994,4(12)
Background: Paired helical filaments (PHFs) are a characteristic pathological feature of Alzheimer's disease; their principal component is the microtubule-associated protein tau. The tau in PHFs (PHF-tau) is hyperphosphorylated, but the cellular mechanisms responsible for this hyperphosphorylation have yet to be elucidated. A number of kinases, including mitogen-activated protein (MAP) kinase, glycogen synthase kinase (GSK)-3α, GSK-3β and cyclin-dependent kinase-5, phosphorylate recombinant tau in vitro so that it resembles PHF-tau as judged by its reactivity with a panel of antibodies capable of discriminating between normal tau and PHF-tau, and by a reduced electrophoretic mobility that is characteristic of PHF-tau. To determine whether MAP kinase, GSK-3α and GSK-3β can also induce Alzheimer's disease-like phosphorylation of tau in mammalian cells, we studied the phosphorylation status of tau in primary neuronal cultures and transfected COS cells following changes in the activities of MAP kinase and GSK-3.Results Activating MAP kinase in cultures of primary neurons or transfected COS cells expressing tau isoforms did not increase the level of phosphorylation for any PHF-tau epitope investigated. But elevating GSK-3 activity in the COS cells by co-transfection with GSK-3α or GSK-3β decreased the electrophoretic mobility of tau so that it resembled that of PHF-tau, and induced reactivity with eight PHF-tau-selective monoclonal antibodies.Conclusion Our data indicate that GSK-3α and/or GSK-3β, but not MAP kinase, are good candidates for generating PHF-type phosphorylation of tau in Alzheimer's disease. The involvement of other kinases in the generation of PHFs cannot, however, be eliminated. Our results suggest that aberrant regulation of GSK-3 may be a pathogenic mechanism in Alzheimer's disease. 相似文献
82.
Hongkui Jin Renhui Yang Gilbert A. Keller Anne Ryan Annie Ko David Finkle Todd A. Swanson WeiLi Diane Pennica William I. Wood Nicholas F. Paoni 《Cytokine》1996,8(12):920-926
Cardiotrophin-1 (CT-1) is a recently discovered cytokine that was isolated based on its ability to induce cardiac myocyte hypertrophy in vitro. In this study, the effects of chronic administration of CT-1 to mice (0.5 or 2 μg by intraperitoneal injection, twice a day for 14 days) were determined. A dose-dependent increase in both the heart weight and ventricular weight to body ratios was observed in the treated groups. The body weights of the animals were unaffected. These results indicate that CT-1 can induce cardiac hypertrophy in vivo. CT-1 was not specific for the heart, however. It stimulated the growth of the liver, kidney, and spleen, and caused atrophy of the thymus. CT-1 administration also increased the platelet counts by 70%, with no change in mean platelet volume. Red blood cell counts were increased in the treated animals, and there was a concomitant increase in haemoglobin concentration. Thus, CT-1 has a broad spectrum of biological activities in vivo. This observation is consistent with previous in-vitro findings showing that the mRNA for CT-1 is expressed in several tissues, and that CT-1 can function through binding to the leukaemia inhibitory factor (LIF) receptor and signalling through the gp130 pathway. 相似文献
83.
Regulation of the mouse sex-limited protein (Slp) gene is unusual in that hormone response is conferred by the 5′ LTR of an upstream inserted provirus, dubbed the imposon
(imp1). In a search for additional genes whose regulation has been affected by retrotransposition events, we isolated two partial
proviral elements by stringent screening of a mouse genomic library. One clone (imp2) contained a portion of the envelope gene and a 3′ LTR that was nearly identical to the 3′ LTR of imp1; this similarity extended to insertion into a B1 repetitive element. The second proviral clone (imp3) contained a 5′ LTR and associated coding sequences, but lacked its 3′ LTR; the LTR of imp3 differed by 12% from the imp1 sequence. To assess potential hormone response, proviral enhancer regions cloned into reporter vectors were tested in transfection.
The imp2 enhancer was similar in behavior to imp1, conferring both androgen and glucocorticoid induction in one fragment context and an androgen-specific response in another.
In contrast, the imp3 enhancer allowed high expression in the absence of hormone and was less responsive to steroids in general and androgen in
particular. These three proviral elements define a small family of steroid responsive proviruses in the mouse genome, and
at least one member has had a lasting impact on an endogenous gene's regulation.
Received: 29 April 1997 / Accepted: 14 July 1997 相似文献
84.
Mycoplasma pneumoniae pneumonia is usually a benign illness, and respiratory complications and extrapulmonary manifestations occur rarely. In this series, patients admitted to a referral hospital with this disorder had unusual symptoms, signs and findings on chest roentgenograms and laboratory studies. Pneumonia was often severe and extrapulmonary manifestations were frequent, resulting in prolonged hospital stays and illnesses. Although this extreme end of the spectrum of disease caused by M pneumoniae is not representative of this type of pneumonia as seen in outpatients, it is important to realize that patients admitted to hospital with severe, complicated pneumonia frequently have unusual manifestations of a common disease. 相似文献
85.
Diane Willenbrock McNees Roscoe W. Lewis Betty J. Ponzio Raymond F. Sis Frank J. Stein 《Primates; journal of primatology》1984,25(1):103-109
Blood chemistry values were collected over a three-year period from at least 10 colony-born and 24 wild-born apparently normal
common marmosets. BUN, SGOT, creatinine, calcium, phosphorus, alkaline phosphatase, protein, albumin, cholesterol, triglycerides,
uric and glucose values were determined. A statistical comparison of baseline values was made between wild-born, colony-born,
male and female marmosets. Also the same comparison was made between common marmosets and cotton-top tamarins, white lipped
tamarins and human subjects. 相似文献
86.
The effect of cycloheximide on the chloroplastic, cytoplasmic and mitochondrial phenylalanyltransferRNA synthetases of Euglena gracilis was studied by growing both logarithmic and stationary phase cultures in the presence of the antibiotic. Enzyme activity was measured relative to untreated control cultures. At very low concentrations of cycloheximide (1 g/ml), all three log phase enzymes showed an increase in activity of 40–50%. At slightly higher concentrations (2.5 g/ml), the phenylalanyl-tRNA synthetase activities were comparable to those of the control cultures. At a cycloheximide concentration of 5g/ml the enzyme activities from stationary phase cultures showed only very slight decreases (5–20%). The cytoplasmic and mitochondrial enzymes behaved similarly in log phase cultures at this concentration. However, the chloroplastic phenylalanyl-tRNA synthetase from log phase cultures treated with 5g/ml cycloheximide showed a marked decrease in activity (70%). A further increase in antibiotic concentration to 10g/ml resulted in significant losses of activity of all three enzymes, from both growth stages. The implications of the data with regard to identification of the site(s) of chloroplast enzyme synthesis are discussed. 相似文献
87.
88.
Perin L. Donnini M. Diomede L. Romano M. Tacconi M. T. Luisetti M. Salmona M. 《Cytotechnology》1991,7(1):25-32
An expression vector for G-CSF, pASLB3-3, was constructed and introduced into Namalwa KJM-1 cells (Hosoi et al., 1988), and cells resistant to 100 nM of methotrexate (MTX) were obtained. Among them, the highest producer, clone SC57, was selected and the productivity of this clone was further characterized. The maximal production of G-CSF was at the most 1.8 g/ml/day using a 25 cm2 tissue culture flask, even though the cell number was above 7×105 cells/ml. The limiting factors at high density were analyzed as the deficiency of nutrients, such as glucose, cysteine and serine, and pH control. The depression of specific G-CSF productivity per cell under the batch culture conditions was overcome by using a perfusion culture system, BiofermenterTM (Sato, 1983) with modifications of nutrients supplementation by a dialysis membrane and/or dissolved oxygen (DO) supplementation by microsilicone fibers. ITPSGF medium was modified to elevate concentrations of amino acids and glucose by 2.0- and 2.5-times, respectively. Under the control of pH at 7.4 and DO at 3 ppm, the specific G-CSF productivity was not depressed even at high cell density (above 1×107 cells/ml), and the amount of G-CSF reached 41 g/ml. These results indicated the possibility of finding the optimum culture conditions for the production of recombinant proteins by Namalwa KJM-1 cells.Abbreviations ABTS
2,2-Azino-di-(3-ethylbenzothiazoline)-6-sulfonic acid
- BSA
Bovine Serum Albumin
- BSA-PBS
Phosphate-buffered Saline without Ca2+ and Mg2+ containing Bovine Serum Albumin
- dhfr
Dihydrofolate Reductase
- DO
Dissolved Oxygen
- G-CSF
Granulocyte Colony-stimulating Factor
- HEPES
4-(2-Hydroxyethyl)-1-piperazineethansulfonic Acid
- IFN
Interferon
- MTX
Methotrexate
- PBS(-)
Phosphate-buffered saline without Ca2+ and Mg2+
- Tween-PBS
Phosphate-buffered saline without Ca2+ and Mg2+ containing 0.05% of Tween 20 相似文献
89.
A comparison was made of the S-070 Pediatric Bubble Oxygenator, which was unreliable above flow rates of approximately 1.5 L/min, with a modified S-070A, which proved to be extremely efficient to flow rates of 2.5 L/min. 相似文献
90.
The Transplantable B-16 melanotic melanoma carried in syngeneic C57B1/6J female mice and the Syrian hamster melanoma cell line, RPMI 3460, were utilized to determine whether steroid-hormone receptors are present in animal melanomas. In the B-16 melanoma, a cytoplasmic-estrogen receptor is detectable, but there is no evidence for androgen or progestin receptors. Some tumors contain a glucocorticoid-binding macromolecule. Sucrosedensity gradient centrifugation of cytosol after incubation with [3H]-estradiol revealed an 8S peak that was suppressed by excess radioinert diethylstilbesterol. Binding varied from 5–35 fmoles per mg cytosol protein. Scatchard analysis of [3H]-estradiol binding in cytosol yielded a single class of high-affinity binding sites; the dissociation constant is 6 × 10?10 M. The receptor molecule is shown to be estrogen-specific by ligand competition assays. In contrast to B-16 melanoma, no estrogen, androgen, or progestin receptor can be found in the Syrian hamster melanoma cell line. However, a substantial level of specific binding is observed using [3H]-dexamethasone. Sucrose-gradient centrifugation of cytosol from this cell line after incubation with [3H]-dexamethasone revealed a 7S peak that was suppressed by excess radioinert dexamethasone. Scatchard analysis indicated a single class of high affinity sites with a dissociation constant of 2 × 10?9 M. Binding levels from 70–610 fmoles per mg cytosol protein were observed. The Syrian hamster melanoma cells also exhibit a biological response to glucocorticoids: Dexamethasone causes both an inhibition of growth and a decrease in final-cell density in these cells. 相似文献