全文获取类型
收费全文 | 18885篇 |
免费 | 1697篇 |
国内免费 | 1436篇 |
专业分类
22018篇 |
出版年
2023年 | 242篇 |
2022年 | 531篇 |
2021年 | 896篇 |
2020年 | 609篇 |
2019年 | 690篇 |
2018年 | 726篇 |
2017年 | 538篇 |
2016年 | 699篇 |
2015年 | 1149篇 |
2014年 | 1266篇 |
2013年 | 1372篇 |
2012年 | 1565篇 |
2011年 | 1503篇 |
2010年 | 984篇 |
2009年 | 834篇 |
2008年 | 910篇 |
2007年 | 881篇 |
2006年 | 771篇 |
2005年 | 656篇 |
2004年 | 618篇 |
2003年 | 546篇 |
2002年 | 518篇 |
2001年 | 393篇 |
2000年 | 390篇 |
1999年 | 361篇 |
1998年 | 169篇 |
1997年 | 161篇 |
1996年 | 158篇 |
1995年 | 119篇 |
1994年 | 146篇 |
1993年 | 90篇 |
1992年 | 157篇 |
1991年 | 148篇 |
1990年 | 122篇 |
1989年 | 100篇 |
1988年 | 84篇 |
1987年 | 97篇 |
1986年 | 83篇 |
1985年 | 92篇 |
1984年 | 49篇 |
1983年 | 48篇 |
1982年 | 49篇 |
1981年 | 36篇 |
1980年 | 37篇 |
1979年 | 52篇 |
1978年 | 41篇 |
1977年 | 45篇 |
1976年 | 33篇 |
1975年 | 35篇 |
1974年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
为了解四川省自贡地区3~5岁幼儿乙型肝炎病毒(HBV)感染情况,并探索与感染有关的因素,1985年调查了1167名幼儿,其HBV总感染率为41.13%,HBsAg阳性率12.68%。幼儿的HBV感染与母亲HBsAg阳性密切相关。共检查母亲409例,38例HBsAg阳性,其幼儿HBsAg阳性率为50%(19/38),HBsAg阴性的母亲371例,其幼儿HBsAg阳性率9.97%(37/371),来自HBsAg阳性母亲的阳性子女占33.3%(19/56)。1986年随访HBV易感幼儿448例,HBV年感染率为12.95%(58/448),HBsAg年阳转率3.79%。HBV年感染率与原幼儿班级HBsAg阳性率的高低有关。 相似文献
72.
To study the effect of SV40 T-antigen in mammary epithelial cells, a rat beta-casein promoter-driven SV40 early-region construct was stably introduced into the clonal mouse mammary epithelial cell line HC11. With the expression of the viral T-antigens under the control of a hormone-inducible promoter, it was possible to dissociate the effects of different levels of T-antigen expression on cell growth, morphology, and gene expression. Following hormonal induction, a rapid but transient induction of T-antigen was observed, followed by a delayed induction of H4 histone mRNA. In T-antigen-positive HC11 cells cultured in the absence of EGF, the expression of basal levels of T-antigen (in the absence of hormonal induction) led to a decreased doubling time and an increased cell density. In the presence of EGF, T-antigen expression resulted additionally in an altered cell morphology. Despite the effects of T-antigen on cell growth and gene expression, the cells were unable to form colonies in soft agar and were nontumorigenic when transplanted into cleared mammary fat pads. They were, however, weakly tumorigenic in nude mice. Relatively high levels of p53 protein synthesis were observed in both the transfected HC11 cells and the parental COMMA-D cells, as compared to 3T3E fibroblasts and another mammary epithelial cell line. The HC11 and COMMA-D cells synthesized approximately equal levels of wild-type and mutated p53 proteins as defined by their reactivities with monoclonal antibodies PAb246 and PAb240, respectively. Interactions between excess p53 and T-antigen may, in part, explain the failure of these cells to display a completely transformed phenotype. 相似文献
73.
J A Ladias M Hadzopoulou-Cladaras D Kardassis P Cardot J Cheng V Zannis C Cladaras 《The Journal of biological chemistry》1992,267(22):15849-15860
74.
Magnesium regulates intracellular free ionized calcium concentration and cell geometry in vascular smooth muscle cells. 总被引:9,自引:0,他引:9
Regulatory effects of extracellular magnesium ions ([Mg2+]o) on intracellular free ionized calcium ([Ca2+]i) were studied in cultured vascular smooth muscle cells (VSMCs) from rat aorta by use of the fluorescent indicator fura-2 and digital imaging microscopy. With normal Mg2+ (1.2 mM)-containing incubation media, [Ca2+]i in VSMCs was 93.6 +/- 7.93 nM with a heterogeneous cellular distribution. Lowering [Mg2+]o to 0 mM or 0.3 mM (the lowest physiological range) resulted in 5.8-fold (579.5 +/- 39.99 nM) and 3.5-fold (348.0 +/- 31.52 nM) increments of [Ca2+]i, respectively, without influencing the cellular distribution of [Ca2+]i. Surprisingly, [Mg2+]o withdrawal induced changes of cell geometry in many VSMCs, i.e., the cells rounded up. However, elevation of [Mg2+]o up to 4.8 mM only induced slight decrements of [Ca2+]i (mean = 72.0 +/- 4.55 nM). The large increment of [Ca2+]i induced by [Mg2+]o withdrawal was totally inhibited when [Ca2+]o was removed. The data suggest that: (1) [Mg2+]o regulates the level of [Ca2+]i in rat aortic smooth muscle cells, and (2) [Mg2+] acts as an important regulatory ion by modulating cell shapes in cultured VSMc and their metabolism to control vascular contractile activities. 相似文献
75.
Na-, K-, Ba-, and Ca-Y were employed for the separation of fructose and glucose in an adsorption column. Effects of temperature, solvent flow rate, amount of mixture injection, and exchangeable cations on the separation were investigated. Efficiency of separation was used as a criterion to characterize the effectiveness of the separation. The transport and kinetic parameters for the column separation were also presented. From simple pulse experiments and moment analysis, the obtained process information of equilibrium and dynamic parameters might be used to design, operate, and control the separation column. (c) 1992 John Wiley & Sons, Inc. 相似文献
76.
To examine the existence of pressure equilibrium between tributary veins and the central vena cava during the mean circulatory filling pressure manoeuvre, pressures in the hepatic portal vein, renal vein, and inferior vena cava were determined at 4-s intervals over a 20-s period of circulatory arrest induced by inflating a right atrial balloon in normal blood volume, 10% volume depletion, and 10% volume expansion states in urethane-anaesthetized rats. Portal vein pressure determined 8 s after arrest during volume depletion and expansion was significantly higher than vena caval pressure (6.2 +/- 0.8 vs. 3.4 +/- 0.2 and 7.7 +/- 0.5 vs. 6.2 +/- 0.4 mmHg (1 mmHg = 133.32 Pa), respectively; p less than 0.01); this pressure disequilibrium continued for 16 s during volume expansion and for the entire 20 s during volume depletion. Renal vein pressure was equal to vena caval pressure during this manoeuvre. Portal vein pressure at normal blood volume was not significantly different from vena caval pressure following circulatory arrest (4.6 +/- 0.3 vs. 3.8 +/- 0.4 mmHg, respectively). Following ganglionic blockade, portal vein pressure was still significantly higher than vena caval pressure for 12 s during volume alterations. At the 8th s of the arrest the portal pressure determined in volume depletion was 3.6 +/- 0.3 mmHg and the inferior vena caval pressure was 2.6 +/- 0.4 mmHg (p less than 0.05). Under the volume expansion condition, the respective values were 6.5 +/- 0.3 and 5.3 +/- 0.4 mmHg (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
77.
Zn+2 is required for specific binding of c-erbA proteins to the hormone response elements of target genes. It is unclear whether Zn+2 is important for the binding of ligand to c-erbA proteins. The present study evaluated the effect of Zn+2 and other divalent cations on the binding of 3,3',5-triiodo-L-thyronine(T3) to the purified human placental c-erbA protein (h-TR beta 1). Zn+2 induced cross-linking of h-TR beta 1 to form aggregates in a dose-dependent manner with an apparent half-maximal concentration of approximately 200 microM at 22 degrees C. Cross-linking was reversible by the addition of 5 microM EDTA or 10 mM dithiothreitol. The cross-linked h-TR beta 1 bound T3. These results indicated Zn+2 had no effect on T3 binding and suggested that the cysteines and histidines involved in cross-linking are not essential for T3 binding. 相似文献
78.
The cytoplasmic and cell wall components of the Gram-positive bacterium Bacillus megaterium and the cytoplasmic and cell envelope components of the Gram-negative bacterium Escherichia coli were assayed for chemotactic activity for the hemocytes of Crassostrea virginica. The cellular components were separated by differential centrifugation and gel filtration was used to determine the approximate molecular weights of the chemoattractant molecules. Active fractions were assayed for glycoproteins and lipoproteins. As a result, it is known that hemocytes are chemotactically attracted to proteins of approximately 10,000 daltons which are associated with the cell wall of B. megaterium and the cell envelope of E. coli. 相似文献
79.
Seiji Suzuki Hiroshi Oka Hiroko Yasuda Masahiro Ikeda Po Yuan Cheng Toshitsugu Oda 《Biochemical and biophysical research communications》1981,99(3):987-993
Isolated rat pancreatic islets, incubated in the presence of extracellular 32Pi to steady state 32P incorporation into cellular phosphopeptides, were exposed to glucose for 10 min. Glucose (16.7 mM) significantly stimulated the phosphorylation of six phosphoproteins with molecular weights of 15,000, 35,000, 49,000, 64,000, 93,000 and 138,000. Mannoheptulose (16.7 mM) markedly inhibited glucose-stimulated phosphorylation of these six phosphoproteins. This protein phosphorylation might be important in mediating glucose-stimulated insulin release. 相似文献
80.
Two basic cell types occur in the hemolymph of Bulinus truncatus rohlfsi: granulocytes and hyalinocytes. Granulocytes are divided into three subtypes: (1) Granulocytes I, which account for 19% of the hemocytes, are small, young amoebocytes with 1–20 filopodia and small numbers of cytoplasmic granules, including some lysosomes; (2) granulocytes II, which account for 78% of the cells, are large, fully developed amoebocytes that possess 1–20 filopodia and many granules, both acidophilic and basophilic, including numerous lysosomes, phagosomes, and mitochondria; and (3) spent granulocytes, which are rare, have few filopodia, large accumulations of glycogen granules and prominent vacuoles in addition to lysosomes in the cytoplasm. These three subtypes of granulocytes probably represent ontogenetic stages within a single cell line. In addition, granulocytes with 40 or more filopodia and little ectoplasm, found in only 1 of 45 snails examined, probably reflect a pathologic condition. Hyalinocytes, which account for 3% of all hemocytes, are similar in size to mature granulocytes, but have few or no cytoplasmic granules and lack filopodia and glycogen granules. Total hemocyte concentration in hemolymph is 328,000 ± 188,000 cells/ml. 相似文献