首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   696篇
  免费   143篇
  2021年   13篇
  2018年   11篇
  2017年   16篇
  2016年   14篇
  2015年   23篇
  2014年   16篇
  2013年   20篇
  2012年   24篇
  2011年   27篇
  2010年   17篇
  2009年   19篇
  2008年   22篇
  2007年   30篇
  2006年   29篇
  2005年   24篇
  2004年   20篇
  2003年   22篇
  2002年   19篇
  2001年   22篇
  2000年   25篇
  1999年   16篇
  1998年   14篇
  1997年   8篇
  1996年   12篇
  1995年   8篇
  1994年   10篇
  1993年   9篇
  1992年   22篇
  1991年   23篇
  1990年   13篇
  1989年   21篇
  1988年   19篇
  1987年   9篇
  1986年   23篇
  1985年   14篇
  1984年   14篇
  1983年   11篇
  1982年   15篇
  1981年   12篇
  1980年   11篇
  1979年   9篇
  1978年   10篇
  1977年   10篇
  1976年   12篇
  1974年   6篇
  1972年   6篇
  1971年   6篇
  1970年   7篇
  1969年   6篇
  1968年   9篇
排序方式: 共有839条查询结果,搜索用时 31 毫秒
51.
Using a battery of seven lectin-ferritin conjugates as probes for cell surface glycoconjugates, we have studied the pattern of plasmalemmal differentiation of cells in the embryonic rat pancreas from day 15 in utero to the early postpartum stage. Our results indicate that differentiation of plasmalemmal glycoconjugates on acinar, endocrine, and centroacinar cells is temporally correlated with development and is unique for each cell type, as indicated by lectin-ferritin binding. Specifically, (a) expression of adult cell surface saccharide phenotype can be detected on presumptive acinar cells as early as 15 d in utero, as indicated by soybean agglutinin binding, and precedes development of intracellular organelles characteristic of mature acinar cells; (b) maturation of the plasmalemma of acinar cells is reached after intracellular cytodifferentiation is completed, as indicated by appearance of Con A and fucoselectin binding sites only at day 19 of development; conversely, maturation of the endocrine cell plasmalemma is accompanied by "loss" (masking) of ricinus communis II agglutinin receptors; and (c) binding sites for fucose lectins and for soybean agglutinin are absent on endocrine and centroacinar cells at all stages examined. We conclude that acinar, centroacinar, and endocrine cells develop from a common progenitor cell(s) whose plasmalemmal carbohydrate composition resembles most closely that of the adult centroacinar cell. Finally, appearance of acinar lumina beginning at approximately 17 d in utero is accompanied by differenetiation of apical and basolateral plasmalemmal domains of epithelial cells, as indicated by enhanced binding of several lectin-ferritin conjugates to the apical plasmalemmal, a pattern that persists from this stage through adult life.  相似文献   
52.
Livers from normally fed male and female rats were perfused in vitro with different amounts of oleate, and the production and properties of the very low density lipoprotein (VLDL) were studied. The mobility of the VLDL in the zonal ultracentrifuge was dependent on the uptake of free fatty acid and on the sex of the animal from which the liver was obtained. A higher proportion of the VLDL secreted by livers from females displayed a more rapid mobility in the zonal ultracentrifuge and, in addition, contained less phospholipid and cholesterol per mole triglyceride than the VLDL from the male, suggestive of larger size of the VLDL secreted by livers from the female rats. Such differences were diminished when the VLDL was compared at equal output of triglyceride but unequal uptake of free fatty acid. These data suggest that the properties of the VLDL are only secondarily modulated by sex, and primarily result from differences in the capacities of livers from either male or female rats to synthesize triglyceride for transport as VLDL. The quantity of triglyceride secreted, regardless of sex, may be an important determinant of both size and number of the VLDL particles. The incorporation of endogenous hepatic fatty acid into VLDL triglyceride was diminished in livers from both sexes by increased uptake of oleate. The greater output of VLDL triglyceride by livers from female animals was dependent on both exogenous and endogenous fatty acids when relatively small quantities of exogenous oleate were available for uptake by the liver. The proportion of palmitate and oleate in the phospholipid of the VLDL secreted by livers from male rats decreased and the content of arachidonate increased with increasing uptake of oleate; no differences were observed in the composition of the phospholipid fatty acids among the various experimental female groups, although these contained more stearate and less oleate and linoleate compared to the male groups. The change of fatty acid composition of the VLDL phospholipid may reflect inclusion of specific types of phospholipid in the VLDL structure for transport of triglyceride from the liver under particular conditions.  相似文献   
53.
The araCi protein differs in stability from araC+ protein and alters the concentration of cyclic adenosine-3', 5'-monophosphate required to maximally stimulate L-arabinose isomerase synthesis in an in vitro protein-synthesizing system.  相似文献   
54.
Rate of electrophoretic migration of prealbumin-2 of mouse blood plasma was found to be controlled by three autosomal codominant alleles. Presence and absence of a component next to albumin (prealbumin-4) was shown to be controlled by autosomal dominant and recessive alleles, respectively, and to be affected by sex. Evidence was presented for close linkage of the Pre-2 and Pre-4 loci, and for the control of albumin conformation by a separate locus.  相似文献   
55.
In 70 obese women no correlation was found between body weight and serum cholesterol or triglyceride concentrations, but there was a significant correlation between weight and blood pressure. Weight reduction by diet or jejunoileal shunt was not accompanied by any significant change in serum lipid concentrations other than the decrease in serum cholesterol expected after intestinal bypass. Twelve months after bypass surgery was carried out on 14 patients, however, both systolic and diastolic blood pressures were significantly reduced and at levels appropriate to the patients'' new weights. These results suggest that obesity in women cannot be taken to indicate the presence of hyperlipidaemia and that sustained weight loss may lower blood pressure.  相似文献   
56.
Nitrosoguanidine induced mutants of Anabaena cylindrica have been obtained, which are altered in heterocyst spacing. In the wild type organism the pattern is composed of single intercalary heterocysts. The mutant patterns fall into several classes: those with only terminal heterocysts, with both terminal and intercalary heterocysts, with groups of heterocysts and those totally lacking heterocysts. The mutants are described in detail, and the various pattern modifications are interpreted in terms of a model we have proposed.  相似文献   
57.
Wetlands Ecology and Management - Gas drilling into the Marcellus Shale play has been linked to environmental issues, including potential impacts on wildlife. In 2009, three separate accidents...  相似文献   
58.
59.
In an era of rapid global change, our ability to understand and predict Earth's natural systems is lagging behind our ability to monitor and measure changes in the biosphere. Bottlenecks to informing models with observations have reduced our capacity to fully exploit the growing volume and variety of available data. Here, we take a critical look at the information infrastructure that connects ecosystem modeling and measurement efforts, and propose a roadmap to community cyberinfrastructure development that can reduce the divisions between empirical research and modeling and accelerate the pace of discovery. A new era of data‐model integration requires investment in accessible, scalable, and transparent tools that integrate the expertise of the whole community, including both modelers and empiricists. This roadmap focuses on five key opportunities for community tools: the underlying foundations of community cyberinfrastructure; data ingest; calibration of models to data; model‐data benchmarking; and data assimilation and ecological forecasting. This community‐driven approach is a key to meeting the pressing needs of science and society in the 21st century.  相似文献   
60.
The aim of this study was to investigate whether the predicted finite element (FE) stiffness of vertebral bone is altered when using images of preserved rather than fresh tissue to generate specimen-specific FE models. Fresh ovine vertebrae were used to represent embalmed (n = 3) and macerated dry-bone (n = 3) specimens and treated accordingly. Specimens were scanned pre- and post-treatment using micro-computed tomography. A constant threshold level derived from these images was used to calculate the respective bone volume fraction (BV/TV) from which the conversion factor validated for fresh tissue was used to determine material properties that were assigned to corresponding FE models. Results showed a definite change in the BV/TV between the fresh and the preserved bone. However, the changes in the predicted FE stiffness were not generally greater than the variations expected from assignment of loading and boundary conditions. In conclusion, images of preserved tissue can be used to generate FE models that are representative of fresh tissue with a tolerable level of error.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号