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   2442篇
  免费   281篇
  2021年   27篇
  2020年   20篇
  2019年   17篇
  2018年   33篇
  2017年   27篇
  2016年   34篇
  2015年   79篇
  2014年   108篇
  2013年   124篇
  2012年   144篇
  2011年   143篇
  2010年   101篇
  2009年   89篇
  2008年   125篇
  2007年   124篇
  2006年   110篇
  2005年   97篇
  2004年   125篇
  2003年   93篇
  2002年   89篇
  2001年   40篇
  2000年   32篇
  1999年   47篇
  1998年   46篇
  1997年   36篇
  1996年   40篇
  1995年   34篇
  1994年   20篇
  1993年   30篇
  1992年   33篇
  1991年   38篇
  1990年   46篇
  1989年   49篇
  1988年   44篇
  1987年   25篇
  1986年   25篇
  1985年   22篇
  1984年   34篇
  1983年   29篇
  1982年   25篇
  1981年   30篇
  1980年   17篇
  1979年   24篇
  1978年   18篇
  1977年   16篇
  1976年   13篇
  1974年   17篇
  1973年   27篇
  1971年   14篇
  1970年   13篇
排序方式: 共有2723条查询结果,搜索用时 31 毫秒
101.
Rapidly adhering cells (RA cells) from the amniotic fluid of a pregnancy with fetal anencephaly were investigated by immunofluorescence assay with an antiserum against glial cells. After 24 hours'' cultivation a high proportion of the cells showed positive glial-specific fluorescence, whereas no staining was seen in cells from samples of normal amniotic fluid. At the 24th week the mother was delivered of a stillborn infant with anencephaly. Immunofluorescence staining of RA cells with glial-specific antiserum may be used for the differential diagnosis of fetal abnormalities associated with a high alpha-fetoprotein concentration in amniotic fluid.  相似文献   
102.
The reported expression of the cell surface-associated, mainly mesenchymal glycoprotein fibronectin by cultured glial cells is in discrepancy with recent work on brain tissue failing to demonstrate any glial or neuronal fibronectin. We have investigated the expression of fibronectin in relation to glial fibrillary acidic protein in cultured human glial and glioma cell lines as well as in cultures derived from newborn rat brain. Using double immunofluorescence technique we found that cells containing glial fibrillary acidic protein do not express fibronectin, and vice versa. The only exception to this rule was the occasional finding of fibronectin at points of cell-to-cell adhesion also in relation to cells containing glial fibrillary acidic protein. The results were also tested by polyacrylamide gel electrophoresis of the culture media of the human cell lines, and by subcultures from the brain of newborn rat, cultures stimulated with dibutyryl cyclic AMP (db-cAMP), and by vinblastine treatment of the cells. The lack of expression of fibronectin in cells containing glial fibrillary acidic protein, a gliospecific cytoskeletal protein, is discussed with reference to glio-mesenchymal interactions and glial markers in vitro.  相似文献   
103.
In this work we report that phosphate anions interact with the anion binding site of alcohol dehydrogenase from horse liver. In protection experiments against the two affinity labels, iodoacetic acid and bromo-imidazolylpropionic acid, the dissociation constant for the enzyme-phosphate complex at pH 7.0 is, based on total phosphate, found to be 20 +/- 5 mM. The 1,4-piperazinediethanesulfonate anion has a lower affinity for the anion binding site, the dissociation at pH 7.0 being 130 +/- 20 mM. The anion-independent dissociation constants for the reversible enzyme-affinity label complexes are at pH 7.0, 1.35 +/- 0.2 mM for iodoacetic acid and 0.39 +/- 0.05 mM for bromo-imidazolylpropionic acid. These findings have important implications with respect to past and future work on this well known enzyme.  相似文献   
104.
Summary Liver glycogen synthase b phosphatase, chromatographically separable from phosphorylase a phosphatase, is decreased in 48-hour alloxan diabetic rats. The phosphatase activities are measured in an in vitro system using exogenous isolated phospho-enzyme as substrates with added phosphatases. Synthase and phosphorylase phosphatases were shown to have differential catalytic properties by their reactivity in the presence of Pi, the heat-stable inhibitor of phosphorylase phosphatase and after incubation with added cAMP-dependent protein kinase.Supported by NIH Grants HD-07788, AM-21149 and, in part, by grants from the Greater St. Louis Diabetic Childrens Welfare Association and the American Diabetes Association, N.Y.  相似文献   
105.
Both neuronal and glial cell differentiation occur in reaggregating cell cultures of mouse cerebellums, as evidenced by electron microscopy and immunofluorescence to the glial fibrillary acidic protein (GFA). However, after the initial 10 days in culture a process occurs in which the neuronal cells degenerate while glial cells predominate. We have found that when poly-l-lysine is added to the culture medium either for the entire culture period or during the latter days of culture, i.e., Days 4 through 10, the neuronal character is stabilized, as evidenced by acetylcholin-esterase levels and electron microscopy, while the gliosis is inhibited. Culturing reaggregating cells in poly-l-lysine containing medium from Days 0 through 4 has no inhibitory influence on the gliosis observed on Day 10. Cerebellar cells cultured as monolayers on plastic surfaces coated with poly-l-lysine express an intense immunofluoresence with antisera to GFA as do cells grown on uncoated flasks. The data suggest that poly-l-lysine in reaggregating cell cultures stabilizes the neuronal cells by some unknown mechanism. It is postulated that a stable neuronal population reduces the trend toward gliosis in cerebellar aggregates.  相似文献   
106.
107.
Compared with human material glial fibrillary acidic protein isolated from bovine, rat and mouse brain was remarkably homogeneous and migrated as a single band at 54 000 mol. wt. on sodium dodecyl sulfate gel electrophoresis. The protein was extremely susceptible to proteolysis and lower molecular weight components were invariably isolated together with the major species when the brain was not rapidly frozen. Further degradation of the 54 000 mol wt. polypeptide in bovine tissues incubated at 24 degrees C resulted in preparations essentially identical to those previously isolated from human autopsy material and separating into a series of immunologically active polypeptides ranging in molecular weight from 54 000 to approximately 40 500. The gel band pattern obtained after progressively longer periods of autolysis suggested that small fragments were cleaved from the original polypeptide in successive steps of degradation. As in human brain, the lower molecular weight products in the 45 000-40 500 range were more resistant to proteolysis and still present after prolonged periods of tissue autolysis. The effect of the pH and of proteinase inhibitors on degradation was studied in homogenates of bovine brain stem incubated at 37 degrees C. At pH 8.0 PROTEOLYSIS OF The glial fibrillary acidic protein followed essentially the same pattern as in tissue. Cleavage of the major species was not prevented by the addition of proteinase inhibitors. At pH 6.0 and 6.5 a different type of degradation was observed, with rapid breakdown of the protein and loss of immunological activity. Increased solubility in buffer solutions was another effect of autolysis. Compared with cerebral cortex and brain stem, where most of the protein was water soluble, only a small fraction was extracted with buffer from bovine white matter. However, the solubility markedly increased following incubation and comparable amounts were extracted in buffer and in 6 M urea.  相似文献   
108.
109.
New nomenclatural combinations are validated for fifteen taxa belonging to the generaDanthonia, Stipa, Lolium, Phippsia, Elymus, Schoenoplectus andAllium.  相似文献   
110.
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