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   109749篇
  免费   5477篇
  国内免费   74篇
  115300篇
  2018年   1066篇
  2017年   1241篇
  2016年   3015篇
  2015年   6384篇
  2014年   6063篇
  2013年   6056篇
  2012年   5217篇
  2011年   2537篇
  2010年   2464篇
  2009年   2363篇
  2008年   1047篇
  2007年   1058篇
  2006年   1066篇
  2005年   7159篇
  2004年   5852篇
  2003年   4102篇
  2002年   1689篇
  2001年   2849篇
  2000年   2109篇
  1999年   2781篇
  1998年   777篇
  1997年   626篇
  1996年   575篇
  1992年   2869篇
  1991年   2967篇
  1990年   3024篇
  1989年   2936篇
  1988年   2818篇
  1987年   2629篇
  1986年   2308篇
  1985年   2329篇
  1984年   1625篇
  1983年   1296篇
  1982年   807篇
  1981年   745篇
  1980年   679篇
  1979年   1591篇
  1978年   1169篇
  1977年   990篇
  1976年   944篇
  1975年   1297篇
  1974年   1425篇
  1973年   1391篇
  1972年   1323篇
  1971年   1217篇
  1970年   1058篇
  1969年   1058篇
  1968年   945篇
  1967年   944篇
  1966年   746篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
991.
Metabolically 35S-labeled proteoglycans were isolated from cell-associated matrices and media of confluent cultures of human normal transitional epithelial cells and HCV-29T transitional carcinoma cells. On Sepharose CL-4B columns, the cell-associated proteoglycans synthesized from both cell types separated into three identical size classes, termed CI, CII, and CIII. Normal epithelial cell C-fractions eluted in a 22:34:45 proportion and contained 64%, 64%, and 72% heparan sulfate, whereas corresponding HCV-29T fractions eluted in a 29:11:60 proportion, and contained 91%, 77%, and 70% heparan sulfate, respectively. Medium proteoglycans from normal cells separated into two size classes in a proportion of 6:94 and were composed of 35% and 50% heparan sulfate. HCV-29T medium contained only one size class of proteoglycans consisting of 23% heparan sulfate. The remaining percentages were accounted for by chondroitin/dermatan sulfate. On isopycnic CsCl gradients, proteoglycan fractions from normal cells had buoyant densities that were higher than the corresponding fractions from HCV-29T cells. DEAE-Sephacel chromatography showed that cell and medium associated heparan sulfate from HCV-29T cells was consistently of lower charge density (undersulfated) than that from normal epithelial cells. In contrast, the chondroitin/dermatan sulfate of HCV-29T was of a charge density similar to that of normal cells. These as well as other structural and compositional differences in the proteoglycan may account, at least in part, for the altered behavioral traits of highly invasive carcinoma cells.  相似文献   
992.
The Prader-Willi syndrome chromosome region on the long arm of human chromosome 15 was microdissected and microcloned from 20 GTG-banded metaphase chromosomes, and 5000 recombinant clones were obtained. Of these clones, 39% identify single-copy human DNA sequences, most of which map to the dissected chromosome region and are evolutionarily conserved in other species. Three of eleven clones studied in detail are deleted in several patients with Prader-Willi syndrome. The microclones will be useful for the physical characterization of the Prader-Willi syndrome chromosome region and the identification of the affected genes in this disease.  相似文献   
993.
994.
Endo-exonuclease of Aspergillus nidulans   总被引:1,自引:0,他引:1  
Endo-exonuclease (EE) has been found in both active and inactive, but trypsin-activatable, forms in Aspergillus nidulans. Active EE was present mainly in nuclei, mitochondria, and vacuoles, while trypsin-activatable EE was mainly in the cytosol. The active form accounts for over 90% of the neutral deoxyribonuclease activity extracted from mycelia. A single strand (ss) DNA-binding EE associated with a 28 kilodalton (kDa) polypeptide was partially purified and characterized. It was found to closely resemble, in size and enzymological properties, the ss-DNA-binding EE previously purified from Neurospora crassa. Aspergillus nidulans EE was also found to be immunochemically related to the N. crassa EE and, like that enzyme, was probably derived from a polypeptide of 90 kDa or larger through proteolysis during extraction and purification. It had divalent metal ion-dependent (Mg2+, Mn2+, or Zn2+) activity on both DNA and RNA, which ultimately produced small 5'-P-terminated oligonucleotides. The nuclease activity was mixed endo- and exo-nucleolytic with ss-DNA as substrate, but largely exonucleolytic with double strand (ds) DNA. Superhelical phi X-174 DNA was nicked by EE to form relaxed circular and then linear ds-DNA, which was rapidly degraded to shorter fragments. Linearized pBR322 DNA was extensively nicked internally under conditions where there was relatively low exonuclease activity, but this nicking required that 5'-P-termini be present on the linear ds-DNA. The levels of active EE found in extracts of two recombination-deficient mutants of A. nidulans, uvsC and uvsE, dit not differ significantly from those in extracts of the wild type.  相似文献   
995.
Photosensory membrane degradation in crayfish occurs at first in multi-vesicular bodies (MVBs) and then, with the aid of lysosomal enzymes, in lysosome related lamellar bodies. In organ culture experiments with the isolated crayfish retina (Orconectes limosus) small screening pigment-like granules became visible under the electron microscope in such lamellar bodies and suggested a possible relation of photosensory membrane degradation and screening pigment granule synthesis. Chloroquine, an inhibitor of lysosomal activity, when added to the culture medium reduced the appearance of screening pigment-like granules in lamellar bodies, but led to the appearance of these granules in mature MVB's, indicating the involvement of lysosomal enzymes in the formation of pigmented lamellar bodies. In a second set of experiments the effect of bright light on the screening pigment granule ultrastructure of crayfish phoreceptors was investigated. It was found that after bright light exposure large numbers of little screening pigment granules (0.15-0.3 microns) were located between or close to rhabdomeral microvilli that were not at these sites in crayfish kept under natural light. MVB's were also reduced in size, and among the little screening pigmentary organelles granules of different electron density and morphology appeared. Additionally, vesicle flux to little screening pigment granules was detected. The screening pigment granules of the little type did not seem to be transported close to or between the microvilli, but appeared to be synthesized at these sites within little MVBs.  相似文献   
996.
Selected cholinergic markers (choline acetyltransferase, acetylcholinesterase, muscarinic acetylcholine receptor, high-affinity choline uptake) were studied in the hindlimb representation areas of the rat somatosensory cortex and within the visual cortex 1 to 63 days after unilateral transection of the sciatic nerve. In the contralateral somatosensory cortex, peripheral deafferentation resulted in a significant reduction of choline acetyltransferase activity (by 15%) 3 days after sciatic nerve injury, and in a significant reduction of high-affinity choline uptake (by 30%) 1 day after nerve transection, in comparison to untreated control rats. Investigations in individual cortical layers revealed that the decrease of both choline acetyltransferase activity and high-affinity choline uptake sites was mainly due to reductions in cortical layer V. Acetylcholinesterase activity and [3H]quinuclidinyl benzilate binding to muscarinic acetylcholine receptors were not affected by unilateral transection of the sciatic nerve. In the ipsilateral somatosensory cortex, as well as in the visual cortex at both cortical hemispheres, no significant changes in the cholinergic parameters studied could be detected. The data indicate that peripheral deafferentation of the somatosensory cortex results in a transient change of presynaptic cholinergic parameters within the affected somatosensory area as early as 1 to 3 days after the lesion; thus, they emphasize the involvement of cholinergic mechanisms in cortical reorganizational events.  相似文献   
997.
998.
Role of oxidatively modified LDL in atherosclerosis   总被引:32,自引:0,他引:32  
Oxidative modification of LDL is accompanied by a number of compositional and structural changes, including increased electrophoretic mobility, increased density, fragmentation of apolipoprotein B, hydrolysis of phosphatidylcholine, derivatization of lysine amino groups, and generation of fluorescent adducts due to covalent binding of lipid oxidation products to apo B. In addition, oxidation of LDL has been shown to result in numerous changes in its biologic properties that could have pathogenetic importance, including accelerated uptake in macrophages, cytotoxicity, and chemotactic activity for monocytes. The present article summarizes very recent developments related to the mechanism of oxidation of LDL by cells, receptor-mediated uptake of oxidized LDL in macrophages, the mechanism of phosphatidylcholine hydrolysis during LDL oxidation, and other biologic actions of oxidized LDL including cytotoxicity, altered eicosanoid metabolism, and effects on the secretion of growth factors and chemotactic factors. In addition, this review will examine the evidence for the presence of oxidized LDL in vivo and the evidence that oxidized LDL plays a pathogenetic role in atherosclerosis.  相似文献   
999.
Growth factors have an important role in the regulation of cell growth, division and differentiation. They are also involved in the regulation of embryonic growth and differentiation. Insulin and insulin-like growth factor I (IGF I) play an important part in these events in the later stages of embryogenesis, when organogenesis is completed. In this study, we are presenting evidence that insulin and IGF I are also secreted by embryonic tissues during the prepancreatic stage of mouse development. We found measurable amounts of insulin and IGF I in 8- to 12-day-old mouse embryos. We also showed that embryonic cells derived from 8-, 9- and 10-day-old mouse embryos secrete insulin, IGF I and/or related molecules. Furthermore, the same growth factors, when added to the culture of 9-day-old mouse embryonic cells, stimulate their proliferation. These results lead to the conclusion that insulin can stimulate the growth of embryonic cells during the period when pancreas is not yet formed, which is indirect evidence for a paracrine (or autocrine) type of action.  相似文献   
1000.
The effects of two specific 5-lipoxygenase inhibitors AA-863 and U-60,257 (piriprost) on the growth of two human glioma cell lines, U-343 MGa and U-251 MG were investigated. Both monolayer cultured cells and spheroids were studied. The results of the monolayer studies showed potent and dose dependent inhibitory effects on the proliferation of glioma cells (IC50/one week treatment/of AA-863: 9.0 microM, IC50 of U-60,257: 40.0 microM). The experiments made on the tumor spheroids suggested an inhibitory effect on proliferation and volume growth already at lower doses (AA-863: 0.4-2.0 microM, U-60,257: 1.0-5.0 microM), a dose range where effects were not found in monolayers. At higher doses (AA-863: 10.0-30.0 microM, U-60,257: 30.0-90.0 microM) the experiments with spheroids failed to demonstrate a further inhibitory effect on spheroid volume, probably attributed to phenomena such as swelling of cells, dissociation of spheroid structure and development of necrosis. The clearly dose dependent inhibitory effect on the proliferation of human glioma cells in monolayer culture and the inhibitory effects on spheroid growth with these specific inhibitors indicate a role for lipoxygenase products in the growth of gliomas.  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