全文获取类型
收费全文 | 12778篇 |
免费 | 1093篇 |
国内免费 | 6篇 |
出版年
2023年 | 72篇 |
2022年 | 143篇 |
2021年 | 275篇 |
2020年 | 147篇 |
2019年 | 178篇 |
2018年 | 266篇 |
2017年 | 235篇 |
2016年 | 402篇 |
2015年 | 625篇 |
2014年 | 797篇 |
2013年 | 866篇 |
2012年 | 1197篇 |
2011年 | 1080篇 |
2010年 | 706篇 |
2009年 | 652篇 |
2008年 | 826篇 |
2007年 | 802篇 |
2006年 | 756篇 |
2005年 | 755篇 |
2004年 | 667篇 |
2003年 | 599篇 |
2002年 | 571篇 |
2001年 | 123篇 |
2000年 | 76篇 |
1999年 | 116篇 |
1998年 | 124篇 |
1997年 | 86篇 |
1996年 | 88篇 |
1995年 | 66篇 |
1994年 | 56篇 |
1993年 | 66篇 |
1992年 | 51篇 |
1991年 | 39篇 |
1990年 | 33篇 |
1989年 | 32篇 |
1988年 | 25篇 |
1987年 | 14篇 |
1986年 | 19篇 |
1985年 | 17篇 |
1984年 | 21篇 |
1983年 | 22篇 |
1982年 | 11篇 |
1981年 | 14篇 |
1980年 | 17篇 |
1979年 | 13篇 |
1978年 | 10篇 |
1977年 | 13篇 |
1976年 | 12篇 |
1973年 | 7篇 |
1972年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
112.
Summary Transplantation of a small fragment of the ileum beneath the kidney capsule in syngenic mice results in the formation of a cyst lined with proliferating intestinal epithelium. The duration of the cell cycle in this epithelium was determined (using tritiated thymidine and the FLM method) as 14.5 h, as compared with 11.5 h in the intestinal epithelium in situ. We conclude that the intestinal content has little effect on the cell cycle of epithelial cells of the small intestine. 相似文献
113.
Cartilage proteoglycan aggregates. Electronmicroscopic studies of native and fragmented molecules
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
1. Proteoglycan aggregates from bovine nasal cartilage were studied by using electron microscopy of proteoglycan/cytochrome c monolayers. 2. The aggregates contained a variably long central filament of hyaluronic acid with an average length of 1037nm. The proteoglycan monomers attached to the hyaluronic acid appeared as side chain filaments varying in length (averaging 249nm). They were distributed along the central filament at an average distance of about 36nm. 3. Chondroitin sulphate side chains were removed from the proteoglycan monomers of the aggregates by partial chondroitinase digestion. The molecules obtained had the same general appearance as intact aggregates. 4. Proteoglycan aggregates were treated with trypsin and the largest fragment, which contains the hyaluronic acid, link protein and hyaluronic acid-binding region, was recovered and studied with electron microscopy. Filaments that lacked the side chain extensions and had the same length as the central filament in the intact aggregate were observed. 5. Hyaluronic acid isolated after papain digestion of cartilage extracts gave filaments with similar length and size distribution as observed for the central filament both in the intact aggregate and in the trypsin digests. 6. Umbilical-cord hyaluronic acid was also studied and gave electron micrographs similar to those described for hyaluronic acid from cartilage. However, the length of the filament was somewhat shorter. 7. The electron micrographs of both intact and selectively degraded proteoglycans corroborate the current model of cartilage proteoglycan structure. 相似文献
114.
Stefan Berking 《Development genes and evolution》1977,181(3):215-225
Summary From crude extracts ofHydra tissue a substance has been purified which prevents or retards the asexual reproduction by budding. The molecular weight is in the range of 300 to 1000 daltons. Inhibition of bud formation can be observed with concentrations equivalent to the extract from one hydra per 4 ml, that is, to a more than 10,000-fold dilution of the initial crude extract of a hydra. The purified inhibitor is active at a concentration of less than 10–8 M.Most of the inhibitor present inHydra is bound to cells. Within the cells the substance is mainly bound to particulate structures which sediment at 10,000 g. Its concentration is highest in the hypostomal region and decreases in the direction of the tentacles and peduncle. A second, lower, peak has been found in the basal disc. Treatment of the animals with a toxic agent (nitrogen mustard) which depletes the animal of interstitial cells, nematocytes and nematoblasts excludes the possibility that the inhibitor is present to any great extent in these cells. In conjunction with cell separation experiments by centrifugation of fixed cells in suspension, these results indicate that nerve cells are the most likely sites of storage of the inhibiting substance, although epithelial cells are not excluded as sources for the inhibitor. 相似文献
115.
Summary Buds originate inHydra attenuata at a position 1/3 of the body length from the basal disc. The position with respect to the vertical axes is determined first and the position of the bud on the circumference of this budding region is specified later.Bud formation in hydra is reversibly prevented by pre-treatment with an inhibitor purified from hydra tissue (Berking, 1977). Some hours after the end of the treatment with the inhibitor, bud formation is resumed. From the starting or restarting point of development after the inhibitory treatment to the visible beginning of bud formation, 4 intermediary stages were distinguished on the basis of different responses to a second treatment with inhibitor. The pre0treatment is followed immediately by a period of maximal sensitivity to the inhibitor, which varies in length. At the conclusion of this phase the time interval required for the visible appearance of buds is fixed (12 h). In this and the following phase another application of inhibitor can cancel the entire preparatory process from the pre-treatment onwards. A transition to near complete resistance to inhibitor is the basis for defining a third phase. In a fourth phase, immediately before the evagination of the bud starts, the proesence of the inhibitor will again hinder the development. Upon removal of the inhibitor the suppressed buds will appear. 相似文献
116.
Gene libraries of the micronucleus and the macronuclear anlagen of the polytene chromosome stage of Stylonychia lemnae were screened for internal C4A4 repeats. The number of these internal repeats was shown to be identical in both kinds of nuclei. Analysis of macronuclear sequences associated with C4A4 in the polytene chromosomes showed that several macronuclear DNA sequences are clustered. However, interspersed between short exons of one gene are located exons of several other genes, i.e. the exon of one gene is an intron for several other genes.by M. Trendelenburg 相似文献
117.
Roger Wigren Hans Elwing Ragnar Erlandsson Stefan Welin Ingemar Lundstrm 《FEBS letters》1991,280(2):225-228
It is shown that scanning force microscopy (SFM), operated in the attractive mode, can be used to obtain high resolution pictures of adsorbed fibrinogen molecules on solid surfaces, without the need for staining or special microscope grids. SFM also reveals the three-dimensional structure of the adsorbed molecules. Two forms of adsorbed fibrinogen are demonstrated on hydrophobic silicone dioxide surfaces; a trinodular about 60 nm long and a globular with about a 40 nm diameter. Polymeric networks formed after storage of the surface with adsorbed fibrinogen in PBS for 11 days are also shown. The SFM-results for the trinodular structure suggest the existence of loops or peptide chains extending outside the basic structure of the fibrinogen molecule. 相似文献
118.
Muscarinic receptor stimulation or depolarization with elevated extracellular K+ induced rapid and sustained increases in mass accumulations of myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] in cerebral cortex slices. Synergistic but transient responses of both inositol polyphosphate second messengers were observed when slices were stimulated with carbachol under depolarizing conditions; this synergy was observed as an increase in the maximal responsiveness, with no significant change in EC50 values for carbachol. Omission of buffer Ca2+ ([Ca2+]e 10-20 microM) reduced basal Ins(1,4,5)P3 and Ins(1,3,4,5)P4 concentrations; the relative stimulatory effects of muscarinic receptor stimulation were maintained, but the effects of depolarization were markedly attenuated under these conditions. A component of the response to depolarization appeared to be indirectly mediated by the release of acetylcholine, because the K(+)-evoked increase in Ins(1,3,4,5)P4 was enhanced by the cholinesterase inhibitor physostigmine, and was partially attenuated by atropine. An additive suppression by nitrendipine suggests that entry of Ca2+ through L-type Ca2+ channels may serve to accelerate phosphorylation of Ins(1,4,5)P3 by 3-kinase. Norepinephrine did not significantly increase Ins(1,4,5)P3 or Ins(1,3,4,5)P4 accumulation; however, in the presence of depolarizing K+, norepinephrine caused a dramatic increase in Ins(1,3,4,5)P4 mass accumulation. In contrast, the excitatory amino acid quisqualate caused significant increases in the mass accumulations of both inositol polyphosphates measured, with no further increase being observed under depolarizing conditions. The results are discussed with respect to the interactive effects of agonist and depolarization stimuli on inositol polyphosphate accumulation which might more accurately reflect the conditions pertaining in vivo. 相似文献
119.
The actions of the excitatory amino acid N-methyl-D-aspartate (NMDA) on the accumulation of 3H-inositol polyphosphate isomers in rat cerebral cortex slices have been examined over short (less than 5 min) incubation periods. NMDA caused the dose-dependent accumulation of only [3H]inositol monophosphate and [3H]inositol bisphosphate (maximal effect between 0.3 and 1 mM), with no increase in [3H]inositol trisphosphate ([3H]InsP3) and [3H]inositol tetrakisphosphate ([3H]InsP4). HPLC analysis confirmed this, showing no increases in the breakdown products of [3H]Ins(1,3,4,5)P4. When present with the muscarinic agonist carbachol (1 mM), high concentrations of NMDA (1 mM) could almost totally inhibit carbachol-induced accumulation of 3H-inositol polyphosphates. In contrast, at lower concentrations of NMDA (10 microM), the inhibitory effect was replaced with a synergistic accumulation of inositol polyphosphates, especially [3H]InsP4 and [3H]InsP3. The inhibitory effects of NMDA were only apparent when extracellular Ca2+ was present, although incubation in media with no added Ca2+ resulted in somewhat reduced stimulatory responses to NMDA alone, but suppressed totally the inhibitory effects of 1 mM NMDA and reduced the synergistic effects of 10 microM NMDA on carbachol responses. These studies, therefore, reveal Ca(2+)-dependent effects of NMDA indicative of indirect mechanisms of action and show that care must be made in interpreting the effects of NMDA on phosphoinositide metabolism unless the inositol polyphosphate composition has been fully characterised. 相似文献
120.
Vanadyl caused a time- and dose-dependent degradation of deoxyribose to carbonyl products detectable with thiobarbituric acid. This process was inhibited by catalase, ethanol or HEPES; whereas superoxide dismutase was without effect. Vanadate did not substitute for vanadyl even in the presence of a source of O2- plus H2 O 2; but it did so in the presence of reductants such as thiols or NADH. It appears that hydrogen peroxide, generated by the autoxidation of vanadyl, is reduced by vanadyl to the hydroxyl radical; which, in turn, was responsible for the degradation of deoxyribose. A similar process might contribute to the toxic and pharmacological effects of vanadium salts. 相似文献