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   886篇
  免费   56篇
  国内免费   1篇
  943篇
  2023年   8篇
  2022年   12篇
  2021年   19篇
  2020年   12篇
  2019年   20篇
  2018年   32篇
  2017年   27篇
  2016年   42篇
  2015年   76篇
  2014年   53篇
  2013年   70篇
  2012年   73篇
  2011年   71篇
  2010年   35篇
  2009年   33篇
  2008年   62篇
  2007年   37篇
  2006年   25篇
  2005年   25篇
  2004年   44篇
  2003年   21篇
  2002年   18篇
  2001年   7篇
  2000年   12篇
  1999年   4篇
  1998年   5篇
  1997年   2篇
  1996年   5篇
  1995年   2篇
  1993年   3篇
  1992年   4篇
  1991年   3篇
  1990年   6篇
  1989年   2篇
  1988年   2篇
  1987年   4篇
  1986年   3篇
  1985年   3篇
  1983年   2篇
  1981年   2篇
  1979年   6篇
  1977年   2篇
  1976年   6篇
  1975年   5篇
  1974年   6篇
  1971年   5篇
  1970年   3篇
  1969年   6篇
  1968年   8篇
  1967年   2篇
排序方式: 共有943条查询结果,搜索用时 15 毫秒
51.
The wing of the chick embryos (the 17th-21st stages of development according to Hamburger--Hamilton) were transplanted on the chorioallantois of the chick embryo-recipients, incubated for 8.5-9.5 days. Differentiation of the joints was studied in serial histological sections and in translucent preparations of the skeleton stained with alcian blue. The transplants for the investigation were taken on the 1st-11th days after transplantation. In the transplants all three segments of the wing always developed. The development of the external form of the extremity, chondrogenesis and osteogenesis of the skeletal anlages were about 24 h late. Histological changes, specific for the early period of the articular interzone and cleft formation corresponded to the control embryos data, but were one day younger. In future the changes did not progress, and passed into regression, demonstrating as fusion of the articular surfaces. In the transplants blood vessels formed networks of irregular form that surrounded the articular zones. Some branches run from them into mesenchyme, situating around the joint. According to the literature data, these vessels are connected with formation of the articular cleft and in the control embryos blood vessels of the articular capsule develop from them. In the transplants they are dilated, twisted (especially in the ulnar joint area) and do not penetrate into the developing prechondral and then into the cartilage bridges of the fusing articular surfaces. Numerous blood accumulations, as well as extravasates are often seen near the deformed anlages of bones. Thus, disturbance of blood supply in the transplants and lack of innervation in them, discussed in the literature, result in fusion of the articular surfaces.  相似文献   
52.
Both Cinnamomi cassiae and Rhodiola rosea extracts are used as anti-diabetic folk medicines. Recently, increased oxidative stress was shown to play an important role in the etiology and pathogenesis of diabetes mellitus and its complications. This study was designed to examine the effects of Cinnamomi cassiae and Rhodiola rosea extracts on blood glucose, lipid peroxidation, the level of reduced glutathione and its related enzymes (glutathione reductase, glutathione S-transferase), and the activity of the antioxidant enzymes (catalase, superoxide dismutase and glutathione peroxidase) in the liver of db/db mice. Diabetic C57BL/Ks db/db mice were used as experimental models. Mice were divided into control (n=10), Cinnamomi cassiae (200 mg/kg/day, n=10), and Rhodiola rosea (200 mg/kg/day, n=10) treated groups for 12 weeks of treatment. These type II diabetic mice were used to investigate the effects of Cinnamomi cassiae and Rhodiola rosea on blood glucose, reduced glutathione, glutathione reductase, glutathione S-transferase, glutathione peroxidase, lipid peroxidation, catalase and superoxide dismutase. Cinnamomi cassiae and Rhodiola rosea extracts significantly decreased on blood glucose, increased levels of reduced glutathione and the activities of glutathione reductase, glutathione S-transferase, glutathione peroxidase, catalase and superoxide dismutase in the liver. Extract treatment also significantly decreased lipid peroxidation. Cinnamomi cassiae and Rhodiola rosea extracts may be effective for correcting hyperglycemia and preventing diabetic complications.  相似文献   
53.
The mechanisms by which mutant variants of Cu/Zn-superoxide dismutase (SOD1) cause familial amyotrophic lateral sclerosis are not clearly understood. Evidence to date suggests that altered conformations of amyotrophic lateral sclerosis mutant SOD1s trigger perturbations of cellular homeostasis that ultimately cause motor neuron degeneration. In this study we correlated the metal contents and disulfide bond status of purified wild-type (WT) and mutant SOD1 proteins to changes in electrophoretic mobility and surface hydrophobicity as detected by 1-anilinonaphthalene-8-sulfonic acid (ANS) fluorescence. As-isolated WT and mutant SOD1s were copper-deficient and exhibited mobilities that correlated with their expected negative charge. However, upon disulfide reduction and demetallation at physiological pH, both WT and mutant SOD1s underwent a conformational change that produced a slower mobility indicative of partial unfolding. Furthermore, although ANS did not bind appreciably to the WT holoenzyme, incubation of metal-deficient WT or mutant SOD1s with ANS increased the ANS fluorescence and shifted its peak toward shorter wavelengths. This increased interaction with ANS was greater for the mutant SOD1s and could be reversed by the addition of metal ions, especially Cu2+, even for SOD1 variants incapable of forming the disulfide bond. Overall, our findings support the notion that misfolding associated with metal deficiency may facilitate aberrant interactions of SOD1 with itself or with other cellular constituents and may thereby contribute to neuronal toxicity.The sequence of events by which more than 100 mutations in the gene encoding Cu/Zn-superoxide dismutase (SOD1)3 cause familial forms of amyotrophic lateral sclerosis (ALS) is unknown. Studies of purified SOD1 proteins and cellular or rodent models of SOD1-linked ALS suggest that impaired metal ion binding or misfolding of mutant SOD1 proteins in the cellular environment may be related to their toxicity (110). Available evidence suggests that partially unfolded mutant SOD1 species could contribute to motor neuron death by promoting abnormal interactions that produce cellular dysfunction (1116).In previous studies we characterized physicochemical properties of 14 different biologically metallated ALS SOD1 mutants (17) and demonstrated altered thermal stabilities of these mutants compared with wild-type (WT) SOD1 (18). These “as-isolated” SOD1 proteins, which contain variable amounts of copper and zinc, were broadly grouped into two classes based on their ability to incorporate and retain metal ions with high affinity. WT-like SOD1 mutants retain the ability to bind copper and zinc ions and exhibit dismutase activity similar to the normal enzyme, whereas metal binding region (MBR) mutants are significantly deficient in copper and/or zinc (17, 19). We also observed that ALS-associated SOD1 mutants were more susceptible than the WT enzyme to reduction of the intrasubunit disulfide bond between Cys-57 and Cys-146 (20). The significance of these results is that even WT-like mutants, which exhibit a nearly normal backbone structure (2123), may be vulnerable to destabilizing influences in vivo. Our group and others subsequently showed that the mutant SOD1 proteins share a susceptibility to increased hydrophobicity under conditions that reduce disulfide bonds and/or chelate metal ions (5) and that similar hydrophobic species exist in tissue lysates from mutant SOD1 transgenic mice (46). One consequence of such hydrophobic exposure could be the facilitation of abnormal interactions between the mutant enzymes and other cellular constituents (e.g. chaperones, mitochondrial components, or other targets), which might influence pathways leading to motor neuron death (15, 16, 2427).Accumulating evidence suggests that metal deficiency of SOD1 is an important factor that can influence SOD1 aggregation or neurotoxicity (4, 2833), but the metal-deficient states of SOD1 that are most relevant to ALS remain unclear. Zinc-deficient, copper-replete SOD1 species, which can be produced in vitro by adding copper to SOD1 that has been stripped of its metal ions at acidic pH, were shown to be toxic to motor neurons in culture (28). However, it has not been shown that zinc-deficient, copper-replete SOD1 is produced in vivo as a consequence of ALS mutations, and loading of copper into SOD1 by the copper chaperone for SOD1 (CCS) is not required for toxicity (34, 35). Furthermore, the MBR mutants have a disrupted copper site and have been found to be severely deficient in both zinc and copper (17, 30), yet expression of these SOD1s still produces motor neuron disease (1, 2, 30, 34, 36, 37).When recombinant human SOD1 was overexpressed in insect cells, we instead observed zinc-replete but copper-deficient species for most WT-like mutants, probably because the capacity of the copper-loading mechanism was exceeded (17). These preparations indicate that zinc can be efficiently incorporated into many WT-like mutants in vivo, and much of it is retained after purification. Furthermore, these copper-deficient biologically metallated proteins may be useful reagents to assess the influence of copper binding upon other properties of SOD1 mutants that may be relevant to their neurotoxicity.We previously observed that reduction of the Cys-57—Cys-146 disulfide bond facilitates the ability of metal chelators to alter the electrophoretic mobility and to increase the hydrophobicity of SOD1 mutants (5). This is consistent with the known properties of this linkage to stabilize the dimeric interface, to orient Arg-143 via a hydrogen bond from the carbonyl oxygen of Cys-57 to Arg-143-NH2, and to prevent metal ion loss (3840). However, it remains unclear whether the Cys-57—Cys-146 bond is required to prevent abnormal SOD1 hydrophobic exposure or whether the aberrant conformational change primarily results from metal ion loss. Ablation of the disulfide bond by the experimental (non-ALS) mutants C57S and C146S provides useful reagents to test the relative influence of the disulfide bond and copper binding upon SOD1 properties.In this study we sought to correlate the consequences of copper deficiency, copper and zinc deficiency, and disulfide reduction upon the hydrodynamic behavior and surface hydrophobicity of WT and representative mutant SOD1 enzymes (Fig. 1A). We quantitated the metal contents of as-isolated SOD1 proteins, detected changes in conformation or metal occupancy using native PAGE to assess their electrophoretic mobility, a measure of global conformational change, and correlated these changes to hydrophobic exposure using 1-anilinonaphthalene-8-sulfonic acid (ANS), which is very sensitive to local conformational changes. ANS is a small amphipathic dye (Fig. 1B) that has been used as a sensitive probe to detect hydrophobic pockets on protein surfaces (4144). Free ANS exhibits only weak fluorescence that is maximal near 520 nm, but when ANS binds to a hydrophobic site in a partially or fully folded protein, the fluorescence peak increases in amplitude and shifts to a shorter wavelength (42). ANS also has an anionic sulfonate group that can interact with cationic groups (e.g. Arg or Lys residues) through ion-pair formation which may be further strengthened by hydrophobic interactions (4346).Open in a separate windowFIGURE 1.A, WT SOD1 structure showing the position of the C57-C146 intrasubunit disulfide bond (S–S, yellow), bound copper and zinc ions, and ALS mutant residues. The residues altered in A4V, G85R, G93A, D124V, and S134N SOD1s are indicated as green spheres. The backbone of the β-barrel core and the loops is shown in a rainbow color, from blue at the amino terminus to red at the carboxyl terminus. The figure was generated using PyMOL (84) and PDB entry 1HL5 (22). B, chemical structure of ANS fluorophore.To evaluate further the importance of metal ion binding, we measured spectral changes related to the binding of cobalt and copper to the same SOD1 proteins. We observed that as-isolated WT-like mutants containing zinc could interact with copper ions to produce an electrophoretic mobility and decreased hydrophobicity resembling that of the fully metalated holo-WT SOD1. In contrast, we saw no evidence for copper binding to MBR mutants in a manner that alters their hydrodynamic properties or their hydrophobicity. Our data suggest that binding of both copper and zinc are important determinants of SOD1 conformation and that perturbation of such binding may be relevant to the ALS disease process.  相似文献   
54.
Spectra of thymocyte nuclear DNAases of control and irradiated (4 Gy) rats have been investigated. Using the method of SDS-electrophoresis of nuclear proteins in DNA-polyacrylamide gel (PAAG) the authors managed to discover a number of polypeptides of 35, 32, 17.7, 17.2, and 16.4 kDA molecular mass possessing a DNAase activity. The enzyme of 35 kDA is only active in the presence of Ca2+ and Mg2+ ions. Nucleases of 32, 17.7, 17.2, and 16.4 kDA are active in the presence of Ca2+ ions and inactive in the presence of Mg2+ ions or in the absence of divalent cations. A simultaneous addition of Ca2+ and Mg2+ ions to the incubation medium causes a synergistic effect with respect to the manifestation of these DNAase activities. Nucleases of 32, 17.7, 17.2, and 16.4 kDa only emerge after the preliminary removal of histones by ion exchange chromatography on a column with CM-sephadex C-50. The enzymic activity of 32 kDA protein increases 60 min after irradiation and drops to the control value in 4 h. At the same time, the postirradiation increase in DNAase activity of a low-molecular weight enzyme group remains invariable throughout the entire period of observation (1-4 h). The preinjection of cycloheximide (CHI) prevents the postirradiation degradation of chromatin and, simultaneously, makes the enzymic activity, corresponding to 35 kDA protein, disappear at the electrophoregrams. The experiments with CHI permit to identify the given enzymic fraction as Ca/Mg-dependent endonuclease. This indicates the participation of normally pre-existing Ca/Mg-dependent endonuclease in implementing the process of chromatin enzymic degradation in the irradiated thymocytes.  相似文献   
55.
Though hypothermia is the only clinically available treatment for neonatal hypoxic-ischemic encephalopathy (HIE), it is not completely effective in severe cases. We hypothesized that combined treatment with hypothermia and transplantation of human umbilical cord blood (UCB)-derived mesenchymal stem cells (MSCs) would synergistically attenuate severe HIE compared to stand-alone therapy. To induce hypoxia-ischemia (HI), male Sprague-Dawley rats were subjected to 8% oxygen for 120 min after unilateral carotid artery ligation on postnatal day (P) 7. After confirmation of severe HIE involving >50% of the ipsilateral hemisphere volume as determined by diffusion-weighted brain magnetic resonance imaging (MRI) within 2 h after HI, intraventricular MSC transplantation (1 × 105 cells) and/or hypothermia with target temperature at 32°C for 24 h were administered 6 h after induction of HI. Follow-up brain MRI at P12 and P42, sensorimotor function tests at P40–42, evaluation of cytokines in the cerebrospinal fluid (CSF) at P42, and histologic analysis of peri-infarct tissues at P42 were performed. Severe HI resulted in progressively increased brain infarction over time as assessed by serial MRI, increased number of cells positive for terminal deoxynucleotidyl transferase nick-end labeling, microgliosis and astrocytosis, increased CSF cytokine levels, and impaired function in behavioral tests such as rotarod and cylinder tests. All of the abnormalities observed in severe HIE showed greater improvement after combined treatment with hypothermia and MSC transplantation than with either therapy alone. Overall, these findings suggest that combined treatment with hypothermia and human UCB-derived MSC transplantation might be a novel therapeutic modality to improve the prognosis of severe HIE, an intractable disease that currently has no effective treatment.  相似文献   
56.
The purpose of this study is to assess health-related quality of life in polio survivors (PS) compared with that in the general population in Korea. Polio survivors (n = 120) from outpatient clinics at two hospitals, healthy controls (HC, n = 121) and members of the general population with activity limitations (AL, n = 121) recruited through a proportional-allocation, systematic sampling strategy from the Fourth Korean National Health and Nutrition Examination Survey were surveyed with self-rated health-related quality of life (Euro QoL five-dimensions). The proportion of participants who reported problems in mobility, usual activity, and symptoms of anxiety/depression were higher in the PS group compared with the HC and AL groups. There was no significant difference in the self-care dimension across the groups. Polio-specific questionnaire, pain, depression, fatigue, Modified Barthel Index (K-MBI) and Short Physical Performance Battery (SPPB) were assessed in the PS group. Those with post-poliomyelitis syndrome had greater problems in mobility, usual activity, and depression/anxiety. Polio survivors, especially those with more pain and fatigue symptoms, and those who did not have access to medical services had poorer health-related quality of life. These findings afford useful information for potential intervention improving quality of life in polio survivors.  相似文献   
57.
Monoclonal antibody (mAb) CO17‐1A binds to GA733, which is a tumor‐associated glycoprotein antigen highly expressed on the colorectal cancer cell surface. Thus, mAb CO17‐1A is considered a useful biomolecule for diagnosis and treatment against colorectal cancer. Previously, we established a baculovirus–insect cell expression system for the production of mAb CO17‐1A. In order to use mAb CO17‐1A as a diagnostic and therapeutic tool, however, the antibody must be properly purified from the insect cells. In this study, our aim was to investigate effective purification processes of mAb CO17‐1A expressed in Spodoptera frugiperda (Sf9) insect cells, using a French press and sonication for cell disruption. SDS‐PAGE confirmed that both mAb CO17‐1A and mAb CO17‐1A fused to the KDEL endoplasmic reticulum (ER) retention signal (mAb CO17‐1AK) were expressed clearly in Sf9 insect cells. Western blot analysis showed that detection levels of mAb CO17‐1A and CO17‐1AK were higher when the insect cells were disrupted two times by the French press and then sonicated, compared to only one French press disruption plus sonication. Optical microscopy confirmed that insect cells treated with both the French press and sonication were properly disrupted. Analysis of gene sequence information on mAb CO17‐1A verified that a signal peptide is present but a transmembrane protein does not exist. These results suggest that cell disruption by the French press twice and sonication once is an effective method for improving purification efficiency.  相似文献   
58.
The Neotropical Region harbors high biodiversity and many studies on mammals, reptiles, amphibians and avifauna have investigated the causes for this pattern. However, there is a paucity of such studies that focus on Neotropical insect groups. Synoeca de Saussure, 1852 is a Neotropical swarm-founding social wasp genus with five described species that is broadly and conspicuously distributed throughout the Neotropics. Here, we infer the phylogenetic relationships, diversification times, and historical biogeography of Synoeca species. We also investigate samples of the disjoint populations of S. septentrionalis that occur in both northwestern parts of South America through Central American and the Brazilian Atlantic rainforests. Our results showed that the interspecific relationships for Synoeca could be described as follows: (S. chalibea + S. virginea) + (S. cyanea + (S. septentrionalis/S. surinama)). Notably, samples of S. septentrionalis and S. surinama collected in the Atlantic Forest were interrelated and may be the result of incomplete lineage sorting and/or mitochondrial introgression among them. Our Bayesian divergence dating analysis revealed recent Plio-Pleistocene diversification in Synoeca. Moreover, our biogeographical analysis suggested an Amazonian origin of Synoeca, with three main dispersal events subsequently occurring during the Plio-Pleistocene.  相似文献   
59.
The aim of this study was to determine the optimal route of mesenchymal stem cell (MSC) transplantation. To this end, gene expression profiling was performed to compare the effects of intratracheal (IT) versus intravenous (IV) MSC administration. Furthermore, the therapeutic efficacy of each route to protect against neonatal hyperoxic lung injury was also determined. Newborn Sprague-Dawley rats were exposed to hyperoxia (90% oxygen) from birth for 14 days. Human umbilical cord blood-derived MSCs labeling with PKH26 were transplanted through either the IT (5×105) or IV (2×106) route at postnatal day (P) 5. At P14, lungs were harvested for histological, biochemical and microarray analyses. Hyperoxic conditions induced an increase in the mean linear intercept and mean alveolar volume (MAV), indicative of impaired alveolarization. The number of ED-1 positive cells was significantly decreased by both IT and IV transplantations. However, IT administration of MSCs resulted in a greater decrease in MAV and ED-1 positive cells compared to IV administration. Moreover, the number of TUNEL-positive cells was significantly decreased in the IT group, but not in the IV group. Although the IT group received only one fourth of the number of MSCs that the IV group did, a significantly higher number of donor cell-derived red PKH 26 positivity were recovered in the IT group. Hyperoxic conditions induced the up regulation of genes associated with the inflammatory response, such as macrophage inflammatory protein-1 α, tumor necrosis factor-α and inter leukin-6; genes associated with cell death, such as p53 and caspases; and genes associated with fibrosis, such as connective tissue growth factor. In contrast, hyperoxic conditions induced the dwon-regulation of vascular endothelial growth factor and hepatocyte growth factor. These hyperoxia-induced changes in gene expression were decreased in the IT group, but not in the IV group. Thus, local IT MSC transplantation was more effective than systemic IV MSC administration in protecting against neonatal hyperoxic lung injury.  相似文献   
60.
Thoracic ultrasound (TUS) is an easy-to-use imaging modality that aids physicians in the differential diagnosis of respiratory diseases. However, no data exist on the TUS findings of acute eosinophilic pneumonia (AEP) or their clinical utility in patients with AEP. Thus, we performed an observational study on TUS findings and their clinical utility for follow-up in patients with AEP. We prospectively screened patients who visited the emergency department for acute respiratory symptoms at the Armed Forces Capital Hospital in South Korea between February 2014 and July 2014. Of them, patients suspected to have AEP underwent an etiological investigation, including flexible bronchoscopy with bronchoalveolar lavage and TUS, and we evaluated TUS findings and serial changes on TUS during the treatment course compared with those from chest radiographs. In total, 22 patients with AEP were identified. The TUS examinations reveled that all patients exhibited multiple diffuse bilateral B-lines and lung sliding, with (n = 5) or without pleural effusion, which was consistent with alveolar-interstitial syndrome. B-line numbers fell during the course of treatment, as the lines became thinner and fainter. A-lines were evident in 19 patients on day 7 of hospitalization, when B-lines had disappeared in 13 patients, and all pleural effusion had resolved. All patients exhibited complete ultrasonic resolution by day 14, along with clinicoradiological improvement. Chest radiographs of five patients taken on day 7 seemed to show complete resolution, but several abnormal B-lines were evident on TUS performed the same day. As a result, our data show common TUS findings of AEP and suggest that AEP may be included as a differential diagnosis when multiple diffuse bilateral B-lines with preserved lung sliding are identified on a TUS examination in patients with acute symptoms, and that TUS is a useful modality for evaluating the treatment response in patients with AEP.  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