全文获取类型
收费全文 | 1904篇 |
免费 | 156篇 |
专业分类
2060篇 |
出版年
2023年 | 9篇 |
2022年 | 17篇 |
2021年 | 36篇 |
2020年 | 18篇 |
2019年 | 38篇 |
2018年 | 43篇 |
2017年 | 48篇 |
2016年 | 51篇 |
2015年 | 77篇 |
2014年 | 93篇 |
2013年 | 121篇 |
2012年 | 144篇 |
2011年 | 110篇 |
2010年 | 93篇 |
2009年 | 63篇 |
2008年 | 80篇 |
2007年 | 85篇 |
2006年 | 85篇 |
2005年 | 75篇 |
2004年 | 96篇 |
2003年 | 74篇 |
2002年 | 73篇 |
2001年 | 54篇 |
2000年 | 41篇 |
1999年 | 35篇 |
1998年 | 20篇 |
1997年 | 19篇 |
1996年 | 17篇 |
1995年 | 20篇 |
1994年 | 16篇 |
1993年 | 15篇 |
1992年 | 28篇 |
1991年 | 17篇 |
1990年 | 19篇 |
1989年 | 16篇 |
1988年 | 14篇 |
1987年 | 13篇 |
1986年 | 16篇 |
1985年 | 19篇 |
1984年 | 21篇 |
1983年 | 14篇 |
1982年 | 14篇 |
1981年 | 9篇 |
1977年 | 8篇 |
1975年 | 8篇 |
1973年 | 12篇 |
1971年 | 7篇 |
1970年 | 6篇 |
1969年 | 9篇 |
1968年 | 6篇 |
排序方式: 共有2060条查询结果,搜索用时 15 毫秒
31.
Angela M. Zanata Willian M. Ohara Osvaldo T. Oyakawa Fernando C. P. Dagosta 《Journal of fish biology》2020,97(5):1343-1353
Characidium iaquira, a new species from the upper rio Juruena, rio Tapajós basin, Brazil, is described. The new species can be promptly distinguished from all congeners by having a unique v-shaped dark mark lying along the caudal-fin extension, in medium- and large-sized specimens, and a remarkable iridescent green colouration in life. Characidium iaquira is closely related to Characidium crandellii and Characidium declivirostre by sharing unambiguous synapomorphies such as branchiostegal membranes united to each other across the isthmus, a scaleless area extending from the isthmus to the pectoral girdle, and dermal flaps surrounding anterior and posterior naris independent, but touching each other distally. Morphological specializations of the paired fins in the three riffle-dwellers species are discussed, including the wing-like shape, robustness, and inclination of the pectoral fin. 相似文献
32.
Osvaldo Loquiha Niel Hens Leonardo Chavane Marleen Temmerman Marc Aerts 《Biometrical journal. Biometrische Zeitschrift》2013,55(5):647-660
Count data are very common in health services research, and very commonly the basic Poisson regression model has to be extended in several ways to accommodate several sources of heterogeneity: (i) an excess number of zeros relative to a Poisson distribution, (ii) hierarchical structures, and correlated data, (iii) remaining “unexplained” sources of overdispersion. In this paper, we propose hierarchical zero‐inflated and overdispersed models with independent, correlated, and shared random effects for both components of the mixture model. We show that all different extensions of the Poisson model can be based on the concept of mixture models, and that they can be combined to account for all different sources of heterogeneity. Expressions for the first two moments are derived and discussed. The models are applied to data on maternal deaths and related risk factors within health facilities in Mozambique. The final model shows that the maternal mortality rate mainly depends on the geographical location of the health facility, the percentage of women admitted with HIV and the percentage of referrals from the health facility. 相似文献
33.
In vitro and in vivo characterization of an interleukin‐15 antagonist peptide by metabolic stability, 99mTc‐labeling,and biological activity assays 下载免费PDF全文
Yunier Rodríguez‐Álvarez Ania Cabrales‐Rico Alejandro Perera‐Pintado Anais Prats‐Capote Hilda E. Garay‐Pérez Osvaldo Reyes‐Acosta Erik Pérez‐García Araceli Chico‐Capote Alicia Santos‐Savio 《Journal of peptide science》2018,24(4-5)
Interleukin (IL)–15 is an inflammatory cytokine that constitutes a validated therapeutic target in some immunopathologies, including rheumatoid arthritis (RA). Previously, we identified an IL‐15 antagonist peptide named [K6T]P8, with potential therapeutic application in RA. In the current work, the metabolic stability of this peptide in synovial fluids from RA patients was studied. Moreover, [K6T]P8 peptide was labeled with 99mTc to investigate its stability in human plasma and its biodistribution pattern in healthy rats. The biological activity of [K6T]P8 peptide and its dimer was evaluated in CTLL‐2 cells, using 3 different additives to improve the solubility of these peptides. The half‐life of [K6T]P8 in human synovial fluid was 5.88 ± 1.73 minutes, and the major chemical modifications included peptide dimerization, cysteinylation, and methionine oxidation. Radiolabeling of [K6T]P8 with 99mTc showed a yield of approximately 99.8%. The 99mTc‐labeled peptide was stable in a 30‐fold molar excess of cysteine and in human plasma, displaying a low affinity to plasma proteins. Preliminary biodistribution studies in healthy Wistar rats suggested a slow elimination of the peptide through the renal and hepatic pathways. Although citric acid, sucrose, and Tween 80 enhanced the solubility of [K6T]P8 peptide and its dimer, only the sucrose did not interfere with the in vitro proliferation assay used to assess their biological activity. The results here presented, reinforce nonclinical characterization of the [K6T]P8 peptide, a potential agent for the treatment of RA and other diseases associated with IL‐15 overexpression. 相似文献
34.
35.
James J. Gilroy Graham W. Prescott Johann S. Cardenas Pamela González del Pliego Castañeda Andrés Sánchez Luis E. Rojas‐Murcia Claudia A. Medina Uribe Torbjørn Haugaasen David P. Edwards 《Global Change Biology》2015,21(4):1531-1540
Oil palm agriculture is rapidly expanding in the Neotropics, at the expense of a range of natural and seminatural habitats. A key question is how this expansion should be managed to reduce negative impacts on biodiversity. Focusing on the Llanos of Colombia, a mixed grassland–forest system identified as a priority zone for future oil palm development, we survey communities of ants, dung beetles, birds and herpetofauna occurring in oil palm plantations and the other principal form of agriculture in the region – improved cattle pasture – together with those of surrounding natural forests. We show that oil palm plantations have similar or higher species richness across all four taxonomic groups than improved pasture. For dung beetles, species richness in oil palm was equal to that of forest, whereas the other three taxa had highest species richness in forests. Hierarchical modelling of species occupancy probabilities indicated that oil palm plantations supported a higher proportion of species characteristic of forests than did cattle pastures. Across the bird community, occupancy probabilities within oil palm were positively influenced by increasing forest cover in a surrounding 250 m radius, whereas surrounding forest cover did not strongly influence the occurrence of other taxonomic groups in oil palm. Overall, our results suggest that the conversion of existing improved pastures to oil palm has limited negative impacts on biodiversity. As such, existing cattle pastures of the Colombian Llanos could offer a key opportunity to meet governmental targets for oil palm development without incurring significant biodiversity costs. Our results also highlight the value of preserving remnant forests within these agricultural landscapes, protecting high biodiversity and exporting avian ‘spill‐over’ effects into oil palm plantations. 相似文献
36.
Paula Pluta Pietro Roversi Ganeko Bernardo-Seisdedos Adriana L. Rojas Jonathan B. Cooper Shuang Gu Richard W. Pickersgill Oscar Millet 《Biochimica et Biophysica Acta (BBA)/General Subjects》2018,1862(9):1948-1955
Human porphobilinogen deaminase (PBGD), the third enzyme in the heme pathway, catalyzes four times a single reaction to convert porphobilinogen into hydroxymethylbilane. Remarkably, PBGD employs a single active site during the process, with a distinct yet chemically equivalent bond formed each time. The four intermediate complexes of the enzyme have been biochemically validated and they can be isolated but they have never been structurally characterized other than the apo- and holo-enzyme bound to the cofactor. We present crystal structures for two human PBGD intermediates: PBGD loaded with the cofactor and with the reaction intermediate containing two additional substrate pyrrole rings. These results, combined with SAXS and NMR experiments, allow us to propose a mechanism for the reaction progression that requires less structural rearrangements than previously suggested: the enzyme slides a flexible loop over the growing-product active site cavity. The structures and the mechanism proposed for this essential reaction explain how a set of missense mutations result in acute intermittent porphyria. 相似文献
37.
Wafa H. Cabrera Olga M. Ibanez Silvio L. Oliveira Osvaldo A. Sant'Anna Maria Siqueira Denise Mouton Guido Biozzi 《Immunogenetics》1982,16(6):583-592
The effect of the selective breeding of mice for high or low antibody production to complex immunogens is largely nonspecific, since it modifies the responsiveness of high (H) and low (L) lines to many antigens unrelated to the selection antigen. However, the nonspecific effect of the polygenic control operating in these lines is not a general feature. For example, the group of genes in selection IV, carried out for responsiveness to somatic antigen of Salmonella, does not modify the responses to sheep erythrocytes (SE). Despite equivalent responses in H and L mice of selection IV, a large variability was found in individual responses of F2 interline hybrids, which demonstrates the presence of alleles with high or low effect on responses to SE. A selective breeding (Selection IV-A) was therefore initiated from this F2 population for responsiveness to SE. A progressive interline divergence occurred during the first seven generations of selection; the interline separation was due to polygenic regulation (about four independent loci from a preliminary estimate).Equivalent responses to the s antigen of Salmonella are observed in the two lines. This constitutes additional evidence for distinct polygenic regulation of responses to SE and to somatic antigen. Moreover, the pattern of responses to several unrelated antigens (nonspecific effect) also differs between Selections IV and IV-A.Abbreviations H
high responder lines
- L
low responder lines
- s
somatic antigen of Salmonella
- f
flagellar antigen of Salmonella
- R
response to selection
- S
selection differential
- F0
foundation population
- h2
heritability (realized)
- RGG
rabbit gamma globulin
- CE
chicken erythrocyte
- HE
human erythrocyte
- PE
pigeon erythrocyte
- SE
sheep erythrocyte 相似文献
38.
Monitoring of black mangrove restoration with nursery-reared seedlings on an arid coastal lagoon 总被引:3,自引:0,他引:3
Black mangrove (Avicennia germinans) seedlings (n=555) were grown from field-collected propagules for 3 months in a new type of terrestrial nursery. They were grown in clusters of five plants, and then they were transplanted to a clear-cut zone in a lagoon fringed by a mangrove forest at Laguna de Balandra, Baja California Sur, Mexico. Survival and plant development of transplants were monitored at 6-monthly intervals for 2 years. After 1 month, the survival of seedlings was 96%, later stabilizing at approximately 77%. After 24 months, 74% of the plants were still alive. The best cluster, showing maximum growth under mangrove swamp conditions in this arid zone, was a two-plant cluster. The lagoon has a low natural regeneration rate of 48 plants per 350 m2 per 6 years of monitoring. This study shows the feasibility of restoring destroyed arid-coast lagoons with black mangroves. 相似文献
39.
K J Sellers M S Smith F J Rojas R H Asch A V Schally A Bartke 《Regulatory peptides》1986,15(3):219-228
We have examined the effects of a single subcutaneous injection of an LHRH agonist, D-Trp-6-LHRH, in biodegradable microcapsules of poly(DL-lactide-co-glycolide) on plasma gonadotropin and prolactin (PRL) levels in castrated and in castrated-hypophysectomized-pituitary grafted (CAST-APX-GRAFT) male rats. The results were compared to the effects of daily injections of the same LHRH agonist dissolved in saline. In castrated rats, there were no significant alterations in plasma LH or PRL levels during the 10 days following the injection of LHRH agonist microcapsules, while FSH levels were generally reduced. In castrated males given daily injections of 6 micrograms of LHRH agonist in saline, plasma LH levels were significantly reduced while plasma PRL levels were not changed. In CAST-APX-GRAFT rats, both D-Trp-6-LHRH microcapsules and daily LHRH agonist injections appeared to increase plasma PRL levels. The pattern of changes in PRL release in both groups was similar, with levels on day 6 being significantly higher than those measured on days 1, 3 and 10 after onset of treatment. As expected, LH and FSH levels in these animals were extremely low. Immunoreactive D-Trp-6-LHRH was consistently detectable in the plasma of CAST-APX-GRAFT animals after microcapsule administration, whereas in animals given daily injections of this agonist in saline, its plasma concentrations were often below the detectability limit of the employed assay. These findings suggest that the LHRH agonist, D-Trp-6-LHRH, is capable of causing a short term stimulation of PRL release from ectopic pituitaries. Elevation of plasma LH levels is apparently not required for this effect. 相似文献
40.
Expression of nonstructural rotavirus protein NSP4 mimics Ca2+ homeostasis changes induced by rotavirus infection in cultured cells 下载免费PDF全文
Díaz Y Chemello ME Peña F Aristimuño OC Zambrano JL Rojas H Bartoli F Salazar L Chwetzoff S Sapin C Trugnan G Michelangeli F Ruiz MC 《Journal of virology》2008,82(22):11331-11343
Rotavirus infection modifies Ca2+ homeostasis, provoking an increase in Ca2+ permeation, the cytoplasmic Ca2+ concentration ([Ca2+]cyto), and total Ca2+ pools and a decrease in Ca2+ response to agonists. A glycosylated viral protein(s), NSP4 and/or VP7, may be responsible for these effects. HT29 or Cos-7 cells were infected by the SA11 clone 28 strain, in which VP7 is not glycosylated, or transiently transfected with plasmids coding for NSP4-enhanced green fluorescent protein (EGFP) or NSP4. The permeability of the plasma membrane to Ca2+ and the amount of Ca2+ sequestered in the endoplasmic reticulum released by carbachol or ATP were measured in fura-2-loaded cells at the single-cell level under a fluorescence microscope or in cell suspensions in a fluorimeter. Total cell Ca2+ pools were evaluated as 45Ca2+ uptake. Infection with SA11 clone 28 induced an increase in Ca2+ permeability and 45Ca2+ uptake similar to that found with the normally glycosylated SA11 strain. These effects were inhibited by tunicamycin, indicating that inhibition of glycosylation of a viral protein other than VP7 affects the changes of Ca2+ homeostasis induced by infection. Expression of NSP4-EGFP or NSP4 in transfected cells induced the same changes observed with rotavirus infection, whereas the expression of EGFP or EGFP-VP4 showed the behavior of uninfected and untransfected cells. Increased 45Ca2+ uptake was also observed in cells expressing NSP4-EGFP or NSP4, as evidenced in rotavirus infection. These results indicate that glycosylated NSP4 is primarily responsible for altering the Ca2+ homeostasis of infected cells through an initial increase of cell membrane permeability to Ca2+. 相似文献