全文获取类型
收费全文 | 17914篇 |
免费 | 1121篇 |
国内免费 | 3篇 |
专业分类
19038篇 |
出版年
2023年 | 139篇 |
2022年 | 235篇 |
2021年 | 500篇 |
2020年 | 389篇 |
2019年 | 426篇 |
2018年 | 515篇 |
2017年 | 459篇 |
2016年 | 662篇 |
2015年 | 1003篇 |
2014年 | 1028篇 |
2013年 | 1202篇 |
2012年 | 1527篇 |
2011年 | 1373篇 |
2010年 | 891篇 |
2009年 | 815篇 |
2008年 | 1071篇 |
2007年 | 1012篇 |
2006年 | 924篇 |
2005年 | 859篇 |
2004年 | 797篇 |
2003年 | 719篇 |
2002年 | 671篇 |
2001年 | 171篇 |
2000年 | 117篇 |
1999年 | 144篇 |
1998年 | 145篇 |
1997年 | 128篇 |
1996年 | 107篇 |
1995年 | 94篇 |
1994年 | 99篇 |
1993年 | 59篇 |
1992年 | 59篇 |
1991年 | 68篇 |
1990年 | 64篇 |
1989年 | 39篇 |
1988年 | 40篇 |
1987年 | 30篇 |
1986年 | 25篇 |
1985年 | 47篇 |
1984年 | 65篇 |
1983年 | 24篇 |
1982年 | 29篇 |
1981年 | 29篇 |
1980年 | 28篇 |
1979年 | 18篇 |
1978年 | 21篇 |
1977年 | 25篇 |
1976年 | 20篇 |
1975年 | 20篇 |
1969年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
951.
Temporal localization of porcine reproductive and respiratory syndrome virus in reproductive tissues of experimentally infected boars 总被引:3,自引:0,他引:3
Porcine reproductive and respiratory syndrome virus (PRRSV) has been reported to be shed in the semen of infected boars. To determine whether the reproductive tissues could be a persistent source of virus and the possible origin of PRRSV found in semen of infected boars, 20 PRRSV-seronegative boars were intranasally inoculated with 5 x 10(6) median tissue culture infective doses (TCID50) of PRRSV and necropsied at different times post-inoculation (p.i.) from Day 2 to Day 37 p.i. Blood samples were collected before experimental inoculation, at necropsy and at different times p.i. At necropsy, epididymal semen and reproductive tissues were collected and the presence of the virus determined by virus isolation. The infection of the boars was demonstrated by the isolation of the virus from the sera of all inoculated boars and by seroconversion. PRRSV was detected in serum samples from Day 2 to Day 23 p.i., although the viremic period was largely dependent on the individual response to infection. Viral replication was proven within different reproductive tissues from Day 2 to Day 23 p.i., being most consistently found in the epididymus. In addition, PRRSV was isolated in semen from Day 4 to Day 10 p.i. The correlation of a diminished viremia and the inability to isolate PRRSV from semen or reproductive tissues may be due to one of two possibilities. First, viremia is responsible for most of the virus isolated from reproductive tissues due to the movement of PRRSV-infected cells out of the blood and into the tissues. Second, viremia may initially seed the reproductive tissues with PRRSV, and then the virus is produced into the reproductive tract and shed into semen at low levels. 相似文献
952.
Reactive oxygen species and induction of lignin peroxidase in Phanerochaete chrysosporium 总被引:2,自引:0,他引:2
Belinky PA Flikshtein N Lechenko S Gepstein S Dosoretz CG 《Applied and environmental microbiology》2003,69(11):6500-6506
We studied oxidative stress in lignin peroxidase (LIP)-producing cultures (cultures flushed with pure O(2)) of Phanerochaete chrysosporium by comparing levels of reactive oxygen species (ROS), cumulative oxidative damage, and antioxidant enzymes with those found in non-LIP-producing cultures (cultures grown with free exchange of atmospheric air [control cultures]). A significant increase in the intracellular peroxide concentration and the degree of oxidative damage to macromolecules, e.g., DNA, lipids, and proteins, was observed when the fungus was exposed to pure O(2) gas. The specific activities of manganese superoxide dismutase, catalase, glutathione reductase, and glutathione peroxidase and the consumption of glutathione were all higher in cultures exposed to pure O(2) (oxygenated cultures) than in cultures grown with atmospheric air. Significantly higher gene expression of the LIP-H2 isozyme occurred in the oxygenated cultures. A hydroxyl radical scavenger, dimethyl sulfoxide (50 mM), added to the culture every 12 h, completely abolished LIP expression at the mRNA and protein levels. This effect was confirmed by in situ generation of hydroxyl radicals via the Fenton reaction, which significantly enhanced LIP expression. The level of intracellular cyclic AMP (cAMP) was correlated with the starvation conditions regardless of the oxygenation regimen applied, and similar cAMP levels were obtained at high O(2) concentrations and in cultures grown with atmospheric air. These results suggest that even though cAMP is a prerequisite for LIP expression, high levels of ROS, preferentially hydroxyl radicals, are required to trigger LIP synthesis. Thus, the induction of LIP expression by O(2) is at least partially mediated by the intracellular ROS. 相似文献
953.
Bernal C Velásquez C García G Uribe G Palacio CM 《Biomédica : revista del Instituto Nacional de Salud》2003,23(1):47-59
A clinical trial was conducted to compare the efficacy of a low-osmolarity solution (245 mOsm/L), and a standard oral rehydration solution (ORS) recommended by WHO for children dehydrated by diarrhea. Group 1 (69 children) received WHO/ORS (311 mOsml/L) and group 2 (71 children) received a low-osmolarity solution (245 mOsm/L). Rehydration was successful in 88.4% in group 1 and 92.9% in group 2 (p = 0.35). Rehydration was completed in 5.2 h (SD +/- 1.8) in group 1 and 5.5 (SD +/- 1.7) in group 2 (p = 0.31). Stool output was 6.3 g/kg/h (SD +/- 5.0) in group 1 and 5.6 g/kg/h (SD +/- 5.1) in group 2 (p = 0.94). Sodium at rehydration-completion was 139.3 mEq/L (SD +/- 7.1) in group 1 and 136.7 mEq/L (SD +/- 4.3) in group 2 (p = 0.014). Group 1 was under observation for 21 hours (SD +/- 5.7) and group 2, for 22 hours (SD +/- 5.6). Stool output in group 1 was 5.2 g/kg/h (SD 4.1) and 4.2 gr./kg/h (SD +/- 4.1) in group 2 (p = 0.16). In group 1, 23.1% required intravenous solutions and 9.8% in group 2 (p = 0.03). In treating dehydrated children, the low-osmolarity solution diminished the need for intravenous solutions, corrected most plasmatic sodium disorders, and produced no-risk of developing hyponatremia. 相似文献
954.
Pharmacovigilance is a health sciences discipline devoted to the data collection, data analysis and decision-making related to adverse drug reactions (ADR). It has played an expanded theoretical and practical role since the 1960's. However, few studies have made a careful analysis of the decision-making costs in evaluating ADRs. Herein, the relevant literature is reviewed concerning the costs generated due to the attention of the drug adverse events in medical practice. Examples are taken from international literature which offer by extrapolation of the potential future costs of ADR in Colombia. The objective is to sensitize and generate insights about the need for implementation and development of a national pharmacovigilance system. 相似文献
955.
Continuous cultures of two strains of Clostridium acetobutylicum were stable for over 70 d when grown on glucose/glycerol mixtures. Butanol was the major fermentation end-product, accounting for 43 to 62% (w/w) of total products. Low-grade glycerol [65% (w/v) purity] could replace commercial glycerol [87% (w/v) purity], leading to a similar fermentation pattern: a butanol yield of 0.34 (mol/mol), a butanol productivity of 0.42 g l–1 h–1 and a 84% (w/w) glycerol consumption were attained when cultures were grown at pH 6 and D = 0.05 h–1; butanol accounted for 94% (w/w) of total solvents. These values are among the highest reported in literature for C. acetobutylicum simple chemostats. 相似文献
956.
The induction of a Crassulacean acid like metabolism (CAM) was evidenced after 21–23 days of drought stress in the C4 succulent plant Portulaca oleracea L. by changes in the CO2 exchange pattern, in malic acid content and in titratable acidity during the day–night cycle. Light microscopy studies also
revealed differences in the leaf structure after the drought treatment. Following the induction of the CAM-like metabolism,
the regulatory properties of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), the enzyme responsible for the diurnal fixation
of CO2 in C4 plants but nocturnal in CAM plants, were studied. The enzyme from stressed plants showed different kinetic properties with
respect to controls, notably its lack of cooperativity, higher sensitivity to L-malate inhibition, higher PEP affinity and
lower enzyme content on a protein basis. In both conditions, PEPC's subunit mass was 110 kDa, although changes in the isoelectric
point and electrophoretic mobility of the native enzyme were observed. In vivo phosphorylation and native isoelectrofocusing studies indicated variations in the phosphorylation status of the enzyme of
samples collected during the night and day, which was clearly different for the control and stressed groups of plants. The
results presented suggest that PEPC activity and regulation are modified upon drought stress treatment in a way that allows
P. oleracea to perform a CAM-like metabolism.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
957.
Engineered control of cell morphology in vivo reveals distinct roles for yeast and filamentous forms of Candida albicans during infection 总被引:3,自引:0,他引:3 下载免费PDF全文
It is widely assumed that the ability of Candida albicans to switch between different morphologies is required for pathogenesis. However, most virulence studies have used mutants that are permanently locked into either the yeast or filamentous forms which are avirulent but unsuitable for discerning the role of morphogenetic conversions at the various stages of the infectious process. We have constructed a strain in which this developmental transition can be externally modulated both in vitro and in vivo. This was achieved by placing one copy of the NRG1 gene (a negative regulator of filamentation) under the control of a tetracycline-regulatable promoter. This modified strain was then tested in an animal model of hematogenously disseminated candidiasis. Mice injected with this strain under conditions permitting hyphal development succumbed to the infection, whereas all of the animals injected under conditions that inhibited this transition survived. Importantly, fungal burdens were almost identical in both sets of animals, indicating that, whereas filament formation appears to be required for the mortality resulting from a deep-seated infection, yeast cells play an important role early in the infectious process by extravasating and disseminating to the target organs. Moreover, these infecting Candida yeast cells still retained their pathogenic potential, as demonstrated by allowing this developmental transition to occur at various time points postinfection. We demonstrate here the importance of morphogenetic conversions in C. albicans pathogenesis. This engineered strain should provide a useful tool in unraveling the individual contributions of the yeast and filamentous forms at various stages of the infectious process. 相似文献
958.
959.
Ito LS Yamashita M Strüssmann CA 《Journal of experimental zoology. Part A, Comparative experimental biology》2003,297(2):169-179
Elevated temperature causes degeneration and disappearance of the germ cells in the males of scrotal mammals. It was recently shown that heat-induced germ cell degeneration occurs also in fish but, unlike in mammals, it occurs not only in males but also in females. The purpose of this study was to clarify the histological process and dynamics of heat-induced germ cell disappearance in pejerrey Odontesthes bonariensis larvae and juveniles. Monosex and mixed-sex fish produced by thermal manipulation of sex (temperature-dependent sex determination) were subjected to 29 degrees C for periods between 1 and 12 weeks, and used to analyze, by histological methods, the changes in gonadal size and the number of normal and degenerating germ cells. Groups exposed to 29 degrees C for 8-12 weeks were subsequently transferred to 24 degrees C to verify if any gonadal damage would be permanent. Germ cell degeneration, histologically characterized by nuclear pyknosis or eosinophilia and cytoplasmic eosinophilia, was observed with increasing frequency at higher temperatures (29>24> 17 degrees C) and more in males than in females. Clear degenerative changes in the germinal epithelium usually began within one week of exposure to 29 degrees C and appeared clearer in females than in males. Complete loss of germ cells was observed only in individuals exposed for periods of 8-12 weeks to 29 degrees C but no treatment produced 100% sterile fish. Germ cells that remained in the gonads after exposure to 29 degrees C retained the capacity to rapidly recolonize germ cell-depleted areas, suggesting that the associated somatic cells in the gonads are little or not affected by this temperature. 相似文献
960.
Decaestecker TN Coopman EM Van Peteghem CH Van Bocxlaer JF 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2003,789(1):19-25
An entire series of SPE sorbents, classified into three different categories (apolar, mixed-mode and polymeric) was evaluated for sample preparation of a data-dependent LC-MS-MS "general unknown" screening procedure. An extraction procedure was formulated for each individual column, in agreement with the enclosed instructions, according to the characteristics of each packing. For conciseness, only neutral and basic compounds were chosen for this sorbent suitability test. Thus, the goal of our research was to select the best sorbent with regard to extraction yield and cleanliness of the extracts, all with respect to data-dependent acquisition (DDA) mediated LC-MS-MS general unknown screening. We conclude that for that purpose an Isolute C(8) sorbent performs best in terms of extraction yield and clean-up potential. 相似文献