全文获取类型
收费全文 | 92956篇 |
免费 | 260篇 |
国内免费 | 884篇 |
专业分类
94100篇 |
出版年
2022年 | 1篇 |
2020年 | 1篇 |
2019年 | 1篇 |
2018年 | 11837篇 |
2017年 | 10669篇 |
2016年 | 7444篇 |
2015年 | 580篇 |
2014年 | 271篇 |
2013年 | 280篇 |
2012年 | 4190篇 |
2011年 | 12795篇 |
2010年 | 11988篇 |
2009年 | 8217篇 |
2008年 | 9770篇 |
2007年 | 11347篇 |
2006年 | 245篇 |
2005年 | 496篇 |
2004年 | 955篇 |
2003年 | 1011篇 |
2002年 | 771篇 |
2001年 | 253篇 |
2000年 | 158篇 |
1999年 | 23篇 |
1998年 | 9篇 |
1997年 | 21篇 |
1996年 | 9篇 |
1994年 | 6篇 |
1993年 | 29篇 |
1992年 | 20篇 |
1991年 | 37篇 |
1990年 | 7篇 |
1989年 | 8篇 |
1988年 | 17篇 |
1987年 | 13篇 |
1984年 | 7篇 |
1983年 | 17篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1975年 | 5篇 |
1972年 | 246篇 |
1971年 | 274篇 |
1970年 | 5篇 |
1965年 | 13篇 |
1962年 | 24篇 |
1956年 | 5篇 |
1944年 | 12篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
No studies on the concentration dependency of the inhibition of Streptococcus mutans with xylitol are available. We studied xylitol-induced growth inhibition of two type strains, S. mutans NCTC 10449 and Ingbritt, and three clinical isolates of S. mutans. The strains were grown in Brain Hearth Infusion Medium in the presence of 0.001% (0.066 mM), 0.005% (0.33 mM), 0.01% (0.66
mM), 0.1% (6.6 mM), and 1% (66 mM) xylitol. Growth was followed by measuring the absorbance at a wavelength of 660 nm. The
highest xylitol concentration tested in this study, 1%, showed mean inhibition percentages ranging from 61% to 76% when the
growth inhibition of the five strains was compared to the control without xylitol at log-phase. For 0.1% xylitol, the inhibition
percentages ranged from 22% to 59%. A concentration dependency was seen in the growth inhibition, with 0.01% xylitol being
the lowest xylitol concentration inhibiting all five strains significantly (p < 0.001). The growth inhibition percentages determined for 0.01% xylitol, however, were low, and the inhibition was significantly
weaker as compared to 0.1% and 1% xylitol. Our results suggest that low xylitol concentrations of 0.1% (6.6 mM) could inhibit
mutans streptococci in vivo but even lower xylitol concentrations may be inhibitory. 相似文献
992.
Local delivery of IL-12 and GM-CSF to advanced primary tumors results in T- and NK-cell-dependent cure of disseminated disease
in a murine spontaneous lung metastasis model. Post-therapy functional dynamics of cytotoxic T- and NK-cells were analyzed
in primary and metastatic tumors to determine the specific roles of each subset in tumor eradication. Time-dependent depletion
of CD8+ T and NK-cells demonstrated that CD8+ T-cells were critical to eradication of metastatic tumors within 3 days of treatment,
but not later. In contrast, NK-cells were found to be essential to tumor regression for at least 10 days after cytokine delivery.
Analysis of tumor-infiltrating lymphocyte populations in post-therapy primary tumors demonstrated that treatment resulted
in the activation of tumor-associated CD8+ T-cells within 24 h as determined by IFNγ and perforin production. T-cell activity
peaked between days 1 and 3 and subsided rapidly thereafter. Activation was not accompanied with an increase in cell numbers
suggesting that treatment mobilized pre-existing T-effector/memory cells without inducing proliferation. In contrast, therapy
resulted in a ≥3-fold enhancement of both the quantity and the cytotoxic activity of NK-cells in primary and metastatic tumors
on day 3 post-therapy. NK-cell activity was also transient and subsided to pre-therapy levels by day 5. Depletion of CD4+
and CD8+ T-cells prior to treatment completely abrogated NK-cell infiltration into primary and metastatic tumors demonstrating
the strict dependence of NK-cell recruitment on pre-existing T-effector/memory cells. Treatment failed to induce significant
NK-cell infiltration in IFNγ-knockout mice establishing the central role of IFNγ in NK-cell chemotaxis to tumors. These data
show that transient activation of tumor-associated T-effector/memory and NK-cells, but not long-term CD8+ T-cell responses,
are critical to suppression of metastatic disease in this model; and reveal a novel role for pre-existing adaptive T-cell
immunity in the recruitment of innate effectors to tumors.
This work was supported by NIH/NCI grant R01-CA100656-01A1 to N.K.E. 相似文献
993.
A non-destructive method for collecting samples for DNA analysis from the mucus of molluscs was successfully adapted for use
with the genus Ischnochiton. DNA was extracted using a Chelex-based method and the COI subunit of the mtDNA was amplified and sequenced. Sequences from the mucus were crosschecked against sequences from the foot
tissue of the same animal and were found to be identical. This method provides a non-destructive way of carrying out larger
studies of the genetics of rare organisms and may be of general use for genetic-based field studies of molluscs. 相似文献
994.
Natàlia Alonso 《Vegetation History and Archaeobotany》2008,17(1):75-84
Various sites in the valley of the Sant Cugat stream in Cerdanyola del Vallès (Catalonia) were subject to systematic archaeobotanical
sampling to obtain an overview of the crops and agriculture of the area during the Iron Age and late antiquity. In all cases,
the most numerous taxa were crop plants. Among these, cereals were clearly predominant at all sites investigated, especially
Hordeum vulgare var. vulgare (hulled barley) and Triticum aestivum/durum (bread or macaroni wheat), both in numbers and frequency. Other cereals, such as Triticum dicoccum (emmer) or Setaria italica (foxtail bristle-grass), were regularly present in considerably lower numbers but in fairly high frequencies. Pulses were
much less numerous, although their presence increases in terms of frequency. Among them, clearly the best represented was
Lens culinaris (lentil). The results show that the agriculture in the period considered was principally based on winter cereals, with a
gradual substitution of hulled barley by bread/hard wheat, accompanied by other cereals of minor importance, led by Triticum dicoccum (emmer), and pulses. The appearance of Vitis (grapevine) in the Iberian period is one of the important characteristics of agriculture in the Iberian world.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
995.
996.
Leuconostoc mesenteroides strain NRRL B-1355 produces the soluble exopolysaccharides alternan and dextran in planktonic cultures. Mutants of this strain
are available that are deficient in the production of alternan, dextran, or both. Another mutant of NRRL B-1355, strain R1510,
produces an insoluble glucan in place of alternan and dextran. To test the effect of exopolysaccharide production on biofilm
formation, these strains were cultured in a biofilm reactor. All strains grew well as biofilms, with comparable cell densities,
including strain NRRL B-21414, which produces neither alternan nor dextran in planktonic cultures. However, the exopolysaccharide
phenotype clearly affected the appearance of the biofilms and the sloughed-off biofilm material produced by these biofilms.
For all strains, soluble glucansucrases and soluble polysaccharides produced by biofilm cultures appeared to be similar to
those produced by planktonic cultures. Biofilms from all strains also contained insoluble polysaccharides. Strain R1510 biofilms
contained an insoluble polysaccharide similar to that produced by planktonic cultures. For most other strains, the insoluble
biofilm polysaccharides resembled a mixture of alternan and dextran.
Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard
of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also
be suitable. 相似文献
997.
Dubsky P Hayden H Sachet M Bachleitner-Hofmann T Hassler M Pfragner R Gnant M Stift A Friedl J 《Cancer immunology, immunotherapy : CII》2008,57(6):859-870
BACKGROUND: Recent preclinical and clinical evidence suggests the use of allogeneic tumor as a source of antigen for DC-based immunotherapy against cancer. We hypothesized that addition of allogeneic tumor lysate to monocyte-derived DC culture could serve a dual purpose: (1) antigen source and (2) protein supplementation of DC culture media. Protein supplementation whether of known origin (human serum/plasma, fetal bovine serum, human serum albumin) or undeclared origin ("serum-free" media) is a source of variability and bias. We addressed the question whether protein supplementation can be omitted in the presence of allogeneic tumor lysate. MATERIALS AND METHODS: Human DC cultured in the presence of lysate from medullary thyroid carcinoma (MTC) cell line SHER-I (TuLy-DC) and DC pulsed with the same lysate but cultured in the presence of FBS (FBS-DC) were assessed for morphology, phenotype, maturation and functional properties. RESULTS: In comparison of FBS-DC/TuLy-DC no significant differences in morphology, phenotype and maturation could be detected. Both culture conditions produced CD1a(high), CD14(low) DC with high expression of costimulatory molecules and CD83 upon stimulation. TuLy-DC gave significantly better yields and produced more IL12p70. DC showed high (allo)stimulatory capacity toward T-cells. TuLy-DC induced more intracellular IFNgamma in CD8+T-cells of vaccinated MTC patients. Both types of DC induced killing of SHER-I after short in vitro restimulation. Tumor lysate from SHER-I can substitute for further protein supplementation in DC culture. Allogeneic tumor lysates should be taken into consideration as both source of antigen and protein supplementation in monocyte-derived DC culture. 相似文献
998.
A bacterial strain capable of utilizing a mixture containing 2-hydroxybenzoic acid (2-HBA), 3-hydroxybenzoic acid (3-HBA)
and 4-hydroxybenzoic (4-HBA) acid was isolated through enrichment from a soil sample. Based on 16SrDNA sequencing, the microorganism
was identified as Acinetobacter calcoaceticus. The sequence of biodegradation of the three isomers when provided as a mixture (0.025%, w/v each) was elucidated. The dihydroxylated
metabolites formed from the degradation of 2-HBA, 3-HBA and 4-HBA were identified as catechol, gentisate and protocatechuate,
respectively, using the cell-free supernatant and cell-free crude extracts. Monooxygenases and dioxygenases that were induced
in the cells of Acinetobacter calcoaceticus in response to growth on mixture containing 2-HBA, 3-HBA and 4-HBA could be detected in cell-free extracts. These data revealed
the pathways operating in Acinetobacter calcoaceticus for the sequential metabolism of monohydroxybenzoate isomers when presented as a mixture. 相似文献
999.
1000.
Kim MH Kim MO Heo JS Kim JS Han HJ 《Apoptosis : an international journal on programmed cell death》2008,13(2):295-304
This study examined the effect of acetylcholine (ACh) on the hypoxia-induced apoptosis of mouse embryonic stem (ES) cells.
Hypoxia (60 h) decreased both the cell viability and level of [3H] thymidine incorporation, which were prevented by a pretreatment with ACh. However, the atropine (ACh receptor [AChR] inhibitor)
treatment blocked the protective effect of ACh. Hypoxia (90 min) increased the intracellular level of reactive oxygen species
(ROS). On the other hand, ACh inhibited the hypoxia-induced increase in ROS, which was blocked by an atropine treatment. Subsequently,
the hypoxia-induced ROS increased the level of p38 mitogen activated protein kinase (MAPK) and Jun-N-terminal kinase (JNK)
phosphorylation, which were inhibited by the ACh pretreatment. Moreover, hypoxic exposure (90 min) increased the level of
nuclear factor-κB (NF-κB) phosphorylation, which was blocked by a pretreatment with SB 203580 (p38 MAPK inhibitor) or SP 600125
(JNK inhibitor). However, hypoxia (60 h) decreased the protein levels of Bcl-2 and c-IAPs (cellular inhibitor of apoptosis
proteins) but increased the level of caspase-3 activation. All these effects were inhibited by a pretreatment with ACh. In
conclusion, ACh prevented the hypoxia-induced apoptosis of mouse ES cells by inhibiting the ROS-mediated p38 MAPK and JNK
activation as well as the regulation of Bcl-2, c-IAPs, and caspase-3.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献