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   962篇
  免费   99篇
  国内免费   1篇
  1062篇
  2021年   11篇
  2020年   19篇
  2019年   43篇
  2018年   36篇
  2017年   12篇
  2016年   20篇
  2015年   19篇
  2014年   18篇
  2013年   51篇
  2012年   32篇
  2011年   22篇
  2010年   22篇
  2009年   19篇
  2008年   27篇
  2007年   32篇
  2006年   35篇
  2005年   30篇
  2004年   22篇
  2003年   32篇
  2002年   25篇
  2001年   35篇
  2000年   19篇
  1999年   24篇
  1998年   29篇
  1997年   17篇
  1996年   28篇
  1995年   24篇
  1994年   12篇
  1993年   20篇
  1992年   21篇
  1991年   17篇
  1990年   20篇
  1989年   30篇
  1988年   15篇
  1987年   18篇
  1986年   17篇
  1985年   12篇
  1984年   11篇
  1983年   14篇
  1980年   8篇
  1979年   16篇
  1978年   5篇
  1976年   6篇
  1974年   12篇
  1973年   14篇
  1972年   7篇
  1971年   9篇
  1970年   11篇
  1969年   5篇
  1968年   11篇
排序方式: 共有1062条查询结果,搜索用时 15 毫秒
71.
Eight Danish Holstein cows were milked with a 1-mm thick specially designed soft liner on their right rear teat and a standard liner mounted under extra high tension on their left rear teat. Four of the animals were overmilked for 5 min. Rear teats were subjected to ultrasound examination on the first day and to infrared thermography on the second day. Teats were submersed in ethanol 20 min post-milking on the second day. Ultrasonography measurements showed that teat canal length increased by 30–41% during milking. Twenty minutes after milking, teats milked with modified standard liners still had elongated teat canals while teats milked with the soft liner were normalized. Overmilking tended to increase teat wall thickness. Approximately 80% of variability in teat canal length, from before teat preparation to after milking, could be explained by changes during teat preparation. Thermography indicated a general drop in teat temperature during teat preparation. Teat temperature increased during milking and continued to increase until the ethanol challenge induced a significant drop. Temperatures approached pre-challenge rather than pre-milking temperatures within 10 minutes after challenge. Teat temperatures were dependent on type of liner. Mid-teat temperatures post-challenge relative to pre-teat preparation were dependent on overmilking. Thermography and ultrasound were considered useful methods to indirectly and non invasively evaluate teat tissue integrity.  相似文献   
72.
73.
Various physiological and biochemical characters of a leaf change with stages of its ontogeny. It is likely that the photosynthetic functions of leaves of different ontogeny have different levels of heat tolerance. This study was initiated to analyze the photosynthetic heat tolerance of fully-developed, nearly-developed (more than 2/3 expanded) and developing (10–12 cm visible) leaves of two maize genotypes, F223 and F250. The results indicate that the photosynthetic CO2 assimilation rate (Pn) of developing leaves was less affected by heat stress (42°C in the dark for 90 min) than that of developed leaves. The impaired Pn recovered within 24 h in the developing leaves, while the Pn of developed and nearly-developed leaves did not reach the non-stress level, even after 72 h. The Pn of the developed leaves of genotype F250 was less affected by heat stress than that of genotype F223. After heat stress, the slightly affected Pn of the developing leaf was associated with the almost unchanged photochemical efficiency of photosystem II (Fv/Fm) and the quantum yield of photosystem II electron transport. The chlorophylls a and b were degraded by heat stress; the degradation was pronounced in the developed leaves. As a result of heat stress, the antheraxanthin and zeaxanthin of the xanthophyll cycle accumulated in both the nearly-developed and developed leaves but not in the developing leaves. Injury to the plasma membrane due to heat stress was much less severe in developing leaves than that in the developed leaves. From the physiological characters which we determined it would appear that the Pn functions of the developing leaves are more resistant to heat stress than those of nearly-developed and developed leaves.  相似文献   
74.
Mutations in the low-density lipoprotein receptor-related protein 5 gene (LRP5) cause autosomal recessive osteoporosis-pseudoglioma syndrome (OPPG). We sequenced the coding exons of LRP5 in 37 probands suspected of having OPPG on the basis of the co-occurrence of severe congenital or childhood-onset visual impairment with bone fragility or osteoporosis recognized by young adulthood. We found two putative mutant alleles in 26 probands, only one mutant allele in 4 probands, and no mutant alleles in 7 probands. Looking for digenic inheritance, we sequenced the genes encoding the functionally related receptor LRP6, an LRP5 coreceptor FZD4, and an LRP5 ligand, NDP, in the four probands with one mutant allele, and, looking for locus heterogeneity, we sequenced FZD4 and NDP in the seven probands with no mutations, but we found no additional mutations. When we compared clinical features between probands with and without LRP5 mutations, we found no difference in the severity of skeletal disease, prevalence of cognitive impairment, or family history of consanguinity. However, four of the seven probands without detectable mutations had eye pathology that differed from pathology previously described for OPPG. Since many LRP5 mutations are missense changes, to differentiate between a disease-causing mutation and a benign variant, we measured the ability of wild-type and mutant LRP5 to transduce Wnt and Norrin signal ex vivo. Each of the seven OPPG mutations tested, had reduced signal transduction compared with wild-type mutations. These results indicate that early bilateral vitreoretinal eye pathology coupled with skeletal fragility is a strong predictor of LRP5 mutation and that mutations in LRP5 cause OPPG by impairing Wnt and Norrin signal transduction.  相似文献   
75.

Background

In patients with genotype 1 chronic hepatitis C infection, telaprevir (TVR) in combination with peginterferon and ribavirin (PR) significantly increased sustained virologic response (SVR) rates compared with PR alone. However, genotypic changes could be observed in TVR-treated patients who did not achieve an SVR.

Methods

Population sequence analysis of the NS3•4A region was performed in patients who did not achieve SVR with TVR-based treatment.

Results

Resistant variants were observed after treatment with a telaprevir-based regimen in 12% of treatment-naïve patients (ADVANCE; T12PR arm), 6% of prior relapsers, 24% of prior partial responders, and 51% of prior null responder patients (REALIZE, T12PR48 arms). NS3 protease variants V36M, R155K, and V36M+R155K emerged frequently in patients with genotype 1a and V36A, T54A, and A156S/T in patients with genotype 1b. Lower-level resistance to telaprevir was conferred by V36A/M, T54A/S, R155K/T, and A156S variants; and higher-level resistance to telaprevir was conferred by A156T and V36M+R155K variants. Virologic failure during telaprevir treatment was more common in patients with genotype 1a and in prior PR nonresponder patients and was associated with higher-level telaprevir-resistant variants. Relapse was usually associated with wild-type or lower-level resistant variants. After treatment, viral populations were wild-type with a median time of 10 months for genotype 1a and 3 weeks for genotype 1b patients.

Conclusions

A consistent, subtype-dependent resistance profile was observed in patients who did not achieve an SVR with telaprevir-based treatment. The primary role of TVR is to inhibit wild-type virus and variants with lower-levels of resistance to telaprevir. The complementary role of PR is to clear any remaining telaprevir-resistant variants, especially higher-level telaprevir-resistant variants. Resistant variants are detectable in most patients who fail to achieve SVR, but their levels decline over time after treatment.  相似文献   
76.
Quorum sensing in Staphylococcus aureus biofilms   总被引:7,自引:0,他引:7       下载免费PDF全文
  相似文献   
77.
A 2 to 1 matched case control study design was used to analyze herd level risk factors for Neospora caninum-associated abortion storms in 47 dairy herds. Data were obtained using a questionnaire regarding the state of affairs at the farms over the 2 years prior to the abortion storm. The questionnaire included 120 variables considered to be potential risk factors for either introduction of infection or recrudescence of chronic infection. The relationship between risk factors and case control pairs was analyzed by conditional logistic regression using a three-steps procedure. In addition, cross sectional serology was used to assess the possible role of concomitant infections. The main factors that were significant in the analysis and that were considered to have potential biological relevance were the presence of dogs, the presence of poultry, and the feeding of moldy maize-silage during summer. For both the presence of dogs and the presence of poultry on the farms, a linear relationship was found between the number of animals and the assessed risk for an abortion storm. These findings suggest a possible role of these species in the transmission of N. caninum. Further evidence for such a role of dogs was the significant association between the presence of dogs and the presence of seropositive cattle in the control herds. The feeding of moldy fodder is considered to be a factor which may induce recrudescence of a latent N. caninum-infection by mycotoxins causing immune suppression. We also found some evidence for a possible influence of management practices around calving and a high prevalence of retained afterbirths. No significant association was found for herd level prevalence of antibodies to bovine viral diarrhea virus, bovine herpesvirus 1, Leptospira hardjo or Salmonella dublin.  相似文献   
78.
The effective treatment of urethral stricture remains a medical problem. The use of proinflammatory cytokines as stimuli to improve the reparative efficacy of mesenchymal stem cells (MSCs) towards damaged tissues represents an evolving field of investigation. However, the therapeutic benefits of this strategy in the treatment of urethral stricture remain unknown. Here, we enriched exosomes derived from human umbilical cord-derived MSCs pretreated with or without tumor necrosis factor alpha (TNF-α) to evaluate their therapeutic effects in an in vivo model of TGFβ1-induced urethral stricture. Male Sprague-Dawley rats received sham (saline) or TGFβ1 injections to urethral tissues followed by incisions in the urethra. Animals in the TGFβ1 injection (urethral fibrosis) cohort were subsequently injected with vehicle control, or with exosomes derived from MSCs cultured with or without TNF-α. After 4 weeks, rats underwent ultrasound evaluation and, following euthanasia, urethral tissues were harvested for histological and molecular analysis. In vitro, the effects of MSC-derived exosomes on fibroblast secretion of collagen and cytokines were studied by enzyme-linked immunosorbent assay (ELISA), quantitative real-time polymerase chain reaction (qRT-PCR), and western blot analysis. Exosomes derived from MSCs pretreated with TNF-α were more effective in suppressing urethral fibrosis and stricture than exosomes from untreated MSCs. We found that miR-146a, an anti-inflammatory miRNA, was strongly upregulated in TNF-α-stimulated MSCs and was selectively packaged into exosomes. Moreover, miR-146a-containing exosomes were taken up by fibroblasts and inhibited fibroblast activation and associated inflammatory responses, a finding that may underlie the therapeutic mechanism for suppression of urethral stricture. Inhibition of miR-146a in TNF-α-treated MSCs partially reduced antifibrotic effects and increased the release of proinflammatory factors of exosomes derived from these cells. Together these findings demonstrate that exosomes derived from TNF-α-treated MSCs are of therapeutic benefit in urethral fibrosis, suggesting that this strategy may have utility as an adjuvant therapy in the treatment of urethral stricture diseases.  相似文献   
79.
A collection of over 130 cDNA clones has been constructed in the bacterial plasmids pPH207 and pBR322 using as template the poly A+ RNA from membrane-bound polysomes of barley endosperm (cv. Sundance). Fifty four B hordein cDNA clones have been identified by cross-hybridization analysis and in vitro translation of plasmid-selected mRNAs. Hybridization of 11 of the B hordein cDNA clones to Northern blots of size-fractionated RNA indicated that the B hordein mRNA is ca. 1300 nucleotides long. One cDNA clone, pHvE-c16, has been partially sequenced and shown by comparison with C-terminal and other peptide sequences to be related to B1 hordein polypeptides. The results obtained from the analysis of the B hordein cDNA clones support the idea that the Hor 2 locus, which specifies the B hordeins, is complex and codes for a family of related mRNA species.  相似文献   
80.
A cancer microenvironment generates strong hydrogen bond network system by the positive feedback loops supporting cancer complexity and robustness. Such network functions through the AKT locus generating high entropic energy supporting cancer metastatic robustness. Charged lepton particle muon follows the rule of Bragg effect during a collision with hydrogen network in cancer cells. Muon beam dismantles hydrogen bond network in cancer by the muon-catalyzed fusion, leading to apoptosis of cancer cells. Muon induces cumulative energy appearance on the hydrogen bond network in a cancer cell with its fast decay to an electron and two neutrinos. Thus, muon beam, muonic atom, muon neutrino shower, and electrons simultaneously cause fast neutralization of the AKT hydrogen bond network by the conversion of hydrogen into deuterium or helium, inactivating the hydrogen bond networks and inducing failure of cancer complexity and robustness with the disappearance of a malignant phenotype.  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