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   11827篇
  免费   277篇
  国内免费   1篇
  2021年   89篇
  2020年   86篇
  2019年   97篇
  2018年   237篇
  2017年   198篇
  2016年   282篇
  2015年   270篇
  2014年   354篇
  2013年   467篇
  2012年   439篇
  2011年   474篇
  2010年   295篇
  2009年   225篇
  2008年   310篇
  2007年   357篇
  2006年   352篇
  2005年   318篇
  2004年   268篇
  2003年   318篇
  2002年   283篇
  2001年   285篇
  2000年   222篇
  1999年   177篇
  1998年   89篇
  1992年   232篇
  1991年   227篇
  1990年   178篇
  1989年   218篇
  1988年   232篇
  1987年   236篇
  1986年   214篇
  1985年   239篇
  1984年   196篇
  1983年   161篇
  1980年   85篇
  1979年   231篇
  1978年   153篇
  1977年   134篇
  1976年   136篇
  1975年   165篇
  1974年   263篇
  1973年   259篇
  1972年   249篇
  1971年   241篇
  1970年   257篇
  1969年   245篇
  1968年   185篇
  1967年   185篇
  1966年   166篇
  1965年   123篇
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
991.
It can be supposed that relation between HLA-G polymorphism and sHLA-G protein expression are associated with successful embryo implantation and pregnancy maintenance. The aim of the study was the estimation specific differences in expression of sHLA-G and sHLA- class I antigens in women with reproductive failure in comparison with fertile women. The study sample enrolled 80 women, divided into 2 groups. The study group (B) enrolled 60 women with reproductive failure including 20 women with 3 recurrent spontaneous abortions in the first trimester of pregnancy (RSA), 20 women with empty sac (ES) and 20 women with 3 consecutive in-vitro fertilization failures (IVFf). The control group (C) enrolled 20 fertile women with at least 2 children. Soluble HLA- class I antigens (sHLA-I) and soluble HLA-G (sHLA-G) were determined using ELISA test kits from IBio Vendor Labolatory Medicine, Inc. HLA-G allele found in individuals in our study were identified by comparing the obtained bp sequences of exon 2., 3. and 4. with bp sequences of HLA-G antigen published at the Nolan Research Institute website. The highest concentration of sHLA-I is noted among women with HLA-G 10401 allele which differed significantly for the mean sHLA-I concentration calculated for all the remaining alleles (p<0.0001). The most prevalent alleles were: HLA-G 10101, 10102 and 10108 with sHLA-I concentrations among women bearing those alleles significantly lower in comparison to the HLA-G 10401 carriers (p<0.001). Allele 10101 and 10102 was related to the lower significantly plasma sHLA-I concentrations than 10108 allele (p<0.02). Lowest mean sHLA-G values were observed in the IVFf group with significant difference from the remaining groups (p<0.05). To conclude, sHLA-G molecules is associated to certain HLA-G alleles and imply that sHLA-G levels are under genetic control. Low concentration sHLA-G seems to be prognostically important in IVF failure.  相似文献   
992.
Thrombin activatable fibrinolysis inhibitor (TAFI) is a plasma zymogene (procarboxypeptidase B) which can decrease fibrinolysis and thus act as a haemostatic factor. TAFI is now extensively studied in many complications as well as in physiological and complicated pregnancy. The question we posed in the present study was whether TAFI antigen is present in cord blood plasma. The study group consisted of 38 parturient women, 26 primiparous and 12 multiparous with normal course of pregnancy and delivery. The cord blood was sampled from the cord vein, and the mother's blood from the antecubital vein. 3.2% sodium citrate was used as an anticoagulant. TAFIa/ai antigen was measured by ELISA method. TAFIa/ai antigen was identified in all samples of cord blood plasma. Its level was 91.50 ng/ml (range: 71.76 - 160.77 ng/ml) vs. 55.46 ng/ml (range: 39.77 - 68.54 ng/ml ) in the mother's blood, which means that the level of TAFIa/ai antigen was significantly higher in fetal blood than in maternal blood (p<0.00001). TAFIa/ai antigen is an integral component of cord blood plasma. The concentration of TAFIa/ai antigen is about two times higher in fetal blood than in maternal blood.  相似文献   
993.
The recognition, internalization and intracellular processing of antigen are the main functions of dendritic cells (DCs). In the course of these processes, DCs differentiate and acquire the ability to produce cytokines responsible for polarization of the immunological response. Therefore, vaccination with tumor antigen-loaded DCs is one of the most promising approaches to induce tumor-specific immune response. The purpose of this study was to analyze the migratory abilities, from an injection site to tumor-draining lymph nodes (tLN), of DCs applied as an anti-tumor vaccine and their capacity for immune response activation. Mouse DCs of the established JAWS II cell line transduced with EGFP gene or ex vivo bone marrow-isolated DCs (BM-DCs) stained with intravital CFDA dye were loaded with MC38 colon carcinoma tumor lysate (TAg) and then administered peritumorally to MC38 tumor-bearing C57BL/6 mice. On the first, third, fifth and seventh days after injection the tumors, tLNs and spleens were examined. The TAg-loaded DCs migrated more effectively to the tLNs than did the unloaded control DCs; however, the majority of them remained in the tumor vicinity. Immunohistological analysis of the tumor tissues demonstrated that only TAg-loaded DCs activated an immune response. Seven days after DCs vaccine administration, numerous necrotic areas and some apoptotic bodies were observed in the tumor tissue. However, the anti-MC38 tumor cytotoxic activity of spleen and tLN cells from mice treated with both TAg-loaded and unloaded DCs reached a maximum on the fifth day after DC injection. Concluding, TAg-loaded DCs migrated more efficiently to tLNs and were more effective activators of local (but not systemic) cellular immune response than were unloaded DCs. We hypothesize that only the application of TAg-loaded DCs to tumor-bearing mice as an adjuvant supporting chemotherapy may activate a more effective anti-tumor response.  相似文献   
994.
Interactive playback experiments were used to study the signal value to the corn bunting, Miliaria calandra, of alternating and overlapping singing. We subjected 15 males to two stimuli that differed in the temporal pattern of song playback (alternating or overlapping). We measured eight characteristics of the males’ response in two categories—song output and movements. Overlapping and alternating playback elicited a similar song response, characteristic of highly aroused males. Song response correlated positively with males’ singing activity before playback, irrespective of stimulus. There were significant differences between latency of approach to the loudspeaker and number of flights. Birds approached the loudspeaker more quickly and spent more time close to it when playback alternated with their songs. The results suggest overlapping song could be interpreted as a stronger threat but elicits a more cautious, rather than stronger, response than the alternating pattern. Males were found to shorten songs during the playback compared with songs sung before and after stimulation. The only predictor of degree of song shortening was song activity before the playback began. It should, therefore, be regarded as a signal which is related to escalated, close-distance counter-singing.  相似文献   
995.
Protein kinase CK2 is a highly conserved Ser/Thr protein kinase that is ubiquitous among eucaryotic organisms and appears to play an important role in many cellular functions. This enzyme in yeast has a tetrameric structure composed of two catalytic (α and/or α′) subunits and two regulatory β and β′ subunits. Previously, we have reported isolation from yeast cells four active forms of CK2, composed of αα′ββ′, α2ββ′, α′2ββ′ and a free α′-catalytic subunit. Now, we report that in Saccharomyces cerevisiae CK2 holoenzyme regulatory β subunit cannot substitute other β′ subunit and only both of them can form fully active enzymatic unit. We have examined the subunit composition of tetrameric complexes of yeast CK2 by transformation of yeast strains containing single deletion of the β or β′ regulatory subunits with vectors carrying lacking CKB1 or CKB2 genes. CK2 holoenzyme activity was restored only in cases when both of them were present in the cell. Additional, co-immunoprecypitation experiments show that polyadenylation factor Fip1 interacts with catalytic α subunits of CK2 and interaction with beta subunits in the holoenzyme decreases CK2 activity towards this protein substrate. These data may help to elucidate the role of yeast protein kinase CK2β/β′ subunits in the regulation of holoenzyme assembly and phosphotransferase activity.  相似文献   
996.
The subject of this report was to investigate headgroup hydration and mobility of two types of mixed lipid vesicles, containing nonionic surfactants; straight chain Brij 98, and polysorbat Tween 80, with the same number of oxyethylene units as Brij, but attached via a sorbitan ring to oleic acid. We used the fluorescence solvent relaxation (SR) approach for the purpose and revealed differences between the two systems. Fluorescent solvent relaxation probes (Prodan, Laurdan, Patman) were found to be localized in mixed lipid vesicles similarly as in pure phospholipid bilayers. The SR parameters (i.e. dynamic Stokes shift, Δν, and the time course of the correlation function, C(t)) of such labels are in the same range in both kinds of systems. Each type of the tested surfactants has its own impact on water organization in the bilayer headgroup region probed by Patman. Brij 98 does not modify the solvation characteristics of the dye. In contrast, Tween 80 apparently dehydrates the headgroup and decreases its mobility. The SR data measured in lipid bilayers in presence of Interferon alfa-2b reveal that this protein, a candidate for non-invasive delivery, affects the bilayer in a different way than the peptide melittin. Interferon alfa-2b binds to mixed lipid bilayers peripherally, whereas melittin is deeply inserted into lipid membranes and affects their headgroup hydration and mobility measurably.  相似文献   
997.
The effects of acute pretreatment of rats with corticosterone (5 and 20 mg/kg, s.c.) on emotional behavior, expression of c-Fos protein in brain structures, and serum concentration of corticosterone were studied to model the short-term glucocorticoid-dependent changes in brain functions. Corticosterone was administered 90 min before training of a conditioned fear reaction (a freezing response), and behavioral, hormonal and immunocytochemical effects were examined 1 day later, on the test day. Pretreatment of rats with corticosterone significantly attenuated the freezing reaction in the conditioned fear test. The effect of the corticosterone was accompanied by a selective enhancement of the aversive context-induced c-Fos expression in some brain structures: the parvocellular and magnocellular neurons of the paraventricular hypothalamic nucleus (pPVN and mPVN), the medial amygdala nucleus (MeA), and the cingulate cortex, area 1 (Cg1), as well as an increase in the concentration of aversive context-induced endogenous serum glucocorticoid, 1.5 h and 10 min after the test session, respectively. It is suggested that the behavioral effects of acute pretreatment of rats with corticosterone could be due to changes in the mnemonic processes in the brain, inhibition of brain corticotropin releasing factor (CRF) synthesis, or stimulation of GABA-A receptor modulating neurosteroids synthesis. It is hypothesized that the enhanced activity of Cg1, MeA, pPVN, and mPVN, and the hypothalamic-pituitary-adrenal axis with concomitant increased serum glucocorticoid concentration, might serve to facilitate active coping behavior in a threatening situation.  相似文献   
998.
The aim of our study was to evaluate the effects of dietary equol, metabolite of a phytoestrogen daidzein, on the secretion of prolactin (PRL) and lutenizing hormone (LH), as well as the expression of estrogen receptors (ERalpha, ERbeta and truncated estrogen receptor-1 (TERP-1) in the pituitary gland of ovariectomized (ovx) female Sprague-Dawley rats. Two doses of equol (50 mg/kg of chow and 400 mg/kg of chow) were used and the results were compared with the effects of estradiol 3-benzoate (E2B), also given at two doses (4.3 mg/kg of chow and 17.3 mg/kg of chow). Treatment period was 3 months. Dietary equol administration at the high dose increased significantly serum PRL levels. This effect was also observed in the E2B group but this difference did not reach statistical significance. Surprisingly, high dose dietary equol treatment also significantly increased serum LH levels, which was in contrast to E2B treatment where serum LH levels were significantly decreased at both doses. Serum LH levels in the equol low group were unaffected. Equol treatment had no effects on pituitary ERalpha or ERbeta gene expression. In contrast, high dose E2B treatment increased significantly pituitary ERalpha mRNA levels but decreased those of ERbeta. Both doses of E2B also increased significantly pituitary TERP-1 mRNA levels. This effect was also observed in the equol high group but at a much smaller magnitude. In conclusion, high dose dietary equol administration to ovx rats exerts estrogenic like effects on the lactotropes and anti-estrogenic on the gonadotropes.  相似文献   
999.
The distribution of rotifer communities between emergent (Typha angustifolia) and submerged (Chara tomentosa) vegetation and a comparatively open water zone were compared during the spring, summer and autumn seasons at three macrophyte-dominated lakes. This survey identified 107 rotifera species of which 58% of the taxonomical structure was common for the three examined lakes. Stoneworts with a more complicated spatial and morphological structure (having a much longer stem length than the narrow leaf cattail), supported higher rotifer densities. The stem length appeared to be the best predictor of all the macrophyte parameters and pH and chlorophyll a for the chemical variables, for explaining the variation of rotifer densities using the stepwise multiple forward regressions. The distribution of pelagic species did not differ between particular sites, which may have reflected the behavioural requirements of those rotifers. Some of them remained in the open water zone while others seeking an anti-predator refuge, gathered within macrophyte stands during the daytime. Moreover, there were nine Chara-associated species recorded and only one Typha-associated species was noted. The similarity of rotifer communities was most strongly influenced by particular habitat and season. Guest editors: S. S. S. Sarma, R. D. Gulati, R. L. Wallace, S. Nandini, H. J. Dumont & R. Rico-Martínez Advances in Rotifer Research  相似文献   
1000.
Our objective was to assess the potential of Cladocera from mountain lakes for climate reconstruction. We related Cladocera from surface sediments of Alpine lakes (1,502–2,309 m asl) to 29 abiotic environmental variables using statistical methods. The environmental dataset included water chemistry, lake depth, and bi-hourly water-temperature logs, which were used to assess mean monthly water temperatures, dates of freezing and breakup, spring and autumn mixing. We found 14 different Cladocera of the families Bosminidae, Daphniidae, and Chydoridae. Lakes without Cladocera (eight lakes) were cold and/or ultra-oligotrophic, whereas lakes with planktonic and littoral Cladocera (19 lakes) were warmer and/or less oligotrophic. Lakes with only littoral Cladocera (18 lakes) had intermediate water temperatures/trophy. Changes in Cladocera assemblages were related to changes in climate, nutrients, and/or alkalinity. We found a climate threshold at which Bosminidae disappeared in 95% of the lakes. For climate-change research, we propose studying Cladocera along transects that include climatic thresholds. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera  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