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   5852篇
  免费   550篇
  国内免费   557篇
  6959篇
  2024年   28篇
  2023年   106篇
  2022年   194篇
  2021年   330篇
  2020年   227篇
  2019年   310篇
  2018年   257篇
  2017年   166篇
  2016年   291篇
  2015年   438篇
  2014年   438篇
  2013年   458篇
  2012年   518篇
  2011年   461篇
  2010年   320篇
  2009年   289篇
  2008年   319篇
  2007年   295篇
  2006年   243篇
  2005年   188篇
  2004年   170篇
  2003年   164篇
  2002年   119篇
  2001年   95篇
  2000年   65篇
  1999年   84篇
  1998年   51篇
  1997年   47篇
  1996年   43篇
  1995年   32篇
  1994年   30篇
  1993年   36篇
  1992年   31篇
  1991年   27篇
  1990年   24篇
  1989年   12篇
  1988年   17篇
  1987年   7篇
  1986年   8篇
  1985年   11篇
  1984年   4篇
  1983年   2篇
  1982年   2篇
  1981年   1篇
  1976年   1篇
排序方式: 共有6959条查询结果,搜索用时 13 毫秒
81.
在青藏高原进行了大范围的群落调查 ,研究高原的两种主要草地群落类型———高寒草甸和高寒草原的植物物种丰富度及其变化。结果表明 :(1)在 5 0个样地 2 5 0个 1m× 1m的样方中 ,共出现 2 6 7种植物 ,其中高寒草甸179种 ,高寒草原 135种。在高寒草甸 ,1m2 样方内物种数最多为 32种 ,最少的仅为 3种 ;在高寒草原 ,物种数最多为 18种 /m2 ,最少的仅为 2种 /m2 。 (2 )物种丰富度随经度和纬度的增加呈增加趋势 ;随海拔的上升呈减少趋势。对物种丰富度与环境因子之间进行逐步回归 ,发现物种丰富度与生长季降水和温暖指数呈显著正相关。 (3)物种丰富度与地上生物量呈显著正相关。  相似文献   
82.
Hu J  McCall CM  Ohta T  Xiong Y 《Nature cell biology》2004,6(10):1003-1009
Cullins assemble a potentially large number of ubiquitin ligases by binding to the RING protein ROC1 to catalyse polyubiquitination, as well as binding to various specificity factors to recruit substrates. The Cul4A gene is amplified in human breast and liver cancers, and loss-of-function of Cul4 results in the accumulation of the replication licensing factor CDT1 in Caenorhabditis elegans embryos and ultraviolet (UV)-irradiated human cells. Here, we report that human UV-damaged DNA-binding protein DDB1 associates stoichiometrically with CUL4A in vivo, and binds to an amino-terminal region in CUL4A in a manner analogous to SKP1, SOCS and BTB binding to CUL1, CUL2 and CUL3, respectively. As with SKP1-CUL1, the DDB1-CUL4A association is negatively regulated by the cullin-associated and neddylation-dissociated protein, CAND1. Recombinant DDB1 and CDT1 bind directly to each other in vitro, and ectopically expressed DDB1 bridges CDT1 to CUL4A in vivo. Silencing DDB1 prevented UV-induced rapid CDT1 degradation in vivo and CUL4A-mediated CDT1 ubiquitination in vitro. We suggest that DDB1 targets CDT1 for ubiquitination by a CUL4A-dependent ubiquitin ligase, CDL4A(DDB1), in response to UV irradiation.  相似文献   
83.
The clinical significance of lymphangiogenesis in cervical cancer remains controversial. Our aim was to investigate the correlation between lymphangiogenesis, lymphatic vessel invasion (LVI) and tumor metastasis, invasion and prognosis in squamous cell cervical cancer. Paraffin sections of 90 patients with FIGO (Fédération Internationale de Gynécologie et d'Obstétrique) Ib1-IIa squamous cell cervical cancer were stained for immunohistochemistry with a D2-40 monoclonal antibody against the carcinoembryonic antigen M2A. The lymphatic vessel density (LVD) and LVI were measured, and their relationship with the clinicopathological data was analyzed. D2-40-positive lymphatic vessels were found in 75 of the 90 patients (83.3 %). All D2-40-positive vessels were located in peritumoral areas. The mean±SD of the peritumoral LVD was 10.08±4.16. The positive rate of LVI was 32.0 % (24/75). The recurrence rate of patients with LVD >10 (62.1 %, 18/29) was significantly higher than that of patients with LVD ≤10 (34.8 %, 16/46, P = 0.021). The 5-year recurrence-free survival rate of patients with LVD >10 (41.0 %) was significantly lower than that of patients with LVD ≤10 (67.0 %, P = 0.045). Univariate analysis showed that the peritumoral LVD (≤10 vs >10) was correlated with LVI (absent vs present, P = 0.016). The peritumoral LVD and LVI showed no correlation with age, FIGO stage, tumor size, tumor grade, depth of invasion, or pelvic lymph node metastasis (all: P > 0.05). Peritumoral lymphangiogenesis was correlated with the recurrence and recurrence-free survival in patients with squamous cell cervical cancer. Examination of peritumoral LVD in these patients might therefore help to estimate the risk of recurrence.  相似文献   
84.
To explain the association of Bacillus thuringiensis (Bt) with animal feces, an ecological analysis in chickens was conducted by introducing a cry ? strain marked by production of green fluorescent protein (GFP). After feeding with the tagged Bt strains, the feces of the tested chickens were collected at different times, isolated, and the morphology of Bt was observed. It was shown that Bt strain HD-73GFP in spore form could be isolated from feces of chickens for a period of 13 d, and then it disappeared thereafter. Bt could be detected only up to day 4 (but not thereafter), when chickens were fed with vegetative cells of HD-73GFP. To confirm the source of newly isolated strains, the gfp gene was examined by polymerase chain reaction (PCR), which showed that all the isolated strains harbored the marker gene. Recent data from isolation and PCR had suggested that fecal Bt strains had originated from food. Chicken tissues were thus dissected to isolate Bt strains and to investigate whether Bt could be located in vivo. Bt was located within the duodenum in spore form. Compared to the morphology of the isolated strains at different growth times, the growth rates of all the tested Bt had little changes when passing through the digestive system to the feces. Dissection of the chickens confirmed that Bt was safe for the tested animal.  相似文献   
85.
The host–guest interaction between sulfobutylether‐β‐cyclodextrin (SBE‐β‐CD) and reserpine (RSP) is described using flow injection‐chemiluminescence (FI‐CL) and site‐directed molecular docking methods. It was found that RSP could inhibit the CL intensity produced by a luminol/SBE‐β‐CD system. The decrease in CL intensity was logarithmic over an RSP concentration range of 0.03 to 700.0 nM, giving a regression equation of ?I = 107.1lgCRES + 186.1 with a detection limit of 10 pM (3σ). The CL assay was successfully applied in the determination of RSP in injection, saliva and urine samples with recoveries in the range 93.5–106.1%. Using the proposed CL model, the binding constant (KCD‐R) and the stoichiometric ratio of SBE‐β‐CD/RSP were calculated to be 7.4 × 106 M‐1 and 1 : 1, respectively. Using molecular docking, it was confirmed that luminol binds to the small cavity of SBE‐β‐CD with a nonpolar interaction, while RSP targeted the larger cavity of SBE‐β‐CD and formed a 1 : 1 complex with hydrogen bonds. The proposed new CL method has the potential to become a powerful tool for revealing the host–guest interaction between CDs and drugs, as well as monitoring drugs with high sensitivity. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
86.
在我国腹泻患儿中发现G9型轮状病毒感染   总被引:10,自引:1,他引:10  
钱渊  关德华 《病毒学报》1994,10(3):263-267
  相似文献   
87.
88.
Three previously undescribed diterpenoids, helioscopnoids A–C, and eight known compounds were isolated from the whole plants of Euphorbia helioscopia. Their structures were established by extensive analysis of spectra and data comparison with previous literatures. Among them, compound 4 was identified as 24,24-dimethoxy-25,26,27-trinoreuphan-3β-ol with revised configurations of C-13, C-14, and C-17 (13R*, 14R*, 17R*). Cytotoxicity assays revealed that all compounds exhibited varying levels of cytotoxicity against H1975 cells, with compound 9 displaying the most potent activity, as indicated by cell viability rates of 18.13 % and 20.76 % at concentrations of 20 μM and 5 μM, respectively. This study expands the understanding of E. helioscopia terpenoids’ structural diversity and biological activities, contributing to the exploration of potential therapeutic applications.  相似文献   
89.

Background

To investigate the expression of Golgi phosphoprotein-3 (GOLPH3) in prostate cancer and determine its prognostic value.

Methods

Immunohistochemical staining for GOLPH3 was performed on tissue microarrays of 342 prostate patients. The correlation between GOLPH3 expression with its clinicopathologic factors was also analyzed in order to determine its prognostic significance.

Results

GOLPH3 expression of normal prostate tissues, benign prostate hyperplasia, high-grade prostatic intraepithelial neoplasia, and hormone-dependent prostate cancer (HDPC) did not show any statistically significant difference. In contrast, statistically significant difference was reported in moderate/intense GOLPH3 expression in cases diagnosed with HDPC and castration resistant prostate cancer (CRPC) (P < 0.0005). Moderate /intense expression of GOLPH3 was associated with androgen independence (P?=?0.012), higher Gleason score (P?=?0.017), bone metastasis (P?=?0.024), higher baseline prostate-specific antigen (PSA) (P?=?0.038), and higher PSA nadir (P?=?0.032). A significantly negative correlation was found between moderate/intense GOLPH3 expression and disease-free survival (DFS) (HR?=?0.28, P?=?0.012) and overall survival (OS) (HR?=?0.42, P?=?0.027). Univariated analysis indicated that moderate/intense GOLPH3 expression created a significantly prognostic impact in patients with CRPC. On the other hand, multivariate analysis indicated that GOLPH3 was a significantly independent prognostic factor of DFS (P?=?0.027) in all prostate cancer patients.

Conclusions

In this study, it was discovered that the overexpression of GOLPH3 is associated with the transition of prostate cancer from hormone sensitive phase to hormone refractory phase. GOLPH3 might be an important prognostic factor of DFS and OS in patients with prostate cancer. In totality, GOLPH3 could be used as a novel candidate in devising a more effective therapeutic strategy to tackle CRPC.

Virtual slides

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1452541171722856.  相似文献   
90.

Objective

Gray matter loss in the limbic structures was found in recent onset post traumatic stress disorder (PTSD) patients. In the present study, we measured regional gray matter volume in trauma survivors to verify the hypothesis that stress may cause different regional gray matter loss in trauma survivors with and without recent onset PTSD.

Method

High resolution T1-weighted magnetic resonance imaging (MRI) were obtained from coal mine flood disaster survivors with (n = 10) and without (n = 10) recent onset PTSD and 20 no trauma exposed normal controls. The voxel-based morphometry (VBM) method was used to measure the regional gray matter volume in three groups, the correlations of PTSD symptom severities with the gray matter volume in trauma survivors were also analyzed by multiple regression.

Results

Compared with normal controls, recent onset PTSD patients had smaller gray matter volume in left dorsal anterior cingulate cortex (ACC), and non PTSD subjects had smaller gray matter volume in the right pulvinar and left pallidum. The gray matter volume of the trauma survivors correlated negatively with CAPS scores in the right frontal lobe, left anterior and middle cingulate cortex, bilateral cuneus cortex, right middle occipital lobe, while in the recent onset PTSD, the gray matter volume correlated negatively with CAPS scores in bilateral superior medial frontal lobe and right ACC.

Conclusion

The present study identified gray matter loss in different regions in recent onset PTSD and non PTSD after a single prolonged trauma exposure. The gray matter volume of left dorsal ACC associated with the development of PTSD, while the gray matter volume of right pulvinar and left pallidum associated with the response to the severe stress. The atrophy of the frontal and limbic cortices predicts the symptom severities of the PTSD.  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