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   403篇
  免费   18篇
  国内免费   181篇
  2020年   4篇
  2019年   5篇
  2018年   9篇
  2017年   7篇
  2016年   6篇
  2015年   10篇
  2014年   15篇
  2013年   15篇
  2012年   22篇
  2011年   16篇
  2010年   7篇
  2009年   10篇
  2008年   19篇
  2007年   30篇
  2006年   19篇
  2005年   36篇
  2004年   24篇
  2003年   20篇
  2002年   26篇
  2001年   13篇
  2000年   22篇
  1999年   31篇
  1998年   17篇
  1997年   14篇
  1996年   13篇
  1995年   19篇
  1994年   6篇
  1993年   8篇
  1992年   9篇
  1991年   9篇
  1990年   11篇
  1989年   10篇
  1988年   5篇
  1987年   4篇
  1986年   6篇
  1984年   3篇
  1983年   3篇
  1982年   4篇
  1981年   6篇
  1979年   6篇
  1978年   10篇
  1977年   13篇
  1975年   5篇
  1974年   4篇
  1960年   3篇
  1957年   9篇
  1956年   3篇
  1954年   4篇
  1951年   6篇
  1950年   4篇
排序方式: 共有602条查询结果,搜索用时 14 毫秒
551.
Temporal expression of c-kit in spermatogenesis of two grasshopper species   总被引:1,自引:0,他引:1  
Two species of grasshoppers, Calliptamus abbreviatus (Ikonn.) and Shirakiacris shirakii (I. Bol.), were collected randomly in the Siping area of Jilin Province, China. By using immunohistochemical methods and statistical analysis, we observed and compared the temporal expression of c-kit protein in four representative stages of spermatogenesis of the two grasshoppers, namely: spermatogonia; primary spermatocyte; secondary spermatocyte; and mature sperm. Results showed that there was c-kit positive temporal expression at each stage of spermatogenesis, but there were different positive expression levels: (i) weak positive expression of c-kit protein appeared in spermatogonia and the positive granules were thinner; (ii) strong positive expression of c-kit protein existed in primary spermatocyte and positive granules became biggest among all developmental stages; (iii) c-kit positive expression stayed stronger in secondary spermatocyte while positive granules became thinner; (iv) there was a strong positive expression of c-kit and thinner positive granules in mature sperm, which distributed on head and tail; (v) the biggest c-kit positive granules had been found massing at the end of spermary; and (vi) significant differences of c-kit positive expression existed in spermatogenesis between two species of grasshoppers. The results indicated that c-kit protein may play a crucial role in spermatogenesis and even retain the physiological action of sperms and fertilization in grasshoppers.  相似文献   
552.
能够把基因功能作为生物体表现来观察的改变了基因的小鼠。即使在基因组信息泛滥的今天,这也是最重要的工具之一,基本原理的研究虽然很早,但目前仍在进行改良法的开发工作。本文由基因控制株式会社的三谷匡董事予以阐述。[编者按]  相似文献   
553.
Kim JW  Kim LU  Kim CK  Cho BH  Kim OY 《Biomacromolecules》2006,7(1):154-160
Many dental restorative dental composites still utilize 2,2-bis[4-(2-hydroxy-3-methacryloyloxy propoxy) phenyl] propane (Bis-GMA) as base resin. The high viscosity of Bis-GMA necessitates dilution with dimethacrylate ethers of low viscosity such as triethylene glycol dimethacrylate (TEGDMA). However, increased amounts of the TEGDMA have adverse effects on properties such as water uptake and curing shrinkage. The viscosity of the base resin should be as low as possible to enable the preparation of dental composites with a minimum content of diluent. To overcome the disadvantage of Bis-GMA, i.e., its high viscosity caused by hydrogen bonding between hydroxyl groups, 2,2-bis[4-(2-methoxy-3-methacryloyloxy propoxy) phenyl propane (Bis-M-GMA) was prepared by substituting methoxy groups for hydroxyl groups in Bis-GMA. The viscosity of Bis-GMA was dramatically decreased from 574 (Pa.s) to 3.7 (Pa.s) by substitution of methoxy group. Consequently, the amount of TEGDMA included in the resin matrix could be minimized. Dental composites were prepared from Bis-M-GMA (or Bis-GMA) mixtures with TEGDMA filled with 75 wt % filler. Comparing the curing shrinkage of dental composite containing Bis-M-GMA with that prepared from Bis-GMA, the reduction in curing shrinkage was about 47%. Dental composites prepared from new resin matrixes also exhibited low water uptake and better properties in mechanical strength.  相似文献   
554.
极端干旱区胡杨根系吸水的二维数学模型   总被引:3,自引:0,他引:3  
依据在极端干旱区额济纳旗的实测资料,对胡杨根系及根区土壤水分特征进行了分析,借鉴国内外成熟的植物根系吸水模型,建立了胡杨根系吸水的二维数学模型。模型结果与实测结果平均相对误差为9.02%,最大相对误差为17.84%。所建模型基本反映了胡杨根系吸水的实际状况,对胡杨根系分布、吸水及胡杨SPAC系统进一步的研究提供了有益的参考。  相似文献   
555.
根据微卫星标记在相近物种中具有高度保守性的特点,采用比较基因组学方法,初步探讨了3个与绵羊、山羊产羔率紧密相关的微卫星标记OarHH35、OarHH55和BM1329在4代85只乐至黑山羊中与其产羔数的相关性.结果显示微卫星标记BM1329在乐至黑山羊中可检测到的等位基因数为3,其片段大小分别为125、147和300 bp;标记OarHH55可检测到的等位基因数为2,片段大小分别为106和181 bp;标记OarHH35可检测到的等位基因数为3,片段大小分别为88、112和175 bp.统计分析表明,BM1329的300 bp等位基因和OarHH35的112 bp等位基因对乐至黑山羊产羔数呈正效应(P<0.05),而BM1329的147 bp等位基因和OarHH35的175 bp等位基因对产羔数呈负效应(P<0.05).因此,微卫星标记OarHH35和BM1329可作为乐至黑山羊高繁殖性能候选标记进行深入研究.  相似文献   
556.
用单细胞阳离子测定系统研究了SeO2-3对巨噬细胞内游离Ca2+和Mg2+的影响.实验结果表明:SeO2-3高于10-4mol/L时,有显著的细胞毒性.SeO2-3对细胞的毒性作用使细胞内游离Ca2+和Mg2+的浓度升高但Ca2+浓度的升高速率比Mg2+快.还有,高于10-4mol/L的SeO2-3对红细胞膜上的Ca2+-ATP酶活性有明显抑制作用.  相似文献   
557.
目的:探讨妊娠合并恶性肿瘤在诊治过程中的临床表现,探讨其对妊娠结局的影响及预后.方法:回顾性查阅2005年至2011年9月于我院住院治疗患者病历,收集妊娠合并恶性肿瘤疾病患者10例在入院诊断、治疗经过中的相关资料.结果:10例患者中9例在终止妊娠前确诊,3例孕妇死亡,5例在随访中,2例失访;10例胎儿中有生机儿5例,其中1例放弃,在可随访的3例新生儿生存状态良好.结论:妊娠合并恶性肿瘤疾病的临床确诊及及时治疗对妊娠结局会造成影响,妊娠结局与肿瘤分期及孕周相关,应在产前进行常见恶性肿瘤疾病的筛查,早期发现,早期干预.  相似文献   
558.
The herbal medicine Tong Luo Jiu Nao(TLJN)contains geniposide(GP)and ginsenoside Rg1 at a molar ratio of 10:1.Rg1 is the major component of another herbal medicine,panax notoginseng saponin(PNS).TLJN has been shown to strengthen brain function in humans,and in animals it improves learning and memory.We have previously shown that TLJN reduces amyloidogenic processing in Alzheimer’s disease(AD)mouse models.Together this suggests TLJN may be a potential treatment for patients with dementia.Because chronic damage of the central nervous system by formaldehyde(FA)has been presented as a risk factor for age-associated cognitive dysfunction,in the present study we investigated the protective effect of both TLJN and GP in neuron-like cells exposed to FA.FA-exposed murine N2a neuroblastoma cells were incubated with TLJN,its main ingredient GP,as well as PNS,to measure cell viability and morphology,the rate of apoptosis and expression of genes encoding Akt,FOXO3,Bcl2 and p53.The CCK-8 assay,cytoskeletal staining and flow cytometry were used to test cell viability,morphology and apoptosis,respectively.Fluorescent quantitative real-time PCR(qRT-PCR)was used to monitor changes in gene expression,and HPLC to determine the rate of FA clearance.Treatment of N2a cells with 0.09 mmol L?1 FA for 24 h significantly reduced cell viability,changed cell morphology and promoted apoptosis.Both TLJN and GP conferred neuroprotection to FA-treated N2a cells,whereas PNS,which had to be used at lower concentrations because of its toxicity,did not.Our data demonstrate that TLJN can rescue neuronal damage caused by FA and that its main ingredient,GP,has a major role in this efficacy.This presents purified GP as a drug or lead compound for the treatment of AD.  相似文献   
559.
冰冻蚀刻电子显微镜技术,在生物医学研究方面应用较多,但是在植物病毒研究方面报道极少。1985年我们对日本国小麦上的北方禾谷花叶病毒(Northern cereal mosaic virus简称NCMV)弹状病毒粒子进行了试验,获得了比较满意的结果。  相似文献   
560.
INTR0DUCTIONVascularsmoothmusclecellsp0ssessanumber0fNa transportsystems,namelyNa channels,Na -Ca' ,Na -H exchange,andNa -K -2Cl-c0-transp0rt[1].Hypothesizedr0lesf0rthesesystemsincluderegulati0n0fvasculart0ne,cellvolumeandcellproliferation.Inb0thexperimentalandhumanhypertensi0naberrationsinsm00thmusclecell,univalenti0ntransportsystemplayanimp0rtantroleinthepathogenesisandmaintenanceofhypertensi0n[2].Angiotensinc0nvertingenzyme(ACE)inhibitorsandCa2 antagonistsarewidelyusedforthetreatm…  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