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   2595篇
  免费   22篇
  国内免费   99篇
  2716篇
  2021年   12篇
  2017年   15篇
  2016年   12篇
  2015年   14篇
  2014年   45篇
  2013年   43篇
  2012年   33篇
  2011年   36篇
  2010年   30篇
  2009年   68篇
  2008年   59篇
  2007年   63篇
  2006年   78篇
  2005年   63篇
  2004年   71篇
  2003年   55篇
  2002年   54篇
  2001年   40篇
  2000年   31篇
  1999年   44篇
  1998年   46篇
  1997年   56篇
  1996年   56篇
  1995年   66篇
  1994年   61篇
  1993年   62篇
  1992年   65篇
  1991年   55篇
  1990年   52篇
  1989年   51篇
  1988年   52篇
  1987年   49篇
  1986年   47篇
  1985年   74篇
  1984年   78篇
  1983年   20篇
  1982年   50篇
  1981年   78篇
  1980年   74篇
  1979年   66篇
  1978年   92篇
  1977年   100篇
  1976年   87篇
  1975年   21篇
  1974年   35篇
  1973年   73篇
  1972年   99篇
  1971年   69篇
  1970年   66篇
  1969年   12篇
排序方式: 共有2716条查询结果,搜索用时 9 毫秒
991.
Nitric oxide is a unique neurotransmitter, which participates in many physiological and pathological processes in the organism. Nevertheless there are little data about the neuronal Nitric Oxide Synthase immunoreactive (nNOS-ir) neurons and fibers in the dorsal claustrum (DC) of a cat. In this respect the aims of this study were: (1) to demonstrate nNOS-ir in the neurons and fibers of the DC; (2) to describe their light microscopic morphology and distribution; (3) to investigate and analyze the ultrastructure of the nNOS-ir neurons, fibers and synaptic terminals; (4) to verify whether the nNOS-ir neurons consist a specific subpopulation of claustral neurons; (5) to verify whether the nNOS-ir neurons have a specific pattern of organization throughout the DC. For demonstration of the nNOS-ir the Avidin-Biotin-Peroxidase Complex method was applied. Immunopositive for nNOS neurons and fibers were present in all parts of DC. On the light microscope level nNOS-ir neurons were different in shape and size. According to the latter they were divided into three groups-small (with diameter under 15 mum), medium-sized (with diameter from 16 to 20 mum) and large (with diameter over 21 mum). Some of nNOS-ir neurons were lightly-stained while others were darkly-stained. On the electron microscope level the immunoproduct was observed in neurons, dendrites and terminal boutons. Different types of nNOS-ir neurons differ according to their ultrastructural features. Three types of nNOS-ir synaptic boutons were found. As a conclusion we hope that the present study will contribute to a better understanding of the functioning of the DC in cat and that some of the data presented could be extrapolated to other mammals, including human.  相似文献   
992.
The family Oocystaceae (Chlorophyta) is a group of morphologically and ultrastructurally distinct green algae that constitute a well‐supported clade in the class Trebouxiophyceae. Despite the family's clear delimitation, which is based on specific cell wall features, only a few members of the Oocystaceae have been examined using data other than morphological. In previous studies of Trebouxiophyceae, after the establishment of molecular phylogeny, the taxonomic status of the family was called into question. The genus Oocystis proved to be paraphyletic and some species were excluded from Oocystaceae, while a few other species were newly redefined as members of this family. We investigated 54 strains assigned to the Oocystaceae using morphological, ultrastructural and molecular data (SSU rRNA and rbcL genes) to clarify the monophyly of and diversity within Oocystaceae. Oonephris obesa and Nephrocytium agardhianum clustered within the Chlorophyceae and thus are no longer members of the Oocystaceae. On the other hand, we transferred the coenobial Willea vilhelmii to the Oocystaceae. Our findings combined with those of previous studies resulted in the most robust definition of the family to date. The division of the family into three subfamilies and five morphological clades was suggested. Taxonomical adjustments in the genera Neglectella, Oocystidium, Oocystis, and Ooplanctella were established based on congruent molecular and morphological data. We expect further taxonomical changes in the genera Crucigeniella, Eremosphaera, Franceia, Lagerheimia, Oocystis, and Willea in the future.  相似文献   
993.
皱纹盘鲍肾脏的组织化学和超微结构   总被引:2,自引:0,他引:2  
以组织学、组织化学和透射电镜观察等方法研究了皱纹盘鲍(Haliotis discus hannai)的肾脏。肾脏由无数肾小管和集合管组成。肾小管上皮细胞游离面有明显的刷状缘,并有少量纤毛,细胞顶端存在大量直径为0.8-1.8μm的折光小体,组化研究显示小体内含铁。基底端质膜高度内折,并呈碱性磷酸酶活性。上皮细胞与血窦仅有基底膜相隔,细胞内最具特征性的结构是“膜囊体”,光镜下它呈嗜酸性团块,组化研究显示含丰富的蛋白质,电镜下观察它由双层单位膜围成的众多的小管及扁囊构成,小管的直径或扁囊的高度为30-80nm,某一局部的小管或扁囊的形状及排列很有规律性。“膜囊体”的性质尚不清楚。除纤毛数量增加和“膜囊体”较小一局部的小管或扁囊的形状主排列很有规律性。“膜囊体”的性质尚不清楚。除纤毛数量增加和“膜囊体”较小外,集合管上皮细胞的组织化学和超微结构类似于肾小管上皮细胞。在集合管和腔上皮还存在少量的粘液细胞。  相似文献   
994.
利用透射电镜研究了安得爱胜蚓(Eisenia anderi)精子的超微结构及其形成过程,精子呈纤毛状,由顶体,核,中段和鞭毛四部分组成,全长92μm左右,储精囊中可见成束的各种发育阶段的生精细胞。文中详细描述了从精子细胞到成熟精子的精子形成过程。  相似文献   
995.
996.
菲律宾蛤仔的精子发生和精子超微结构   总被引:12,自引:0,他引:12  
用透射电镜研究了菲律宾蛤仔(Ruditapes philippinarum)精子结构和精子发生过程中细胞形态结构的变化及细胞器的演变规律。菲律宾蛤仔雄性生殖细胞的形态由椭圆形渐变为辣椒状,细胞核的形态由椭圆形逐渐拉长,渐变为锥形。染色质的凝集经历:小颗粒团块状一较大颗粒均匀状一粗颗粒均匀状的过程。线粒体在演化过程中数量先增多后逐渐减少,嵴数逐渐增多,电子密度和体积逐渐增大。高尔基体在初级精母细胞期已经发育,随后的各期中发育良好,分泌旺盛。精细胞Ⅱ期,高尔基体分泌的潴泡开始融合,形成前顶体囊。精细胞Ⅲ期,高尔基体的分泌物仍不断融合。精细胞分化的后期,前顶体囊逐渐发育形成顶体。菲律宾蛤仔成熟精子呈长辣椒状,为原生型,由头部、中段和尾部构成。头部的顶体为细长柱形,末端渐细,电子密度较小;细胞核为锥形;中段线粒体4个,尾部鞭毛为典型的“9 2”型结构。此外在成熟精子线粒体环横切面有一特殊“风车状”结构。  相似文献   
997.
铅在日本沼虾体内的分布和积累   总被引:2,自引:1,他引:1  
应用组织化学、透射电镜和原子吸收光谱分析等方法,研究了Pb在日本沼虾各主要器官和组织中的分布和积累情况。结果表明,在浓度为0.625mg·L^-1Pb溶液暴露10d后的日本沼虾触角腺内,具有大量的电子密度较高的Pb颗粒.在电镜下细胞内的溶酶体中沉积有大量的Pb颗粒,这些Pb通过积聚,在细胞顶端部位逐渐增多,从而出现外排现象.中肠细胞内含有Pb颗粒,细胞质出现空泡化,核膜和线粒体内嵴部分解体.肝胰脏细胞内除分布有少量Pb颗粒外,细胞结构基本完整.在沼虾鳃细胞内未发现Pb颗粒,但在鳃丝之间发现少量Pb颗粒吸附在鳃丝的表面原子吸收光谱分析结果表明,触角腺中Pb含量最高,达637.6mg·kg^-1;触角腺在Pb的解毒方面起着重要作用。  相似文献   
998.
以被甲栅藻(Scenedesmusarmatus)为材料研究极高浓度CO2对其生理活性和细胞结构的影响。研究表明,被甲栅藻能在60%的CO2浓度下快速生长,在5%、20%、40%、60%、80%、100%CO2浓度下的平均增长率分别是1.228、0.925、0.741、0.305、0.042、0.001g·L-1·d-1DW。通入极高浓度CO2(20%、40%)后,被甲栅藻细胞的光系统Ⅱ(PSⅡ)最大光化学效率(Fv/Fm)在24h内明显下降,对PSⅡ抑制作用较明显;其后,随培养时间的增长而逐渐恢复正常。显微结构和亚显微结构显示极高CO2浓度下培养了6d的藻细胞体积稍膨大、颗粒化,色素体结构相对不完整,类囊体膜结构略显松散,蛋白核消失,细胞内的液泡数目增多。  相似文献   
999.
In most ascidian species the oocytes are surrounded by two types of accessory cells called follicle cells and test cells. Test cells are located on the periphery of oocytes and remain in the perivitelline space during egg development until hatching. Heparin and histamine were previously described in the test cells of the ascidian Styela plicata. In the present study, electron microscopy techniques were used to characterize the ultrastructure of the S. plicata test cells and to localize heparin and histamine in these cells. Test cells contain several intracellular granules with unique ultrastructural features. They are formed by elongated filaments composed of serial globules with an electron-lucent circle, containing a central electron-dense spot. Immunocytochemistry showed that heparin and histamine colocalize at the border of granule filaments in the test cell. Compound 48/80, a potent secretagogue of heparin-containing mast cells, also induced degranulation of test cells. According to these results, we suggest that test cells represent ancient effector cells of the innate immunity in primitive chordates.  相似文献   
1000.
苯并(a)芘对大弹涂鱼肝细胞超微结构的影响   总被引:13,自引:0,他引:13  
在实验生态条件下,研究不同浓度苯并(a)芘(BaP)暴露下大弹涂鱼肝脏细胞超微结构的变化。结果表明,暴露于低浓度(0.5mg·L-1)BaP 7d,大弹涂鱼肝脏细胞内的细胞器受到不同程度的损伤,其中线粒体和内质网是受BaP暴露影响最明显的细胞器,细胞核也受到不同程度的影响,细胞质中脂滴也增加;而暴露于高浓度(5mg·L-1)BaP 2h,不仅是线粒体和内质网,几乎所有细胞器都受到严重影响,细胞器严重退化,细胞结构遭到严重破坏。研究结果证实,BaP可对大弹涂鱼肝细胞内多种细胞器造成损伤,并且BaP浓度越高,损伤程度越严重。  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