小麦小孢子发生过程的超微结构

采用透镜电镜技术对小麦（Ｔｒｉｔｉｃｕｍ ａｅｓｔｉｖｕｍ）小孢子发生过程进行了超微结构观察。造孢细胞时期细胞质中含有丰富的核糖体,质体和线粒体。在小孢子母细胞减数分裂过程中,有核糖体数量存在逐渐减少的现象,质体和线袜体结构在双线期／终变期和中期１简化,二分体时期,质体和线粒体形态结构基本恢复正常。结果表明,啵麦小孢子母细胞减数分裂过程中存在质体和线粒体的脱人化与再分化过程,但该过程与核糖体数量增     

麻疯树小孢子发育的研究

用透射电镜观察了麻疯树(Jatropha curcas L.)小孢子发育的超微结构。小孢子母细胞时期内质网和质体较多;减数分裂和四分体时期,细胞处于明显的代谢活跃状态,细胞器丰富,主要有内质网、线粒体、质体、高尔基体和球状体;在小孢子发育早期和晚期,线粒体和内质网仍较丰富;小孢子经过高度的不对称分裂后,形成较大的营养细胞和较小的生殖细胞,营养细胞中细胞器数量明显减少,含大量的淀粉和脂类物质,生殖细胞中脂类物质丰富;表皮、药室内壁和中层细胞在小孢子母细胞和四分体时期淀粉粒丰富,小孢子时期明显减少,绒毡层从小孢子母细胞至小孢子发育晚期的细胞器都很丰富,主要为内质网、质体和线粒体,为二胞花粉发育奠定基础。     

牡丹小孢子发生与雄配子体发育的超微结构研究

牡丹（Paeonia suffruticosa Andr.）花粉母细胞在减数分裂前期Ⅰ出现核液泡,其具有消化和转移细胞核中降解产物的功能。细胞发生了规律性的变化：前期Ⅰ,核糖体数量减少,质体、线粒体结构简化;末期Ⅰ和前期Ⅱ,出现细胞器带,四分体时期,细胞器分散开,结构较清晰,核糖体密度最大。小孢子时期,各结构简化,数量减少,至成熟二胞花粉时,细胞器丰富,结构恢复清晰。牡丹生殖细胞初期具壁,游离在营养细胞质内后壁消失,始终不含质体。花粉成熟时,生殖细胞和营养构成“雄性生殖单位“（MGU）。     

大葱小孢子母细胞至二胞早期花粉发育的超微结构观察

用电镜观察了章丘大葱 (AlliumfistulosumL .)从小孢子母细胞至二胞早期花粉发育的超微结构。终变期的花粉母细胞 ,胼胝壁外方的相邻初生壁间及胞间隙内 ,存在胞间物质 ,四分体期 ,此物质尚部分存在。小孢子母细胞减数分裂前 ,细胞质内含有脂滴 ,小孢子有丝分裂以后 ,脂滴增多增大。小孢子分裂后期 ,质体已积累淀粉粒 1至多个。二胞早期花粉之营养细胞质内 ,有些含淀粉质体亦含脂滴。各发育期 ,核糖体及多聚合糖体丰富 ,并有很多的粗面内质网、高尔基体及小泡、线粒体 ,显示蛋白质、糖类及其它物质合成及运输作用的活跃。小孢子缺中央大液泡。有丝分裂后期 ,细胞器集中于未来的营养细胞极。小孢子胞质分裂期 ,有些内质网贴近或与花粉质膜相连 ,它们或有可能互相融合 ,扩大质膜面积而适应花粉的生长。还讨论了不同时期高尔基体小泡的作用。     

枸杞花粉发育的超微结构变化

用JEM-100CXⅡ透射电子显微镜对宁夏枸杞(Ly cium barbarum L.)花粉发育过程做了超微结构观察.结果表明,在花粉母细胞细线和偶线期,核糖体数量减少、线粒体结构简化;之后核糖体数量逐渐回升,后期Ⅰ线粒体结构恢复正常.小孢子液泡化过程中核糖体再次减少,同时线粒体和质体结构简化;在早期二胞花粉中,核糖体数量增加、线粒体和质体结构再度分化.花粉母细胞和小孢子发育过程中都存在“细胞质改组”现象,且这两次“细胞质改组”与细胞功能的转变密切相关.在次生造孢细胞、花粉母细胞和小孢子后期都有液泡数量明显增加或体积增大的过程.前两次液泡的增加可能参与了其胼胝质壁的构建;而小孢子晚期中的大液泡则创造了一种极性为其不等分裂作好了生理和结构上的准备.     

西瓜小孢子发育过程中几种细胞器的变化

用透射电子显微镜观察了西瓜（Ｃｉｔｒｕｌｌｕｓ ｖｕｌｇａｒｉｓ Ｓｃｈｒａｄ．）小孢子发育过程中核糖体、线粒体、质体、内质网、高尔基体等细胞器的变化。核糖体和线粒体变化都有明显的规律性：核糖体密度在四分体时期高,液泡化时期降低,液泡化结束后再回升;线粒体经历一个脱分化与再分化周期,液泡化时期该细胞器数量及其内嵴数目减少而脱分化,液泡化结束后形状多样化、内嵴重新增多而再分化;质体结构不变,但体积变     

桔梗小孢子发生和雄配子体发育过程超微结构变化

对桔梗小孢子发生和雄配子体发育过程超微结构的观察表明,减数分裂过程中发生第一次细胞质改组,表现为：粗线期／双线期,核糖体数量消减,质体和线粒体结构简化;末期Ｉ,核糖体数量恢复,四分体时期,质体和线粒体获得正常结构。单核靠边期到二细胞花粉时期,发生第二次细胞质改组,表现同第一次细胞入组相。两次核糖体消减过程,均涉及到粗面内质网的。在和分前和有丝分裂前期存在核周腔膨大形成核液泡现象。结果表明,核糖体数     

贮藏洋葱抽苔时鳞片叶表皮细胞的细胞学变化

利用透射电镜观察了洋葱抽苔时其鳞片叶表皮细胞的亚显微结构变化。幼嫩鳞片叶表皮细胞结构正常：液泡在细胞中央,细胞质在靠近细胞壁的边缘;细胞质中富含质体、线粒体和核糖体等细胞器;胞间连丝直径约为５０ｎｍ。伴随着细胞的衰退,细胞质变得松散,在液泡中出现大量絮状物,细胞器逐渐解体。少数胞间连丝直径扩大,达到８０ｎｍ左右,它可能在大分子胞间转移中起重要作用。在衰老细胞中,核和质体已解体但多数胞间连丝仍维持正常状态。     

棉花小孢子发生过程中细胞质的超微结构变化：着重“细胞质改组”问题

棉花(Gossypium hirsutum L.)小孢子发生过程中,细胞质超微结构发生显著而有规律的变化。这些变化主要涉及细胞质中核糖体、质体和线粒体。减数分裂前期Ⅰ,细胞质中核糖体密度逐渐降低,质体和线粒体结构变得不明显。粗线期至双线期,细胞质中核糖体密度降至极低水平,同时质体和线粒体呈衰退结构状态。中期Ⅰ,细胞质中核糖体恢复致密,质体和线粒体也恢复了正常的形态和结构。来自细胞核的类核仁进入细胞质并扩散。这是恢复中期Ⅰ细胞质中核糖体密度的主要原因。内质网在核糖体数量变化中显示出有重要作用。这些细胞质超微结构的变化可认为与世代转变有关。     

芡实绒毡层细胞发育的超微结构变化

芡实（ Ｅｕｒｙａｌｅｆｅｒｏｘ Ｓａｌｉｓｂ） 绒毡层细胞在小孢子母细胞时期, 质体出现明显的变形期,细胞中二核常相互贴近或呈嵌合状态, 细胞壁间层中胞间连丝发达。减数分裂期, 绒毡层细胞壁融解消失, 胞间连丝断离, 细胞间发育出现不同步现象。质体开始积累淀粉, 部分质体呈空泡状, 并出现质体膜内陷, 这与液泡具相似的功能。四分体时期, 绒毡层细胞内部结构开始解体。单核小孢子时期, 绒毡层细胞解体消失, 使小孢子后期发育的营养来源受到影响,作者认为这是生产上成熟花粉囊中花粉粒少而且发育不正常的主要原因之一。     

Ultrastructural Studies of Somatic Embryogenesis in Wheat

Electron microscopic observation of somatic embryogenesis from cultured immature wheat embryos revealed the presence of a lot of small vacuoles, a large nucleus, clear nucleolus and polynucleoli. The electron density of cytoplasm was strengthened during somatic embryogenesis. Quantity and type of organelles—plastid, ribosome and mitochondrion were increased; thickened cell wall, disappeared plasmodesmata, increased organelles andstarch accumulation in the embryogenic cells. Nucleolus vacuoles, autophagic vacuoles and secretory vesicles were present in the embryogenic cells with thickened cell walls. The multicellular proembryos, globular embryoid and pear-shaped embryoid were surrounded by an envelop, but plasmodesmata existed extensively between cells of somatic embryoid. The membranous structures appeared in the plastid which underwent transformation into chloroplast in the cells of growing point in almost mature embryoid. The relation of the above-mentioned structureal changes to somatic embryogenesis is also discussed.     

Ultrastructure and function of mitochondria in gametocytic stage of Plasmodium falciparum

Morphological properties of the mitochondrial organelles in the asexual and sexual gametocytic stages of Plasmodium falciparum have been analyzed and found to be markedly different. From in vitro cultures of both stages in human erythrocytes, it has been demonstrated that the asexual stages contained a defined double-membrane organelle having a few tubular-like cristae. The numbers of mitochondria in the gametocytes were found to be approximately 6 organelles per parasite, and they showed a greater density of the cristae than that of the asexual stage parasite. The organelles of the gametocytes were successfully purified by differential centrifugation following Percoll density gradient separation with the results of approximately 7% yields and approximately 5 folds. The gametocytic organelles contained much more activities of mitochondrial electron transporting enzymes (i.e., cytochrome c reductase, cytochrome c oxidase) than the asexual stage organelles. Mitochondrial function as measured by oxygen consumption were found to be different between these two stages organelles. Their rates of oxygen consumption were relatively low, as compared to those of human leukocyte and mouse liver mitochondria. In contrast to the coupled mammalian mitochondria, the gametocytic organelles were in the uncoupling state between oxidation and phosphorylation reactions during their respiration. However, they were sensitive to inhibitors of the electron transport system, e.g., antimycin A, cyanide. Our results suggest that the mitochondria of the gametocytic stages are metabolically active and still underdeveloped, although their inner membranes are extensively folded. The biochemical significance of the unique structure of the mitochondria in these developing stages in host erythrocytes remains to be elucidated.     

On the rate of ribosome translocation anisotropy and ribosome saturation in the polysome

This study examines the rate of ribosome translocation in the mammalian polysome engaged in protein synthesis by utilizing our knowledge of the hydrodynamic behavior of the rat liver polysomes, sedimenting in a linear sucrose density gradient. The average distance between adjacent ribosomes in the polysome was estimated assuming an extended linear configuration of the polysomes during sedimentation. Based on this estimate, the velocity of ribosome movement along the messenger RNA appears to be non-uniform and inversely related to the ribosome content of the polysome. Such non-uniformity prevails at stages of translation prior to ribosome “saturation” of the polysome. A correlation has been made between the results reported herein and previously published evidence on the rate of polypeptide chain synthesis. The steady-state condition for the polypeptide chain assembly is viewed as representing the state of ribosome “saturation”, characterized by a minimal ribosome velocity and a maximum density of ribosome distribution, both functions being uniform throughout the entire length of the polysome.     

Pollen Ontogeny in Catananche caerulea L. (Compositae: Lactuceae) I. Premeiotic Phase to Establishment of Tetrads

The SEM has been used to observe microsporogenesis in Catananchecaerulea (Compositae: Lactuceae) from the time of mother cellformation to the early tetrad stage. The three main ontogeneticfeatures discussed are: intercellular cytoplasmic connectionsbetween meiocytes, the deposition of the callose special cellwall and changes in cytoplasmic organelles that are associatedwith the cycle of ribosome elimination occurring during thetransition to the gametophyte generation. The results are comparedwith other flowering plants and with members of tribe Lactuceaethat have morphologically different mature pollen grains. Catananche caerulea, Compositae: Lactuceae, meiosis, microsporogenesis, ontogeny, scanning electron microscopy, special cell wall     

文冠果果实韧皮部及其周围薄壁细胞的超微结构观察及功能分析

该研究应用透射电镜技术,对生长发育过程中的文冠果果实的韧皮部及其周围薄壁细胞的超微结构进行了观察,以探讨文冠果果实同化物韧皮部卸载的细胞学路径及其机理。结果显示:(1)文冠果果实发育过程中,筛分子细胞胞腔较空,几乎没有细胞器,但有类似于囊泡的丝状不定型物存在;伴胞胞质浓密且细胞器丰富,液泡化程度不一,大多数存在多个小液泡;薄壁细胞具有中央大液泡,发育中期富含线粒体、高尔基体、内质网等细胞器,并存在囊泡运输现象,发育后期细胞器发生降解,说明随着果实生长发育,果实内物质代谢和转运活跃程度逐渐下降。(2)果实发育过程中筛分子和伴胞之间始终有胞间连丝,薄壁细胞之间也一直存在大量的胞间连丝,而筛分子-伴胞复合体与薄壁细胞之间只有在果实发育前期和后期存在一定数量的胞间连丝,发育中期却几乎没有胞间连丝。研究结果表明,文冠果果实发育过程中同化物韧皮部卸载路径可能发生了共质体途径-质外体途径-共质体途径的转变。     

Ribosome Production and Protein Synthesis in the Preimplantation Rabbit Embryo

Biochemical and radioautographic data show that protein synthesis is increased markedly at the morula stage of rabbit development (60 h embryo). In the late morula an increase in cytoplasmic ribosomes is observed, suggesting that ribosome availability may be rate-limiting for protein synthesis during cleavage. Incorporated ³H-amino acids become highly localized within the nucleoli of late morulae which have been pulse-labelled for 10 min. This localization suggests that ribosomal protein synthesis is increased at the same time as ribosomal RNA synthesis has been shown to increase. Changes in both the incorporation of ³H-amino acids and cytoplasmic ribosome density were found to occur 'synchronously' in all embryonic cells during the cleavage and early blastocyst period (84 h of development). Between 84 h and 108 h, considerable differences in the number of ribosomes per unit area of cytoplasm become apparent among the cells of the blastocyst.     

Ribosome profiling reveals sequence-independent post-initiation pausing as a signature of translation

The journey of a newly synthesized polypeptide starts in the peptidyltransferase center of the ribosome, from where it traverses the exit tunnel. The interior of the ribosome exit tunnel is neither straight nor smooth. How the ribosome dynamics in vivo is influenced by the exit tunnel is poorly understood. Genome-wide ribosome profiling in mammalian cells reveals elevated ribosome density at the start codon and surprisingly the downstream 5th codon position as well. We found that the highly focused ribosomal pausing shortly after initiation is attributed to the geometry of the exit tunnel, as deletion of the loop region from ribosome protein L4 diminishes translational pausing at the 5th codon position. Unexpectedly, the ribosome variant undergoes translational abandonment shortly after initiation, suggesting that there exists an obligatory step between initiation and elongation commitment. We propose that the post-initiation pausing of ribosomes represents an inherent signature of the translation machinery to ensure productive translation.     

Structure of the mammalian 80S ribosome at 8.7 A resolution

In this paper, we present a structure of the mammalian ribosome determined at approximately 8.7 A resolution by electron cryomicroscopy and single-particle methods. A model of the ribosome was created by docking homology models of subunit rRNAs and conserved proteins into the density map. We then modeled expansion segments in the subunit rRNAs and found unclaimed density for approximately 20 proteins. In general, many conserved proteins and novel proteins interact with expansion segments to form an integrated framework that may stabilize the mature ribosome. Our structure provides a snapshot of the mammalian ribosome at the beginning of translation and lends support to current models in which large movements of the small subunit and L1 stalk occur during tRNA translocation. Finally, details are presented for intersubunit bridges that are specific to the eukaryotic ribosome. We suggest that these bridges may help reset the conformation of the ribosome to prepare for the next cycle of chain elongation.     

Multiple stages in codon-anticodon recognition: double-trigger mechanisms and geometric constraints

Thirty years of kinetic studies on tRNA selection in the elongation cycle are reviewed, and confronted with results derived from various sources, including structural studies on the ribosome, genetic observations on ribosome and EF-Tu accuracy mutants, and codon-specific elongation rates. A coherent framework is proposed, which gives meaning to many puzzling effects. Ribosomal accuracy would be governed by a "double-trigger" principle, according to which the ribosome uses energy in the forward direction to create new configurations for tRNA selection, and energy in the backward direction to regain its initial configuration, in particular after a premature dissociation event. The conformation energy would come in part, in Hopfield's mode, from GTP cleavage on the ternary complex (TC). The reset energy would be provided in part, in the author's mode, from GTP cleavage on a binary EF-Tu.GTP complex (BC). There would be several paths for amino acid incorporation. The path of highest accuracy would involve TC binding followed by BC binding, followed either by GTP hydrolysis on the TC, or by TC dissociation and GTP hydrolysis on the BC. Codon-anticodon recognition would occur in at least three kinetically and geometrically distinct stages. In a first stage, there would be a very rapid sorting of the TCs with unstrained anticodons contacting a loosely held mRNA. This stage ends with the anchoring of the codon-anticodon complex by a cluster of three nucleotides of 16S RNA. The second stage would be the most discriminative one. It would operate on the 5 ms time scale and terminate with GTP cleavage on the TC. The third stage would provide a last, crude selection involving "naked" aa-tRNA, partially held back by steric hindrance. Streptomycin and most EF-Tu mutants as well as high accuracy ribosomal mutants would produce specific alterations at stage 2, which are mapped on the stage 2 kinetic mechanism. The ram ribosomal ambiguity mutants, and anticodon position 37 modifications could be markers of stages 1 and 3 selection. Dissociation events at stage 2 or stage 3, when they are not immediately followed by reset events create a leaky state favorable to shortcut incorporation events. These events are equivalent to an "error-prone codon-anticodon mismatch repair". From the recent evidence on ribosome structure, it is conjectured that the L7/L12 flexible stalk of the large ribosome subunit acts as a proofreading gate, and that the alternation of its GTPase activation center between "TCase" competence and "BCase" competence is a main factor in the control of accuracy.