BRIDGES BETWEEN MICROTUBULES

Bridges between microtubules have been studied with the electron microscope in the axostyle of Saccinobaculus and in various tubule systems of chicken testis, including the helix of tubules surrounding the elongating spermatid nucleus and the flagellum of the sperm tail. In addition to the previously described periodic bridges, evidence is presented that nonperiodic bridges exist between certain tubules. An analysis of axial spacing between adjacent nonperiodic bridges suggests that these structures are attached to periodic binding sites on the microtubule wall, but that not all the binding sites are filled. The bridges appear nonperiodic as a result of random occupancy of some fraction of the periodic sites. The distribution of these binding sites is related to the substructure of the microtubule wall as seen with negative staining and optical diffraction.     

ON THE ROLE OF MICROTUBULES IN MOVEMENT AND ALIGNMENT OF NUCLEI IN VIRUS-INDUCED SYNCYTIA

Infection of baby hamster kidney (BHK21-F) cells with the parainfluenza virus SV5 causes rapid and extensive cell fusion. Time-lapse cinematography shows that when cells fuse, their nuclei migrate straight to the center of the syncytium at rates of 1–2 µ/min. Nuclei are often arranged in long, tightly packed, parallel rows in syncytia derived from the fibroblastic BHK21-F cells. Polarization microscopy shows birefringent material between and parallel to these rows of nuclei, and electron microscopy shows bundles of cytoplasmic microtubules, ~250 A in diameter, and filaments, ~80 A in diameter, parallel to and between the rows of nuclei. Colchicine treatment causes disappearance of microtubules from BHK21-F cells and an apparent increase in the number of 80-A filaments. Although colchicine-treated, SV5-infected cells fuse, their nuclei do not migrate or form rows but remain randomly scattered through the syncytial cytoplasm. Incubation at 4°C does not disrupt microtubules in BHK21-F cells. Rows of nuclei have been isolated from SV5-induced syncytia, and the nuclei in them have been found to be intimately associated with microtubules but not with other cytoplasmic structures. These results suggest that microtubules demarcate cytoplasmic channels through which nuclei migrate and that they may also be involved in the mechanism of nuclear movement.     

花粉管中的微丝和微管

对花粉管中的微丝和微管研究的几个问题的进展进行了综述, 包括微丝和微管在花粉管中的分布;微丝和微管在花粉管胞质流动、细胞器运动以及花粉管生长中的作用等几个方面。并对今后这些方面研究的重要问题进行了论述。     

MICROTUBULES AND EARLY STAGES OF CELL-PLATE FORMATION IN THE ENDOSPERM OF HAEMANTHUS KATHERINAE BAKER

A fine structure study of the phragmoplast and developing cell plate has been made on glutaraldehyde-osmium tetroxide-fixed, dividing, cultured cells of the liquid endosperm of Haemanthus katherinae Baker. The phragmoplast arises between the telophase nuclei, usually in association with a remnant strand of spindle elements, and consists of an accumulation of microtubules oriented at right angles to the plane of the future cell plate. The microtubules, which are 200–240 A in diameter, occur in small clusters spaced at approximately 0.2–0.3 µ intervals along the plate. Short interconnections interpreted as "cross-bridges" have been observed between individual microtubules. Within each cluster there is an electron-opaque zone about 0.3 µ in width which can be attributed in part to an overlap of microtubules from both sides of the plate and in part to a local accumulation of an amorphous electron-opaque material. During development these dense zones become aligned in a plane which itself defines the plane of the plate. Vesicles, commonly observed in long files, are derived from a cytoplasmic matrix rich in elements of the endoplasmic reticulum and sparse in dictyosomes. They aggregate between the clusters of microtubules and eventually coalesce to form the cell plate.     

SUBPLASMALEMMAL MICROFILAMENTS AND MICROTUBULES IN RESTING AND PHAGOCYTIZING CULTIVATED MACROPHAGES

The subplasmalemmal organization of the free and glass-attached surfaces of resting and phagocytizing cultivated macrophages were examined in an attempt to define specific membrane-associated structures related to phagocytosis. From analysis of serial thin sections of oriented cells it was found that the subplasmalemmal region of the attached cell surface has a complex microfilament and microtubule organization relative to the subplasmalemmal area of the free surface. A filamentous network composed of 40–50-Å microfilaments extended for a depth of 400–600 Å from the attached plasma membrane. Immediately subjacent to the filamentous network was a zone of oriented bundles of 40–50-Å microfilaments and a zone of microtubules. Additional microtubules were found to extend from the plasma membrane to the interior of the cell in close association with electron-dense, channellike structures. In contrast, the free aspect of the cultivated macrophage contained only the subplasmalemmal filamentous network. However, after a phagocytic pulse with polystyrene particles (14 µm diam) microtubules and oriented filaments similar to those found on the attached surface were observed surrounding the ingested particles. The observations reported in this paper provide support for the hypothesis that microfilaments and/or microtubules play a role in the translocation of plasma membrane required for the functionally similar processes of phagocytosis and cell attachment to glass.     

SUSCEPTIBILITY AND PROTECTIVE MECHANISMS OF MOTILE AND NON MOTILE CELLS OF HAEMATOCOCCUS PLUVIALIS (CHLOROPHYCEAE) TO PHOTOOXIDATIVE STRESS1

The life cycle of the unicellular green alga Haematococcus pluvialis consists of motile and nonmotile stages under typical growing conditions. In this study, we observed that motile cells were more susceptible than nonmotile cells to high light, resulting in a decrease in population density and photo‐bleaching. Using two Haematococcus strains, CCAP 34/12 (a motile cell dominated strain) and SAG 34/1b (a nonmotile cell dominated strain), as model systems we investigated the cause of cell death and the protective mechanisms of the cells that survived high light. The death of motile cells under high light was attributed to the generation of excess reactive oxygen species (ROS), which caused severe damage to the photosynthetic components and the membrane system. Motile cells were able to dissipate excess light energy by nonphotochemical quenching and to relax ROS production by a partially up‐regulated scavenging enzyme system. However, these strategies were not sufficient to protect the motile cells from high light stress. In contrast, nonmotile cells were able to cope with and survive under high light by (i) relaxing the over‐reduced photosynthetic electron transport chain (PETC), thereby effectively utilizing PETC‐generated NADPH to produce storage starch, neutral lipid, and astaxanthin, and thus preventing formation of excess ROS; (ii) down‐regulating the linear electron transport by decreasing the level of cytochrome f; and (iii) consuming excess electrons produced by PSII via a significantly enhanced plastid terminal oxidase pathway.     

镉离子对微管、管蛋白巯基和在体轴浆转运的作用

大鼠腹角注入~3H亮氨酸,3h后坐骨神经内注射 Cd~(2 )30μl,50mM Cd~(2 )轻度减小标记蛋白的轴浆转运距离,75或 100mM 阻断转运在注射点。巯基抑制剂马来酰胺与 50mM Cd~(2 )伍用的效应相加,使转运也阻断在注射点。巯基供应剂二巯基丁二钠则完全抵消100mMCd~(2 )对转运的阻断作用。Cd~(2 )还使坐骨神经匀浆上清液的巯基含量减少,马来酰胺与Cd~(2 )伍用则巯基进一步降低。自兔脑提取了微管的亚基管蛋白,Cd~(2 )降低管蛋白巯基的程度与其浓度呈线性关系。电镜的验证证明,Cd~(2 )使在体神经的微管解聚、微管减少或基本消失。由于巯基、微管和转运的变化程度均与Cd~(2 )的浓度大小有关,并且同一浓度的Cd~(2 )使微管和转运的受累程度相互对应,因此Cd~(2 )可能借络合管蛋白的巯基使微管解聚而阻断转运。实验进一步证明以前的论断,微管解聚可能是Cd~(2 )等药物阻断转运的一个中间环节,微管可能参与在体轴浆转运的机制。     

THE FINE STRUCTURE AND FUNCTION OF THE CONTRACTILE AXOSTYLES OF CERTAIN FLAGELLATES

The axostyles of the flagellates Oxymonas, Saccinobaculus, and Notila are large ribbon-shaped structures which undulate actively in the cytoplasm. The form of their movements is described and illustrated. Axostyles consist of regular arrays of longitudinal fibres, the number of which varies between 100 and 5000 in different species. The fibres are about 240 A in diameter, apparently hollow, regularly cross-banded with a periodicity of about 150 A, and connected by delicate cross-links, also at regular intervals of about 150 A. They resemble very closely the central fibres of cilia and flagella. No other structural components are present, except at the anterior end, where the fibres are attached to one or more basal bodies, and at the posterior tip, where they are anchored to the plasma membrane. The relevance of the findings to an understanding of the mechanism of ciliary and flagellar movements is discussed.     

MICROTUBULES IN THE SPERMATIDS OF THE DOMESTIC FOWL

Spermiogenesis in chicken has been examined in order to see whether the radical changes observed in cell shape can be related to the presence of cytoplasmic microtubules. A highly ordered array of tubules has been found which surrounds the nucleus as it elongates from a sphere to a slender cylinder. The structure of the array has been determined by following the tubules through 12–14 adjacent serial sections, and it is a left-handed double helix. Faint cross-bridges connect consecutive turns of the two helices. After the change in nuclear shape is complete, the helical system of microtubules disappears and is replaced by a set of almost straight tubules which run parallel to the long axis of the nucleus. These tubules remain while the spermatid nucleus condenses isotropically to its final size. We suggest that the helix is the agent which effects nuclear elongation and that the subsequent system of paraxial tubules determines the curvature of the final sperm head. Evidence for these suggestions is found in the form of spermatids which have failed to develop properly. In an appendix we consider the kinematics of single and multiple helix systems and discuss the revelance of these models to the morphogenesis of chicken spermatids.     

洋葱鳞茎内表皮细胞中的微管

用超薄切片和原生质体负染的方法,在电镜下观察到洋葱鳞茎内表皮细胞周质微管的存在。其出现时间、分布状况以及排列形态,基本上与考马斯亮蓝染色法观察到的网状结构物相一致。讨论了包被囊泡和微管组织中心可能的作用。     

MICROTUBULES IN BIOLOGICAL CELLS AS CIRCULAR WAVEGUIDES AND RESONATORS

The microtubules in the cellular cytoskeleton have a fundamental role in the living processes of biological cells. They are hollow cylinders which resemble circular waveguides or cylindrical resonators. The cutoff and resonant frequencies of the transverse magnetic and transverse electric modes of the microtubule cavities are in the band of soft x-rays. This suggests the possibility of interaction of electromagnetic cavity modes with inner electrons in atoms (e.g., in carbon, nitrogen, and oxygen). Biological cells (e.g., the yeast cells of spherical shape) may also represent cavity resonators. In this case, the resonant frequencies may be in the infrared region.     

百合生殖细胞分裂过程中微管分布的显微荧光观察

用抗微管蛋白抗体和荧光标记技术,观察了百合生殖细胞经有丝分裂形成精细胞过程中微管的变化。生殖细胞在分裂的前期,存在于核外围以及细胞两端胞质内的微管大都以微管束的形式沿细胞长轴方向平行排列。在靠近核的部位,有些微管有时会斜向排列。分裂进入中期后,染色体集中排列在赤道面。在染色体周围可以见到有多束与细胞长轴平行排列着的微管,但这些微管束是在分裂中期时新形成的或是在前期已存在,尚难以断定。这些微管束有一个特点,就是当它们延伸至赤道板部位时,在每一条微管束上都有一个无荧光的小圆区;这个小圆区可能代表着丝粒的位置。细胞分裂进入后期,姊妹染色单体分别向两极移动形成两组染色体。在它们之间近赤道板位置出现了一个具有强烈荧光的区域,显示在这一部位,微管相当浓密。从这一强烈荧光区向两极分别伸出多条微管束。因此,在这一强烈荧光区内可能有多个微管束重叠。到细胞分裂末期,在这一强烈的荧光区的中央出现了一条横向的无荧光区。这一区域有可能为胞质完成分裂后新形成的细胞板所在的部位。     

微管在气孔运动中的作用

用植物微管专一性解聚剂甲基胺草磷(APM)预处理蚕豆(Vicia faba L.)下表皮,再用诱导气孔运动的因子处理,在显微镜下观察气孔孔径的变化。结果显示,用50mg/L APM预处理开放或关闭状态气孔,虽胞质微管被解聚,但气孔孔径没有发生明显的变化,表明胞质微管与开放或关闭状态气孔的维持无关;而去掉APM后,CaCl_2可在4h内诱导气孔关闭,气孔的运动功能又可恢复。进一步的研究表明,开放气孔经APM预处理60min后,再用ABA、Ca~(2 )及暗处理均不能诱导气孔关闭,表明微管可能参与了ABA、Ca~(2 )及暗诱导的气孔关闭过程;关闭气孔经50mg/L APM预处理后,光诱导气孔开度较不经 APM处理的有明显差异,且随着APM预处理时间和浓度的变化,气孔开放程度亦不同,表明微管也参与了光诱导的气孔开放过程。     

玉米类囊体膜的超分子结构及LHCP在个体发育中的变化

对玉米(Zea mays)营养生长期中的下位叶(第5叶)和生殖生长时期的中位叶(果穗叶)和上位叶(顶生叶)的成熟叶片的冰冻撕裂电镜观察,发现叶绿体类囊体膜所有撕裂面上各种功能蛋白颗粒的密度均以果穗叶中的最大,依次是顶生叶和第5叶的。以果穗叶与顶生叶相比,其类囊体膜中包含有绝大多数 LHCP 的 EFs 颗粒增加28%;包含有 PSI 反应中心与LHCP 相结合的 PFu 颗粒增加20%;包含有 PSII 反应中心与 LHCP 相结合的 EFs 颗粒增加19%。这一超分子结构的电镜观察结果与其 SDS-聚丙烯酰胺梯度凝胶板电泳解析的结果相一致。即 SDS-聚丙烯酰胺梯度凝胶板电泳解析的色素带上,同样是果穗叶类囊体膜上呈现的21kD(LHCP Ⅰ)和25 kD(LHCPⅡ)多肽的色素带相应地也比顶生叶的加宽,表明果穗叶叶绿体类囊体膜上镶嵌的叶绿素 a/b 蛋白复合体等比顶生叶的显著地增多,这有利于果穗叶光合作用中光能的吸收、传递、分配和转化。