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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Protoplasma 204 (1998), S. 94-102 
    ISSN: 1615-6102
    Keywords: Ascidian ; Cellulose microfibril ; Hemocoel ; Polyandrocarpa misakiensis ; Tunic cord
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary A specialized structure of tunic cord inPolyandrocarpa misakiensis is investigated by electron microscopy. The tunic cord is a cord-like coiled structure of 5–30 μm in diameter and 0.1–9.0 mm in length. The tunic cords originate and elongate from the dorsal tunic, and their termini have a swollen and ornamented structure. Scanning and transmission electron micrographs and the electron diffractogram show that the tunic cords are composed of bundled microfibrils of cellulose I with high crystallinity. The tunic cord is completely surrounded by single-layered epidermal cells, which have been found as the site of cellulose biosynthesis. A number of tunic cords are connected to the internal tunic of the siphon by forming “eyelet” structures at their termini. These observations suggest that the tunic cords act as a connector between dorsal and internal tunic of the siphon.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1615-6102
    Keywords: Cellulose microfibril ; Freeze-fracture ; Terminal complex ; Tunic ; Tunicate ; Ascidian
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Cellulose synthesizing enzyme complexes (terminal complexes, TCs) have been found in the plasma membrane of epidermal cells in the tunicateMetandrocarpa uedai by using freeze-fracture replication techniques for electron microscopy. Assembly of cellulose microfibrils by TCs is a universal phenomenon in the biological kingdoms. The TCs are locally distributed in the plasma membrane of the epidermal cells facing the tunic, and no TCs are observed on the lateral membranes bordered by tight junctions. The TCs consist of two types of membrane subunits: large particles (14.5 nm in diameter) on the periphery and small subunit particles (7.2 nm) filling the center; the latter are hypothesized to be involved in cellulose synthesis. The TCs are the linear type (ca. 195 nm in length and 78 nm in width). Direct connections of TCs with the termini of microfibrils were observed. Amorphous regions, which were hypothesized the nascent microfibrils, were associated with the depressions of the TCs. The distortion of microfibrils on their terminus indicates that the crystallization may occur at the margin of TCs from which the microfibrils are discharged. This report provides evidence that: (1) The outer cell membrane of epidermis is the site for the assembly of cellulose microfibrils in the tunic; (2) a new type of TC is involved in the biosynthesis of cellulose microfibrils in the tunicates; (3) disorganized glucan chains may be synthesized in the depression of TCs and crystallized outside the E-surface of the epidermal cell membrane.
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  • 3
    ISSN: 1615-6102
    Keywords: Cellulose microfibril ; Electron diffraction ; Glomerulocyte ; Metandrocarpa uedai ; Tunic ; Vacuole-like structure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The tunicate,Metandrocarpa uedai, contains a large quantity of cellulose; however, it is not known how and where the cellulose is synthesized. Based on evidence from electron diffraction and conventional thin-sectioning for electron microscopy, this study shows that the glomerulocyte is involved in the synthesis of cellulose. The bundles of microfibrils in the glomerulocyte as well as the tunic were identified as cellulose I using selected area electron diffraction analysis. The diffraction pattern of cellulose in the glomerulocyte was similar to that from the tunic, suggesting that the crystallization of cellulose already is initiated in the glomerulocyte. The diameter of cellulose microfibrils, both in the glomerulocyte and the tunic was the same, about 16 nm. These results suggest that the glomerulocyte is the most probable site for the synthesis of cellulose in the tunic ofM. uedai. Using thin-sectioning techniques, a series of observations showed that individual microfibrils are primarily assembled in structures tentatively identified as vacuole-like structures, then they are bundled by a tapering region within the vacuole-like structures. These bundles of microfibrils are deposited in a continuously circular arrangement. The microtubules are oriented parallel to the bundles of microfibrils at the tapering vacuole-like structure, and they may be involved in the tapering of these structures (perhaps controlling the shape). This study also provides the first account for the involvement of a vacuole-like structure in the synthesis of cellulose microfibrils among living organisms.
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  • 4
    ISSN: 1615-6102
    Keywords: Cellulose microfibril ; Cross-sectional shape ; Lattice image ; Lattice orientation ; Glomerulocyte
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Glomerulocyte cellulosic bundles ofPolyzoa vesiculiphora were investigated by microdiffraction and high-resolution electron microscopy. In each bundle, hundreds of cellulose microfibrils, having a rectangular cross-sectional shape, are packed regularly with their 0.6 nm lattice planes parallel to each other. Lattice images reveal that the 0.6 nm plane is parallel to the longer edge of the cross section which is similar to the lattice organization of cellulose with a squarish cross section inValonia spp. More interestingly, all the microfibrils in a bundle have the same directionality of crystallographic c-axis, which suggests that the biosynthesis of the microfibrils within particular bundle occurs unidirectionally.
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  • 5
    ISSN: 1432-2048
    Keywords: Cell wall ; Cellulose microfibril ; Chaetomorpha ; Cytoskeleton ; Microtubule ; Plasma membrane
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The functions of the microtubule (MT) cytoskeleton in changing the orientation of microfibrils (MFs) in the cell walls of the coenocytic green alga Chaetomorpha moniligera Kjellman were investigated by electron microscopy. The cortical MT cytoskeleton in Chaetomorpha was comprised of longitudinally oriented MTs. Cellulose MFs, however, alternately changed their orientation longitudinally and transversely to form crisscross MF textures. Microtubules were parallel to longitudinally oriented MFs but never to those that were transversely oriented. The average density of MTs during the formation of longitudinally oriented MFs was 216 per 50 μm of wall and that of transversely oriented MFs 170/50 μm. To determine exactly the MT-density dependency of each MF orientation, changes in MF orientation were examined by changing MT density after treating and removing amiprophos-methyl (APM). Microtubules were reduced in number by a half (100/50 μm) after 2 h and by 3/4 (50/50 μm) after 3 h of treatment with APM (3 mM). This reduction was caused by the disappearance of alternating MTs. Microtubules retained this density (50/ 50 μm) up to 6 h, and then gradually disappeared within 24 h. Microfibril orientation in the innermost cell wall was transverse after treatment with APM for 2 h but was helicoidal after 6 h. Polymerization of MTs occurred in the longitudinal direction following the removal of APM after treatment for 48 h. Microtubule density rose to about 100/50 μm and 200/50 μm after 6 h and 24 h, respectively. The orientation of MTs changed from helicoidal to transverse and transverse to longitudinal after 6 h and 24 h, respectively. When APM was removed prior to formation of the helicoidal texture, longitudinally oriented MFs appeared within 6 h. There is thus an alternating cycle of formation of longitudinally and transversely oriented MFs within a 12-h period. Formation of transversely oriented MFs as a result of APM treatment started in the middle of a cell as hoops which then extended in the apical and basal directions. Formation of longitudinally oriented MFs as a result of the removal of APM started from the apical end and proceeded toward the base. It follows from these results that: (1) the point of formation of longitudinally oriented MFs differs from that for transversely oriented MFs, (2) MF orientation in each case depends on a separately functioning mechanism, (3) MT density changes rhythmically to trigger a switch for crisscross orientation of MFs.
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  • 6
    ISSN: 0005-2736
    Keywords: Ion channel ; Lipid bilayer ; Melittin ; Membrane
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology , Chemistry and Pharmacology , Medicine , Physics
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 0005-2736
    Keywords: Fluorescence ; Glycopeptide ; Helix orientation ; Ion channel ; Lectin ; Lipid bilayer
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology , Chemistry and Pharmacology , Medicine , Physics
    Type of Medium: Electronic Resource
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