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991.
Summary A model is proposed for the structure of the plasmodesmata ofAzolla root primordia, based on micrographs obtained by a combination of fixation in glutaraldehyde/p-formaldehyde/tannic acid/ferric chloride, digestion of cell walls and the use of stereo pairs. Unlike the model for plasmodesmatal structure proposed byRobards (1971), the desmotubule is depicted as a virtually closed cylindrical bilayer providing little or no open pathway for transport. In this respect it is similar to the model ofLópez-Sáez
et al. (1966). An analysis of the molecular packing of types of lipids found in endoplasmic reticulum (of which the desmotubule is an extension) indicates that the model is geometrically feasible. Details cannot be discerned with accuracy, but material, possibly particulate, occupies much of the space between desmotubule and plasma membrane, the cytoplasmic lumen being reduced to inter-particle spaces of cross-sectional area comparable to that of the bore in a gap junction connexon. Implications for intercellular transport are discussed. 相似文献
992.
Two glandular systems were discovered that secrete their products onto the cuticular surface in ants. The first, the subepithelial gland, was previously undescribed in ants, and is found throughout the body just beneath the epithelium. This gland consists of independent secretory units, each made up of a single gland cell and an associated duct cell that penetrates the cuticle. Its ultrastructural appearance is consistent with possible hydrocarbon production. Examining 84 ant species, the subepithelial gland was found in eight subfamilies (out of 13), although not necessarily in all species. In a single ant species, Harpegnathos saltator, it was the epithelium itself that was enlarged and functioned as a gland. The enlarged epithelial cells secrete their products directly onto the cuticle through distinct cuticular crevasses. 相似文献
993.
994.
Telecommunications and Travel: The Case for Complementarity 总被引:2,自引:0,他引:2
Patricia L. Mokhtarian 《Journal of Industrial Ecology》2002,6(2):43-57
This article examines the conceptual, theoretical, and empirical evidence with respect to the impact of telecommunications on travel. The primary focus is on passenger travel, but goods movement is addressed briefly. I argue that although direct, short-term studies focusing on a single application (such as telecommuting) have often found substitution effects, such studies are likely to miss the more subtle, indirect, and longer-term complementarity effects that are typically observed in more comprehensive analyses. Overall, substitution, complementarity, modification, and neutrality within and across communication modes are all happening simultaneously. The net outcome of these partially counteracting effects, if current trends continue, is likely to be faster growth in telecommunications than in travel, resulting in an increasing share of interactions falling to telecommunications, but with continued growth in travel in absolute terms. The empirical evidence to date is quite limited in its ability to assess the extent of true causality between telecommunications and travel, and more research is needed in that area. At this point, what we can say with confidence is that the empirical evidence for net complementarity is substantial, although not definitive, and the empirical evidence for net substitution appears to be virtually nonexistent. 相似文献
995.
Andreas Stumpner Otto von Heiversen 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1992,171(3):405-412
Summary Males of the grasshopper Chorthippus dorsatus produce songphrases which contain two differently structured elements — pulsed syllables in the first part (A) and ongoing noise in the second part (B). Females of Ch. dorsatus answer to artificial song models only if both elements A and B are present. Females strongly prefer song models in which the order of elements is A preceding B. Females discriminate between the two elements mainly by the existence of gaps within A-syllables. Pulses of 5–8 ms separated by gaps of 8–15 ms make most effective A-syllables, while syllable duration and syllable intervals are less critical parameters. Females respond to models which contain more than 3 A-syllables with high probability. Female model preferences lie well in the range of parameter values produced by singing males, except for B-parts which must be longer than those of any natural song to be most effective. In ancestors of Ch. dorsatus the two elements of the songs might have been directed towards females (part A) and males (part B). 相似文献
996.
Eric C Beyer Karen E Reed Eileen M Westphale H Lee Kanter David M Larson 《The Journal of membrane biology》1992,127(1):69-76
Summary Gap junctions contain intercellular channels which are formed by members of a group of related proteins called connexins. Connexins contain conserved transmembrane and extracellular domains, but unique cytoplasmic regions which may provide connexin-specific physiologic properties. We used polymerase chain reaction (PCR) amplification and cDNA library screening to clone DNA encoding a novel member of this gene family, rat connexin40 (Cx40). The derived rat Cx40 polypeptide contains 356 amino acids, with a predicted molecular mass of 40,233 Da. Sequence comparisons suggest that Cx40 is the mammalian homologue of chick connexin42, but it has predicted cytoplasmic regions that differ from previously described mammalian connexins. Southern blots of rat genomic DNA suggest that Cx40 is encoded by a single copy gene containing no introns within its coding region. Northern blots demonstrate that Cx40 is expressed in multiple tissues (including lung, heart, uterus, ovary, and blood vessels) and in primary cultures and established lines of vascular smooth muscle cells. Cx40 is coexpressed with connexin43 in several cell types, including A7r5 cells, which contain two physiologically distinct gap junctional channels. To demonstrate that Cx40 could form functional channels, we stably transfected communication-deficient Neuro2A cells with Cx40 DNA. These Cx40-transfected cells showed intercellular passage of microinjected Lucifer yellow CH. The expression of multiple connexins (such as Cx40 and Cx43) by a single cell may provide a mechanism by which cells regulate intercellular coupling through the formation of multiple channels 相似文献
997.
Summary Junctional conductance between the epidermal cells of the beetle Tenebrio molitor is raised after exposure to the hormone 20-hydroxyecdysone and lowered reversibly by exposure to chlorpromazine. Gap Junctional particle size, density and arrangement associated with these conductance changes were studied. We found no significant difference in particle density in gap junctions of control (2456±471 particles/m2, mean ±S.D.) and hormone-treated epidermis (2490±315); however, a significant increase in packing density occurred in chlorpromazine-uncoupled epidermis (3133±665). The particles are randomly arranged in all three states of conductance. Particle size measurements show that the E-face gap junctional particles are heterogeneous with a mean diameter ±S.D. of 15.2±2.0 nm. No significant difference in particle size between controls and experimentals was detected. Although glutaraldehyde irreversibly uncoupled these cells, the absence of glutaraldehyde fixation but presence of glycerol induced marked alterations in the appearance of the gap junctions such that quantification was no longer possible. From this particle-packing data and our previous thin-section data, we estimate that there are 90000 gap junctional particles per cell (within junctional plaques). The conductance of a single gap junctional channel (assuming one population) changes from 94 pS to 213 pS after hormone treatment. 相似文献
998.
Julian J. Edney Christopher S. Harper 《Human ecology: an interdisciplinary journal》1978,6(4):387-395
A resource management simulation was devised in which players could harvest points for individual short-term gain, causing the premature destruction of the resource pool, or they could curb their own individual harvesting to preserve the pool for a longer overall supply. Although the first strategy was explained to be self-defeating, most groups opted for it, as they did even when an optimal harvesting strategy was provided that would avoid pool destruction. Groups whose members were allowed to communicate generally made better resource managers and achieved larger individual harvests. 相似文献
999.
The role of junctional communication in animal tissues 总被引:2,自引:0,他引:2
John D. Pitts 《In vitro cellular & developmental biology. Plant》1980,16(12):1049-1056
Summary Permeable intercellular junctions are a common feature of most animal tissues. These junctions allow the free exchange of
small ions and molecules between all the cells in coupled populations. Such limited syncytial interaction contributes to the
integration of individual cells into organized tissues.
Presented in the symposium on Molecular and Morphological Aspects of Cell-Cell Communication at the 31st Annual Meeting of
the Tissue Culture Association, St. Louis, Missouri, June 1–5, 1980.
This symposium was supported in part by Contract 263-MD-025754 from the National Cancer Institute and the Fogarty International
Center. 相似文献
1000.
Elliot L. Hertzberg 《In vitro cellular & developmental biology. Plant》1980,16(12):1057-1067
Summary Studies on gap junctions isolated from rat liver by a procedure that avoids exogenous proteolysis (Hertzberg, E. L.; Gilula,
N. B.; J. Biol. Chem. 254: 2138–2147; 1979) are described. The original isolation procedure was modified to increase the yield
and has been extended to the preparation of gap junctions from mouse and bovine liver. Peptide map studies showed that the
27,000-dalton polypeptides present in liver gap junction preparations from all three sources are homologous and are not derived
from other polypeptides of higher molecular weight that are observed in cruder preparations. Similar studies with lens fiber
junctions demonstrated no homology between liver and lens junction polypeptides. Antibodies to the lens junction polypeptide
did not cross-react with the liver gap junction polypeptide, further supporting this conclusion.
Presented in the symposium on Molecular and Morphological Aspects of Cell-Cell Communication at the 31st Annual Meeting of
the Tissue Culture Association, St. Louis, Missouri, June 1–5, 1980.
This symposium was supported in part by Contract 263-MD-025754 from the National Cancer Institute and the Fogarty International
Center.
Research in the laboratory was supported by grants to Dr. Gilula from the National Institute of Health (HL 16507 and GM 24753). 相似文献