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21.
Summary The structure of thezonula occludens in the secretory epithelium of the salt gland of the domestic duck was determined by thin section and freeze-fracture electron microscopy. These glands secrete an effluent with a NaCl concentration four times that of plasma, and thus maintain a steep ionic gradient across their secretory epithelium. Freezefracture replicas from salt stressed ducks demonstrate that thezonula occludens is surprisingly shallow in depth (20–25 nm) and generally consists of two parallel junctional strands which are juxaposed along their entire length. In addition to the simplicity of the junction separating mucosal and serosal compartments, the ratio of junctional length to apical surface area is large since luminal surfaces of secretory cells are narrow and intermesh with one another. Thezonula occludens in nonsecreting fresh water-adapted birds is similar to the salt stressed group except that two sets of double strand junctions are seen in addition to junctions consisting of a single set. Based on previous ultrastructural, cytochemical and physiological studies in salt glands and in other epithelia, a model for salt secretion was suggested in which intercellular space Na+, generated by basolateral ouabain-sensitive Na+ pumps, reaches the lumen via a paracellular route (Ernst & Mills, 1977,J. Cell Biol.
75:74). The simplicity of the morphological appearance of thezonula occludens in the salt gland, which resembles that described for several epithelia known to be leaky to ions, is consistent with this hypothesis. 相似文献
22.
Clara B. Jones 《Primates; journal of primatology》1982,23(1):130-134
To test the hypothesis that dominance rank is reflected in spatial relationships and rates of vocalization, a field manipulation
was performed removing the third-ranked male from a three-male group of mantled howler monkeys, significantly decreasing the
interindividual distance (IID) between the first- and second-ranked males and increasing the rates of vocalization (VR) between
them. After five days, the third-ranked male returned to his group, and IID's and VR's returned to premanipulation levels.
During the manipulation phase, the second-ranked male was able to achieve closer proximity to females than during the premanipulation
or postmanipulation periods, suggesting that he found it less difficult to control relations with the first-ranked male when
the third-ranked male was absent. All statistically significant results were a function of behavioral changes by the second-ranked
male. The study's findings may reflect the importance of male coalitions in the regulation of male-male competition and the
relationship between spatial and acoustic variables and sexual selection. 相似文献
23.
Clara B. Jones 《Primates; journal of primatology》1981,22(1):70-83
Dominance hierarchies are presumed to evolve by individual selection from an evolutionary compromise between intraspecific
competition for resources and for mates. The hypothesis is put forward that when competition in “stable” habitats leads to
“niche breadth,” a species is preadapted to life in heterogeneous environments and the consequent selection for fecundity.
Status patterns are viewed as systems of signals communicating differential tendencies among individuals to attack or retreat,
and a simple graphical model is presented which relates the costs or benefits to fitness of aggressive or appeasement behavior
and interindividual distance. Primate societies are classified on the basis of their dominance hierarchies, and the ecological
correlates of these patterns are discussed. Based on hypotheses presented in the paper, topics for future research are suggested. 相似文献
24.
Six of the eight transfer RNAs coded by bacteriophage T4 are synthesized via three dimeric precursor molecules. The sequences of two of these have been determined. Both of these precursors give rise to equimolar amounts of the cognate tRNA molecules in vivo. In contrast, even in wild-type infections, tRNAIle is present in ≤ 30% the amount of tRNAThr, with which it is processed from a common dimeric precursor.We have now determined the sequence of this dimer. In addition to the nucleotides present in tRNAThr and tRNAIle, it contains nine precursor-specific residues, located at the 5′ and 3′ termini and at the interstitial junction of the two tRNA sequences. While the three dimers share the majority of structural features in common, pre-tRNAThr + Ile is the only case in which an encoded tRNA 3′ -C-C-A terminus is present in the interstitial region.The processing of this dimer in various biosynthetic mutants has been analyzed in vivo and in vitro and shown to be anomalous in several respects. These results suggest that the apparent underproduction of tRNAIle can be explained by a novel processing pathway that generates a metabolically unstable tRNAIle product. Data from DNA sequence analysis of the T4 tRNA gene cluster (Fukada & Abelson, 1980) support the conclusion that the asymmetric maturation of this precursor is a consequence of the unique disposition of the -C-C-A sequence. These results argue that gene expression can be modulated at the level of RNA processing. The biological significance of this phenomenon is discussed in relation to evidence that tRNAIle has a unique physiological role. 相似文献
25.
26.
Gloria N. Sando Pauline Titus-Dillon Clara W. Hall Elizabeth F. Neufeld 《Experimental cell research》1979,119(2):359-364
Cultured human skin fibroblasts take up α-
-iduronidase by receptor-mediated pinocytosis. Certain lysosomotropic amines such as chloroquine, ammonia and procaine inhibit this process, without affecting the fluid endocytosis of dextran. In contrast to the competitive inhibition by mannose 6-phosphate, the inhibition by amines is non-competitive and is therefore presumed not to affect binding of the enzyme to receptors. The dose response curves are very steep, and equations that best fit the data use a power of inhibitor concentration (i2 for procaine, i4 for chloroquine), indicating interaction of several amine molecules at the inhibitory site(s). The inhibition is reversed by removal of the amine from the medium and does not result from accelerated efflux of endocytosed enzyme. We suggest that the amines interfere with delivery of receptor-bound enzyme to lysosomes. 相似文献
27.
Plant Ecology - Due to increased reserves available to face adverse conditions, drought tolerance should increase with seed size. This has been confirmed in the laboratory, but under field... 相似文献
28.
29.
Espinoza Dulce O. Molina-Freaner Francisco Tinoco-Ojanguren Clara 《Plant Ecology》2020,221(4):255-264
Plant Ecology - Buffel grass (Cenchrus ciliaris L.) is an African grass used as forage for livestock that is also invasive. Several methods have been evaluated to remove invasive buffel grass, but... 相似文献