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排序方式: 共有146条查询结果,搜索用时 15 毫秒
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An electron microscopical study of the third eye of the Western Fence Lizard, Sceloporus occidentalis, fixed with 1 per cent osmium tetroxide, pH 7.4–7.6, for 16 to 20 hours at 0°C., revealed the following new facts. The fibrillar system of the retinal photoreceptor consists of nine double fibrils enclosed in a sheath. Pigment cells and lens cells possess similar systems. Two short cylindrical centrioles are associated with the fibrillar apparatus: one, from which striated rootlets extend inward, lies at the base of the fibrils, with the other at an oblique angle to the axis of the system. A Golgi complex, whorls of endoplasmic reticulum, lipid (?) droplets, and other organelles and inclusions in the photoreceptors are described. An axon leads from the base of the photoreceptor into the nervous layer of the retina which consists of many nerve fibers and large ganglion cells. Although the pattern of neural connections has not yet been determined, some synapses were found. The parietal nerve consists of about 250 non-medullated fibers. The capsule of the eye usually has a layer of iridocytes, which contain rows of guanine (?) rods. A few parietal eyes of the Granite Night Lizard, Xantusia henshawi, were also examined. Large lipid (?) droplets occur in the bases of their receptoral processes. 相似文献
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Summary The ocelli of trochophore and segmented larvae of the archiannelid Polygordius cf. appendiculatus were studied by electron microscopy. An eye consists of two pigmented supportive cells forming an eyecup that encloses one sensory cell bearing one (trochophore) or two (segmented larva) ranks of microvilli and one adventitious cilium. Remarkably abundant tubules (submicrovillar endoplasmic reticulum) radiate from the perinuclear region of the sensory cell, which lies outside the ocellus, toward its receptoral end. Possible functions of the tubules are proposed: carriers of ions, metabolites and photopigments; pinocytic uptake of products resulting from photoreception; storage of membrane; and light guides. Finally, the eyes of Polygordius larvae are believed to have evolutionary significance and provide further support for Eakin's theory of diphyletic origin of photoreceptors. 相似文献
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Carbonyl complexes of horse cytochrome c and various carboxymethylated derivatives have been examined using 13C NMR (carbon-13 nuclear magnetic resonance) spectroscopy. The multiplicity and chemical shift of the 13CO resonance were found to be functions of pH and the extent of alkylation. Correlations have been made among prominent features of the chemical shift titration curves and changes in the environment of the heme. A simple model compatible with the bulk of previous observations has been suggested to account for the several carbonyl resonance peaks and the complex behavior of the chemical shift with changes in pH. 相似文献
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Felipe Murtinho Christina Tague Bert de Bievre Hallie Eakin David Lopez-Carr 《Human ecology: an interdisciplinary journal》2013,41(5):667-681
In the Andean region of South America, understanding communities’ water perceptions is particularly important for water management as many rural communities must decide by themselves if and how they will protect their micro-watersheds and distribute their water. In this study we examine how Water User Associations in the Eastern Andes of Colombia perceive water scarcity and the relationship between this perception and observed climate, land use, and demographic changes. Results demonstrate a complex relationship between perceptions and observed changes. On the one hand, observed changes in land cover match perceptions of deforestation as the primary cause of increasing water scarcity. On the other hand, perceptions of climate driven changes in water availability are not reflected in observed precipitation data. Furthermore, water scarcity was perceived in regions where seasonal rainfall variability is higher but not in regions where annual rainfall is lower. We discuss how these results contribute to our understanding of adaptation to climate change and the implications of possible mismatches between environmental changes and local perceptions. 相似文献
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Stephen GS Vreden Jeetendra K Jitan Rakesh D Bansie Malti R Adhin 《Memórias do Instituto Oswaldo Cruz》2013,108(8):968-973
The emerging resistance to artemisinin derivatives that has been reported in
South-East Asia led us to assess the efficacy of artemether-lumefantrine as the first
line therapy for uncomplicated Plasmodium falciparum infections in
Suriname. This drug assessment was performed according to the recommendations of the
World Health Organization in 2011. The decreasing number of malaria cases in
Suriname, which are currently limited to migrating populations and gold miners,
precludes any conclusions on artemether efficacy because adequate numbers of patients
with 28-day follow-up data are difficult to obtain. Therefore, a comparison of day 3
parasitaemia in a 2011 study and in a 2005/2006 study was used to detect the
emergence of resistance to artemether. The prevalence of day 3 parasitaemia was
assessed in a study in 2011 and was compared to that in a study in 2005/2006. The
same protocol was used in both studies and artemether-lumefantrine was the study
drug. Of 48 evaluable patients in 2011, 15 (31%) still had parasitaemia on day 3
compared to one (2%) out of 45 evaluable patients in 2005/2006. Overall, 11 evaluable
patients in the 2011 study who were followed up until day 28 had negative slides and
similar findings were obtained in all 38 evaluable patients in the 2005/2006 study.
The significantly increased incidence of parasite persistence on day 3 may be an
indication of emerging resistance to artemether. 相似文献
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A debilitating complication of long-term hemodialysis is the deposition of beta-2-microglobulin (beta2m) as amyloid plaques in the joint space. We have recently shown that Cu(2+) can be a contributing, if not causal, factor at concentrations encountered during dialysis therapy. The basis for this effect is destabilization and incorporation of beta2m into amyloid fibers upon binding of Cu(2+). In this work, we demonstrate that while beta2m binds Cu(2+) specifically in the native state, it is binding of Cu(2+) by non-native states of beta2m which is responsible for destabilization. Mutagenesis of potential coordinating groups for Cu(2+) shows that native state binding of Cu(2+) is mediated by residues and structures that are different than those which bind in non-native states. An increased affinity for copper by non-native states compared to that of the native state gives rise to overall destabilization. Using mass spectrometry, NMR, and fluorescence techniques, we show that native state binding is localized to H31 and W60 and is highly specific for Cu(2+) over Zn(2+) and Ni(2+). Binding of Cu(2+) in non-native states of beta2m is mediated by residues H13, H51, and H84, but not H31. Although denatured beta2m has characteristics of a globally unfolded state, it nevertheless demonstrates the following strong specificity of binding: Cu(2+) > Zn(2+) > Ni(2+). This requires the existence of a well-defined structure in the unfolded state of this protein. As Cu(2+) effects are reported in many other amyloidoses, e.g., PrP, alpha-synuclein, and Abeta, our results may be extended to the emerging field of divalent ion-associated amyloidosis. 相似文献