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991.
低氧对大鼠体液免疫的抑制作用 总被引:3,自引:1,他引:2
本实验以模拟高原低氧的方法,探讨了低氧对大鼠体液免疫的作用,并与高原鼠兔比较,体液免疫以溶血素和IgG产生为指标。实验结果:与对照组相比,大鼠低氧10d,5km海拔抑制溶血素形成10.3%,7km海拔抑制溶血素形成21.9%;经再次免疫后又低氧10d,5km海拔抑制溶血素形成4.2%,7km海拔抑制溶血素形成4.6%,高原土著动物高原鼠兔则不表现上述的抑制现象;大鼠经SRBC腹腔致敏形成2d后低氧 相似文献
992.
Wim D.F. Meutermans Paul F. Alewood Greg T. Bourne Bernadette Hawkins Mark L. Smythe 《Letters in Peptide Science》1997,4(2):79-84
-Aspartyl-containing cyclic pentapeptides were synthesised in high yields using astrategy that maintained fluorenylmethyl protection on the aspartic acid side chain during chainassembly, resin cleavage and cyclisation of the linear precursors. Tetra-n-butylammoniumfluoride treatment of the fluorenylmethyl-protected cyclic peptides catalysed imide formation,whereas piperidine-induced deprotection resulted in good yields of the target cyclic peptides. 相似文献
993.
The relationship between the cessation of cell expansion and formation of the secondary wall was investigated in the early-wood
tracheids of Abies sachalinensis Masters by image analysis and field emission scanning electron microscopy. The area of the lumen and the length of the perimeter
of the lumen of differentiating tracheids increased from the cambium towards the xylem. These increases had just ceased in
the case of tracheids closest to the cambium in which birefringence was first detected by observations with a polarizing light
microscope. Cellulose microfibrils (MFs) deposited on the innermost surfaces of radial walls were not well ordered during
the expansion of cells, but well ordered MFs were deposited at the subsequent stage of cell wall formation. The first well
ordered MFs were oriented in an S-helix. The well ordered MFs had already been deposited at the tracheids where birefringence was first detected under the
polarizing light microscope. These results indicate that the deposition of the well ordered MFs, namely, the formation of
the secondary wall, begins before the cessation of cell expansion of tracheids. Therefore, it seems that the expansion of
tracheids is restricted by the deposition of the secondary wall because the cell walls become rigid simultaneously with the
development of the secondary wall and, therefore, the yield point of cell walls exceeds the turgor pressure of the cell.
Received: 3 July 1996 / Accepted: 24 September 1996 相似文献
994.
Planarian pharynx regeneration in regenerating tail fragments monitored with cell-specific monoclonal antibodies 总被引:2,自引:0,他引:2
D. Bueno Lluis Espinosa Jaume Baguñà Rafael Romero 《Development genes and evolution》1997,206(7):425-434
The special morphological features of freshwater planarians make them an attractive and informative model for studying the
processes of regeneration and pattern formation. In this work, we investigate pattern formation and maturation of the planarian
pharynx during regeneration in tail fragments. Using three monoclonal antibodies (TCAV-1, TF-26 and TMUS-13) specific for
epithelial, secretory and muscle cells, respectively, we followed the sequence and timing of differentiation and maturation
of these three cell types within the regenerating pharynx. Two of these monoclonal antibodies, TCAV-1 and TMUS-13, also labelled
morphologically immature cells that appear to be committed to the differentiation pathway leading to their respective adult
cell types. Our results show that the cells forming the new pharynx come from undifferentiated cells through proliferation
and differentiation processes rather than from differentiated cells of the old stump. We describe three stages of pharynx
regeneration according to the immunoreactivity shown: (1) no immunoreactivity, corresponding to the accumulation of undifferentiated
cells that form the pharynx primordium; (2) immunoreactivity to TCAV-1 and TMUS-13, corresponding to the re-building of the
pharynx; and (3) immunoreactivity to TF-26, corresponding to a fully mature and functional pharynx. The sequence of differentiation
of these three cell types suggests that the pharynx grows by intercalation of new undifferentiated cells coming from the parenchyma
between the older pharyngeal cells, in agreement with existing models of pharynx regeneration. Finally, our results suggest
an intercalary model for pharynx epithelial cell renewal.
Received: 30 September 1996 / Accepted: 6 December 1996 相似文献
995.
A key step in the evolution of multicellular organisms is the formation of cooperative multicellular groups. It has been suggested that predation pressure may promote multicellular group formation in some algae and bacteria, with cells forming groups to lower their chance of being eaten. We use the green alga Chlorella vulgaris and the protist Tetrahymena thermophila to test whether predation pressure can initiate the formation of colonies. We found that: (1) either predators or just predator exoproducts promote colony formation; (2) higher predator densities cause more colonies to form; and (3) colony formation in this system is facultative, with populations returning to being unicellular when the predation pressure is removed. These results provide empirical support for the hypothesis that predation pressure promotes multicellular group formation. The speed of the reversion of populations to unicellularity suggests that this response is due to phenotypic plasticity and not evolutionary change. 相似文献
996.
The formation of distinct subdomains of the cell surface is crucial for multicellular organism development. The most striking example of this is apical‐basal polarization. What is much less appreciated is that underpinning an asymmetric cell surface is an equally dramatic intracellular endosome rearrangement. Here, we review the interplay between classical cell polarity proteins and membrane trafficking pathways, and discuss how this marriage gives rise to cell polarization. We focus on those mechanisms that regulate apical polarization, as this is providing a number of insights into how membrane traffic and polarity are regulated at the tissue level. 相似文献
997.
Nial Rau Gursanscky Virginie Jouannet Karin Grünwald Pablo Sanchez Martina Laaber‐Schwarz Thomas Greb 《The Plant journal : for cell and molecular biology》2016,86(3):210-220
Plants maintain pools of pluripotent stem cells which allow them to constantly produce new tissues and organs. Stem cell homeostasis in shoot and root tips depends on negative regulation by ligand–receptor pairs of the CLE peptide and leucine‐rich repeat receptor‐like kinase (LRR‐RLK) families. However, regulation of the cambium, the stem cell niche required for lateral growth of shoots and roots, is poorly characterized. Here we show that the LRR‐RLK MOL1 is necessary for cambium homeostasis in Arabidopsis thaliana. By employing promoter reporter lines, we reveal that MOL1 is active in a domain that is distinct from the domain of the positively acting CLE41/PXY signaling module. In particular, we show that MOL1 acts in an opposing manner to the CLE41/PXY module and that changing the domain or level of MOL1 expression both result in disturbed cambium organization. Underlining discrete roles of MOL1 and PXY, both LRR‐RLKs are not able to replace each other when their expression domains are interchanged. Furthermore, MOL1 but not PXY is able to rescue CLV1 deficiency in the shoot apical meristem. By identifying genes mis‐expressed in mol1 mutants, we demonstrate that MOL1 represses genes associated with stress‐related ethylene and jasmonic acid hormone signaling pathways which have known roles in coordinating lateral growth of the Arabidopsis stem. Our findings provide evidence that common regulatory mechanisms in different plant stem cell niches are adapted to specific niche anatomies and emphasize the importance of a complex spatial organization of intercellular signaling cascades for a strictly bidirectional tissue production. 相似文献
998.
Lukas Rigger Sandro Neuner Byung‐Sik Shin Gulnara Yusupova Thomas E Dever Ronald Micura Marat Yusupov 《EMBO reports》2016,17(12):1776-1784
Proline is an amino acid with a unique cyclic structure that facilitates the folding of many proteins, but also impedes the rate of peptide bond formation by the ribosome. As a ribosome substrate, proline reacts markedly slower when compared with other amino acids both as a donor and as an acceptor of the nascent peptide. Furthermore, synthesis of peptides with consecutive proline residues triggers ribosome stalling. Here, we report crystal structures of the eukaryotic ribosome bound to analogs of mono‐ and diprolyl‐tRNAs. These structures provide a high‐resolution insight into unique properties of proline as a ribosome substrate. They show that the cyclic structure of proline residue prevents proline positioning in the amino acid binding pocket and affects the nascent peptide chain position in the ribosomal peptide exit tunnel. These observations extend current knowledge of the protein synthesis mechanism. They also revise an old dogma that amino acids bind the ribosomal active site in a uniform way by showing that proline has a binding mode distinct from other amino acids. 相似文献
999.
Ethylene‐dependent aerenchyma formation in adventitious roots is regulated differently in rice and maize 下载免费PDF全文
Takaki Yamauchi Akihiro Tanaka Hitoshi Mori Itsuro Takamure Kiyoaki Kato Mikio Nakazono 《Plant, cell & environment》2016,39(10):2145-2157
In roots of gramineous plants, lysigenous aerenchyma is created by the death and lysis of cortical cells. Rice (Oryza sativa) constitutively forms aerenchyma under aerobic conditions, and its formation is further induced under oxygen‐deficient conditions. However, maize (Zea mays) develops aerenchyma only under oxygen‐deficient conditions. Ethylene is involved in lysigenous aerenchyma formation. Here, we investigated how ethylene‐dependent aerenchyma formation is differently regulated between rice and maize. For this purpose, in rice, we used the reduced culm number1 (rcn1) mutant, in which ethylene biosynthesis is suppressed. Ethylene is converted from 1‐aminocyclopropane‐1‐carboxylic acid (ACC) by the action of ACC oxidase (ACO). We found that OsACO5 was highly expressed in the wild type, but not in rcn1, under aerobic conditions, suggesting that OsACO5 contributes to aerenchyma formation in aerated rice roots. By contrast, the ACO genes in maize roots were weakly expressed under aerobic conditions, and thus ACC treatment did not effectively induce ethylene production or aerenchyma formation, unlike in rice. Aerenchyma formation in rice roots after the initiation of oxygen‐deficient conditions was faster and greater than that in maize. These results suggest that the difference in aerenchyma formation in rice and maize is due to their different mechanisms for regulating ethylene biosynthesis. 相似文献
1000.
Amina Zarrugh 《Ethnic and racial studies》2016,39(15):2722-2739
This article explores significant factors influencing the process of Arab American racial formation. I bring into conversation theories of racial formation and ‘political shock’ in social movement scholarship to develop the notion of ‘racialized political shock’ as an important factor in how racial and ethnic groups mobilize and organize. Many moments of political shock are highly racialized and have the potential to reorder the racial and ethnic landscape in ways that can open opportunities or introduce constraints to mobilizations around racial formation. Drawing on existing studies of Arab Americans, this paper highlights how Arab American racial formation has been galvanized during moments of racialized political shock. In the Arab American case, these moments have led to a call for recognition outside the category of white. I conclude by outlining ways forward in the study of Arab Americans, who have been overlooked in studies of race and ethnicity in the US. 相似文献