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51.
Cell recognition during neuronal development 总被引:20,自引:0,他引:20
C S Goodman M J Bastiani C Q Doe S du Lac S L Helfand J Y Kuwada J B Thomas 《Science (New York, N.Y.)》1984,225(4668):1271-1279
Insect embryos, with their relatively simple nervous systems, provide a model system with which to study the cellular and molecular mechanisms underlying cell recognition during neuronal development. Such an approach can take advantage of the accessible cells of the grasshopper embryo and the accessible genes of Drosophila. The growth cones of identified neurons express selective affinities for specific axonal surfaces; such specificities give rise to the stereotyped patterns of selective fasciculation common to both species. These and other results suggest that early in development cell lineage and cell interactions lead to the differential expression of cell recognition molecules on the surfaces of small subsets of embryonic neurons whose axons selectively fasciculate with one another. Monoclonal antibodies reveal surface molecules in the Drosophila embryo whose expression correlates with this prediction. It should now be possible to isolate the genes encoding these potential cell recognition molecules and to test their function through the use of molecular genetic approaches in Drosophila. 相似文献
52.
Genetic diversity in the Lima bean (P. lunatus L.) was assessed bymeans of two chloroplast DNA probes. Based on data obtained from 152accessions including wild forms and landraces, the two separateMesoamerican and Andean groups were confirmed and a transition poolof genetic diversity was observed in the two botanical formsconsistent with their distribution range. Three primary centres ofgenetic diversity and one secondary diversification spot are proposedfor the wild Lima bean whereas only two domestication sites areunderlined for the landraces. The importance of human intervention inwidening the distribution range and the genetic diversity inlandraces is discussed. 相似文献
53.
The modeling of wavelet coefficient for still image compression is studied.Firstly,through simulation experiments on three classical wavelet coefficient models(i.e.,EZW model,SPIHT model/and EBCOT model),it is shown that SPIHT is superior to EZW in terms of compression performance(0.6 dB higher)and the compression performance of EBCOT is significantly better than that of SPIHT on average. Then the cause for such performance differences is explained through the analysis of models.Finally a new wavelet coefficients model is proposed which combines the advantages of the three classical models.Experimental results show that our model is better than the above mentioned three models with respect to compression ratio. 相似文献
54.
Narindra H. Roopnarine Sunil K. Gupta Lizette du Plessis Tom A. Aire 《Anatomia, histologia, embryologia》2020,49(6):814-819
It remains a major concern that sperm structure has continued to be poorly investigated and reported in avian species. To our knowledge, sperm structure in the order Pelecaniforme has not been reported. Although McFarlane (1963; Proceedings of the XIII International Ornithological Congress; Ithaca, NY; American Ornithologists’ Union) reported the study of spermatozoa in two genera and two species of the family Ardeidae, he did not provide an account, or the names of the species examined. The present report on the sperm structure of the cattle egret, Bubulcus ibis, is, thus, the first in the order Pelecaniformes (this bird has been placed variably under the order Ciconiiformes, or the order Pelecaniformes). Five sexually mature and reproductively active male cattle egrets were obtained from the wild, humanely euthanized, the reproductive organs dissected out, and tissues from the ducti deferentia were prepared for transmission electron microscopy. The sperm structure of this bird is generally similar to that described for most non-passerine birds. However, the acrosome is a short, conical or bullet-shaped, blunt-ending organelle that lacks a perforatorium. The base of the acrosome is flat and makes contact with the nucleus along, a correspondingly flat plane. The nucleus, thus, ends anteriorly in a flat plane devoid of a concavity or a rostrum, and an endonuclear canal. The acrosomal and nuclear features of this bird are, therefore, main deviations from the situation in the non-passerine clade of birds. 相似文献
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56.
J. A. Tarr M. W. van Rooyen J. du P. Bothma 《Land Degradation \u0026amp; Development》2004,15(5):487-497
Phragmites australis (Cav.) Trin. ex Steud. has been harvested in the Muzi Swamp in Maputaland, South Africa for generations. Over the last ten years, however, a flourishing trade in this reed has developed. Concern has now been expressed that at the current levels of utilization the ecological integrity of the Muzi Swamp is being compromised, and that the current harvesting rates are not sustainable in the long term. The hypothesis was put forward that a degradation gradient exists with the most severe degradation occurring the closest to where community members enter the park, and the least degradation the furthest from this point. The results of this study, however, show no distinct degradation gradient. Yet the overall condition of the reeds in the harvesting area is poorer than in the non‐utilized area. Expansion of the current harvesting area, coupled with adaptive harvesting systems and yearly monitoring will improve the quality of the reeds within the harvesting area without affecting the harvesting quotas. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
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J L du Plessis L van Gas F J Labuschagne S Wijma 《The Onderstepoort journal of veterinary research》1990,57(3):145-149
Lyophilized tissues of mice and blood of sheep, infected with either the Kümm, the Welgevonden or the Ball 3 stock of Cowdria ruminantium, remain infective to mice and sheep after storage at 4 degree C for 90 days. Freeze-dried tissues stored at -18 degrees C and -28 degrees C are still infective after 6 months and 2 years, respectively. 相似文献
59.
J L du Plessis F T Potgieter L van Gas 《The Onderstepoort journal of veterinary research》1990,57(3):205-208
Neither sheep immune to the Ball 3, the Kwanyanga or the Mara stocks of Cowdria ruminantium nor those immunized with combinations of these 3 stocks were protected against challenge with the Mali stock. Against challenge with the Welgevonden stock, however, immunization with each of the 3 combinations and with single stocks effected a protective immunity. 相似文献
60.
Oxidative browning is one of the reasons why Protea cynaroides is not being extensively propagated via tissue culture techniques. The reduction of oxidative browning was best achieved by stirring the explants for 1 h in an antioxidant solution mixture containing 100 mg l−1 ascorbic acid and 1500 mg l−1 citric acid before inserting them into the medium. In combination with this antioxidant solution, in vitro establishment was highly successful where 100% bud growth was achieved by explants growing under 16 h photoperiod. 相似文献