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481.
482.
Maternal age specific rates for all major chromosome aberrations have been determined in 52 965 pregnancies in mothers 35 years of age and over at the time of amniocentesis. Rates increase exponentially with advancing maternal age for trisomies 21, 18 and 13, and for the XXX and XXY syndromes, but in the autosomal trisomies this rise appears to be followed by a levelling off at the upper end of the age range. A significant inverse relationship with maternal age is found for 45,X cases. It is postulated that these various patterns are the result of the interaction of three principal factors: a maternal age effect acting particularly on first meiotic nondisjunction: a higher spontaneous abortion rate with advancing maternal age for aneuploid as compared to euploid conceptions; and an increased probability of spontaneous abortion before the time of amniocentesis for conceptions with more extensive chromosome imbalance. A stepwise logistic regression analysis of 13 299 pregnancies in which both parental ages are known shows that the father's age does not influence these maternal age specific rates, with the possible exception of the 47,XXY syndrome. 相似文献
483.
In this paper we deal with the problem of identifying environmental principles for the design and operation of supply chains. The operations that are included in supply chains are briefly described along with the approaches that are applied in order to improve their environmental performance. A background of environmental principles for achieving eco-efficiency and building of environmentally friendly organizational systems is presented and emphasis is put on the application of such principles “from cradle to grave”. Then, environmental principles applicable to particular objects of logistics networks planning are identified and commented upon. In addition, selective case studies from the literature, which show the applicability of the formulated principles and their relevance to practice, are discussed. The paper concludes with some remarks regarding the benefits for companies and societies, in general, that occur as a result of the application of the formulated principles. 相似文献
484.
485.
Jörg Pfleiderer 《Die Naturwissenschaften》1989,76(7):297-300
Die Idee zu dieser Arbeit geht auf Diskussionen mit W. Priester über die Reduktion von Radiobeobachtungen ausgedehnter Galaxien im Jahre 1974 zurück. Die wesentlichen Teile wurden während eines Aufenthalts des Autors am Department of Astronomy, University of Florida, Gainesville, im Jahre 1987 entworfen. Ich habe ferner vielen Kollegen für hilfreiche Hinweise zu danken, darunter E. E. Baart, J. Condon, T. Cornwell, L. Erickson, S. Gottesman, J. Högbom, J. Skilling. 相似文献
486.
Circulating placental [human chorionic gonadotrophin (hCG), Schwangerschafts protein 1 (SP1), pregnancy-associated plasma protein A (PAPP-A), decidual (pregnancy protein) 12 (PP12), and fetal alphafetoprotein (AFP)] proteins were measured immediately before and within 1 h in 18 women undergoing diagnostic chorionic villus sampling (CVS) in the first trimester. An elevation of serum AFP levels was consistently seen, while fluctuations in excess of 10 per cent of the pre-CVS levels of SP1 and PP12 were seen in the majority of patients. Fluctuations in hCG and PAPP-A were consistently less than 10 per cent of pre-CVS values. Post-CVS changes in levels were not apparently associated with any feature of the technique, the pregnancy, or its outcome (one missed abortion). As feto-maternal haemorrhage is a common event, anti-D should be offered to rhesus-negative women undergoing CVS. In the prediction of subsequent miscarriage, only hCG and PAPP-A measurements should be considered. 相似文献
487.
488.
489.
近来巴西大豆生产的增长引发了环境保护团体以及一些学者对于亚马孙盆地的热带自然环境正在向大豆种植地转换的关注.大豆生产的支持者则认为大豆是区域的一种可行性农产品;对于环境的关注是没有根据的,因为新的大豆种植地正在替代已经被砍伐的林地或者是转换了的土地.两种观点虽被提出却都没有对大豆生产扩充区的土地利用与土地覆被(LULC)状况进行过综合研究与估量.本案例研究是在巴西的朗多尼亚州,亚马孙盆地西南部的维列纳市(Vimena),通过遥感手段对1996~2001年以来该市大豆种植面积的大幅度增加所伴随的土地利用与土地覆被变化状况进行评估.结果表明虽然森林变成了大豆生产基地,但是大部分的大豆生产的增长归功于对已有土地的微量扩充、已伐林地的转换以及更高的单位产量. 相似文献
490.
In Hydra vulgaris at the level of dissociated single cells endodermal cells adhere to each other more readily than to ectodermal cells at the
initial adhesion. The time required for adhesion to occur between two adjacent cells is shorter for both endodermal and ectodermal
homotypic cell adhesions once the initial adhesion of the first pair of cells has been established. It is confirmed that contact
of an aggregated pair with additional homotypic cells facilitates the occurrence of homotypic adhesions; heterotypic adhesions
are discouraged. This suggests that adhesion of homotypic cells contributes to an increased readiness for subsequent homotypic
cells to adhere.
Received: 25 August 1999 / Accepted in revised form: 11 January 2000 相似文献