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Barbara S. Parris 《Brittonia》2001,53(2):270-283
Four major austral continental distribution patterns are evident in pteridophytes. Twenty-two species are completely circum-Antarctic.
Another 39 species are partially circum-Antarctic, occurring in Australasia (Australia and New Zealand) and Africa (including
Madagascar) but not South America, while 29 are in Africa and South America but not Australasia, and 13 are in South America
and Australasia but not Africa. Two hypotheses are considered as explanations for the patterns: continental drift following
the breakup of Gondwana and long-distance dispersal. Fossil evidence indicates that the majority of pteridophyte families
involved appeared after the southern continents had drifted apart, so long-distance dispersal is likely to explain the distribution
of species in these families on now widely separated continents. For those families extant before the break-up, there is no
indication in the fossil record that the species involved were present in Gondwana. Aspects of the ecology of the species
that are partly or completely circum-Antarctic indicate that long-distance dispersal, rather than continental drift, is a
likely explanation for the patterns. 相似文献
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The contractile indices Vmax (maximum shortening velocity of the contractile element) and ARPD (power averaged rate of power density generation) which have been shown to be unaffected by alterations in preload and afterload were computed from isovolumic left ventricular pressure data of dogs. The two indices were tested for their ability to detect changes in contractility induced by a positive inotropic drug (Isoprenalin). Whereas a good correlation was found between ARPD and Vmax (coefficient of correlation 0,895) the index ARPD was more sensitive to augmentation of myocardial contractility; also because it is simpler to obtain computationally and more appropriate for the intact heart from a theoretical point of view. ARPD should be useful especially for quantification of acute changes in myocardial contractility. 相似文献
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Barbara D. Abbott Robert M. Pratt 《In vitro cellular & developmental biology. Plant》1988,24(4):343-352
Summary Retinoids and growth factors seem to be important for normal mammalian reproduction and development. High levels of retinoic
acid are teratogenic and induce cleft palate in the mouse. Little is known concerning the mechanisms through which retinoids
induce cleft palate. Palatal epithelia from CD-1 embryonic mice on Day 12 of gestation were isolated from the mesenchyme and
cultured in serum-free media, with all-trans retinoic acid or 13-cis retinoic acid, with or without epidermal growth factor
(EGF). The epithelia attached and grew, and the cells differentiated over a 72-h culture period. Binding of [125I]EGF was observed in all cultures in a pattern that correlated with thymidine (TdR) uptake by the epithelia. EGF enhanced
growth and [3H]TdR incorporation of the oral cells, but nasal cells generally did not proliferate. In this culture system, both retinoids
suppressed [3H]TdR incorporation in a concentration-dependent manner for epithelia cultured with or without EGF. Medial cells are important
to normal palatogenesis as they play a role in fusion of opposing shelves and subsequently many of these cells undergo programmed
cell death. Death of medial cells in vitro is prevented by EGF and by the retinoids, either with or without EGF. This response
occurs in the absence of a mesenchymal interaction, suggesting that the medial cell response to EGF and retinoids is not mediated
by or dependent on the mesenchymal tissues. The survival of medial cells may be responsible for the failure of opposing shelves
to fuse. 相似文献
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