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THE eyes Of all birds have a black pleated structure, known as the pecten, which projects from the retinal surface into the vitreal space, towards the lens. This remarkable structure has provoked many speculations about its function1,2, but only one has so far been widely accepted : the pecten has a copious blood supply and this is thought to function as a replacement for the capillary bed that, in many other vertebrates, supplies the vitreal surface of the retina3,4. Its size, shape and heavy black pigmentation have suggested optical functions, some of which centre around its shadow casting potentialities5,6, but many of these theories do not seem to take account of the way the pecten is placed in the optical system. It is oriented nearly parallel to light entering the centre of the pupil, so that it presents the minimum area to the converging rays forming the retinal image and the folded free border that is its most striking gross anatomical feature approaches close behind the lens. Thus the shadow cast by the incoming light, which we here call the “primary shadow”, will be as small as possible and very blurred. It is, furthermore, centred on the pecten's own base, which is the elongated optic disk or “blind spot”, devoid of receptors, where optic nerve fibres leave the eye. Thus we think the primary shadow has no beneficial functional significance; on the contrary, it is remarkable that such a large nutrient structure can be inserted into a small optical system with so little disturbance to its normal image forming function. There is, however, an optical function that can be performed by an opaque screen in the position of the pecten.  相似文献   

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The suggestions from the author's group over the past 25 years for how steps in catalysis by ATP synthase occur are reviewed. Whether rapid ATP hydrolysis requires the binding of ATP to a second site (bi-site activation) or to a second and third site (tri-site activation) is considered. Present evidence is regarded as strongly favoring bi-site activation. Presence of nucleotides at three sites during rapid ATP hydrolysis can be largely accounted for by the retention of ADP formed and/or by the rebinding of ADP formed. Menz, Leslie and Walker ((2001) FEBS Lett., 494, 11-14) recently attained an X-ray structure of a partially closed enzyme form that binds ADP better than ATP. This accomplishment and other considerations form the base for a revised reaction sequence. Three types of catalytic sites are suggested, similar to those proposed before the X-ray data became available. During net ATP synthesis a partially closed site readily binds ADP and Pi but not ATP. At a closed site, tightly bound ADP and Pi are reversibly converted to tightly bound ATP. ATP is released from a partially closed site that can readily bind ATP or ADP. ATP hydrolysis when protonmotive force is low or lacking occurs simply by reversal of all steps with the opposite rotation of the subunit. Each type of site can exist in various conformations or forms as they are interconverted during a 120° rotation. The conformational changes with the ATP synthase, including the vital change when bound ADP and Pi are converted to bound ATP, are correlated with those that occur in enzyme catalysis in general, as illustrated by recent studies of Rose with fumarase. The B structure of Walker's group is regarded as an unlikely, or only quite transient, intermediate. Other X-ray structures are regarded as closely resembling but not identical with certain forms participating in catalysis. Correlation of the suggested reaction scheme with other present information is considered.  相似文献   

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Induction of the Eye Lens   总被引:11,自引:0,他引:11  
In general terms embryonic induction involves the association of embryonic tissues and leads to tissue differentiation. It is one of the known essential processes leading to the normal development of embryos. However, despite its importance, very little is known about the mechanisms of inductive interactions. For example, what is the nature of communication between tissues, how does this communication effect the synthetic activity of the cells, and once a new pattern of synthesis has been established how is the sequence of events leading to tissue differentiation co-ordinated? The answers to these questions will come only from the intensive study of inductive interactions and tissue differentiation at all levels from the morphological to the molecular.
One of the best known examples of induction, at least superficially, is the differentiation of lens from head ectoderm after its interaction with optic vesicle. The popularity of this tissue with embryologists may be attributed to its accessibility of manipulation because of its position on the outside of the embryo. In addition, its distinct morphology and specific biochemical composition make it relatively easy to determine whether the lens differentiates after experimental treatment. About the turn of this century lens differentiation was thought to depend on the specific interaction of just two embryonic tissues, head ectoderm and neuro-ectoderm (optic vesicle). However, experimental analysis since then has revealed that this oversimplified view of lens induction is incorrect. In fact there is evidence that a large number of other tissues besides embryonic head ectoderm can differentiate into lens and that other conditions besides the presence of optic vesicle can induce lens differentiation. The purpose of this work is to review the evidence on lens induction and based on this, to determine what we know about the mechanism(s) controlling this process.  相似文献   

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