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Growth characteristics of ganglion cell axons in the developing and regenerating retino-tectal projection of the rat
No full textRetinal ganglion cell (RGC) axons from the temporal retina project to the anterior superior colliculus (SC), whereas nasal retinal axons project to the posterior SC. The stripe assay has shown that temporal retinal axons avoid growing on membrane stripes from the posterior SC but nasal retinal axons show no growth preference. Several putative guidance molecules have been identified in target tissues for these axons during development in vertebrates. Regenerating axons from adult rat retinae also possess the capacity to recognize appropriate target cells and to form functional connections in vivo and in vitro. However, the expression of information for axonal guidance and target recognition in mammals and birds seems to be limited to the period when central nervous system projections develop during embryogenesis. Nevertheless, the deafferented adult rat SC re-establishes target information recognizable by embryonic rat retinae, although the re-expression of guidance factors after deafferentation does not seem to be a mere recapitulation of their normal developmental expression pattern. The roles of astrocytes and microglia in axon guidance are also discussed
Retinal ganglion cell axons recognize specific guidance cues present in the deafferented adult rat superior colliculus
No full textDuring development, retinal ganglion cell axons establish a topographically ordered projection from the retina to the superior colliculus (SC). The putative guidance activities for retinal axons that operate during embryonic development are not detectable in the normal adult SC. However, these cues reappear upon transection of the optic nerve of adult rats. In the present study, we used a modified version of the 'stripe assay,' in which membranes from either anterior or posterior SC alternated with laminin stripes. Temporal embryonic retinal axons consistently avoid membranes from embryonic posterior SC, but only rarely from adult deafferented SC. However, they are attracted to membranes from both embryonic and adult deafferented anterior SC. Nasal retinal axons only show a significant preference for membranes from posterior SC after deafferentation. When retinal axons were offered a choice to grow on membranes either from their embryonic or their deafferented target regions, they showed a preference for the deafferented SC. On carpets consisting of laminin and membranes from normal SC (not deafferented) or nontarget regions (inferior colliculus), temporal and nasal axons grow either in a random fashion or show preferences for the laminin stripes. Our modified version of the classic stripe assay shows specific growth preferences of embryonic retinal axons for membrane lanes from their appropriate embryonic or deafferented adult target regions. These findings suggest that the deafferentation of the SC in adult rats triggers the reexpression of specific guidance activities for retinal axons. Those 'attractive' guidance cues appear to be differentially expressed in the developing and deafferented SC
Retinal ganglion cell axons recognize specific guidance cues present in the deafferented adult rat superior colliculus
No full textDuring development, retinal ganglion cell axons establish a topographically ordered projection from the retina to the superior colliculus (SC). The putative guidance activities for retinal axons that operate during embryonic development are not detectable in the normal adult SC. However, these cues reappear upon transection of the optic nerve of adult rats. In the present study, we used a modified version of the 'stripe assay,' in which membranes from either anterior or posterior SC alternated with laminin stripes. Temporal embryonic retinal axons consistently avoid membranes from embryonic posterior SC, but only rarely from adult deafferented SC. However, they are attracted to membranes from both embryonic and adult deafferented anterior SC. Nasal retinal axons only show a significant preference for membranes from posterior SC after deafferentation. When retinal axons were offered a choice to grow on membranes either from their embryonic or their deafferented target regions, they showed a preference for the deafferented SC. On carpets consisting of laminin and membranes from normal SC (not deafferented) or nontarget regions (inferior colliculus), temporal and nasal axons grow either in a random fashion or show preferences for the laminin stripes. Our modified version of the classic stripe assay shows specific growth preferences of embryonic retinal axons for membrane lanes from their appropriate embryonic or deafferented adult target regions. These findings suggest that the deafferentation of the SC in adult rats triggers the reexpression of specific guidance activities for retinal axons. Those 'attractive' guidance cues appear to be differentially expressed in the developing and deafferented SC
Growth preferences of adult rat retinal ganglion cell axons in retinotectal cocultures
No full textWe examined whether regenerating axons from adult rat ganglion cells are able to recognize their appropriate target region in vitro. Explants from adult rat retina were cocultured with embryonic sagittal midbrain slices in Matrigel((R)). The midbrain sections contained the superior colliculus, the main target for retinal ganglion cell axons in rats, and the inferior colliculus. We observed a statistically significant preference of both temporal and nasal retinal axons to grow toward their appropriate target region (anterior and posterior superior colliculus, respectively). No preferential growth of retinal ganglion cell axons was detected in controls, for which retinal explants were cultured on their own. When retinal ganglion cell axons were given a choice between superior colliculus and inferior colliculus, axons from nasal retina preferentially grew toward the posterior superior colliculus and avoided the inferior colliculus. In contrast, temporal axons in the same assay did not show preference for either of the colliculi. These findings suggest that regenerating axons from adult rat retina are able to recognize target-specific guidance cues released from embryonic midbrain targets in vitro. (C) 1998 John Wiley & Sons, Inc
Appearance of target-specific guidance information for regenerating axons after CNS lesions
No full textDuring development of the vertebrate visual system, an orderly projection of ganglion cells from the retina onto the superior colliculus (SC) is established. Mechanisms that might govern this process include the coordinated action of guidance and corresponding receptor molecules that are specifically distributed on the axons and their targets. In birds and mammals, information for axonal guidance and targeting appears to be confined to the time when the retinocollicular projection is being formed. Here we show that putative guidance activities for temporal and nasal retinal axons, which are not detectable in the normal adult SC, appear after optic nerve transection in adult rats. Both embryonic and adult retinal axons are able to respond to these guiding cues, although the guidance activities detectable in the deafferented adult rat SC might be different from those found during development. These findings imply that it might be possible to reestablish an ordered projection after lesions in the adult mammalian visual system
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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