Retinotopic order in the absence of axon competition

The retinotectal projection has long been studied experimentally and theoretically, as a model for the formation of topographic brain maps. Neighbouring retinal ganglion cells (RGCs) project their axons to neighbouring positions in the optic tectum, thus re-establishing a continuous neural represent...

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Bibliographic Details
Published in:Nature 2008-04, Vol.452 (7189), p.892-895
Main Authors: Baier, Herwig, Gosse, Nathan J, Nevin, Linda M
Format: Article
Language:English
Subjects:
Animals
Axons - physiology
Biological and medical sciences
Cellular biology
Danio rerio
DNA-Binding Proteins - genetics
DNA-Binding Proteins - metabolism
Eye and associated structures. Visual pathways and centers. Vision
Fish
Freshwater
Fundamental and applied biological sciences. Psychology
Models, Neurological
Mutation
Neurosciences
Retina
Retinal Ganglion Cells - cytology
Retinal Ganglion Cells - physiology
Superior Colliculi - cytology
Superior Colliculi - physiology
Vertebrates: nervous system and sense organs
Zebrafish - physiology
Zebrafish Proteins - genetics
Zebrafish Proteins - metabolism
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Summary:The retinotectal projection has long been studied experimentally and theoretically, as a model for the formation of topographic brain maps. Neighbouring retinal ganglion cells (RGCs) project their axons to neighbouring positions in the optic tectum, thus re-establishing a continuous neural representation of visual space. Mapping along this axis requires chemorepellent signalling from tectal cells, expressing ephrin-A ligands, to retinal growth cones, expressing EphA receptors. High concentrations of ephrin A, increasing from anterior to posterior, prevent temporal axons from invading the posterior tectum. However, the force that drives nasal axons to extend past the anterior tectum and terminate in posterior regions remains to be identified. We tested whether axon-axon interactions, such as competition, are required for posterior tectum innervation. By transplanting blastomeres from a wild-type (WT) zebrafish into a lakritz (lak) mutant, which lacks all RGCs, we created chimaeras with eyes that contained single RGCs. These solitary RGCs often extended axons into the tectum, where they branched to form a terminal arbor. Here we show that the distal tips of these arbors were positioned at retinotopically appropriate positions, ruling out an essential role for competition in innervation of the ephrin-A-rich posterior tectum. However, solitary arbors were larger and more complex than under normal, crowded conditions, owing to a lack of pruning of proximal branches during refinement of the retinotectal projection. We conclude that dense innervation is not required for targeting of retinal axons within the zebrafish tectum but serves to restrict arbor size and shape.
ISSN:0028-0836
1476-4687
1476-4679
DOI:10.1038/nature06816