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Fig. 2. The role of axon-axon interactions in the formation of the retinotectal
map. (A) Dorsal view of the head of a wild-type zebrafish larva,
showing the projection of retinal ganglion cells (colored circles) from the
eye to the optic tectum (broken outlines). The terminal arbors of
ganglion-cell axons are arranged in a topographic fashion, preserving their
neighborhood relationships in the retina. Axons from the temporal retina (red,
T) are connected to the anterior (A) region of the tectum; nasal axons (green,
N) project to the posterior (P) tectum; and axons from intermediate positions
in the retina (yellow) terminate in the center of the tectum. (B)
Dorsal view of a lakritz (atoh7) mutant zebrafish in which a
single ganglion cell has developed from a transplanted clone of wild-type
cells. The single ganglion cell (green; an example of a nasal cell is shown
here) sends a solitary axon into the tectum, where it projects to its
topographically appropriate zone, but forms a larger arbor than is normal (red
arrowhead). This experiment rules out a requirement for axon competition in
retinotectal mapping along the anterior-posterior axis. However, axon-axon
interactions strongly influence the stability of branches on the
proximal/anterior side of the arbor. Images courtesy of Nathan Gosse and
Herwig Baier (University of California at San Francisco, CA, USA).
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