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).