ABSTRACT
In the normal goldfish, optic axons are distributed between the two arms (brachia) of each optic tract, in such a way that each axon enters the tectum close to its retinotopic termination site. We have shown previously that regenerating axons at first express little or no preference for their normal brachium. Later, however, a partial refinement of the brachial pathway takes place, implying that some axons must have sent out divergent collateral branches and then eliminated the least appropriate.
We have now studied the formation and subsequent loss of axon collaterals in regeneration using retro–gradely transported fluorescent dyes. We labelled the axons in the medial brachium with Fast Blue and those in the lateral brachium with Diamidino Yellow in a way that avoided cross–contamination. In normal fish, yellow–labelled ganglion cells dominated the dorsal retina and blue–labelled ganglion cells the ventral, with only a narrow zone of overlap. Double–labelled cells were not found.
In fish labelled early in regeneration, however, both dyes were spread over the entire retina in single– and double–labelled ganglion cells. As regeneration progressed, each dye again came to dominate its appropriate retinal region; but much less strongly, confirming previous results. At the same time, doublelabelled cells became harder to find. From 60 days after nerve cut onwards they were rare, and largely confined to the boundary zone between dorsal and ventral retina. Their distribution was consistent with our previous proposal that excess collaterals may be eliminated competitively. Since axons from the boundary zone terminate in a tectal strip equidistant from the two brachia, neither route would confer any clear competitive advantage over the other.
