Abstract
Optic and nonoptic fibers and synapses were counted in the primary optic innervation layer (S-SO-SFGS) in anteromedial tectum in normal goldfish and in fish 30, 60, and 240 d after the optic nerve was crushed. A newly developed “cold-fill” HRP-labeling protocol was used to label optic afferents for electron microscopy, and counts were then made on EM photomontages of columns through the HRP-labeled S-SO-SFGS. Normal numbers of retinotectal synapses were present at 30 d regeneration, at a time when activity-dependent refinement of the optic projection is incomplete. Normal numbers were also found at 60 and 240 d, when refinement is largely completed. In contrast to this constancy in optic synapse numbers, there was nearly 10 times the normal number of optic fibers in the SFGS at 30 d, and these were reduced by 50% at 60 d, remaining over 4 times normal at 240 d. These findings imply extensive rearrangement of optic synapses during map refinement. They also indicate that synapse rearrangement is associated with the elimination of optic collaterals.