Recovery of regenerating goldfish retinal ganglion cells is slowed in the absence of the topographically correct synaptic target.

When the axons of goldfish retinal ganglion cells are severed the cell bodies undergo a series of changes as the axons regenerate. These changes begin to reverse when the axons start to innervate the tectum and by 3 months after the lesion the cell bodies have nearly returned to normal. When the axons projecting to the caudal tectum were severed by a mediolateral transection of the tectum, only retinal ganglion cells in the nasal portion of the contralateral retina underwent the changes normally associated with regeneration, followed by a speedy return to normal. Because the injured fibers probably did not fully retract from the tectum, these results indicated that: (1) the complete removal of the axons from the tectal milieu was not essential for initiating the cell body changes, and (2) close proximity to the target sites would speed the recovery of the cells. When the caudal portion of the tectum was ablated the retinal ganglion cells of the nasal retina remained enlarged significantly longer than after tectal transection. During the time the cells remained enlarged the electrophysiological projection onto the remaining rostral part of the tectum revealed no significant 'compression' of the visual field. Compression of the visual field onto the rostral portion of the tectum can be accelerated if the caudal tectal ablation is accompanied by an optic nerve crush. However, under this condition the recovery of ganglion cells in the nasal retina was significantly slower than the recovery of cells in the temporal retina. This may reflect an element of topographical specificity in the regulation of the recovery of the cell body from axonal injury.