Changes in retinal projections and ganglion cell morphology after unilateral enucleation in the common carp

Changes in retinal projections and ganglion cell morphology were studied in one-eyed individuals of the common carp, Cyprinus carpio, which were enucleated at a juvenile stage (within 6 months after hatching) and kept for 18 months after the operation. Gross examination of the brains showed a marked atrophy of the contralateral optic tectum and a fine attenuated optic tract ipsilateral to the remaining eye. All retinal recipient areas were bilateral, but numerous projections were heavier contralaterally. Terminal branches in the recipient areas showed more complex patterns with tortuous courses and larger numbers of terminal swellings than in normal animals. Total numbers and distribution patterns of ganglion cells in Nissl-stained retinal whole mounts of one-eyed carp were compared with those in normal carp. The total number of ganglion cells was estimated to be 14 x 10(4)-18 x 10(4) in both one-eyed and normal carp. No difference was observed in isodensity maps and soma area histograms between one-eyed and normal carp. Following injections of horseradish peroxidase and nuclear yellow into the optic tectum of each side, three different types of tectal projecting ganglion cells were observed in the remaining retina: contralaterally projecting (CP) cells, ipsilaterally projecting (IP) cells, and bilaterally projecting (BP) cells. The distribution pattern of CP and BP cells in the retina suggested normal retinotopy. However, BP cells were found in a more restricted zone within the CP cell distribution area. The IP cells had a tendency to be scattered sparsely in a wide central area and a dorsal quadrant of the retina. No IP or BP cells were found in the peripheral retina. The time course and morphological changes in axons of these cells are discussed.