Organization of retinopetal axons in the optic nerve of the cichlid fish, Herotilapia multispinosa

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Abstract
We have studied the position and numbers of retinopetal axons in the rainbow cichlid fish, Herotilapia multispinosa, to determine the response of related parts of the brain of fish to the continual addition of new neurons in the retina. The retinopetal axons were traced by using the retrograde tracers HRP and cobaltous lysine and an immunocytochemical probe, antibodies to FMRFamide, the molluscan cardioexcitatory peptide. One population of cells with retinopetal axons was found in the telencephalon (in the nucleus olfactoretinalis) and the other was scattered in the diencephalon. Some of the cells in the nucleus olfactoretinalis with retinopetal axons were FMRFamide positive; antibodies were used to trace the axons of these cells into the retina. All the retinopetal axons, from the nucleus olfactoretinalis and the diencephalon, were confined to the portion of the optic nerve that contains axons from the central retinal ganglion cells, that is, the oldest ganglion cells. Theis result suggests that the retinopetal axons grow into the optic nerve and retina early in the life of the fish, and no new ones are added later in life despite the extensive addition of cells in the retina. Counts of the cells in the nucleus olfactoretinalis that project to the retina in 3‐month‐old and adult fish support this interpretation. We conclude that retinopetal axons grow into the retina early in the life of the fish and respond to the formation of new retina by extending their arbors toward the new retina.