Cone photoreceptor regeneration in adult fish retina: phenotypic determination and mosaic pattern formation

The retina of anamniotes (fish and amphibia), unlike the CNS of most vertebrates, can regenerate neurons following injury. Using the highly ordered mosaic of single and double cones in the retina of the adult green sunfish (Lepomis cyanellus) as our model system, we examined the events that followed the surgical excision of a small patch of central retina. After surgery there was a transient elevation in the number, and a change in the distribution, of proliferative cells within the retina. The wound was filled in two ways: a proliferative regeneration of new retina and a nonproliferative movement of the wound boundaries toward the center of the lesion. The nonproliferative movement stretched the surrounding, intact retina. In stretched retina the basic pattern of the cone mosaic was maintained, but it was augmented by new cones, even though cones are not normally generated in intact central retina. The stretch itself likely triggered the anomalous cone production. The new and preexisting cones in stretched retina had their morphological phenotypes influenced by mutual contact, often resulting in atypical morphologies (triple and quadruple cones). In the center of the lesioned area, the regenerated cone mosaic was disordered, had a higher than normal cone density, and contained atypical morphologies. The presence of outer segments and synaptic pedicles suggested that the new cones in regenerated and stretched retina were functional. We interpret these results to mean (1) a stretch-induced decrease in cell density can trigger a compensatory, adaptive neurogenesis, (2) cone morphological phenotypes in fish retina are plastic throughout life, and are influenced by cone-cone contacts, (3) the mechanisms that spatially regulate cone production during normal growth are disrupted regeneration.