Growth zone of the juvenile goldfish retina revealed by fluorescent flat mounts

The retinas of juvenile goldfish (Carassius auratus) were processed for fluorescence microscope studies with flat mounts. The fish were reared in a laboratory aquarium for 1-3 months after either one class of retinal dopaminergic (DA) cells or indoleamine-accumulating (IA) cells had been destroyed by intravitreal injection of 6-hydroxydopamine (6-OHDA; 2.5 micrograms) or 5,7-dihydroxytryptamine (5,7-DHT; 2.5 micrograms). The fish grew 10-12 cm in body length on the average for 3 months. The retinas were isolated at various survival times after pretreatment with 6-OHDA or 5,7-DHT. To improve detection of the cells, a mixture of noradrenaline (NA; 0.5 microgram) and 5,6-DHT (0.5 microgram) was intravitreally injected into the eyes 2 hr before enucleation. Retinal growth occurred by cell proliferation at the circumferential margin distal to the retinal center. In this process, DA and IA cells were added in concentric rings, with a rate of cell accretion of approximately 12 cells per day, estimated from the mean cell density in the growth zone (about 252 micrograms width) over a 3-month period. In order to compensate for loss of dopaminergic innervation induced by intravitreal 6-OHDA, the proliferation rate appeared to be facilitated significantly compared to that in normal retinas, and long and centrally extending processes were found to emerge from the DA cells formed earliest after intravitreal injection of 6-OHDA.