Growth of the adult goldfish eye. III. Source of the new retinal cells.

The manner in which new cells are added to the growing adult goldfish retina was examined using 3H-thymidine radioautography. Cell proliferation leading to the formation of neurons is restricted to the retinal margin at the ora terminalis. New retina is added in concentric rings, with slightly more growth dorsonasally. The rate of cell addition is variable, averaging 12,000 cells/day. These new cells account for about 20% of the total increase in retinal area; the remaining 80% is due to hypertrophy, or expansion, of the retina. In contrast to all of the other retinal cells, the rods do not appear to participate in the retinal expansion. This hypothesized immobility of the rods would create a shearing strain between the retinal layers resulting in a shift in their position relative to the other cells. Were they to maintain synaptic contacts with the same horizontal and bipolar cells, the rod axons would have to be elongated peripherally or the post-synaptic cell dendrites displaced centrally. Since neurons with this morphology have not been found in the goldfish retina, these observations suggest that the rods must be changing their synaptic connections as the retina grows.