Formation of new rod photoreceptor synapses onto differentiated bipolar cells in goldfish retina.

In goldfish new rods are continuously added to the entire retina at a rate that assures stable rod density, while the densities of other neurons decrease. The b1 bipolar, known to contact every rod within its dendritic domain, was used to determine the fate of these newly formed rods. Golgi-stained b1 bipolars were sectioned serially at 0.5 micron in the plane of the receptor terminals and reconstructions of their rod and cone contacts were prepared from camera lucida drawings. The newly formed rods are accommodated within the dendritic trees of already-formed b1 bipolars at a rate of about one new rod synapse/bipolar/month. During growth from about 6 months to 5 years of age the number of synapses onto each b1 bipolar increases by 50%. Concomitantly the dendritic tree area increases by about 50%, and the density of rod-b1 synapses remains constant at about one synapse/11 micron 2. Assuming a dendritic coverage factor of 1, the b1 bipolars will contact every retinal rod. The numbers of cones contacted and not contacted do not significantly change. The overall dimensions of b1 bipolars increase with retinal growth and new branches are added to their dendritic trees. These observations show that new rods added to adult goldfish retina form synapses with old bipolars. Some functional inferences are also made.