Dendritic differentiation in the periphery of the growing zebrafish retina.

In the retina of teleost fish, new ganglion cells are generated from a circumferential peripheral growth zone at the edge of the eye throughout life. Addressing the question how new cells are fitted into the existing retina, we investigated newly formed ganglion cells in the zebrafish retina morphologically, by tracing them from the cut optic nerve with rhodamine dextran. We identified proliferating cells by antibody detection against proliferating cell nuclear antigen. In addition, newly formed bipolar cell and amacrine cell dendrites were investigated by antibodies against protein kinase C (PKC) and choline-acetyl-transferase (ChaT), respectively, and analyzed in sections or wholemount preparations using confocal microscopy. In retinal sections we observed that ganglion cell dendritic branches in the inner plexiform layer were in close apposition to dividing cells. In the periphery of retinal wholemounts, we detected rhodamine traced ganglion cells adjacent to the growth zone, extending dendrites in proximity to the growth zone, typically branching off in opposite directions running parallel to the retinal rim over more then 100 microm. Ganglion cells with similar dendritic branching patterns were not found in more central retinal areas. Similarly, the dendrites of ChaT-positive amacrine cells showed a preference for running parallel to the circumference in the periphery. Dendritic branches of PKC-positive bipolar cells did not show similar preferred orientation. The change in shape of the dendritic tree with distance from the periphery was studied for the Ma type ganglion cell. The data are consistent with the idea that existing ganglion cells might control differentiation of new ganglion cells. Moreover, ganglion cells with specific branching patterns towards the retinal periphery undergo a restructuring of their dendritic trees.