Disappearance of Displaced Amacrine Cells in the Central Retina of Adult Fish

Abstract

Abstract: : Purpose: We analyzed the distribution of different cell types in the ganglion cell layer of growing teleost fish and their relationship to existing retinal mosaics, in order to understand how neuronal organization copes with continued growth. Here we report on the proportions of non-ganglion cells (displaced amacrine cells, DAC) in the ganglion cell layer (GCL) during retinal growth. Methods: To characterize ganglion cells (GC) and distinguish them from DAC in the GCL we labeled both cell types in adult cichlid fish (Aequidens pulcher). GC were labeled with rhodamine dextran in isolated eyes. They were cultured for 24 hours, fixed and further processed for immunocytochemistry as wholemounts or cryostat sections. Antibodies against parvalbumin stained a subset of amacrine cells in the inner nuclear layer and DAC. Results: Virtually all cells in the GCL were stained by either rhodamine dextran taken up by the ganglion cell axons and transported to the cell body, or by antibodies binding to parvalbumin. This was confirmed by Sytox Green nuclear counter stains. We observed that in the retinal periphery more cells were parvalbumin-positive compared to central retinal areas. To corroborate this observation we compared the density of GC to the density of DAC across the retina in small, medium, and large fish (standard length ca. 2, 5, and 79 cm, respectively). As expected the absolute densities for either cell type decreased with increasing eye size. However, the GC/DAC ratio did not change with eye size but with retinal location: we found that this ratio is lowest (approximately 2:1) in the retinal periphery of all fish and highest (up to 4:1) in the central retina of large fish. Conclusion: Due to the continued growth, the retinal periphery of a small fish will be located more centrally as the animal grows. Therefore a decreasing centro-peripheral gradient in the GC/DAC ratio implies the elimination of DAC from the GCL. This is remarkable because most neuronal elements in the retina of cichlid fish are arranged in a regular proportional fashion. Although the mechanism of DAC elimination remains unknown the results suggest an adaptation process of cellular plasticity to changing visual properties.

Keywords: 312 amacrine cells • 520 plasticity • 564 retinal development