Development of cell density gradients in the retinal ganglion cell layer of marsupials.

An area centralis and visual streak, specializations of high cell density in the mammalian retinal ganglion cell layer, develop from a more uniform cell distribution. In the wallaby and kangaroo, this transition is first seen 60 days postnatally. Total live cell numbers in this layer fall by approximately one-third as these specializations start to appear and dying cells are seen. However, dying cells have already become concentrated at the retinal rim prior to the emergence of live cell density gradients. Our results suggest that cell death may partially sculpt patterns of live cells, particularly by lowering densities around the entire far periphery. Studies of cell division demonstrate that ganglion cells are generated and enter the ganglion cell layer before the area centralis and visual streak are formed. However, cell addition to the inner and outer nuclear layers continues as cell density gradients become apparent in the ganglion cell layer. Furthermore, this late cell generation ceases first in areas adjacent to the presumptive area centralis. The differential distribution of such prolonged cell addition to the inner and outer nuclear layers may result in an asymmetric expansion of the retina. This process would partially explain the changing topography of cells in the ganglion cell layer. Mathematical analysis of patterns of overall retinal growth support this interpretation.