Variations in cell density in the ganglion cell layer of the retina as a function of ocular pigmentation

Ocular melanin regulates retinal development, including cell density gradients in the central retina, a region essential for normal visual acuity. In albinos this region is underdeveloped and peak cell numbers are reduced. It is not known whether there is a dosage relationship between pigmentation and the degree of this underdevelopment, as studies of the retinal effects of albinism have commonly used rodents. These have poorly developed central regions even in the wild type. Rabbits, however, have a unique, highly specialized, visual streak in the central retina where cell density gradients are very steep and these are reduced in albinos. Here, cell densities in the ganglion cell layer of separate groups of rabbits, with different levels of ocular pigmentation and known mutations of the tyrosinase gene coding sequence, were examined. These revealed reductions in peak cell densities and/or in the regions over which high cell densities were maintained in all hypopigmented phenotypes. There was no dosage relationship between levels of pigmentation and deficits in the ganglion cell layer as animals with relatively small reductions in retinal pigment had deficits comparable to those found in albinos. The greatest variability between pigmentation phenotypes was between the two completely unpigmented strains. Consequently, although pigment may regulate the development of the central retina, this study failed to show that it does so in a dose-dependent manner.