Nasotemporal asymmetry during teleost retinal growth: preserving an area of specialization.

Teleost fish retinas grow throughout adult life through both cell addition and stretching. Cell division occurs at the periphery of the retina, resulting in annular addition of all cell types except rod photoreceptors, which are added in the central retina. Since many teleosts have a region of high cellular density at the temporal pole of the eye, we analyzed whether and how this specialized region of high visual acuity maintained its relative topographical position through asymmetric circumferential growth. To do this, we measured the pattern of long-term retinal growth in the African cichlid Haplochromis burtoni. We found that the retina expands asymmetrically along the nasotemporal axis, with the nasal retina growing at a higher rate than the temporal, dorsal, or ventral retinae, whose growth rates are equal. This nasotemporal asymmetry is produced via significantly greater expansion of retinal tissue at the nasal pole rather than through differential cell proliferation. The mechanisms responsible for this differential retinal enlargement are unknown; however, such asymmetric expansion very likely minimizes disruption in vision during rapid growth.