Nasotemporal Asymmetry during Teleost Retinal Growth: Preserving an Area of Specialization Carol A. Zygar, Marc J. Lee, Russell D. Fernald Program in Neurosciences and Department of Psychology, Jordan Hall, Bldg. 420, Stanford University, Stanford, California 94305-2130 Received 10 February 1999; accepted 27 April 1999 ABSTRACT: 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 photore- ceptors, 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 main- tained its relative topographical position through asym- metric 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 prolifer- ation. The mechanisms responsible for this differential retinal enlargement are unknown; however, such asym- metric expansion very likely minimizes disruption in vision during rapid growth. © 1999 John Wiley & Sons, Inc. J Neurobiol 41: 435– 442, 1999 Keywords: teleost; fish; retina; asymmetry; growth; RHCD Although all vertebrate eyes share a similar structure arising from common developmental programs, the eyes of cold-blooded animals are unique because they grow throughout life. For example, the eyes of teleost fish enlarge in proportion to increasing body size into adulthood through both stretching and cell addition (Mu ¨ller, 1952; Lyall, 1957; Scholes, 1976; Johns and Easter, 1977; Meyer, 1978). This ongoing cell addi- tion occurs at two locations in the retina. Rod photo- receptors exclusively are intercalated throughout the central retina, arising from the division of precursor cells in the outer nuclear layer (Johns and Fernald, 1981). All other cell types, including cone photore- ceptors, are added at the marginal germinal zone where the retina meets the iris. Since the physical properties of light tightly constrain the structure of a functioning eye (Fernald, 1988), its continuous growth requires precise control. For these reasons, the adult teleost eye has proven to be a useful model system for analyses of retinal development and, more generally, for study of central nervous system assem- bly. A particularly useful fish because of its rapid growth and predominant reliance on vision for social communication and survival is the African cichlid Haplochromis burtoni (Fernald, 1984). Because of the lateral location of fish eyes, their continued growth poses particularly complicated problems, because images from the front of the ani- mal fall near the edge of the temporal pole of the retina. In H. burtoni, the temporal pole maintains a fourfold greater density of cone photoreceptors than the nasal pole of the retina throughout the life of the animal (Fernald, 1983). All other cell types (e.g., bipolar, amacrine, and ganglion cells) except rods also maintain this fixed density ratio between temporal and Correspondence to: R. D. Fernald Contract grant sponsor: NIH; contract grant number: EY 05051 Contract grant sponsor: Stanford Undergraduate Research Op- portunity © 1999 John Wiley & Sons, Inc. CCC 0022-3034/99/030435-08 435