Video-enhanced imaging of retinal wholemounts reveals an abrupt change in the composition of the photoreceptor mosaic at the edge of the human retina. Cone densities rise threefold and rod densities fall tenfold in a 1-mm-wide peripheral band. Antibodies directed against cones confirm the identification of the major subtypes of photoreceptors within this peripheral band. The cone-enriched rim is most highly developed along the nasal retinal margin, an area where the extreme lateral periphery of the visual field is imaged. This rim of cones may function as part of a rapid-acting alert mechanism under conditions of moderate and bright illumination.

We studied the development of multibank rod retinae by monitoring the size-related addition of new layers of rod inner and outer segments in four species of deep-sea fishes and found two different growth paradigms. In the mesopelagic Chauliodus sloani, new banks of rod inner and outer segments are added as long as the fish increases in size, as observed earlier by Locket (1980). By contrast, in three bathybenthic species (Antimora rostrata, Coryphaenoides (Coryphaenoides) guentheri, and Coryphaenoides (Nematonurus) armatus), the final complement of banks is reached when the specimens have grown to between 20 and 47% of their maximal size, suggesting that the visual system is mature only after this stage. Increase in retinal area, density of rod nuclei, and densities of rod inner and outer segments were also studied in these and additional species. Taken together with previous data on rod proliferation patterns and outer segment membrane synthesis, our findings indicate that at least in species with no continual addition of new banks, there is no major functional difference between the innermost and outermost banks of rod inner and outer segments. While Chauliodus spends all its life in the mesopelagic environment, the three bathybenthic species live in this environment during early development and descend towards greater depths only upon maturation. We speculate that this coincides with the stage when the full complement of rod banks is formed in the retina, as a possible prerequisite for a life outside the reach of sunlight.