Abstract
Abstract This article reviews evidence about the phylogenetic and ontogenetic origins of rod photoreceptors. Studies of morphology, molecular phylogeny of the opsin molecules, and retinal wiring indicate cone photoreceptors are the more ancient of the two kinds of photosensitive cells in the retina. Consistent with this view, rod photoreceptors arise later in ontogenesis in all vertebrates. The most dramatic example of this ontogenetic sequence is retinal development in cold-blooded vertebrates, particularly teleost fish, in which rods continue to be added throughout life. Because of this extension of ontogenesis into adulthood, examination of the teleost eye offers a unique model system for studying retinal development. Control of retinal rod neurogenesis requires exquisite coordination of both genetic and extrinsic factors for appropriate temporal and spatial expression.
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