Development of the pattern of photoreceptors in the chick retina.

The various classes of photoreceptor cells found in vertebrate retinae are organized in specific patterns, which are important for visual function. It is not known how these patterns are achieved during development. The chick retina provides an excellent model system in which to investigate this issue, containing cone opsins red, green, blue, and violet, as well as the rod-specific opsin rhodopsin. In this study, whole-mount in situ hybridization has revealed striking differences among opsins in both spatial and temporal aspects of expression. The long-wavelength cone opsins, red and green, were first detected in a small spot within the area centralis at embryonic day 14 (E14). In contrast, the short-wavelength cone opsins, blue and violet, were not detected until 2 d later and showed domains of expression both within the area centralis and in temporal retina. The first rhodopsin transcripts were seen at E15 in inferior retina. When opsin expression was first detected, there were differences in the localization of RNA within the inner segment of cone photoreceptors, suggesting that morphological differentiation preceded the expression of photopigment molecules. Marked differences in the distribution of rods and cones were also found. Within the area centralis, a circular rod-free zone bisected by a narrow rod-sparse region along the nasal-temporal axis was evident as soon as rhodopsin RNA could be detected. Such specialized regions appear to be set aside soon after photoreceptor cells become postmitotic, as evidenced by a spatially restricted pattern of visinin RNA observed at E7. The onset of particular opsins in restricted regions of the retina suggest an underlying pattern related to visual function in the chick.