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Abstract The importance of the pigmented retinal epithelium (PRE) for photoreceptor maturation during retinal development was evaluated in Rana pipiens. Retinal rudiments with or without an enveloping PRE were isolated from stage 18 embryos and allowed to develop in a non‐nutrient culture medium for up to 19 days. A set of morphological features with which to evaluate the degree of differentiation occurring in cultured material was obtained through examination of eyes of normal embryos, stages 18–25. Some photoreceptor outer segment material first appeared after 7 days in culture. The number of well‐ordered outer segment discs per receptor increased progressively with age in PRE‐covered retinas. However, photoreceptors of retinas lacking PRE contained little disc membrane material and this quickly lost proper orientation. Electroretinograms could first be elicited from PRE‐containing cultures after 7 days, but none were produced by retinas lacking a PRE. In PRE‐covered retinas a progressive differentiation of photoreceptor synaptic complexes was seen between 7–19 days; in retinas lacking a PRE, synaptic ribbons appeared after 7 days in culture, but no further maturation of the synapse was observed. The PRE itself underwent sequential differentiation and achieved the tadpole appearance after 17 days in culture. Both PRE and retina developed at the same rate in cultured material, and this rate was about one‐third that observed in eyes of embryos raised at the same temperature. These findings indicate that the PRE plays a critical role in photoreceptor development.
Hollyfield et al. (Sun,) studied this question.