DISTRIBUTION OF 3 RETINAL PROTEINS IN DEVELOPING OCTOPUS PHOTORECEPTORS

The expression of proteins unique to plasma membrane domains of develo ping photoreceptors is used as a marker for retinal differentiation in vertebrates. Invertebrate photoreceptors are also compartmentalized, but little information is available on the development of these compar tments or the expression of retinal proteins specific to these cellula r regions. Using routine electron microscopy techniques, we have made observations on the formation of photoreceptor organelles, including m yeloid bodies and rhabdomeres, in embryonic octopus eyes from an early stage in development through hatching. Immunocytochemical experiments on the embryos demonstrate a timed expression of three retinal protei ns during development, and the early separation of the octopus photore ceptor plasma membrane into distinct domains. Using polyclonal antibod ies for opsin, retinochrome and retinal binding protein we have shown that opsin appears first and is confined to the distal end of the phot oreceptor that will eventually differentiate into rhabdomeres. This me mbrane domain is separated from the proximal/inner segment plasma memb rane by a septate junction. Retinochrome is expressed later when the m yeloid bodies appear in the inner segments, and retinal binding protei n is apparently not synthesized until sometime after hatching. These r esults suggest that, in the cephalopod retina, protein components of t he retinoid cycling apparatus appear in a specific developmental seque nce during the differentiation of this tissue.