A phylogenetic structural analysis in molecular dimensions of visual photoreceptors in a variety of animals is being made. These photoreceptors include structures from the eyespot-flagellum of the protozoan flagellates, to the more elaborate sensory cells of flatworms, the retinal cells of the compound eyes of anthropods, the molluscs, and the retinal rods and cones of the eyes of higher vertebrates, including man. In this study, the geometry of these photoreceptors and their fine structure has been analyzed by light and electron microscopy. These microscopic studies show that all of the photoreceptors are highly ordered, crystalline-like disks or tubes, which when cut in the right plane, are observed as lamellar structures with dimensions of <100 to >400 Å. Microanalysis employing microspectrophotometry was then used to determine the concentration of the photosensitive pigment-complexes within a single retinal rod. The geometrical structural data and the concentration of rhodopsin was then used to calculate the surface area that would be required for all of the pigment molecules if they were associated with the lamellar surfaces. These calculations indicate that the rhodopsin molecules could all fit on all the lamellar surfaces, and that each molecule would have a diameter of the order of 50 Å. The molecular weight of rhodopsin was also calculated from these data for frog and cattle rhodopsin, which give values of the order of 60 000 and 40 000, respectively. From these microscopic observations, it is possible to trace a phylogenetic structural development for a retinal rod, with flagellum-like processes which are a unique part of all of these photoreceptor structures. Since the photosensitive pigment-complex, rhodopsin, is an aqueous digitonin (1–2%) extract of the retinal rods, it has been used as a model system to study the photochemistry of the rhodopsin–digitonin complex. Such pigment-complexes form micelles, macromolecular aggregates, which possess the photochemistry and indicate a molecular organization similar to that of the retinal rod. These studies of the in situ, isolated, and solubilized photoreceptors, suggest that we are dealing with phase transitions as exhibited by liquid–crystalline states.
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