Abstract
ABSTRACTOil droplets are spherical organelles found in the cone photoreceptors of vertebrates. They are generally assumed to focus incident light into the outer segment, and thereby improve light catch because of the droplets' spherical lens-like shape. However, using full-wave optical simulations of physiologically realistic cone photoreceptors from birds, frogs and turtles, we find that pigmented oil droplets actually drastically reduce the transmission of light into the outer segment integrated across the full visible wavelength range of each species. Only transparent oil droplets improve light catch into the outer segments, and any enhancement is critically dependent on the refractive index, diameter of the oil droplet, and diameter and length of the outer segment. Furthermore, oil droplets are not the only optical elements found in cone inner segments. The ellipsoid, a dense aggregation of mitochondria situated immediately prior to the oil droplet, mitigates the loss of light at the oil droplet surface. We describe a framework for integrating these optical phenomena into simple models of receptor sensitivity, and the relevance of these observations to evolutionary appearance and loss of oil droplets is discussed.
Highlights
The cone photoreceptors of approximately half of the orders of vertebrates contain spherical structures composed of lipids and carotenoid pigment, known as oil droplets (Walls, 1942; Jacobs and Rowe, 2004)
First, we present the results of how variation in the geometry and refractive index of the oil droplets affect the light catch in model outer segments
We use the models of cone photoreceptors from G. g. domesticus, T. s. elegans and X. laevis that are based on geometrical and optical measurements of real photoreceptor cells to investigate how oil droplet pigments affect optical enhancement and absorption
Summary
The cone photoreceptors of approximately half of the orders of vertebrates contain spherical structures composed of lipids and carotenoid pigment, known as oil droplets (Walls, 1942; Jacobs and Rowe, 2004). Many oil droplets contain mixtures of carotenoid pigment (Johnston and Hudson, 1976; Toomey et al, 2015) and have been studied predominantly for their spectral filtering properties The oil droplets of ultraviolet- and violet-sensitive (UVS and VS, respectively) cones are transparent across the visible spectrum (Bowmaker et al, 1997), containing no pigment. Numerous species (Fig. 1) only have transparent oil droplets in all of their cone types. This widespread presence of transparency indicates that
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