Optical parameters of the tunable Bragg reflectors in squid

Abstract

squid and cuttlefish) dynamically tune the colour and brightness of their skin for camouflage and communication using specialized skin cells called iridocytes. We use high-resolution microspectrophotometry to investigate individual tunable Bragg structures (consisting of alternating reflectin protein-containing, high-refractive index lamellae and low-refractive index inter-lamellar spaces) in live and chemically fixed iridocytes of the California market squid, Doryteuthis opalescens. This subcellular, single-stack microspectrophotometry allows for spectral normalization, permitting use of a transfer-matrix model of Bragg reflectance to calculate all the parameters of the Bragg stack-the refractive indices, dimensions and numbers of the lamellae and inter-lamellar spaces. Results of the fitting analyses show that eight or nine pairs of lowand high-index layers typically contribute to the observed reflectivity in live cells, whereas six or seven pairs of low-and high-index layers typically contribute to the reflectivity in chemically fixed cells. The reflectin-containing, high-index lamellae of live cells have a refractive index proportional to the peak reflectivity, with an average of 1.405 + 0.012 and a maximum around 1.44, while the reflectin-containing lamellae in fixed tissue have a refractive index of 1.413 + 0.015 suggesting a slight increase of refractive index in the process of fixation. As expected, incremental changes in refractive index contribute to the greatest incremental changes in reflectivity for those Bragg stacks with the most layers. The excursions in dimensions required to tune the measured reflected wavelength from 675 (red) to 425 nm (blue) are a decrease from ca 150 to 80 nm for the high-index lamellae and from ca 120 to 50 nm for the low-index inter-lamellar spaces. Fixation-induced dimensional changes also are quantified, leading us to suggest that further microspectrophotometric analyses of this iridocyte system can be used as a model system to quantify the effects of various methods of tissue fixation. The microspectrophotometry technique described can be expected to provide deeper insights into the molecular and physical mechanisms governing other biophotonically active cells and structures.

Dynamic biophotonics: female squid exhibit sexually dimorphic tunable leucophores and iridocytes

“…The especially bright iridocytes from the iridescent stripes yield a high reflectance signal, enabling our use of a specialized microspectrophotometer to obtain optical measurements of a single stack of high-index, reflectin-containing lamellae within a single cell. This has allowed us to directly calculate the parameters of the sub-cellular multilayer reflectors, including the number of lamellae and their thickness, spacing and refractive index (Ghoshal et al, 2013). The adaptive leucophores offer an interesting comparison to the adaptive iridocytes at an ultrastructural and protein level, providing further insights into the recently elucidated mechanism of reversible, ACh-dependent assembly of reflectins driving the changes in optical properties of the cells .…”

“…We speculate that differences in gene expression and the resulting protein composition (potentially including the unidentified major protein bands and the unique relative ratios of the known reflectins we observe in electropherograms of the leucophore tissue) help direct the morphology of the high-index material controlling the optical properties of these otherwise similar cells. We suggest that it is this mesoscale assembly of the variety of reflectin protein subtypes (and their associated membrane structures) found throughout cephalopods that differentially creates distinct sub-cellular structures with specific optical properties ranging from specular to diffuse reflectance (Holt et al, 2011;Sutherland et al, 2008), from static to adaptive reflectance Kramer et al, 2007;Tao et al, 2010) and from narrowband to broadband reflectance (Mäthger et al, 2009;Mäthger et al, 2013), with colors tunable from red to blue and from transparent to highly reflective (Ghoshal et al, 2013;Kramer et al, 2007). This multitude of reflectin-based structures and their photonic properties is increasingly recognized as an interesting model of mesoscale assembly and its control for the design of functional materials.…”

“…The female stripe iridocytes contain a particularly high number of lamellae in each multilayer stack relative to other iridocytes (including the dorsal iridocytes typically studied; supplementary material Fig. S3) (Table1), making each individual cell brighter (Ghoshal et al, 2013;Land, 1972). The iridocyte stripe also has more cells in cross-section than the iridocyte patches in the dermis.…”

Structures, Organization, and Function of Reflectin Proteins in Dynamically Tunable Reflective Cells

“…Recent optical analysis of individual iridocyte reflectors determined that the lamellae containing the condensed reflec-tins have refractive indices of ϳ1.405 (25). To achieve this refractive index, the reflectin proteins must be concentrated to several hundred mg/ml (15,26,27).…”

Wavelength-specific forward scattering of light by Bragg-reflective iridocytes in giant clams

“…Normalized spectra, obtained as described above, were then fitted using a transfer matrix model [29] to obtain the following parameters of the Bragg stack, with the only assumption being that the low refractive index was that of cytoplasm (¼1.35 [31 -34]): N, the number of pairs of layers of alternating high and low refractive index; n H , the refractive index of the high-index layer; and t L and t H , the thicknesses of the low-and high-index layers, respectively. A total of 282 spectra were obtained, analysed and fitted from 11 specimens (seven T. derasa, one T. maxima and three T. crocea).…”