Institute for Collaborative Biotechnologies, University of California, Santa Barbara, CA 93106-5100, USA.
J R Soc Interface. 2013 Jun 5;10(85):20130386. doi: 10.1098/rsif.2013.0386. Print 2013 Aug 6.
Cephalopods (e.g. octopus, 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 low- and 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.
头足类动物（如章鱼、鱿鱼和乌贼）通过专门的皮肤细胞——虹细胞来动态调节皮肤的颜色和亮度，以实现伪装和交流。我们使用高分辨率微分光光度法研究加利福尼亚市场鱿鱼（Doryteuthis opalescens）活体和化学固定虹细胞中的单个可调布拉格结构（由交替的反射蛋白、高折射率层和低折射率层间空间组成）。这种亚细胞的单堆栈微分光光度法允许进行光谱归一化，从而可以使用布拉格反射率的传递矩阵模型来计算布拉格堆栈的所有参数——折射率、层和层间空间的尺寸和数量。拟合分析的结果表明，在活体细胞中，通常有八到九对低折射率和高折射率层对观察到的反射率有贡献，而在化学固定细胞中，通常有六到七对低折射率和高折射率层对反射率有贡献。活体细胞中含反射蛋白的高折射率层的折射率与峰值反射率成正比，平均值为 1.405 ± 0.012，最大值约为 1.44，而固定组织中的含反射蛋白的层的折射率为 1.413 ± 0.015，表明在固定过程中折射率略有增加。正如预期的那样，折射率的增量变化对具有最多层的布拉格堆栈的反射率的增量变化贡献最大。为了将测量的反射波长从 675nm（红色）调谐到 425nm（蓝色），需要高折射率层的尺寸减小约 150nm 至 80nm，低折射率层间空间的尺寸减小约 120nm 至 50nm。还量化了固定引起的尺寸变化，这使我们得出结论，进一步对这种虹细胞系统进行微分光光度分析可以用作量化各种组织固定方法影响的模型系统。所描述的微分光光度技术有望为其他生物光子活性细胞和结构的分子和物理机制提供更深入的见解。
