Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, MO, United States.
Front Neural Circuits. 2017 Dec 8;11:97. doi: 10.3389/fncir.2017.00097. eCollection 2017.
To distinguish colors, the nervous system must compare the activity of distinct subtypes of photoreceptors that are maximally sensitive to different portions of the light spectrum. In vertebrates, a variety of adaptations have arisen to refine the spectral sensitivity of cone photoreceptors and improve color vision. In this review article, we focus on one such adaptation, the oil droplet, a unique optical organelle found within the inner segment of cone photoreceptors of a diverse array of vertebrate species, from fish to mammals. These droplets, which consist of neutral lipids and carotenoid pigments, are interposed in the path of light through the photoreceptor and modify the intensity and spectrum of light reaching the photosensitive outer segment. In the course of evolution, the optical function of oil droplets has been fine-tuned through changes in carotenoid content. Species active in dim light reduce or eliminate carotenoids to enhance sensitivity, whereas species active in bright light precisely modulate carotenoid double bond conjugation and concentration among cone subtypes to optimize color discrimination and color constancy. Cone oil droplets have sparked the curiosity of vision scientists for more than a century. Accordingly, we begin by briefly reviewing the history of research on oil droplets. We then discuss what is known about the developmental origins of oil droplets. Next, we describe recent advances in understanding the function of oil droplets based on biochemical and optical analyses. Finally, we survey the occurrence and properties of oil droplets across the diversity of vertebrate species and discuss what these patterns indicate about the evolutionary history and function of this intriguing organelle.
为了区分颜色，神经系统必须比较对光频谱不同部分最敏感的不同感光器亚型的活动。在脊椎动物中，出现了多种适应机制来完善视锥细胞的光谱敏感性并改善色觉。在这篇综述文章中，我们专注于一种这样的适应机制，即油滴，这是一种独特的光学细胞器，存在于多种脊椎动物物种的视锥细胞内节中，从鱼类到哺乳动物。这些由中性脂质和类胡萝卜素色素组成的液滴位于光通过感光器的路径中，并改变到达光敏感外节的光的强度和光谱。在进化过程中，油滴的光学功能通过类胡萝卜素含量的变化得到了微调。在暗光中活跃的物种会减少或消除类胡萝卜素来提高灵敏度，而在强光中活跃的物种则精确地调节视锥细胞亚型中单键共轭和浓度的类胡萝卜素，以优化颜色分辨和颜色恒常性。视锥细胞油滴已经激发了视觉科学家一个多世纪的好奇心。因此，我们首先简要回顾一下关于油滴的研究历史。然后，我们讨论了关于油滴发育起源的已知信息。接下来，我们根据生化和光学分析描述了对油滴功能的最新理解进展。最后，我们调查了油滴在多样性的脊椎动物物种中的出现和特性，并讨论了这些模式表明了这个有趣的细胞器的进化历史和功能。
