Structural flexibility in the optical design of the arthopod cornea

节肢动物角质层是由嵌入蛋白质基质中的几丁质纤维构成的多功能生物复合材料。角质层还形成了节肢动物复眼的角膜，这是节肢动物复眼的一个特征，在很大程度上促成了其令人难以置信的进化成功。在复眼结构中，每个光感受器单元（眼状膜）都有自己的光学元件。节肢动物复眼是同源的，具有相似的总体结构。然而，特别是基本的屈光单位经历了相当大的进化修改。在这里，我们提出研究不同的结构方案观察到的角膜角质层，所有成功的动物在各自的生活方式。目的是提高我们对角膜的光学性能是如何从它们的形状和潜在的几丁质结构中产生的理解。以前的大量工作集中在理解复眼光学与动物栖息地、昼夜适应、个体发生和进化适应的关系上。对于角质层结构和几丁质纤维组织与新出现的光学特性之间的基本关系的研究较少。在我们的方法中，我们选择了三种物种：马蹄蟹、小龙虾和片脚类动物，我们描述了它们的结构和组成，并利用合作小组开发的各种先进技术监测角膜的光学和机械特性。在鲎（鲎属）的小眼中，每个角膜都有一个圆锥形的过程，完全由几丁质构成。相比之下，在下颚目动物（多足类、甲壳类、六足类）中，这个过程被细胞内的结晶锥体所取代，而原眼角膜要么是透镜状的，要么是光滑的，如所选的小龙虾和片脚类动物。角膜必须同时具有光学和机械功能，从而在这些限制之间调整结构。我们希望得出潜在的几丁质结构、角膜角质层组成和新兴材料性能之间的相关性，以便得出与高分子材料设计相关的结论。我们将利用合作伙伴在生物表征和组织学方面的各种专业知识，以及漫射X射线散射和微衍射、拉曼光谱、冷冻电子显微镜等分析技术。该提案的意义在于试图探索生物学中观察到的结构灵活性的边界，作为生物灵感材料合成设计策略的来源。

The arthropod cuticle is a versatile biocomposite made of chitin fibers embedded in a protein matrix. The cuticle also forms the cornea of the arthropod compound eye, a characteristic feature of the phylum that has largely contributed to its incredible evolutionary success. In the compound eye structure each photoreceptor unit (ommatidium) carries its own optics. Arthropod compound eyes are homologous and show a similar general structure. However, in particular the basic dioptric unit underwent considerable evolutionary modifications. Here we propose to study the different structural solutions observed in the cornea cuticle, all of which were successful for the animals in their respective lifestyle. The goal is to improve our understanding of how the corneas optical performance emerges from their shape and the underlying chitin architecture. A large body of previous work has focused on understanding the compound eye optics in relation to the animals habitat, diurnal adaptations, ontogeny and evolutionary adaptations. Much less effort has been taken to understand the fundamental relation between the cuticle structure and chitin fiber organization and the emerging optical properties. In our approach, we have selected three species: a horseshoe crab, a crayfish and an amphipod, and we characterize their structure and composition and monitor the optical and mechanical properties of the cornea exploiting diverse advanced techniques developed within the collaborating groups. In the ommatidia found in the horseshoe crab Limulus polyphemus (Chelicerata) each cornea possesses a cone-like process, made entirely of chitin-based cuticle. In the Mandibulata (myriapods, crustaceans, hexapods), in contrast, this process is replaced by an intracellular crystalline cone and the ommatidial cornea is either lentiform or smooth, as exemplified by the chosen crayfish and amphipod. The cornea has to serve both an optical and a mechanical function leading to adjusted structures between these constraints. We wish to draw correlations between the underlying chitin-structure, the cuticle composition of the cornea and the emerging materials properties in order to draw conclusions relevant for polymer materials design. We shall exploit the diverse expertise of the partners in biological characterization and histology, as well as analytical techniques such as diffuse X ray scattering and microdiffraction, Raman spectroscopy, cryo electron microscopy and more. The significance of this proposal is in the attempt to explore the boundaries of structural flexibility as observed in biology as a source of design strategies for bioinspired material synthesis.