Structural optimization and amorphous calcium phosphate mineralization in sensory setae of a terrestrial crustacean (Isopoda: Oniscidea)

• Published in: Micron (Oxford, England : 1993) England : 1993), 2018-09, Vol.112, p.26-34
• Main Authors: Vittori, Miloš, Srot, Vesna, Bussmann, Birgit, Predel, Felicitas, van Aken, Peter A., Štrus, Jasna
• Format: Article
• Language: English
• Subjects: Arthropod; Biomineralization; Chitin; Composite; Cuticle
• ISSN: 0968-4328; 1878-4291
• DOI: 10.1016/j.micron.2018.06.009

•Sensory setae on Porcellio legs are thin and stiff, necessitating great fracture resistance.•The stiff shaft of each seta is linked to the exoskeleton by a flexible joint membrane.•The shaft is mineralized with amorphous calcium phosphate.•Interchanging layers of axially aligned and helicoidally arranged chitin fibers form the shaft.•The orientation of fibers and amorphous calcium phosphate may increase fracture resistance. Terrestrial isopods possess large sensory setae on their walking legs. Increased fracture resistance of these elongated structures is of crucial importance, making the exoskeleton forming the setae an interesting durable material that may inspire biomimetic designs. We studied the cuticle of the sensory setae with analytical electron microscopy in order to gain detailed insights into its structure and composition at the nanometer scale and identify features that increase the fracture resistance of these minute skeletal elements. The setae are stiff structures formed by mineralized cuticle that are connected to the leg exoskeleton by a non-mineralized joint membrane. Our results demonstrate that different layers of the setal cuticle display contrasting organizations of the chitin-protein fibers and mineral particles. While in the externally positioned exocuticle organic fibers shift their orientation helicoidally in sequential layers, the fibers are aligned axially in the internally positioned endocuticle. In the setal cuticle, layers of structurally anisotropic cuticle likely providing strength in the axial direction are combined with layers of isotropic cuticle which may allow the setae to better resist perpendicular loading. They are further strengthened with amorphous calcium phosphate, a highly fracture resistant mineral rarely observed in invertebrate skeletons.