Mesostructure of the Exoskeleton of the Lobster Homarus Americanus
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Mesostructure of the Exoskeleton of the Lobster Homarus americanus D. Raabe1, P. Romano1, A. Al-Sawalmih1, C. Sachs1, G. Servos2, H.G. Hartwig2 Max-Planck-Institut für Eisenforschung, Max-Planck-Str. 1, 40237 Düsseldorf, Germany 2 Anatomy Department, Düsseldorf University, 40225 Düsseldorf, Germany 1
ABSTRACT The exoskeleton of the lobster Homarus americanus is a multiphase bio-composite which consists of a fibrous organic matrix (crystalline α-chitin and various types of non-crystalline proteins) and embedded biominerals (mainly calcite). In this study we present experimental data about the microscopic and mesoscopic structure of this material. INTRODUCTION TO THE EXOSKELETON OF HOMARUS AMERICANUS Lobsters are large arthropods (joint-limb animals), which belong to the class of the crustaceans and the order of the decapods [1,2]. Their body is segmented into three main parts: the head (cephalon), various numbers of body segments (thorax and abdomen), and the end of the body (telson), but the physical characteristics of the different species of crustaceans may vary widely. The outer covering of these decapods is referred to as exoskeleton or cuticle. It consists – like that of most related crustaceans – of several layers with specific structural and functional properties. It covers the entire body of the animal and is secreted by a single layered epithelium. The exoskeleton provides mechanical support to the body, armor against loads that are externally imposed by predators, and attachment sites for muscles. Its functional role lies in preventing the inner organs from dehydration as it is water resistant but penetrable to gas [1,2]. The cuticle is divided in three layers, referred to as epicuticle, exocuticle, and endocuticle (figure 1). The epicuticle is a thin waxy layer [1-3].
Figure 1. Schematic drawing of the epicuticle, the exocuticle, and the endocuticle of the exoskeleton of Homarus americanus [1-3.] In contrast, the composite material of both, the exo- and the endocuticle is chiefly designed to resist mechanical loads [4-12]. It is a multilayered chitin-protein tissue, which is usually hardened by embedded calcium carbonate minerals (typically crystalline or amorphous calcite) of nanoscopic size [13-20]. Chitin is a sugar molecule, more specific, an insoluble linear polymer
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of ß-1,4-linked N-acetylglucosamine residues. Chitin occurs in three different crystallographic structures. It is the most abundant nitrogen-bearing organic compound in nature and the second most abundant natural polymer on earth after cellulose. Chitin is a common constituent not only of decapods but also of the insect exoskeleton and of fungal cell walls [21-25]. The crystalline αchitin that occurs in the exoskeleton of large crustaceans has an antiparallel arrangement of the molecule chains. Variations in the composition and in the topological and crystallographic arrangement of the various structural constituents found in the lobster cuticle can lead either to a rigid material, which serves as protection or to a hi
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