Liquid crystalline order of biopolymers in cuticles and bones

Abstract

The two major polymers of animal skeletal structures are chitin, a polysaccharide associated with proteins in arthropod cuticles, and collagen the main protein of bone. Electron microscopic studies, with the use of a goniometric stage, have shown that in biological tissues, the three-dimentional arrangement of these biopolymers often follow liquid crystalline geometries. Moreover highly concentrated solutions of purified collagen molecules spontaneously form close assemblies known as cholesteric phases. These results suggest that the determinism of supramolecular order in many extracellular matrices could well correspond to liquid crystalline assembly principles.The crab cuticle first allowed to establish an analogy between the fibrous chitin-protein network and the arrangement of molecules in cholesteric liquid crystals. The demonstration was obtained by tilting, in the electronic microscope, sections showing a typical pattern consisting of series of nested arcs (Fig.1). The suppression, or even better the inversion of arcs, validates the cholesteric model where the fibrillar directions, all parallel to the epidermis, rotate continuously from one plane to the next when moving outwards (LIVOLANT et al, 1978).