Liquid crystalline order of biopolymers in cuticles and bones

Microscopic studies at different scales have shown that in biological tissues the three-dimensional arrangement of chitin-protein or of collagen fibrillar networks can follow the same spatial distributions as those described in certain liquid crystals. The present work reviews the structural analogies established between the dense fibrillar organic matrix found in two materials: crab cuticles and compact bones. In both systems mobile fringes are described in polarizing microscopy, periodic cleavage aspects in scanning electron microscopy (SEM), and arched patterns in transmission electron microscopy (TEM). In parallel to these structural data, results obtained in vitro are recalled which corroborate the relationship established between ordered arrays of biopolymers and liquid crystalline assembly principles, highly concentrated solutions of purified collagen molecules spontaneously form ordered assemblies characterized in polarizing microscopy as cholesteric phases. This particular state of matter joins both fluidity and order and could correspond to a transient state of collagen or chitin secretion, before the stiffening of these skeletal structures, bone or cuticle, by molecular cross-links and crystalline deposition.