Liquid crystals and biological morphogenesis: Ancient and new questions

Superposed series of arcs were observed in extracellular matrices of diverse tissues in plants and animals, and also within certain chromosomes. These bow-shaped patterns were shown to originate from oblique sections in a three-dimensional arrangement of fibrils, which was reminiscent of a well-known structure present in a specific type of liquid crystal, called cholesteric, since it was first described in several cholesterol esters. These liquid crystals were also obtained with numerous chiral polymers, such as double-stranded nucleic acids, diverse polypeptides and polysaccharides. However, the fibrous matrices with arced patterns are not at all fluid in general and are considered as stabilized analogues of cholesteric liquid crystals. These geometrical similarities between fibrous tissues and liquid crystals were at the origin of numerous works defining a new type of self-assembly involving two successive steps: a phase transition from an isotropic liquid phase to a liquid crystalline one, followed by a sol-gel stabilization of the liquid crystal. One is tempted to apply to these biological materials the main concepts introduced by physicists in the field of liquid crystals, but many aspects require a new discussion, especially because living cells are present in such systems, and this opens considerable perspectives for research.