Possible transient liquid crystal phase during the laying out of connective tissues:α-chitin and collagen as models

Abstract

Morphogenesis of extracellular matrices can be considered from different perspectives. Oneis that of ontogenesis, i.e., an organism’s development, which is mostly concerned with thespatiotemporal regulation of genes, cell differentiation and migration. Complementary tothis purely biological point of view, a physico-chemical approach can help in understandingcomplex mechanisms by highlighting specific events that do not require direct cellularcontrol. Because of a structural similarity between some biological systems and liquidcrystals, it was supposed that similar mechanisms could be involved. In this respect, it isimportant to determine the intrinsic self-assembly properties driving the ordering ofbiological macromolecules. Here we review in vitro studies of the condensed state of majorbiological macromolecules from extracellular matrices and related theories describing amesophase transition in suspensions of rodlike particles. Dilute suspensions of collagen orchitin are isotropic, i.e., the macromolecules can take on any orientation in thefluid. Beyond a critical concentration, an ordered nematic phase appears with ahigher volume fraction. The two-phase coexistence can be seen between crossedpolarizers since the nematic phase is strongly birefringent and appears bright,whereas the isotropic phase remains dark. A widespread property of these structuralmacromolecular scaffolds is their chirality. Although the origin of chirality incolloidal suspensions is still a subject of debate, the helical nature of the cholestericphase can be quantified. Small angle x-ray scattering performed on shear-alignedsamples can help demonstrate the cholesteric nature of the anisotropic phase,inferred from optical observations. Liquid-like positional local order is revealed bythe presence of broad interference peaks at low angle. The azimuthal profiles ofthese patterns are fitted to determine the value of the nematic order parameterat the transition. A few physico-chemistry experiments can assess the natureof the transition, and in turn, applying theoretical models can prove useful inpredicting and controlling the structure of assemblies of biological macromolecules.