The Role of Liquid-crystalline Structures in the Morphogenesis of Animal Fibers

The role of liquid-crystalline (mesophase) structures in extra-cellular morphogenesis is widely recognized. This paper summarizes a model for the more unusual case of intra-cellular mesophases. In the nascent mammalian hair cortex, cell differentiation is correlated with different mesophase textures within tactoids that are composed of intermediate filaments (IFs), and which form by a concerted process of unit-length-filament (ULF) polymerization and phase separation. Nematic and double-twist textures arise from differences in mesogen orientation and length in apposed tactoids. The model explains features of mature structures such as the fibril-matrix ratios in different cell types. The rapidity of IF formation suggests that a sudden-transition equilibrium polymerization, involving a high-energy initiating species, obeying the same statistical model as several other biological transitions, may be involved. This leads to an appealing symmetry, with the key factor in both polymerization and mesophase stability being the retention of protein head-group entropy.