Helical twist direction in the macrofibrils of keratin fibres is left handed.

Macrofibrils, the main structural features within the cortical cells of mammalian hair shafts, are long composite bundles of keratin intermediate filaments (KIFs) embedded in a matrix of keratin-associated proteins. The KIFs can be helically arranged around the macrofibril central axis, making a cylinder within which KIF helical angle relative to macrofibril axis increases approximately linearly from macrofibril centre to edge. Mesophase-based self-assembly has been implicated in the early formation of macrofibrils, which first appear as liquid-crystal tactoids in the bulb of hair follicles. Formation appears to be driven initially by interactions between pre-keratinized KIFs. Differences in the nature of these KIF-KIF interactions could result in all macrofibrils being internally twisted in a single handedness, or a 50:50 mixture of handedness within each cortical cell. We data-mined 41 electron tomograms containing three-dimensional macrofibril data from previously published studies of hair and wool. In all 644 macrofibrils examined we found that within each tomogram all macrofibrils had the same handedness. We concluded that earlier reports of left- and right-handed macrofibrils were due to artefacts of imaging or data processing. A handedness marker was used to confirm (using re-imaged sections from earlier studies) that, in both human and sheep, all macrofibrils are left-handed around the macrofibril axis. We conclude that this state is universal within mammalian hair. This also supports the conclusion that the origin of macrofibril twist is the expression of chiral twisting forces between adjacent KIFs, rather than mesophase splay and bending forces relaxing to twisting forces acting within a confined space.

Macrofibril Formation.

Macrofibrils are the main structural component of the hair cortex, and are a composite material in which trichokeratin intermediate filaments (IFs) are arranged as organised arrays embedded in a matrix composed of keratin-associated proteins (KAPs) and keratin head groups. Various architecture of macrofibrils is possible, with many having a central core around which IFs are helically arranged, an organisation most accurately described as a double-twist arrangement. In this chapter we describe the architecture of macrofibrils and then cover their formation, with most of the material focusing on the theory that the initial stages of macrofibril formation are as liquid crystals.

A reciprocal lattice approach to assessing the luster of hair fibers, based on scattering by periodically deformed cylinders.

In a recent prize-winning study (1), Nagase et al. presented an analysis, using geometrical optics, of the perception of luster of hair based on the different directions of light reflection from the front and internal rear surfaces of the fiber. These two reflections, from cuticle cells inclined to the fiber axis, lead to a specular peak and an associated bright zone, displaced in reflection angle, which is associated with luster perception. This work built upon the experimental observations of Bustard and Smith (2). Both these papers employed a model of the fiber which may be described as a linear stack of cone frusta, defined by the exposed axial length and angle of inclination to the fiber axis of the cuticle scales. This fiber model is readily amenable to an alternative treatment, in which the model is recognized as a convolution of a cone frustum with a one-dimensional lattice. The scattering properties are then given in reciprocal (scattering) space as the product of the scattering function of a single frustum and that of the one-dimensional lattice. This problem was addressed, in principle, long ago by Bear and Bolduan (3,4) in work on the scattering of periodically distorted collagen fibrils. The author presents a related theory based on conical shells. It is demonstrated that the scattering from such a model extends over a number of non-equatorial reciprocal lattice planes and is able to reproduce, in a crudely quantitative fashion, several of the features of the experimentally observed scattering. A major benefit of this approach is that it gives a three-dimensional appreciation of light scattering by fibers.

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.