Shinobu Nagase

Cortical cell types and intermediate filament arrangements correlate with fiber curvature in Japanese human hair

Naturally straight and curved human scalp hairs were examined using fluorescence and electron microscopy techniques to determine morphological and ultrastructural features contributing to single fiber curvature. The study excluded cuticle and medulla, which lack known bilateral structural asymmetry and therefore potential to form curved fibers. The cortex contained four classifiable cell types, two of which were always present in much greater abundance than the remaining two types. In straight hair, these cell types were arranged annularly and evenly within the cortex, implying that the averaging of differing structural features would maintain a straight fiber conformation. In curved fibers, the cell types were bilaterally distributed approximately perpendicular to fiber curvature direction with one dominant cell type predominantly located closest to the convex fiber side and the other, closest to the concave side. Electron tomography confirmed that the dominant cell type closest to the convex fiber side contained discrete macrofibrils composed of helically arranged intermediate filaments, while the dominant cell type closest to the concave side contained larger fused macrofibrils composed of intermediate filament arrangements varying from helical to hexagonal arrays approximately parallel to the longitudinal fiber axis. These findings concur with the current hypothesis of hair curvature formation and behavior.

Intrinsic curvature in wool fibres is determined by the relative length of orthocortical and paracortical cells

ABSTRACT Hair curvature underpins structural diversity and function in mammalian coats, but what causes curl in keratin hair fibres? To obtain structural data to determine one aspect of this question, we used confocal microscopy to provide in situ measurements of the two cell types that make up the cortex of merino wool fibres, which was chosen as a well-characterised model system representative of narrow diameter hairs, such as underhairs. We measured orthocortical and paracortical cross-sectional areas, and cortical cell lengths, within individual fibre snippets of defined uniplanar curvature. This allowed a direct test of two long-standing theories of the mechanism of curvature in hairs. We found evidence contradicting the theory that curvature results from there being more cells on the side of the fibre closest to the outside, or convex edge, of curvature. In all cases, the orthocortical cells close to the outside of curvature were longer than paracortical cells close to the inside of the curvature, which supports the theory that curvature is underpinned by differences in cell type length. However, the latter theory also implies that, for all fibres, curvature should correlate with the proportions of orthocortical and paracortical cells, and we found no evidence for this. In merino wool, it appears that the absolute length of cells of each type and proportion of cells varies from fibre to fibre, and only the difference between the length of the two cell types is important. Implications for curvature in higher diameter hairs, such as guard hairs and those on the human scalp, are discussed. Highlighted Article: Curvature in mammalian hairs is underpinned by the relative difference in length between orthocortical and paracortical cells rather than their proportion or number along each side of the fibre.

Like Follicle, like Fibre? Diameter and not Follicle Type Correlates with Fibre Ultrastructure

The hair follicles of most mammals are of two types, primary and secondary. Primary follicles develop earlier and have a prominent arrectorpili muscle. Secondary follicles have less prominent muscles and are often clumped, sharing a common opening from which fibres emerge. It is not entirely clear what types of follicles occur in human scalps. Partly this is because human hairs have a uniform appearance, unlike many mammals in which robust primary hairs differ markedly from narrow secondary fibres. Some sheep breeds are an exception because like humans, wool fibres have a similar macro-scale appearance irrespective of follicle type. How deep does this similarity go Using electron microscopy, we examined wool primary fibres from different breeds and contrasted them to secondary fibres. For fibres of similar diameter, there was no significant difference in the ultrastructure or proportion and distribution of cortex cell types in primary and secondary fibres. We conclude that fibre diameter is the most important fibre parameter with respect to structural differences between fibres, not whether the fibres originate from primary or secondary follicles.