Why is hair curly?—Deductions from the structure and the biomechanics of the mature hair shaft

Abstract

The final shape of a head hair is predetermined through a variety of factors during its formation in the follicle. These are genetic pathways, specific growth factors, cell differentiation and segregation, etc, with spatial as well as chronological dynamics. The cortex of hair consists of two major cell groups. These are characterized by parallel (para‐type) or roughly helical arrangements (ortho‐type) of the intermediate filaments (IF). There are also cell‐specific differences in the disulphide content, that is, of the cross‐link density of the IF‐associated matrix proteins. Given the current state of the academic discussion, we consider it as timely to support and broaden the view that, the structural differences of the cell types together with their lateral segregation are the main driving factor of curl formation. The mechanical effects, which derive thereof, are triggered in the transition zone of the follicle, that is, upon formation of the mature hair shaft. Furthermore, an irregular, “flat” cross section of the hair shaft is shown to be a synergistic but not determining factor of curl formation. The degree of cell type segregation along the mature hair shaft together with dynamic changes of the location of the plane of segregation, namely in a non‐circular cross section can account for very complex curl patterns. Against the background of these views, we argue that contributions to hair curl are implausible, if they relate to physical mechanisms which are active below the transition zone from the living to the mature (dead) hair.

High-throughput phenotyping methods for quantifying hair fiber morphology

“…the effect of cross-sectional eccentricity on curvature). In this sample, we were able to show that eccentricity does not predict hair curvature, which is in line with the findings of other studies 8,13,21 . In fact, we explain this correlation as arising due to uncorrected population structure, which induces correlations between traits that are not necessarily genetically or physically linked.…”

“…West African) and the presence of round cross-sections in populations with straight hair (i.e. East Asian) has been interpreted as evidence that cross-sectional shape of hair dictates its curvature 8,9 . Whether these two traits are genetically and/or developmentally correlated is unknown because of population structure, which induces spurious correlations between traits 10 .…”

High-throughput phenotyping methods for quantifying hair fiber morphology

“…West African) and the presence of round cross-sections in populations with straight hair (i.e. East Asian) has been interpreted as evidence that cross-sectional shape of hair dictates its curvature 2 , although this has been refuted by others 12 , 13 , with some suggesting a synergistic link rather than a causal one 14 . Whether these two traits are genetically and/or developmentally correlated is unknown because of population structure, which induces spurious correlations between traits 15 .…”

Mapping the Chemistry of Hair Strands by Mass Spectrometry Imaging—A Review

“…Compared to cross-sectioning, longitudinal sectioning of hair makes it possible to analyze a broader cortex surface area, providing more information per analysis. This method of sectioning also circumvents the issue of analytical uncertainty related to the fact that the shape of cross-sections along the length of the hair can vary between circular, triangular, irregular, or flattened [102][103][104]. Studies have demonstrated that longitudinal sectioning of hair enables the identification of analyte accumulation patterns which may reveal whether compounds are contaminants or endogenous to the hair medulla [50,105].…”