Human hair keratin network and curvature

In a study from 2007, researchers investigated the structural elements related to the curl pattern of human hair by examining a range of hair samples from straight to tightly curled. They found that at the macroscopic level, the frequency and amplitude of cross-sectional rotations were key to describing curliness. At the cellular level, transmission electron microscopy revealed that macrofibril organization correlated with hair curliness, with curly hair follicles showing retrocurvature regardless of ethnic origin. The study also discovered an intrinsic asymmetry in the proliferative compartment of curly hair follicles, which extended above the Auber line on the convex side, leading to delayed differentiation of the root sheaths. The hair cortex was found to be elliptical and asymmetric, with hHa8 keratin accumulating on the concave side in curly hair, while being evenly distributed in straight hair. The conclusion was that the curly shape of the hair shaft is due to asymmetric differentiation of the precortex, suggesting that hair fiber behaves like a shape memory material.