Background  In human hair, very little is known about the substructures relating to the curl pattern. The interpretation of the macroscopic shape of the fiber at the molecular and cellular scales is still unclear.

Methods  A comparative and multiscale study was carried out on a set of human hair samples, ranging in shape from straight to tightly curled, in order to investigate structural elements that might be related to hair curl pattern.

Results  At the macroscopic level, the frequency and amplitude of cross-sectional rotations were found to be crucial for an accurate description of curliness. At the cellular level, transmission electron microscopy experiments made it possible to confirm that macrofibril organization was strongly related to hair curliness. The curly hair follicles exhibited retrocurvature, independent of ethnic origin. A direct comparison of straight hair and curly hair highlighted an intrinsic asymmetry in the proliferative compartment that clearly extended above the Auber line on the convex side of the curvature. This phenomenon caused delayed differentiation of both inner and outer root sheaths. The hair cortex itself was elliptical and asymmetric, as evidenced by hHa8 keratin. In curly hair, this hair keratin accumulated on the concave side of the curvature, whereas, in straight hair, positive cortical cells were evenly distributed throughout the circular fiber.

Conclusion  The curly shape of the hair shaft seems to result from the asymmetric differentiation of the precortex. Hair fiber can thus be considered as a shape memory material.