title: Keratin network modifications lead to the mechanical stiffening of the hair follicle fiber.
abstract: The complex mechanical properties of biomaterials such as hair, horn, skin, or bone are determined by the architecture of the underlying fibrous bionetworks. Although much is known about the influence of the cytoskeleton on the mechanics of isolated cells, this has been less studied in tridimensional tissues. We used the hair follicle as a model to link changes in the keratin network composition and architecture to the mechanical properties of the nascent hair. We show using atomic force microscopy that the soft keratinocyte matrix at the base of the follicle stiffens by a factor of ∼360, from 30 kPa to 11 MPa along the first millimeter of the follicle. The early mechanical stiffening is concomitant to an increase in diameter of the keratin macrofibrils, their continuous compaction, and increasingly parallel orientation. The related stiffening of the material follows a power law, typical of the mechanics of nonthermal bending-dominated fiber networks. In addition, we used X-ray diffraction to monitor changes in the (supra)molecular organization within the keratin fibers. At later keratinization stages, the inner mechanical properties of the macrofibrils dominate the stiffening due to the progressive setting up of the cystine network. Our findings corroborate existing models on the sequence of biological and structural events during hair keratinization.
authors.affiliation: L'Oreal Research & Innovation, F93600 Aulnay-sous-Bois, France;
authors.forename: Thomas
authors.initials: T
authors.lastname: Bornschlögl
authors.affiliation: L'Oreal Research & Innovation, F93600 Aulnay-sous-Bois, France; lbildstein@rd.loreal.com.
authors.forename: Lucien
authors.initials: L
authors.lastname: Bildstein
authors.affiliation: L'Oreal Research & Innovation, F93600 Aulnay-sous-Bois, France;
authors.forename: Sébastien
authors.initials: S
authors.lastname: Thibaut
authors.affiliation: L'Oreal Research & Innovation, F93600 Aulnay-sous-Bois, France;
authors.forename: Roberto
authors.initials: R
authors.lastname: Santoprete
authors.affiliation: L'Oreal Research & Innovation, F93600 Aulnay-sous-Bois, France;
authors.forename: Françoise
authors.initials: F
authors.lastname: Fiat
authors.affiliation: L'Oreal Research & Innovation, F93600 Aulnay-sous-Bois, France;
authors.forename: Gustavo S
authors.initials: GS
authors.lastname: Luengo
authors.affiliation: Laboratoire de Physique des Solides, Paris-Sud University, F91400 Orsay, France.
authors.forename: Jean
authors.initials: J
authors.lastname: Doucet
authors.affiliation: L'Oreal Research & Innovation, F93600 Aulnay-sous-Bois, France;
authors.forename: Bruno A
authors.initials: BA
authors.lastname: Bernard
authors.affiliation: L'Oreal Research & Innovation, F93600 Aulnay-sous-Bois, France;
authors.forename: Nawel
authors.initials: N
authors.lastname: Baghdadli
mesh_terms: D018859:Hair Follicle / Q000737:chemistry* / Q000378:metabolism*; D006801:Humans; D007633:Keratins / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D007633:Keratins
keywords: X-ray diffraction; atomic force microscopy; biomechanics; elastic modulus; human hair follicle