Cortex
The cortex accounts for most of the keratinized hair shaft and is the main component responsible for its mechanical properties (hair strength and elasticity) and for hair color.
It is therefore the site of most cosmetic changes. Permanent hair coloring, waving, and straightening all cause physicaland chemical changes to the cortex.15The cortex is made up of densely packed elongated cor-tical cells containing keratin filaments that run parallel tothe hair shaft16as well as an amorphous matrix of high-sulphur proteins in which the -helical keratin chains areembedded.19Keratins are a family of complex proteins.Acidic ker-atin polypeptides pair with basic keratin polypeptidesforming polypeptide chains of hard keratin known asprotofilaments.Multiple protofilaments make up a ker-atin strand with a large number of sulfur-containing cysteineresidues that form covalent disulfide bonds to adja-cent chains, so that neighboring keratin chains are strongly crosslinked,thus conferring high physical and chemical stability.These disulfide bonds are largely responsible for hairshape, stability, and texture,and confer a degree of elasticity that allows hair wet with water to be stretched without damage up to 30% beyond its original length.1Keratin chains are also more weakly bonded to each other by van der Waals forces, hydrogen bonds, and saline bonds.