Keratin is the essential component of hair. It is a protein formed by the combination of 18 amino acids, among which is cysteine, which being rich in sulphur plays an important role in the cohesion of the hair.
It is produced by the keratinocytes. These cells, situated in the bottom of the dermal papilla, multiply and differentiate: while some spread to the periphery of the hair follicle to form the internal and external epithelial sheaths, others become elongated to form the hair shaft. During this journey they fill with keratin fibres. As soon as they have filled with keratin, the keratinocytes die. Thus, after a journey of about 0.5 mm inside the root, the hair is definitively formed, and during the remainder of its life does not receive any further supply from the tissue which created it