Hair gets its colour through a process of synthesis and distribution or transfer of pigments. These pigments are melanins which are also responsible for the colour of the skin. They are polymers of phenol radicals (indole 5–6 quinone), and two types exist:
- Eumelanins are highly polymerised brown or black molecules containing few sulphur atoms. They occur in the cortex of the hair in the form of granules.
- Phaeomelanins are yellow or red and contain a large number of sulphur atoms in the form of cysteine (S-S-cysteine-DOPA). Less polymerised than eumelanins, they occur in a more diffuse form in the cortex and are particularly present in ginger hair.
How the pigments form
Synthesis of these pigments occurs in the melanocytes, cells located at the bottom of the hair follicle, more precisely in organelles derived from the Golgi vesicles and the endoplasmic reticulum called melanosomes. This synthesis starts with tyrosine, an amino-acid, and requires an enzyme called tyrosinase. The latter, synthesised in the form of inactive pro-tyrosinase, is stored in the dictyosomes of the Golgi apparatus. Tyrosinase catalyses the oxidation of tyrosine to dihydroxyphenylalanine (DOPA) then into DOPA-quinone. This process requires the presence of copper ions.
By a process of successive oxidation, cyclisation, and polymerisation reactions, the DOPA-quinone is transformed into eumelanin. This series of reactions occurs not only spontaneously but also through the involvement of other enzymes which have a tyrosinase activity (tyrosinase-related protein).
As for phaeomelanins, their synthesis occurs through the addition of two sulphur-containing compounds called glutathion and cysteine to DOPA-quinone.
How gradations of colour occur
A series of gradations is possible, producing mixtures of these two different melanins and the final colour of the hair will depend on the nature and proportion of each of the pigments. This genetically controlled pigmentation does not depend on the number of melanocytes but on the presence and type of the tyrosinase enzymes, on the chemical nature of the pigments and on the distribution of the melanosomes. At the two extremities of this palette of colour, ginger hair contains mainly phaeomelanin while the black hair of Asian peoples contains up to 100% eumelanin. If melanocytes are not present above the dermal papilla, no melanin synthesis is possible: the hair is no longer pigmented and grows white. This absence of pigmentation is known as canitia.