Normal skin pigmentation is a complex process that, in the epidermis as in the hair follicles, begins with the synthesis of melanin within melanosomes in the melanocytes, followed by melanosome transfer to neighboring basal and suprabasal keratinocytes. In basal cells, melanin granules are translocated to the upper pole of the nucleus, forming a melanin cap that protect DNA from UV rays. Melanin granules are eventually degraded as the keratinocyte undergoes terminal differentiation.
In humans, the melanocytes are localised either in the basal layers of the epidermis or in the hair follicles. Whatever is their localisation in skin, the melanocytes are derived from precursor cells (called melanoblasts) that originate from the neural crest.
Mammalian melanocytes produce in their melanosomes, two chemically distinct types of melanin pigments: black-brown eumelanin and yellow-reddish pheomelanin. In the melanocytes, eumelanosomes and pheomelanosomes cohabit.
Tyrosinase is the key-enzyme which regulates the first steps of eumelanin and pheomelanin synthesis: the transformation of L-tyrosines into L-3,4 dihydroxyphénylalanine (DOPAs) and then into DOPAquinones. From DOPAquinones, the synthesis pathways are different for the pheomelanin or eumelanin.
In human, as in the others mammals, the skin and hair color is mainly determined by the number, the size, the type, and the mode of repartition of the melanosomes. It is particularly interesting to note that in normal conditions, the racial differences in skin pigmentation in human do not depend of the melanocyte number in the epidermis. For a specific area, the number of epidermal melanocytes is nearly the same in Caucasoids, Negroids and Mongoloids.