Disorders of Pigmentation

The colour of normal skin comes from a mixture of pigments. Skin pigmentation is darkest near the equator and correlates with latitude and ultraviolet radiation (UVR). Hair colour is determined by the relative amounts of the different types of melanin. Melanocytes are the only cells in the epidermis to contain tyrosinase (dopa oxidase), the rate‐limiting enzyme in melanogenesis. Genetic differences determine the pigmentation of the different races. Abnormal skin colours may be caused by an imbalance of the normal pigments (e.g. in cyanosis, chloasma and carotenaemia) or by the presence of abnormal pigments. Decreased melanin pigmentation can be caused by the absence of melanocytes (vitiligo) or abnormalities in melanin synthesis (albinism). Increased melanin is seen in the basal layer of the epidermis without any increase in the number of melanocytes, and without elongation of the rete ridges. Melasma is an acquired symmetrical hypermelanosis occuring on sun‐exposed skin, especially the face.