Pigmentation

Coloration of the skin, hair, and iris of the eyes by melanin. The more melanin present, the darker the coloration. Blood pigments can also colour skin (such as in a bruise). There are many abnormalities of pigmentation. Patches of pale skin occur in psoriasis, pityriasis alba, pityriasis versicolor, and vitiligo. Albinism is caused by generalized melanin deficiency. Phenylketonuria results in a reduced melanin level, making sufferers pale-skinned and fair-haired. Areas of dark skin may be caused by disorders such as eczema or psoriasis, pityriasis versicolor, chloasma, or by some perfumes and cosmetics containing chemicals that cause photosensitivity. Permanent areas of deep pigmentation, such as freckles and moles (see naevus), are usually due to an abnormality of melanocytes. Acanthosis nigricans is characterized by dark patches of velvet-like, thickened skin. Blood pigments may lead to abnormal colouring. Excess of the bile pigment bilirubin in jaundice turns the skin yellow, and haemochromatosis turns the skin bronze.