Melanin is the brownish pigment responsible for the color of skin, hair, and the iris of the eyes. It is produced by melanocytes. Most people have similar numbers of melanocytes, and the wide range of color shades of human skin is due to the amount of melanin that is produced rather than the number of melanocytes. There are different subtypes of melanin, and the main ones in the skin are
Eumelanin (brown and black types)
Pheomelanin, which has a reddish hue
Ultraviolet radiation, as in sunlight, stimulates melanin production, as do a number of pathologic processes. Other factors can interfere with melanin production.
Pigmentation disorders involve hypopigmentation, depigmentation, or hyperpigmentation. Affected areas may be focal or diffuse.
In hypopigmentation, pigment is decreased, whereas in depigmentation, pigment is completely lost, leaving white skin.
Focal hypopigmentation is most commonly a consequence of
Injury
Inflammatory dermatoses (eg, atopic dermatitis, psoriasis)
Chemical exposure (especially to hydroquinones and phenols)
Focal hypopigmentation or depigmentation is also a feature of vitiligo (which may involve large areas of skin), leprosy, nutritional deficiencies (eg, kwashiorkor), genetic conditions (eg, tuberous sclerosis complex, piebaldism, Waardenburg syndrome), morphea (localized scleroderma, in which skin is usually sclerotic), lichen sclerosus, pityriasis versicolor (or tinea versicolor), idiopathic guttate hypomelanosis, progressive macular hypomelanosis, postinflammatory hypopigmentation, and pityriasis alba.
Diffuse hypopigmentation is most often caused by
Hyperpigmentation typically occurs after inflammation resulting from various causes. This postinflammatory hyperpigmentation is usually focal in distribution. Hyperpigmentation may also be caused by a systemic disorder, drug or substance, or cancer; in such cases, distribution is usually more diffuse.
