Exogenous Pigments

The pigments that contribute to the skin's normal color— melanin and hemoglobin—may be accentuated within benign skin growths such as moles and hemangiomas. A mole (the medical term is "nevus") is a collection of melanocyte-like cells, usually within the dermis. These nevus cells generally produce increased amounts of melanin, imparting a brown color to the lesion. (Nevi are discussed in greater detail in the section on Birthmarks, below.)

A hemangioma is a bright red bump that is composed of capillaries packed closely together. Although these lesions are raised above the skin surface, they are composed of dermal tissue (blood vessels) and are covered with normal epidermis. Hemangiomas are red simply because they contain so many red blood cells.

In certain abnormal or disease states, the skin may contain other pigments that produce unusual colors. Hemosiderin may make the skin appear brown or orange after a bruise or other injury heals, or after an injury that causes red blood cells to leak out of the capillaries (for example, a bruise). Macrophages, a type of white blood cell, are responsible for clearing out any substances that are normally not present in the dermis. Red blood cells that appear in the dermis after an injury die and then deteriorate; macrophages ingest the debris. Enzymes within the macrophages convert hemoglobin to hemosiderin, a protein that, like hemoglobin, contains iron. This substance may be present until the macrophages are able to physically remove it from the skin. The cleanup function of macrophages seems to be more efficient in areas of the body with good circulation (such as the face) than in areas with poor circulation (such as the lower leg or the feet). Hemosiderin is rarely encountered in the face but may persist for years in the ankle area.

Melanin may also get displaced as a result of an injury or inflammatory skin disease. Melanin is normally confined to the epidermis, but with an injury some of the melanin may drop down into the dermis. This displaced melanin will appear as a darker area of skin. A common cause of such post-inflammatory hyperpigmentation in the facial area is acne. Many people attribute this discoloration to scarring, but a true scar is a permanent alteration in the skin's texture (see Scars, below), with or without a change in skin color. Post-inflammatory hyperpigmentation is only a change in color and is not permanent; it will eventually disappear as macrophages remove it.

Hemosiderin and melanin are both endogenous protein products that appear naturally in the skin. There are also exogenous pigments that are the result of deposition into the skin of a manufactured or artificial material. An example of such material would be pigments that develop from topically applied or ingested drugs or medications. An intentional example of exogenous pigmentation would be a tattoo.

Certain drugs can accumulate in the skin directly or in an altered form due to changes in the drug molecule caused by sunlight or by the body's metabolism of the drug. Minocycline, an antibiotic used to treat acne, becomes chemically altered and forms slate gray deposits in the skin of some people. Amiadarone, a heart medication, causes a similar discoloration in nearly all patients who take it. Chlorpromazine, a drug used to treat mental illness, reacts with sunlight and causes discoloration of facial skin.

People have adorned their bodies with decorative tattoos since prehistoric times. Tattoos are produced by placing exogenous pigments into the dermis. Any pigment placed into the epidermis would quickly disappear by being carried away in the constant flow of keratinocytes migrating to the surface of the epidermis as they differentiate and finally flake off into the environment. Exogenous pigment material placed into the dermis will stay there, seemingly indefinitely. Tattoos are essentially permanent but may fade over many years as macrophages gradually remove those pigment particles that are small enough to be ingested.

The most common tattoo pigment, used throughout history and even in modern times, is carbon-based. Soot or charcoal from fires was used in ancient times as the carbon source and was implanted into the skin with small sharp objects such as bone chips. In more recent times the most commonly used black pigment has been India ink, which is composed of graphite (pure carbon) particles suspended in water. Tattoo ink is implanted into the dermis with needles. Because of the optical qualities of the skin, the black graphite particles frequently reflect a dark blue color to the viewer.

Other common pigments used for tattoos include cobalt (blue), cinnabar (red), and chromium (green). Recently, a variety of intense organic pigments have been used to produce tattoos with greater color saturation.

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