M

macule A small SKIN LESIONthat is flat, smooth,
and discolored. Macules are common and may
appear as the presenting symptom for numerous
dermatologic and other health conditions. Often
the only symptom they present themselves is dis-
coloration, though some macules itch or hurt. The
discoloration may be hyperpigmentation (darker
than the surrounding skin) or hypopigmentation
(lighter than the surrounding skin). A macule is
the same texture and thickness as the adjacent
skin and generally no larger than two inches in
length, width, or diameter. The most common
macule is a lentigo, or freckle.
See also BIRTHMARK; LENTIGINES; NODULE; PAPULE;
PUSTULE; VITILIGO.

malignant melanoma See SKIN CANCER.

melanocyte A type of cell prominent in the der-
mis (middle layer of the SKIN) that produces
melanin, the pigment that gives color to the skin
as well as protects the skin from ultraviolet light
damage. There are two types of melanin: the dark
brown pigment eumelanin and the red/yellow
pigment pheomelanin. The skin contains the same
number of melanocytes no matter what the indi-
vidual’s natural skin color. The melanocytes in
darker skin are more active than those in lighter
skin. The eyes and HAIRalso contain melanocytes.

Melanogenesis
The exclusive role of melanocytes is to produce
melanin (melanogenesis), a somewhat sequential
process. To prepare for melanogenesis, the body
produces the enzyme tyrosinase. Genetic encoding
regulates this process. Tyrosinase initiates conver-
sion of the amino acid tyrosine, which the body

synthesizes from dietary proteins such as meats

and which the melanocytes store, into dopa-

quinone. The dopaquinone forms the pigments

eumelanin and pheomelanin, which collectively

comprise mel-anin.

Exposure to ultraviolet light, notably sunlight,

initiates a sequence of hormonal and chemical

events that stimulate melanocytes to produce

melanin (melanogenesis):

1. Sunlight (or other ultraviolet light exposure)
   damages the cells of the skin. The damage acti-
   vates the natural repair mechanisms within the
   cells, which releases chemicals into the blood-
   stream that travel to the PITUITARY GLAND.


2. In response the pituitary gland releases
   melanocyte-stimulating hormone (MSH), to
   bind with melanocytes.


3. Melanocytes pass packets of melanin molecules
   to the keratocytes, which carry them to the
   outer layer of the epidermis as they migrate
   upward.

The resulting skin color depends on the mix of

eumelanin and pheomelanin the melanin con-

tains. The melanin in light skin contains more

pheomelanin than eumelanin. In darker skin the

balance tips the other way with the melanin in

dark skin containing more eumelanin than

pheomelanin. In the epidermis, melanin protects

the skin from damage by absorbing ultraviolet

light. The darker the skin, the less ultraviolet light

penetrates the epidermis. In general, it takes about

a week of regular sun exposure to generate a tan

adequate to begin protecting the skin from further

sun damage, though the tan itself signals sun

damage.

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