Encyclopedia of Health and Medicine

M
macule A small SKIN LESION that 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 discoloration,
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 dermis
(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 individual’s
natural skin color. The melanocytes in
darker skin are more active than those in lighter
skin. The eyes and HAIR also 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 conversion
of the amino acid tyrosine, which the body
synthesizes from dietary proteins such as meats
and which the melanocytes store, into dopaquinone.
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 activates
the natural repair mechanisms within the
cells, which releases chemicals into the bloodstream
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 contains.
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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