or one suspected of being skin cancer,
can be removed surgically.
melanoma, malignantThe most seri-
ous of the 3 types of skin cancer, the
other 2 being basal cell carcinomaand
squamous cell carcinoma. Malignant mel-
anoma is a tumour of melanocytes, the
cells that produce melanin, and is due to
long-term exposure to strong sunlight.
There are an increasing number of new
cases and deaths in the UK each year
from this skin cancer.
Tumours usually develop on exposed
skin but may occur anywhere on the
body. A melanoma usually grows from
an existing mole, which may enlarge,
become lumpy, bleed or crust over,
change colour, develop an irregular
edge, turn into a scab, or become itchy.
Occasionally, a melanoma develops in
normal skin. The tumour often spreads
to other parts of the body. Diagnosis is
by a skinbiopsyand the melanoma
is removed surgically. Radiotherapyor
anticancer drugsmay also be necessary.melanosis coliBlack or brown discol-
oration of the colon lining, associated
with chronic constipationand prolonged
use of certain laxative drugs, such as
senna, rhubarb, and cascara.
The discoloration is most common in
elderly people and is usually symptom-
less, clearing up when the laxatives are
stopped. Rarely, it is associated with
colon cancer (see colon, cancer of).
melasmaSee chloasma.
melatoninA hormonesecreted by the
pineal glandthat is thought to play a
part in controlling daily body rhythms.
melphalanAn anticancer drugused to
treat multiple myelomaas well as certainnausea, dizziness, headache, rash, and,
rarely, raised blood calcium level.
meibomian cystSee chalazion.
Meig’s syndromeA rare condition in
which a tumour of an ovaryis accompa-
nied by ascitesand a pleural effusion.
The fluid usually disappears when the
tumour is removed.
meiosisA type of cell division that
occurs in the ovariesand testesduring
the production of egg and sperm cells.
During meiosis in humans, a cell con-
taining 23 pairs of chromosomes(46 in
total) divides to form 4 sperm or egg
cells, each with 23 single chromosomes.
First, the chromosomes are duplicated
to produce 4 copies of each chromosome
(92 in total). Matching pairs of chromo-
somes line up and exchange genetic
material. The cell then divides twice to
form 4 daughter cells, with each taking
1 copy of each chromosome. Egg and
sperm cells therefore have only half the
usual chromosome content of a body
cell, so that each parent contributes
half of the child’s genetic material. The
exchange between chromosomes means
that each daughter cell has a unique
genetic make-up. (See also mitosis).
melaenaBlack, tarry faecescaused by
bleeding, usually in the upper gastroin-
testinal tract. The blood is blackened
by the action of secretions during diges-
tion. Melaena is usually caused by a
peptic ulcerbut may indicate cancer.
melancholiaFormer term for depression.
melaninThe brown or black pigment
that gives skin, hair, and the iris of the
eyes their colouring. Melanin is pro-
duced by cells called melanocytes.
Exposure to sunlight increases the
production of melanin, which protects
the skin from the harmful effects of
ultraviolet rays and causes the skin to
darken. Localized overproduction of
melanin in the skin can result in a pig-
mented spot, most commonly a freckle
or mole (see naevus).
melanocyteA specialized skin cell that
produces the pigment melanin.
melanoma, juvenileA raised, reddish-
brown skin blemish which sometimes
appears on the face or legs in early
childhood (see naevus). Although they
are usually harmless, an unsightly growth,
MEIBOMIAN CYST MELPHALAN
MELANOMA, MALIGNANT M
Irregular edge Crusted surface