MEIBOMIAN CYST
MELPHALAN
nausea, dizziness, headache, rash, and,
rarely, raised blood calcium level.
meibomian cyst
See
chalazion.
Meig's syndrome
A rare condition in
which a tumour of an
ovary
is accompa-
nied by
ascites
and a
pleural effusion.
The fluid usually disappears when the
tumour is removed.
meiosis
A type of cell division that
occurs in the
ovaries
and
testes
during
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).
melaena Black, tarry
faeces
caused 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 ulcer
but may indicate cancer,
melancholia Former term for
depression.
melanin The 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
).
melanocyte
A specialized skin cell that
produces the pigment
melanin.
melanoma, juvenile
A 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,
or one suspected of being skin cancer,
can be removed surgically.
melanoma, malignant
The most seri-
ous of the 3 types of skin cancer, the
other
2
being
basal cell carcinoma
and
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
skin biopsy
and the melanoma
is removed surgically.
Radiotherapy
or
anticancer drugs
may also be necessary.
MELANOMA, MALIGNANT
melanosis coli Black or brown discol-
oration of the colon lining, associated
with chronic
constipation
and 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).
melasma
See
chloasma.
melatonin
A
hormone
secreted by the
pineal gland
that is thought to play a
part in controlling daily body rhythms.
melphalan
An
anticancer drug
used to
treat
multiple myeloma
as well as certain
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