Bma Illustrated Medical Dictionary

androgen hormonesA group of hor-

mones that stimulate the development

of male sexual characteristics.

Androgens are produced by special-

ized cells in the testes in males and in

the adrenal glands in both sexes. The

ovaries secrete very small quantities of

androgens until the menopause. The

most active androgen is testosterone,

which is produced in the testes. The

production of androgens by the testes

is controlled by certain pituitary hor-

mones, called gonadotrophins. Adrenal

androgens are controlled by ACTH,

another pituitary hormone.

Androgens stimulate male secondary

sexual characteristics at puberty, such

as the growth of facial hair and deepen-

ing of the voice. They have an anabolic

effect (they raise the rate of protein syn-

thesis and lower the rate at which it is

broken down). This increases muscle

bulk and accelerates growth. At the end

of puberty, androgens cause the long

bones to stop growing. They stimulate

sebum secretion, which, if excessive,

causes acne. In early adult life, andro-

gens promote male-pattern baldness.

Androgen deficiency may occur if the

testes are diseased or if the pituitary

gland fails to secrete gonadotrophins.

Typical effects include decreased body

and facial hair, a high-pitched voice,

underdevelopment of the genitalia, and

poor muscle development.

Overproduction of androgens may be

the result of adrenal disorders (see

adrenal tumours; adrenal hyperplasia,

congenital), of testicular tumours (see

testis,cancer of), or, rarely, of androgen-

secreting ovarian tumours (see ovary,

cancer of). In men, excess androgens

accentuate male characteristics; in boys,

they cause premature sexual develop-

ment. In women, excess androgens cause

virilization, the development of mascu-

line features such as an increase in

body hair, deepening of the voice, cli-

toral enlargement, and amenorrhoea.

anencephalyAbsence of the brain and

cranial vault (top of the skull) at birth.

Most infants with anencephaly are still-

born or survive only a few hours.

Anencephaly is detectable early in preg-

nancy by measurement of the maternal

alpha-fetoprotein, by ultrasound scan-

ning, by amniocentesis, or by fetoscopy;

if anencephaly is detected, termination

of the pregnancy may be considered.

Anencephaly is due to a failure in the

development of the neural tube, which

is the nerve tissue in the embryo that

normally develops into the spinal cord

and brain. (See also neural tube defects.)

aneurysmAbnormal dilation (balloon-

ing) of an arterycaused by the pressure

of blood flowing through a weakened

area. Aneurysms most commonly affect

the aortaand arteries supplying the brain.

The most common cause of an aneu-

rysm is atherosclerosis, a condition in

which fatty deposits weaken the artery

wall. The aorta is the usual site of

atherosclerotic aneurysms.

Less commonly, aneurysms may be

due to a congenital weakness of the

artery walls. Most cerebral aneurysms,

known as berry aneurysms because of

their appearance, are congenital. Mar-

fan’s syndrome, an inherited disorder in

which the muscular layer of the aorta

is defective, is often associated with

aneurysms just above the heart. The

arterial wall can

also be weakened

by inflammation,

as occurs in poly-

arteritis nodosa.

Most aneurysms

are symptomless

and remain un-

detected, but if

the aneurysm ex-

pands rapidly and

causes pain, or it

is very large, the

symptoms are due

to pressure on

nearby structures.

Aneurysms may

eventually rupture, cause fatal blood

loss, or, in the case of a cerebral

aneurysm, loss of consciousness (see

subarachnoid haemorrhage).

In some cases, only the inner layer of

the artery wall ruptures, which allows

blood to track along the length of the

artery and block any branching arteries.

There is usually severe pain and high

risk of rupture occurring.

ANDROGEN HORMONES ANEURYSM

A

Fatty

deposit

Weakened, bulging

artery wall

ANEURYSM