Hair growth comes follicles located in the top
layer, of the skin. At the base of each
follicle is the papilla, containing nerves and
blood vessels that are responsible for
maintaining hair growth. Surrounding the
papilla is the bulb. It was once believe that
the bulb contained immature cells that make
new hair, but in 1990 found that new hair is
developed in a region just above the bulb,
called the hair bulge. Above the bulge the
hair is surrounded by a case of cells with
openings for the sebaceous gland, which makes
a waxy coating to prevent the hair from drying
out, and the sweat gland, which makes a watery
secretion to lubricate the hair. An erector
muscle, attached to the side of the follicle,
is responsible for the "goose pimples" seen
when people are cold or frightened.
Hair is composed of three concentric layers.
The outermost is the cuticle, which is made up
of thin overlapping cells like shingles; the
next is the cortex, made of many elongated
cells; and in the center is the medulla, with
its rectangular-shaped cells.
To form a new hair, the papilla sends a signal
to the immature cells in the bulge, directing
them to migrate toward the papilla. These
cells are triggered by the papilla to divide
and mature. The new hair eventually grows up
beyond the surface of the skin. Hair is not a
permanent body structure. Follicles produce
hairs during a growth phase called the anagen
phase, which alternates with a rest phase
called the catagen phase. A follicle then
sheds its hair and replaces it with a new one
during a phase called the telogen phase.
Growth and shedding patterns vary widely. In
humans each follicle seems to have its own
cycle independent of the rest. A human scalp
hair usually lasts about four years.
Baldness, or alopecia, is total or partial
loss of scalp hair. The condition may be
temporary or permanent. The most common type
of alopecia is pattern baldness, a hereditary
trait that is expressed more often in males
than in females because it depends on the
influence of the male hormone testosterone.
Pattern baldness in males extends until only a
sparse growth of hair remains on the back and
sides of the head. In women, the baldness
usually extends until only a sparse growth
remains on the crown. Neither hair tonics nor
any other medical measure can prevent or
reverse such baldness.
Premature baldness may partly result from an
imbalance of sex hormones. Sudden temporary
hair loss sometimes occurs as a result of
typhoid fever, influenza, pneumonia, or
stress. Gradual thinning of the hair may be
caused by severe nutritional deficiency,
tuberculosis, cancer, and disorders of the
thyroid gland or pituitary gland. Temporary
baldness also may be caused by exposure to
nuclear radiation or X rays or by the internal
use of certain anticancer drugs. Plugs of
hair-containing skin from the back of a bald
person's head are sometimes successfully
transplanted on bare areas of the scalp; more
painstakingly, individual hair follicles may
be transplanted. The hypertension drug
minoxidil has been found to restore hair
growth.
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