Integumentary System
bout 1.4 billion people worldwide are
bald. The most common type of baldness
in adults is pattern baldness, in which
the top of the head loses hair. Pattern baldness
ects 35 million men and 20 million women in
the United States and is also common elsewhere.
The women tend to be past menopause, when
lowered amounts of the hormone estrogen con-
tribute to hair loss, which occurs more evenly on
the scalp than it does in men. Pattern baldness is
androgenic alopecia
because it is associated
with testosterone, an androgenic (male) hormone.
Variations in the androgen receptor gene, which
determines the activity of androgens in hair fol-
licles, may lie behind susceptibility to pattern
baldness. Abnormal hormone levels that mimic
menopause may cause hair loss in young women.
Another type of baldness is
alopecia areata,
in which the body manufactures antibodies that
attack the hair follicles. This results in oval bald
spots in mild cases but complete loss of scalp and
body hair in severe cases. About 2.5 million peo-
ple in the United States have alopecia areata.
Temporary hair loss has several causes.
Lowered estrogen levels shortly before and after
giving birth may cause a woman’s hair to fall out
in clumps. Taking birth control pills, cough medi-
cations, certain antibiotics, vitamin A derivatives,
antidepressants, and many other medications
can also cause temporary hair loss. A sustained
high fever may prompt hair loss six weeks to
three months later.
Many people losing their hair seek treatment
g. 6B). One treatment is minoxidil (Rogaine), a
drug originally used to lower high blood pressure.
Rogaine causes new hair to grow in 10% to 14%
of cases, and in 90% of people, it slows hair loss.
However, when a person stops taking it, any new
hair falls out. Hair transplants move hair follicles
from a hairy body part to a bald part. They work.
Several other approaches, however, can damage
the scalp or lead to infection. These include sutur-
ing on hair pieces and implants of high-density
cial ±
bers. Products called “thinning hair sup-
plements” are ordinary conditioners that make
hair feel thicker. They are concoctions of herbs
and the carbohydrate polysorbate.
A future approach to treating baldness may
harness the ability of stem cells to divide and dif-
ferentiate to give rise to new hair follicles. Stem
cells that can produce hair as well as epidermal
cells and sebaceous glands lie just above the
“bulge” region at the base of a hair follicle. The
rst clue to the existence of these cells was that
new skin in burn patients arises from hair fol-
licles. Then, experiments in mice that mark stem
cells and their descendants showed that the cells
give rise to hair and skin. Manipulating stem cells
could someday treat extreme hairiness (hirsut-
ism) as well as baldness.
Hair Loss
A single gene controls the proportions of eumelanin and pheomel-
anin in hair. Analysis of this gene in cells from arm bones of
Neanderthals from about 45,000 years ago indicates that some of
them had reddish hair and pale skin, in contrast to the common view
of Neanderthals as having dark pigmentation.
A bundle of smooth muscle cells, forming the
pili muscle
(see F gs. 6.2
and 6.7
), attaches to each hair
follicle. This muscle is positioned so that a short hair in the
follicle stands on end when the muscle contracts. If a person
is emotionally upset or very cold, nerve impulses may stimu-
late the arrector pili muscles to contract, raising goosefl esh,
or goose bumps. Each hair follicle also has associated with it
one or more sebaceous (oil-producing) glands.
Skin Glands
Sebaceous glands
shus glandz) (see F
g. 6.2) contain
groups of specialized epithelial cells and are usually asso-
ciated with hair follicles. They are holocrine glands (see
chapter 5, p. 150), and their cells produce globules of a fatty
Keratinized cells
of hair shaft
squamous cells
of epidermis
Being bald can be beautiful,
but many people with hair loss seek ways to
grow hair.
Scanning electron micrograph of a hair emerging from
the epidermis (875×).
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