Chapter 5
The Integumentary System
159
5
sheath, which has external and internal parts, thins as it ap-
proaches the hair bulb, so that only a single layer of epithelial
cells covers the papilla.
Te cells that compose the
hair matrix
, or actively divid-
ing area of the hair bulb that produces the hair, originate in a
region called the
hair bulge
located a fraction of a millimeter
above the hair bulb. When chemical signals diffusing from
the hair papilla reach the hair bulge, some of its cells migrate
toward the papilla, where they divide to produce the hair cells.
As the matrix produces new hair cells, the older part of the
hair is pushed upward, and its fused cells become increasingly
keratinized and die.
Associated with each hair follicle is a bundle of smooth mus-
cle cells called an
arrector pili
(ah-rek
9
tor pi
9
li; “raiser of hair”)
muscle. As you can see in Figure 5.1, most hair follicles approach
the skin surface at a slight angle. Te arrector pili muscle is at-
tached in such a way that its contraction pulls the hair follicle
upright and dimples the skin surface to produce goose bumps
in response to cold temperatures or fear. Tis “hair-raising” re-
sponse is not very useful to humans, with our short sparse hairs,
but it is an important way for other animals to retain heat and
protect themselves. Furry animals stay warmer by trapping a
layer of insulating air in their fur; and a scared animal with its
hair on end looks larger and more formidable to its enemy. Te
more important role of the arrector pili in humans is that its
contractions force sebum out of hair follicles to the skin surface
where it acts as a skin lubricant.
Types and Growth of Hair
Hairs come in various sizes and shapes, but as a rule they can
be classified as vellus or terminal. Te body hair of children
and adult females is pale, fine
vellus hair
(vel
9
us;
vell
5
wool,
fleece). Te coarser, longer hair of the eyebrows and scalp is
terminal hair
, which may also be darker.
At puberty, terminal hairs appear in the axillary and pubic re-
gions of both sexes and on the face and chest (and typically the
arms and legs) of males. Tese terminal hairs grow in response
to the stimulating effects of androgens (of which
testosterone
is
the most important), and when male hormones are present in
large amounts, terminal hair growth is luxuriant.
Many factors influence hair growth and density, especially
nutrition and hormones. Poor nutrition means poor hair
growth, whereas conditions that increase local dermal blood
flow (such as chronic physical irritation or inflammation) may
enhance local hair growth. Many old-time bricklayers who
carried their hod on one shoulder all the time developed one
hairy shoulder. Undesirable hair growth (such as on a woman’s
upper lip) may be arrested by
electrolysis
or laser treatments,
which use electricity or light energy, respectively, to destroy
the hair roots.
Homeostatic Imbalance
5.4
In women, both the ovaries and the adrenal glands normally
produce small amounts of androgens. Excessive hairiness, or
hirsutism
(her
9
soot-izm;
hirsut
5
hairy), as well as other signs
of masculinization, may result from an adrenal gland or ovarian
tumor that secretes abnormally large amounts of androgens.
Since few women want a beard or hairy chest, such tumors are
surgically removed as soon as possible.
Te rate of hair growth varies from one body region to another
and with sex and age, but it averages 2.5 mm per week. Each follicle
goes through
growth cycles.
In each cycle, an active growth phase,
ranging from weeks to years, is followed by a regressive phase.
During the regressive phase, the hair matrix cells die and the fol-
licle base and hair bulb shrivel somewhat, dragging the hair papilla
upward to abut the region of the follicle that does not regress. Te
follicle then enters a resting phase for one to three months. A±er
the resting phase, the cycling part of the follicle regenerates and
activated bulge cells migrate toward the papilla. As a result, the
matrix proliferates again and forms a new hair to replace the old
one that has fallen out or will be pushed out by the new hair.
Te life span of hairs varies and appears to be controlled by
a slew of proteins. Te follicles of the scalp remain active for six
to ten years before becoming inactive for a few months. Because
only a small percentage of the hair is shed at any one time, we lose
an average of 90 scalp hairs daily. Te follicles of the eyebrow hairs
remain active for only three to four months, which explains why
your eyebrows are never as long as the hairs on your head.
Hair Thinning and Baldness
A follicle has only a limited number of cycles in it. Given ideal
conditions, hair grows fastest from the teen years to the 40s,
and then its growth slows. Te fact that hairs are shed faster
than they are replaced leads to hair thinning and some degree
of baldness, or
alopecia
(al
0
o-pe
9
she-ah), in both sexes. By age
35, noticeable hair loss occurs in 40% of men, and by age 60
that number jumps to 85%. Much less dramatic in women, the
process usually begins at the anterior hairline and progresses
posteriorly. Coarse terminal hairs are replaced by vellus hairs,
and the hair becomes increasingly wispy.
True
, or
frank
,
baldness
is a different story entirely. Te most
common type,
male pattern baldness
, is a genetically deter-
mined, sex-influenced condition. It is thought to be caused
by a delayed-action gene that “switches on” in adulthood and
changes the response of the hair follicles to DH² (dihydrotes-
tosterone), a metabolite of testosterone. As a result, the follicu-
lar growth cycles become so short that many hairs never even
emerge from their follicles before shedding, and those that do
are fine vellus hairs that look like peach fuzz in the “bald” area.
Until recently, the only cure for male pattern baldness was
drugs that inhibit testosterone production, but they also cause
loss of sex drive—a trade-off few men would choose. Quite by
accident, it was discovered that minoxidil (Rogaine), a drug
used to reduce high blood pressure, has the interesting side ef-
fect in some bald men (and balding women) of stimulating hair
regrowth. Although its results are variable, minoxidil is available
over the counter in dropper bottles or spray form for application
to the scalp. Finasteride (better known as Propecia), according
to some the most promising cure for male pattern baldness, has
had moderate success. Available only by prescription in once-a-
day pill form, it must be taken for the rest of a person’s life. Once
the patient stops taking it, all of the new growth falls out.
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