Hair root
Hair root
Hair an ato m y • Figure 4.3
Hairs are composed of dead, keratinized cells.
Hair shaft
Hair follicle
External root sheath
Arrector pili
muscle
Hair -
matrix
Bulb
Melanocyte
Papilla
of the hair
Blood — =
Sebaceous
gland
Hair root
plexus
Bulb
Papilla
of the hair
Apocrine
sweat gland
Blood
vessels
a. Hair
and surrounding structures
Dermal root
sheath
(dense dermis)
Hair follicle
Hair root
vessels
b.
Frontal and transverse sections of the
hair
root
causes “goose bumps” because the skin around the shaft
forms slight elevations.
The color of hair is due to melanin being synthesized
by melanocytes in the matrix of the bulb and passing into
cells of the root and shaft. Dark-colored hair contains
mostly brown to black melanin. Blond and red hairs con-
tain variants of yellow to red melanin in which there is
iron and more sulfur. Gray hair is the result of sporadic
production of melanin as the melanocytes begin to die off,
usually with aging. White hair is generally due to an ab-
sence of melanin in the hair.
Scalp and body hair grow continuously throughout most
of your life. However, at puberty,
hormones
stimulate
a
different
type of hair growth, which varies
with gender: •
Males
—When the testes begin
secreting significant quantities
of androgens, males develop
the
typical
male
pattern
of
hair growth, including facial,
chest, pubic, and axillary hair.
Females
—The ovaries
and the adrenal glands produce
androgen
(AN-dro-
jen) A masculinizing
sex hormone pro-
duced by the testes in
males and the adrenal
cortex in both genders.
axillary
(ak-SIL-ary)
Of the axilla (plural
is axillae), the small
hollow beneath the
arm where it joins the
body at the shoulders.
c. Transverse section
of hair root
small quantities
of androgens, which
promote hair growth in the axillary
and pubic regions.
Occasionally, a tumor of the adrenal glands, testes, or
ovaries produces an excessive amount of androgens. This
results in a condition of excessive body hair called hirsut-
ism (
HER-soo-tizm
;
hirsut-
= shaggy), which is most com-
mon in females or prepubertal males.
In genetically predisposed adults, androgens inhibit
hair growth, thereby producing the most common form of
baldness, androgenic alopecia, or male-pattern bald-
ness. Men tend to lose hair at the temples and crown,
while women are more likely to have thinning of hair
on top of the head. To treat baldness and enhance hair
growth, the Food and Drug Ad-
ministration approved the use of
minoxidil (sold under the brand
name Rogaine). This drug causes
vasodilation, thus increasing cir-
culation that stimulates scalp fol-
licles to enlarge and lengthens
the growth cycle. While minoxidil
improves hair growth in about
vasodilation
(vaz'-o-DT-la-shun) An
increase in the size of
the lumen of a blood
vessel caused by re-
laxation of the smooth
muscle in the wall of
the vessel.
Accessory Structures Provide Protection and Help Regulate Body Temperature 97
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