tIssUes, orGans, and orGan systeMs 79What is the integumentary system?- The integumentary system is the body’s covering. It consists of
skin and structures that develop from it. - With its layers of keratinized and melanin-shielded epidermal
cells, skin helps the body conserve water, limit damage from
ultraviolet radiation, and resist mechanical stress. - Hair, oil glands, sweat glands, and nails are derived from the
skin’s epidermis.
taKe-hoMe MessageThe epidermis is stratified squamous epithelium. Its
cells arise in deeper layers and are pushed toward the
surface as new cells arise beneath them. (This efficient
replacement is one reason why the skin can mend minor
damage so quickly.) As cells move upward, they become
flattened, lose their nucleus, and die. Eventually they rub
off or flake away.
Most cells of the epidermis are keratinocytes. These
cells make keratin, a tough, water-insoluble protein. By
the time they reach the skin surface and have died, all that
remains are the keratin fibers inside plasma membranes.
This helps make the skin’s outermost layer—the stratum
corneum—tough and waterproof.
In the deepest layer of epidermis, cells called
melano cytes produce a brown-black pigment called
melanin. The pigment is transferred to keratinocytes and
helps give skin its color. Human skin color varies due to
differences in the distribution and activity of those cells.
A yellow-orange pigment in the dermis, called carotene,
also contributes some color. Pale Cauca sian skin has only
a little mela nin, so the pigment hemoglobin inside red
blood cells shows through thin-walled blood vessels and
the epidermis itself, both of which are transparent. Natu-
rally brown or black skin contains more melanin.
The epidermis also contains some defensive cells. Lang-
erhans cells are phagocytes (“cell eaters”). They consume
bacteria or viruses, mobilizing the immune system in the
process. Granstein cells may help control immune responses
in the skin.
Small blood vessels and sensitive nerve endings lace
through the dermis, and hair follicles, sweat glands, and oil
glands are embedded in it. On the palms and soles of the
feet it also has ridges that push up corresponding ridges on
the epidermis. These ridges loop and curve in the patterns
we call fingerprints. Determined mainly by genes, the pat-
tern is different for each of us, even identical twins.
sweat glands and other structures
develop from epidermis
The body has about 2.5 million sweat glands. Sweat is
99 percent water; it also contains dissolved salts, traces
of ammonia and other wastes, vitamin C, and other sub-
stances. A subset of sweat glands that are in the palms,
soles of the feet, forehead, and armpits is important for
cooling the body when it becomes overheated. Another
type of sweat gland is abundant in the skin around the
genitals. Stress, pain, and sexual foreplay all can increase
the amount of sweat they secrete.
Oil glands (or sebaceous glands) are everywhere except on
the palms and the soles of the feet. The oily substance they
release, called sebum, softens and lubricates the hair and
skin. Other secretions kill harmful bacteria.
A hair is mostly keratinized cells, rooted in skin with a
shaft above its surface. As cells divide near the root’s base,
older cells are pushed upward, then flatten and die. Flat-
tened cells of the shaft’s outer layer overlap (Figure 4.11C)
and may frizz out as “split ends.” On average the scalp has
F i g u r e 4.12 vitiligo is a
disorder caused by the
death of melanocytes.
Lee Thomas is an African
American television
reporter who has vitiligo.
The disorder has turned
his hands white and
produced white blotches
on his face and arms.about 100,000 hairs. However, genes, nutrition, hormones,
and stress affect hair growth and density.skin disorders are common
The dense connective tissue of the dermis makes it quite
tough, but this protection has limits. For example, steady
abrasion—as might happen if you wear a too-tight shoe—
separates the epidermis from the dermis, the gap fills with
a watery fluid, and you get a blister.
Acne is a skin inflammation that develops when bac-
teria infect oil glands. Cold sores are caused by a type of
herpes virus. In the disorder vitiligo (Figure 4.12), melano-
cytes die and white patches form on the skin. The cause is
not known, but people of all races are affected.
Ultraviolet (UV) radiation stimulates the melanin-
producing cells of the epidermis. Prolonged sun exposure
increases melanin levels and light-
skinned people become tanned.
Tanning gives some protection
against UV radiation, but over the
years elastin fibers in the dermis
clump. The skin loses its resiliency
and begins to look leathery and
wrinkled.
UV radiation, including from
tanning lamps, also can trig-
ger cancer. The squamous cell
carcinoma shown above is a
common and easily treatable form
of skin cancer. Much more serious
is malignant melanoma, which
forms a dark, uneven, raised lesion
(right). It is a grave threat because in
its later stages it spreads quickly to
other parts of the body. Malignant melanomaBiophoto Associates/
Science SourceJames Stevenson/
Science SourceSquamous cell carcinoma© Michael Shore PhotographyCopyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).