186
UNIT TWO
The epidermis thins as the decades pass. As the cell
cycle slows, epidermal cells grow larger and more irregular
in shape, but are fewer. Skin may appear scaly because, at
the microscopic level, more sulfur–sulfur bonds form within
keratin molecules. Patches of pigment commonly called “age
spots” or “liver spots” appear and grow
(f
g. 6.15)
. These are
sites of oxidation of fats in the secretory cells of apocrine and
eccrine glands and refl
ect formation of oxygen free radicals.
The dermis becomes reduced as synthesis of the connec-
tive tissue proteins collagen and elastin slows. The combi-
nation of a shrinking dermis and loss of some fat from the
subcutaneous layer results in wrinkling and sagging of the
skin. Fewer ± broblasts delay wound healing. Some of the
changes in the skin’s appearance result from speci±
c de±
cits.
The decrease in oil from sebaceous glands dries the skin.
Various treatments temporarily smooth facial wrinkles. “Botox” is
injection of a very dilute solution of botulinum toxin. Produced by
the bacterium
Clostridium botulinum,
the toxin causes food poison-
ing. It also blocks nerve activation of the facial muscles that control
smiling, frowning, and squinting. After three months, though, the
facial nerves contact the muscles at diF
erent points, and the wrinkles
return. (Botox used at higher doses to treat neuromuscular conditions
can cause adverse effects.) Other anti-wrinkle treatments include
chemical peels and dermabrasion to reveal new skin surface; collagen
injections; and transplants of fat from the buttocks to the face.
The skin’s accessory structures also show signs of
aging. Slowed melanin production whitens hair as the fol-
licle becomes increasingly transparent. Hair growth slows,
the hairs thin, and the number of follicles decreases. Males
may develop pattern baldness—hereditary, but not often
expressed in females. A diminished blood supply to the nail
beds impairs their growth, dulling and hardening them.
Sensitivity to pain and pressure diminishes with age as
the number of receptors falls. A ninety-year-old’s skin has
only one-third the number of such receptors as the skin of a
young adult.
The ability to control temperature falters as the number
of sweat glands in the skin falls, as the capillary beds that
surround sweat glands and hair follicles shrink, and as the
ability to shiver declines. In addition, the number of blood
vessels in the deeper layers decreases, as does the ability
to shunt blood toward the body’s interior to conserve heat.
As a result, an older person is less able to tolerate the cold
and cannot regulate heat. In the winter, an older person
might set the thermostat ten to ±
fteen degrees higher than a
younger person would. Fewer blood vessels in and underly-
ing the skin account for the pale complexions of some older
individuals. Changes in the distribution of blood vessels also
contribute to development of pressure sores in a bedridden
person whose skin does not receive adequate circulation.
Aging of the skin is also related to skeletal health. The
skin is the site of activation of vitamin D, which requires
A burn that destroys the epidermis, dermis, and the
accessory structures of the skin is called a
full-thickness
(third-degree)
burn.
The injured skin becomes dry and leath-
ery, and it may vary in color from red to black to white.
A full-thickness burn usually occurs as a result of immer-
sion in hot liquids or prolonged exposure to hot objects,
fl ames, or corrosive chemicals. Most of the epithelial cells in
the affected region are likely to be destroyed, so spontaneous
healing can occur only by growth of epithelial cells inward
from the margin of the burn. If the injury is extensive,
treatment may involve removing a thin layer of skin from
an unburned region of the body and transplanting it to the
injured area. This procedure is an example of an
autograft,
a
transplant within the same individual.
If a burn is too extensive to replace with skin from other
parts of the body, cadaveric skin from a skin bank may be
used to cover the injury. In this case, the transplant, an
example of an
allograft
(from person to person) is a tempo-
rary covering that shrinks the wound, helps prevent infec-
tion, and preserves deeper tissues. In time, after healing
has begun, the temporary covering may be replaced with
an autograft, as skin becomes available in areas that have
healed. However, skin grafts can leave extensive scars.
Various skin substitutes also may be used to temporar-
ily cover extensive burns. These include amniotic membrane
that surrounded a human fetus and artificial membranes
composed of silicone, polyurethane, or nylon together with
a network of collagenous ±
bers. Another type of skin substi-
tute comes from cultured human epithelial cells. In a labo-
ratory, a bit of human skin the size of a postage stamp can
grow to the size of a bathmat in about three weeks. Skin
substitutes are a major focus of tissue engineering, discussed
in From Science to Technology 5.2 (p. 166).
The treatment of a burn patient requires estimating the
extent of the body’s affected surface. Physicians use the
“rule of nines,” subdividing the skin’s surface into regions,
each accounting for 9% (or some multiple of 9%) of the total
surface area
(f g. 6.14)
. This estimate is important in plan-
ning to replace body fl
uids and electrolytes lost from injured
tissues and for covering the burned area with skin or skin
substitutes.
PRACTICE
26
What is the tissue response to in±
ammation?
27
How does a scab slough oF
?
28
Which type of burn is most likely to leave a scar? Why?
6.6
LIFE-SPAN CHANGES
We are more aware of aging-related changes in skin than in
other organ systems, because we can easily see them. Aging
skin affects appearance, temperature regulation, and vitamin
D activation.
previous page 216 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 218 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off