156
UNIT 2
Covering, Support, and Movement of the Body
5
One condition sun exposure does not cause, regardless of
its name, is
sunspots
, small circular white spots on the skin that
stand out like a sore thumb in tanned skin. Actually a fungus in-
fection, this condition—more accurately called
tinea versicolor
(
vers
5
change)—occurs mainly on the arms, legs, upper chest,
and back. Largely a cosmetic problem, its growth is encouraged
by oily, sweaty skin. Antifungal medication works, but it takes
a long time for the white patches to become pigmented again.
Many chemicals induce photosensitivity; that is, they
heighten the skin’s sensitivity to UV radiation and can cause
an unsightly skin rash. Such substances include some antibiotic
and antihistamine drugs, and many chemicals in perfumes and
detergents. Small, itchy blisters erupt all over the body. Ten the
peeling begins—in sheets!
Carotene
(kar
9
o-tēn) is a yellow to orange pigment found in
certain plant products such as carrots. It tends to accumulate in
the stratum corneum and in fatty tissue of the hypodermis. Its
color is most obvious in the palms and soles, where the stratum
corneum is thickest, and most intense when large amounts of
carotene-rich foods are eaten. However, the yellowish tinge of
the skin of some Asian peoples is due to variations in melanin,
as well as to carotene. In the body, carotene can be converted to
vitamin A, a vitamin that is essential for normal vision, as well
as for epidermal health.
Te pinkish hue of fair skin reflects the crimson color of the
oxygenated pigment
hemoglobin
(he
9
mo-glo
0
bin) in the red
blood cells circulating through the dermal capillaries. Because
Caucasian skin contains only small amounts of melanin, the
epidermis is nearly transparent and allows hemoglobin’s color
to show through.
Homeostatic Imbalance
5.3
When hemoglobin is poorly oxygenated, both the blood and
the skin of Caucasians appear blue, a condition called
cya-
nosis
(si
0
ah-no
9
sis;
cyan
5
dark blue). Skin oFen becomes
cyanotic during heart failure and severe respiratory disor-
ders. In dark-skinned individuals, the skin does not appear
cyanotic because of the masking effects of melanin, but cya-
nosis is apparent in their mucous membranes and nail beds
(the same sites where the red cast of oxygenated blood is
normally visible).
Many alterations in skin color signal certain disease states or
even emotional states:
Redness
, or
erythema
(er
0
ĭ-the
9
mah): Reddened skin may
indicate embarrassment (blushing), fever, hypertension, in-
flammation, or allergy.
Pallor
, or
blanching
: During fear, anger, and certain other
types of emotional stress, some people become pale. Pale
skin may also signify anemia or low blood pressure.
Jaundice
(jawn
9
dis), or
yellow cast
: An abnormal yellow skin
tone usually signifies a liver disorder, in which yellow bile
pigments accumulate in the blood and are deposited in body
tissues. [Normally, the liver cells secrete the bile pigments
(bilirubin) as a component of bile.]
Check Your Understanding
5.
Which layer of the dermis is responsible for producing
fingerprint patterns?
6.
Which tissue of the hypodermis makes it a good shock
absorber?
7.
You have just gotten a paper cut. It is very painful, but it
doesn’t bleed. Has the cut penetrated into the dermis or just
the epidermis?
For answers, see Appendix H.
Skin Color
Tree pigments contribute to skin color: melanin, carotene, and
hemoglobin. Of these, only melanin is made in the skin.
Melanin
is a polymer made of tyrosine amino acids. Its two
forms range in color from reddish yellow to brownish black. Its
synthesis depends on an enzyme in melanocytes called tyrosi-
nase (ti-ro
9
sĭ-nās) and, as noted earlier, it passes from melano-
cytes to the basal keratinocytes. Eventually, lysosomes break
down the melanosomes, so melanin pigment is found only in
the deeper layers of the epidermis.
Human skin comes in different colors. However, distribution
of those colors is not random—populations of darker-skinned
people tend to be found nearer the equator (where greater pro-
tection from the sun is needed), and those with the lightest
skin are found closer to the poles. Since all humans have the
same relative number of melanocytes, differences in skin color-
ing reflect the kind and amount of melanin made and retained.
Melanocytes of black- and brown-skinned people produce
many more and darker melanosomes than those of fair-skinned
individuals, and their keratinocytes retain it longer.
Freckles
and
pigmented nevi
(moles) are local accumulations of melanin.
When we expose our skin to sunlight, keratinocytes secrete
chemicals that stimulate melanocytes. Prolonged sun exposure
causes a substantial melanin buildup, which helps protect the
DNA of viable skin cells from UV radiation by absorbing the
rays and dissipating the energy as heat. Indeed, the initial signal
for speeding up melanin synthesis seems to be a faster repair
rate of DNA that has suffered photodamage (
photo
5
light). In
all but the darkest people, this defensive response causes skin to
darken visibly (tanning occurs).
Homeostatic Imbalance
5.2
Despite melanin’s protective effects, excessive sun exposure eventu-
ally damages the skin. It causes elastic fibers to clump, which results
in leathery skin; temporarily depresses the immune system; and
can alter the DNA of skin cells leading to skin cancer. Te fact that
dark-skinned people get skin cancer less oFen than fair-skinned
people and get it in areas with less pigment—the soles of the feet and
nail beds—attests to melanin’s effectiveness as a natural sunscreen.
Ultraviolet radiation has other consequences as well, such as
destroying the body’s folic acid stores necessary for DNA syn-
thesis. Tis can have serious consequences, particularly in preg-
nant women because the deficit may impair the development of
the embryo’s nervous system.
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