175
CHAPTER SIX
Integumentary System
L
ike cigarette smoking, a deep, dark tan was
once desirable. In the 1960s, a teenager
might have spent hours on a beach, skin
glistening with oil, maybe even using a ref
ecting
device to concentrate sun exposure on the Face.
Today, as they lather on sunblock, many oF these
people realize that the tans oF yesterday may
cause cancer tomorrow.
Tanning
Usually the DNA damage response, discussed in
Chapter 4 (p. 137), protects against sun exposure.
The solar radiation activates a gene that encodes
a protein called p53 that normally mediates harm-
Ful e±
ects oF environmental insults in various tis-
sues. In the skin, p53 stimulates a series oF Familiar
responses to sunning: keratinocytes produce sig-
naling molecules that promote the redness (ery-
thema) and swelling oF inf
ammation. Meanwhile,
melanocytes Further diFFerentiate and increase
their production oF melanin, which melanosomes
transFer to keratinocytes. The result is tanning.
Researchers hypothesize that the tanning
response evolved about a million years ago, as
our ancestors ventured From the Forests onto
the plains oF AFrica. Biology may also explain
why we like to sunbathe—it stimulates kerati-
nocytes to release beta endorphin, a molecule
related to opiates that promotes a sense oF well-
being. However, like anything else, sun exposure
should be done in moderation. Use oF tanning
booths is particularly dangerous because it
bathes the skin in doses oF ultraviolet radia-
tion that can overwhelm the natural protection
against cancer.
Skin Cancer
Cancer begins when the sun exposure over-
whelms the ability oF p53 to protect the skin.
Usually, skin cancer arises in nonpigmented epi-
thelial cells in the deep layer oF the epidermis or
From pigmented melanocytes. Skin cancers origi-
nating From epithelial cells are called
cutaneous
carcinomas
(basal cell carcinoma or squamous
cell carcinoma); those arising From melanocytes
are
cutaneous melanomas
(melanocarcinomas or
malignant melanomas) (²
g. 6A).
Cutaneous carcinomas are the most com-
mon type oF skin cancer, aFFecting mostly light-
skinned people over Forty years oF age regularly
exposed to sunlight. Such a cancer usually devel-
ops From a hard, dry, scaly growth with a reddish
base. The lesion may be f
at or raised and usually
²
rmly adheres to the skin, appearing most oFten
on the neck, Face, or scalp. ³ortunately, cutaneous
carcinomas are typically slow growing and can
usually be cured completely by surgical removal
or radiation treatment.
A cutaneous melanoma is pigmented
with melanin, oFten with a variety oF colored
areas—variegated brown, black, gray, or blue.
A melanoma usually has irregular rather than
smooth outlines and may Feel bumpy. Melanoma
accounts For only 4% oF skin cancers but For 80%
oF skin cancer deaths.
People oF any age may develop a cutaneous
melanoma. These cancers seem to be caused by
short, intermittent exposure to high-intensity
sunlight. Thus, risk oF melanoma increases in per-
sons who stay indoors but occasionally sustain
blistering sunburns.
Light-skinned people who burn rather than
tan are at higher risk oF developing a cutane-
ous melanoma. The cancer usually appears in
the skin oF the trunk, especially the back, or the
limbs, arising From normal-appearing skin or
From a mole (nevus). The lesion spreads hori-
zontally through the skin, but eventually may
thicken and grow downward into the skin, invad-
ing deeper tissues. Surgical removal during the
horizontal growth phase can arrest the cancer. But
once the lesion thickens and spreads into deeper
tissues, it becomes more di´
cult to treat, and the
survival rate is low. To reduce risk, avoid exposure
to high-intensity sunlight, use sunscreens and sun-
blocks, and examine the skin regularly. Report any
unusual lesions—particularly those that change in
color, shape, or surFace texture—to a physician.
6.1
Clinical Application
Tanning and Skin Cancer
(a)
(b)
(c)
FIGURE 6A
Skin cancer. (
a
) Squamous cell carcinoma. (
b
) Basal cell carcinoma. (
c
) Malignant melanoma.
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