164
UNIT 2
Covering, Support, and Movement of the Body
5
Interestingly, sunburned skin accelerates its production
of Fas, a protein that causes genetically damaged skin cells to
commit suicide, reducing the risk of mutations that will cause
sun-linked skin cancer. Te death of these gene-damaged cells
causes the skin to peel a±er a sunburn.
Tere is no such thing as a “healthy tan,” but the good news
for sun worshippers is the newly developed skin lotions that can
fix damaged DNA before the involved cells become cancerous.
Tese lotions contain tiny oily vesicles (liposomes) filled with en-
zymes that initiate repair of the DNA mutations most commonly
caused by sunlight. Te liposomes penetrate the epidermis and
enter the keratinocytes, ultimately making their way into the nu-
clei to bind to specific sites where two DNA bases have fused.
Tere, by selectively cutting the DNA strands, they begin a DNA
repair process that is completed by cellular enzymes.
Te three major forms of skin cancer are basal cell carci-
noma, squamous cell carcinoma, and melanoma.
Basal Cell Carcinoma
Basal cell carcinoma
(kar
0
sĭ-no
9
mah), the least malignant and
most common, accounts for nearly 80% of cases. Stratum basale
cells proliferate, invading the dermis and hypodermis. Te can-
cer lesions occur most o±en on sun-exposed areas of the face
and appear as shiny, dome-shaped nodules that later develop
a central ulcer with a pearly, beaded edge
(Figure 5.8a)
. Basal
cell carcinoma is relatively slow-growing, and metastasis sel-
dom occurs before it is noticed. Full cure by surgical excision is
the rule in 99% of cases.
Squamous Cell Carcinoma
Squamous cell carcinoma
, the second most common skin can-
cer, arises from the keratinocytes of the stratum spinosum. Te
lesion appears as a scaly reddened papule (small, rounded eleva-
tion) that arises most o±en on the head (scalp, ears, and lower
lip), and hands (Figure 5.8b). It tends to grow rapidly and metas-
tasize if not removed. If it is caught early and removed surgically
or by radiation therapy, the chance of complete cure is good.
Melanoma
Melanoma
(mel
0
ah-no
9
mah), cancer of melanocytes, is the
most dangerous skin cancer because it is highly metastatic and
resistant to chemotherapy. It accounts for only 2–3% of skin
Check Your Understanding
21.
What chemicals produced in the skin help provide barriers to
bacteria? List at least three and explain how the chemicals
are protective.
22.
Which epidermal cells play a role in body immunity?
23.
How is sunlight important to bone health?
24.
How does the skin contribute to body metabolism?
For answers, see Appendix H.
Homeostatic Imbalances
of Skin
Summarize the characteristics of the three major types of
skin cancers.
Explain why serious burns are life threatening. Describe
how to determine the extent of a burn and differentiate
first-, second-, and third-degree burns.
Loss of homeostasis in body cells and organs reveals itself on the
skin, sometimes in startling ways. Te skin can develop more
than 1000 different conditions and ailments. Te most common
skin disorders are bacterial, viral, or yeast infections. Related
Clinical ²erms on pp. 171–172 summarizes a number of these.
Less common, but far more damaging to body well-being, are
skin cancer and burns, considered next.
Skin Cancer
One in five Americans develops skin cancer at some point. Most
tumors that arise in the skin are benign and do not spread (me-
tastasize) to other body areas. (A wart, a neoplasm caused by a
virus, is one example.) However, some skin tumors are malig-
nant, or cancerous, and invade other body areas.
Te single most important risk factor for skin cancer is
overexposure to the UV radiation in sunlight, which damages
DNA bases. Adjacent pyrimidine bases o±en respond by fusing,
forming lesions called
dimers
. UV radiation also appears to dis-
able a tumor suppressor gene [
p53
or the patched (
ptc
) gene].
In limited numbers of cases, however, frequent irritation of the
skin by infections, chemicals, or physical trauma seems to be a
predisposing factor.
(a) Basal cell carcinoma
(c) Melanoma
(b) Squamous cell carcinoma
Figure 5.8
Photographs of skin cancers.
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