176
UNIT TWO
Blood in the dermal vessels adds color to the skin. When
blood is well oxygenated, the blood pigment hemoglobin is
bright red, making the skins of light-complexioned people
appear pinkish. When the blood oxygen concentration is
low, hemoglobin is dark red, and the skin appears bluish—a
condition called
cyanosis.
The state of the blood vessels also affects skin color. If
the vessels are dilated, more blood enters the dermis, red-
dening the skin of a light-complexioned person. This may
happen when a person is overheated, embarrassed, or under
the infl uence of alcohol. Conversely, conditions that con-
strict blood vessels cause the skin to lose this reddish color.
Thus, if body temperature drops abnormally or if a person is
frightened, the skin may appear pale.
A yellow-orange plant pigment called
carotene,
found in
yellow vegetables, can give skin a yellowish cast if a person
consumes too much. This results from accumulation of caro-
tene in the adipose tissue of the subcutaneous layer. Illnesses
may also affect skin color. A yellowish skin tone can indicate
jaundice,
a consequence of liver malfunction.
PRACTICE
9
What is the function of melanin?
10
How do genetic factors inF
uence skin color?
11
Which environmental factors inF
uence skin color?
12
How do physiological factors inF
uence skin color?
rapidly darken existing melanin, and they stimulate mel-
anocytes to produce more pigment and transfer it to nearby
epidermal cells within a few days. Unless exposure to sun-
light continues, the tan fades as pigmented keratinocytes
wear away.
(a)
Pigment
granule
Nucleus
Cell
membrane
Epidermis
Dermis
(b)
Cellular
extension
of melanocyte
Pigment
granules
Golgi
apparatus
Melanocyte
nucleus
Basement
membrane
FIGURE 6.4
±Melanocyte.±(
a
) Transmission electron micrograph
of a melanocyte with pigment-containing granules (10,600×).
(
b
) A melanocyte may have pigment-containing extensions that pass
between epidermal cells and transfer pigment into them. Much of
the melanin is deposited above the nucleus, where the pigment can
absorb UV radiation from outside before the DNA is damaged.
FIGURE 6.5
The pale or red eyes, skin, and hair of a person with
albinism reF
ect lack of melanin. Albinism is inherited.
Worldwide, 1 in 110,000 people has albinism. Among the native Hopi
people in Arizona, however, the incidence is 1 in 200. The reason
for this is as much sociological as it is biological. Men with albinism
help the women rather than risk severe sunburn in the ²
elds with the
other men. These men disproportionately contribute to the next gen-
eration because they have more sexual contact with women.
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