174
UNIT TWO
more melanin than melanocytes
(f
g. 6.4
b
)
. Certain keratino-
cytes, called pigment recipient cells, attract melanocytes and
stimulate them to release melanin, causing pigment deposi-
tion. The pigment recipient cells, recently discovered, are
thought to act like the outlines in a children’s coloring book,
delineating areas to be f
lled in with color.
Humans come in a wide variety oF hues. Heredity
and the environment determine skin color. Regardless oF
racial origin, all people have about the same number oF
melanocytes in their skin. DiFFerences in skin color result
From diFFerences in the amount oF melanin these cells pro-
duce. This is controlled by several genes. The more mela-
nin, the darker the skin. The distribution and the size oF
pigment granules within melanocytes also infl
uence skin
color. The granules in very dark skin are single and large;
those in lighter skin occur in clusters oF two to Four gran-
ules and are smaller. People who inherit mutant melanin
genes have nonpigmented skin. This white skin is part oF
albinism.
It aFFects people oF all races and also many other
species
(f
g. 6.5)
.
Environmental Factors such as sunlight, ultraviolet light
From sunlamps, and X rays aFFect skin color. These Factors
PRACTICE
6
Explain how the epidermis is formed.
7
Distinguish between the stratum basale and the stratum
corneum.
8
List the protective functions of the epidermis.
Specialized cells in the epidermis called
melanocytes
produce the dark pigment
melanin
(mel
ah-nin) From the
amino acid tyrosine in organelles called melanosomes.
Melanin provides skin color
(f
g. 6.4
a
)
. Melanin also absorbs
ultraviolet radiation in sunlight, which would otherwise
cause mutations in the DNA oF skin cells and other damaging
eFFects. Clinical Application 6.1 discusses one consequence
oF excess sun exposure—skin cancer.
Melanocytes lie in the stratum basale oF the epidermis.
They are the only cells that can produce melanin, but the
pigment gets into nearby epidermal cells. This happens
because melanocytes have long, pigment-containing cellular
extensions that pass upward between neighboring epidermal
cells, and the extensions can transFer granules oF melanin
in melanosomes into keratinocytes, which may accumulate
Stratum corneum
Stratum lucidum
Stratum granulosum
Stratum spinosum
Stratum basale
Basement
membrane
Dermis
Dermal papilla
(a)
(b)
FIGURE 6.3
Epidermis of thick skin. (
a
) The layers of the epidermis are distinguished by changes in cells as they are pushed toward the surface of
the skin. (
b
) Light micrograph of skin (120×).
TABLE
6.1
|
Layers of the Epidermis
Layer
Location
Characteristics
Stratum corneum
Outermost layer
Many layers of keratinized, dead epithelial cells that are F
attened and nonnucleated
Stratum lucidum
Between stratum corneum and stratum
granulosum on soles and palms
Cells appear clear; nuclei, organelles, and cell membranes are no longer visible
Stratum granulosum
Beneath the stratum corneum
Three to ±
ve layers of F
attened granular cells that contain shrunken ±
bers of keratin and
shriveled nuclei
Stratum spinosum
Beneath the stratum granulosum
Many layers of cells with centrally located, large, oval nuclei and developing ±
bers of keratin;
cells becoming F
attened
Stratum basale (basal
cell layer)
Deepest layer
A single row of cuboidal or columnar cells that divide and grow; this layer also includes
melanocytes
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