melanocytes, Langerhans cells, and Merkel cells (Figure
4.2). Several distinct layers of keratinocytes in various
stages of development form the bulk of the epidermis.
This is called thin skin. The epidermis has five layers—
stratum basale, stratum spinosum, stratum granulosum,
stratum lucidum, and a thick stratum corneum, it is called
thick skin.
The layers of the epidermis are as follows (from deep-
est to most superficial):
Stratum
basale
(ba-SA-le;
basal
= base)—A single row of
cuboidal or columnar keratino-
cytes. Some cells in this layer
are stem cells that undergo cell
division to continually produce
new keratinocytes.
Stratum spinosum (spi-NO-sum;
spinos
= thornlike)—
8 to 10 layers of many-sided keratinocytes that fit closely
together. This layer provides strength and flexibility to
the skin. Cells in the more superficial portions of this
layer are beginning to flatten.
Stratum
granulosum
(gran-
u-LO-sum;
granulos
=
little
grains)—3
to
5
layers
of
flattened
keratinocytes
that
are undergoing apoptosis. A
distinctive feature of cells in
this layer is the presence of the
protein
keratin
(KER-a-tin)
and membrane-enclosed
lamellar granules.
Stratum lucidum (LOO-si-dum;
lucid
= clear)— 4-6
layers of flattened clear, dead keratinocytes
that
contain
large
amounts
of keratin.
This
layer
is
found only in the areas of the body (thick skin)
where exposure to friction is greatest (for example,
fingertips, palms, soles).
Stratum corneum (COR-ne-um;
corne-
= horn or
horny)—25 to 30 layers of flattened dead cells from
the deeper strata. The interior of the cells contains
mostly keratin. Its multiple layers of dead cells help
to protect deeper layers from injury and microbial
invasion.
stem cell
An unspe-
cialized cell that has
the ability to divide for
indefinite periods and
give rise to specialized
cells.
apoptosis
(ap-o-TO-
sis
or
ap'-op-TO-sis)
Programmed cell
death in which a cell
goes through a limited
number of cell divi-
sions and then dies.
Layers and cells o f th e skin
• Figure 4.2
The various layers of skin cells consist of mostly keratinocytes intermixed
with other cell types.
Superficial
A
Y
Deep
Stratum
corneum
Stratum
lucidum
Stratum
granulosum
Stratum
spinosum
Stratum
basale
Dead keratinocytes
Lamellar
granules
release a water repellent.
Keratinocytes
comprise ~90% of epidermal cells.
They produce the protein keratin and lamellar
granules.
Langerhans cells
(LANG-er-hans) participate in
immune responses mounted against microbes that
invade the skin and are easily damaged by UV light.
Merkel cells
contact the flattened process of a
sensory neuron, a structure called a
tactile (Merkel)
disc. Merkel cells and tactile discs detect touch
sensations.
Melanocytes
(MEL-a-no-sitz) comprise ~5% of
epidermal cells. They produce the pigment melanin.
Melanocytes are susceptible to UV damage.
94
CHAPTER 4
The Integumentary System
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