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
The layers of the epidermis are as follows (from deep-
est to most superficial):
= 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
Stratum spinosum (spi-NO-sum;
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.
are undergoing apoptosis. A
distinctive feature of cells in
this layer is the presence of the
Stratum lucidum (LOO-si-dum;
= clear)— 4-6
layers of flattened clear, dead keratinocytes
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;
= 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
cialized cell that has
the ability to divide for
indefinite periods and
give rise to specialized
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.
release a water repellent.
comprise ~90% of epidermal cells.
They produce the protein keratin and lamellar
(LANG-er-hans) participate in
immune responses mounted against microbes that
invade the skin and are easily damaged by UV light.
contact the flattened process of a
sensory neuron, a structure called a
disc. Merkel cells and tactile discs detect touch
(MEL-a-no-sitz) comprise ~5% of
epidermal cells. They produce the pigment melanin.
Melanocytes are susceptible to UV damage.
The Integumentary System