152
UNIT 2
Covering, Support, and Movement of the Body
5
Cells of the Epidermis
Te cells populating the epidermis include
keratinocytes
,
melanocytes
,
dendritic cells
, and
tactile cells
.
Keratinocytes
Te chief role of
keratinocytes
(kĕ-rat
9
ĭ-no-
sītz
0
; “keratin cells”) is to produce
keratin
, the fibrous protein
that helps give the epidermis its protective properties (Greek
kera
5
horn) (
Figure 5.2b
, orange cells). Most epidermal cells
are keratinocytes.
±ightly connected to one another by desmosomes, the ke-
ratinocytes arise in the deepest part of the epidermis from a cell
layer called the stratum basale. Tese cells undergo almost con-
tinuous mitosis in response to prompting by epidermal growth
factor, a peptide produced by various cells throughout the body.
As these cells are pushed upward by the production of new cells
beneath them, they make the keratin that eventually dominates
their cell contents. By the time the keratinocytes reach the skin
surface, they are dead, scalelike structures that are little more
than keratin-filled plasma membranes.
Millions of dead keratinocytes rub off every day, giving us a
totally new epidermis every 25 to 45 days, but cell production
and keratin formation is accelerated in body areas regularly sub-
jected to friction, such as the hands and feet. Persistent friction
(from a poorly fitting shoe, for example) causes a thickening of
the epidermis called a
callus
.
Melanocytes
Melanocytes
(mel
9
ah-no-sītz), the spider-
shaped epithelial cells that synthesize the pigment
melanin
(mel
9
ah-nin;
melan
5
black), are found in the deepest layer of
the epidermis (Figure 5.2b, gray cells). As melanin is made, it
accumulates in membrane-bound granules called
melanosomes
that motor proteins move along actin filaments to the ends of
the melanocyte’s processes (the “spider arms”). From there they
are transferred to a number of nearby keratinocytes (4 to 10
depending on body area). Te melanin granules accumulate
on the superficial, or “sunny,” side of the keratinocyte nucleus,
forming a pigment shield that protects the nucleus from the
damaging effects of ultraviolet (UV) radiation in sunlight.
Dendritic Cells
Te star-shaped
dendritic cells
arise from
bone marrow and migrate to the epidermis. Also called
Lang-
erhans cells
(lahng
9
er-hanz) a²er a German anatomist, they
ingest foreign substances and are key activators of our immune
system, as described later in this chapter. Teir slender pro-
cesses extend among the surrounding keratinocytes, forming a
more or less continuous network (Figure 5.2b, purple cell).
Tactile Cells
Occasional
tactile (Merkel) cells
are present at
the epidermal-dermal junction. Shaped like a spiky hemisphere
(Figure 5.2b, blue cell), each tactile cell is intimately associated
with a disclike sensory nerve ending. Te combination, called a
tactile
or
Merkel disc
, functions as a sensory receptor for touch.
Layers of the Epidermis
Variation in epidermal thickness determines if skin is
thick
or
thin
.
In
thick skin
, which covers areas subject to abrasion—the palms,
fingertips, and soles of the feet—the epidermis consists of five lay-
ers, or strata (stra
9
tah; “bed sheets”). From deep to superficial,
these layers are stratum basale, stratum spinosum, stratum gran-
ulosum, stratum lucidum, and stratum corneum. In thin skin,
which covers the rest of the body, the stratum lucidum appears to
be absent and the other strata are thinner (Figure 5.2a, b).
Note that the terms “thick skin” and “thin skin” are really
misnomers because they refer to the epidermis only. Indeed, the
thickest skin in the body is on the upper back.
Stratum Basale (Basal Layer)
Te
stratum basale
(stra
9
tum
bah-sa
9
le), the deepest epidermal layer, is attached to the underly-
ing dermis along a wavy borderline that reminds one of corrugated
cardboard. For the most part, it consists of a single row of stem
cells—a continually renewing cell population—representing the
youngest keratinocytes. Te many mitotic nuclei seen in this layer
reflect the rapid division of these cells and account for its alternate
name,
stratum germinativum
(jer
9
mĭ-nă
0
tiv-um; “germinating
layer”). Each time one of these basal cells divides, one daughter cell
is pushed into the cell layer just above to begin its specialization into
a mature keratinocyte. Te other daughter cell remains in the basal
layer to continue the process of producing new keratinocytes.
Some 10–25% of the cells in the stratum basale are melano-
cytes, and their branching processes extend among the surround-
ing cells, reaching well into the more superficial stratum spinosum
layer. Occasional tactile cells also occur in this stratum.
Stratum Spinosum (Prickly Layer)
Te
stratum spinosum
(spi
9
no-sum; “prickly”) is several cell layers thick. Tese cells
contain a weblike system of intermediate filaments, mainly
tension-resisting bundles of pre-keratin filaments, which span
their cytosol to attach to desmosomes. Looking like tiny ver-
sions of the spiked iron balls used in medieval warfare, the ke-
ratinocytes in this layer appear to have spines, causing them
to be called
prickle cells
. Te spines do not exist in the living
cells; they are artifacts that arise during tissue preparation when
these cells shrink but their numerous desmosomes hold tight.
Scattered among the keratinocytes are melanin granules and
dendritic cells, which are most abundant in this epidermal layer.
Stratum Granulosum (Granular Layer)
Te thin
stratum
granulosum
(gran
0
u-lo
9
sum) consists of four to six cell layers
in which keratinocyte appearance changes drastically, and the
process of
keratinization
(in which the cells fill with the protein
keratin) begins. Tese cells flatten, their nuclei and organelles
begin to disintegrate, and they accumulate two types of gran-
ules. Te
keratohyaline granules
(ker
0
ah-to-hi
9
ah-lin) help to
form keratin in the upper layers, as we will see.
Te
lamellar granules
(lam
9
el-ar; “a small plate”) contain a
water-resistant glycolipid that is spewed into the extracellular
space and is a major factor in slowing water loss across the epi-
dermis. Te plasma membranes of these cells thicken as cytosol
proteins bind to the inner membrane face and lipids released by
the lamellar granules coat their external surfaces. Tese events
produce an epidermal water barrier and make the cells more
resistant to destruction. So, you might say that keratinocytes
“toughen up” to make the outer strata the strongest skin region.
Like all epithelia, the epidermis relies on capillaries in the
underlying connective tissue (the dermis in this case) for its nu-
trients. Above the stratum granulosum, the epidermal cells are
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