The Integumentary System
and resiliency that prevent most jabs and scrapes from penetrating
the dermis. In addition, collagen binds water, helping to keep skin
hydrated. Elastic ﬁbers provide the stretch-recoil properties of skin.
, a third type of skin marking, are dermal folds
that occur at or near joints, where the dermis is tightly secured
to deeper structures. (Notice the deep creases on your palms.)
Since the skin cannot slide easily to accommodate joint move-
ment in such regions, the dermis folds and deep skin creases
form (Figure 5.4c). Flexure lines are also visible on the wrists,
ﬁngers, soles, and toes.
Extreme stretching of the skin, such as during pregnancy, can
tear the dermis, leaving silvery white scars called
“streaks”), commonly called “stretch marks.” Short-term but
acute trauma (as from a burn or wielding a hoe) can cause a
, a ﬂuid-ﬁlled pocket that separates the epidermal and
Friction ridge patterns are genetically determined and unique
to each of us. Because sweat pores open along their crests, our
ﬁngertips leave identifying ﬁlms of sweat called
almost anything we touch.
, accounting for
about 80% of the thickness of the dermis, is coarse, irregularly
arranged, dense ﬁbrous connective tissue (Figure 5.3). Te net-
work of blood vessels that nourishes this layer, the
, lies between this layer and the hypodermis. Its extracel-
lular matrix contains pockets of adipose cells and thick bundles
of interlacing collagen ﬁbers. Te collagen ﬁbers run in various
planes, but most run parallel to the skin surface. Separations, or
less dense regions, between these bundles form
in the skin. Tese externally invisible lines tend to
run longitudinally in the skin of the head and limbs and in cir-
cular patterns around the neck and trunk (Figure 5.4b).
Cleavage lines are important to surgeons because when an inci-
sion is made
to these lines, the skin gapes less and heals
more readily. Te collagen ﬁbers of the dermis give skin strength
(a) Friction ridges of fingertip (SEM 12
(b) Cleavage lines in the
(c) Flexure lines of the hand
sweat gland ducts
on the palm
Dermal modiﬁcations result
in characteristic skin markings.
Scanning electron micrograph of friction
ridges (epidermal ridges topping the deeper
dermal papillary ridges). Notice the sweat
duct openings along the crests of the ridges,
which are responsible for ﬁngerprints.
Cleavage (tension) lines represent
separations between underlying collagen
ﬁber bundles in the reticular region of the
dermis. They tend to run circularly around the
trunk and longitudinally in the limbs.
Flexure lines form where the dermis is
closely attached to the underlying fascia.