C h a p t e r 4 5
Structure and Function of the Integumentum
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the dermis is richly vascularized. It contains capillaries,
end arterioles, and venules that nourish the epidermal
layers of the skin. Lymph vessels and nerve tissue also
are found in this layer.
Reticular Dermis
The reticular layer of the dermis is the thicker area of
the dermis and forms the bulk of the dermal layer. This
is the tough layer in animal hides from which leather is
made. The reticular dermis is characterized by a com-
plex meshwork of dense collagen bundles intercon-
nected with large elastic fibers and ground substance, a
viscid gel that is rich in mucopolysaccharides. The col-
lagen fibers are oriented parallel to the body’s surface
in any given area. Collagen bundles may be organized
lengthwise, as on the abdomen, or in round clusters, as
on the heel. The direction of surgical incisions is often
determined by this organizational pattern.
Immune Cells
Once thought to be composed primarily of fibroblasts,
it is now believed that the dermis is mainly composed of
dendritic cells called dermal dendrocytes. Believed to be
one of the main cell types of the dermis, dermal dendro-
cytes are spindle-shaped cells that have both phagocytic
and antigen-presenting functions and play an important
part in the immunobiology of the dermis. In addition, it
is possible that dermal dendrocytes may be able to initi-
ate or respond to immunologic events in the epidermis.
Dermal dendrocytes also are thought to be involved in
processes such as wound healing, blood clotting, and
inflammation.
The dermis also contains macrophages, T cells and
mast cells. Dermal macrophages and venular epithelial
cells may present antigen to T cells in the dermis, most of
which are previously activated or memory T cells. T-cell
responses to macrophage- or endothelium-associated
antigens in the dermis are probably more important in
generating an immune response to antigen challenge in
previously exposed persons than in initiating a response
to a new antigen. The major type of T-cell–mediated
immune response in the skin is delayed-type hypersensi-
tivity (see Chapter 16).
Mast cells, which have a prominent role in immu-
noglobulin E–mediated immediate hypersensitivity, also
are present in the dermis. These cells are strategically
located at body interfaces such as the skin and mucous
membranes and are thought to interact with antigens
that come in contact with the skin.
BloodVessels
The arteries that nourish the skin form two plexuses
(i.e., collections of blood vessels), one located between
the dermis and the subcutaneous tissue and the other
between the papillary and reticular layers of the dermis.
The pink color of light skin results primarily from blood
in the vessels of this latter plexus. Capillary flow that
arises from vessels in this plexus also extends up and
nourishes the overlying epidermis by diffusion. Blood
leaves the skin through small veins that accompany the
subcutaneous arteries. The lymphatic system of the skin,
which aids in combating certain skin infections, also is
limited to the dermis.
The skin is richly supplied with arteriovenous anasto-
moses in which blood flows directly between an artery
and a vein, bypassing the capillary circulation. These
anastomoses are important for temperature regulation.
They can open up, letting blood flow through the skin
vessels when there is a need to dissipate body heat, and
close off, conserving body heat if the environmental
temperature is cold.
Innervation
The innervation of the skin is complex. The dermis is
well supplied with sensory receptors for temperature,
pain, and touch (see Chapter 35), as well as nerves that
supply the blood vessels, sweat glands, and arrector pili
muscles.
The papillary layer of the dermis is supplied with
free nerve endings that serve as nociceptors (i.e., pain
receptors) and thermoreceptors. The dermis also con-
tains encapsulated pressure-sensitive receptors that
detect pressure and touch. The largest of these are the
pacinian corpuscles,
which are widely distributed in
the dermis and subcutaneous tissue in the digits of the
hands and breasts. These mechanoreceptors are spe-
cialized to perceive pressure, touch, and vibration.
Meissner corpuscles
are encapsulated mechanorecep-
tors specialized for tactile discrimination. They are
concentrated on the fingertips and palms of the hands,
where they account for about half of the tactile recep-
tors. They are also located on the eyelids, lips, tongue,
nipples, and skin of the foot and forearm. The deep
dermis is supplied with small, oval mechanorecep-
tors called
Ruffini corpuscles.
They are slowly adapt-
ing receptors, responding to heavy pressure and joint
movement. They are also believed to detect cold. The
skin is also supplied by
Krause end bulbs
, nerve end-
ings contained in a cylindrical or oval capsule. They
are found most frequently in the oral cavity, conjunc-
tiva, and genitalia. Although their function is uncer-
tain, they are thought to act as mechanoreceptors and
heat detectors.
Most of the skin’s blood vessels are innervated by
the sympathetic nervous system. The sweat glands are
innervated by cholinergic fibers but controlled by the
sympathetic nervous system. Likewise, the sympa-
thetic nervous system controls the arrector pili (pilo-
motor) muscles that cause elevation of hairs on the
skin (
pilo-
means hair). Contraction of these muscles
tends to cause the skin to dimple, producing “goose
bumps.”
Basement Membrane Zone
The basement membrane zone is a layer of intercellu-
lar and extracellular matrices that serves as an inter-
face between the dermis and the epidermis. It separates
the epithelium from the underlying connective tissue,
anchors the epithelium to the loose connective tissue
underneath, and serves as a selective filter for molecules
