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U N I T 1 3
Integumentary Function
increases the production of eumelanin, a brownish-black
pigment, which causes tanning to occur. The primary
function of such melanin is to protect the skin by
absorbing and scattering harmful ultraviolet rays, which
are implicated in skin cancers. Localized concentrations
of eumelanin are also responsible for the formation of
freckles and moles. Pheomelanin, a reddish-yellow pig-
ment, is particularly concentrated in the lips, nipples,
glans penis, and vagina. Both types of melanin are found
in hair, particularly red hair. It has been suggested that
the reason fair-haired individuals are more susceptible
to skin cancers may be due to the enhanced photoreac-
tivity of pheomelanin, as compared to eumelanin.
Langerhans Cells
Langerhans cells are dendritic cells that reside princi-
pally in the stratum spinosum of the epidermis and play
a major role in the functioning of the skin’s immune
system (Fig. 45-4). They are antigen-presenting cells
that originate in the bone marrow and are part of the
mononuclear phagocytic system. Like macrophages,
Langerhans cells express both major histocompatibil-
ity class (MHC) I and II molecules as well as receptors
for immunoglobulins (see Chapter 15). Their major
function is to phagocytose and process foreign anti-
gens. They then migrate into regional lymph nodes
in the dermis, where they present the processed anti-
gens to T lymphocytes (see Fig. 45-4). As antigen-
presenting cells, the Langerhans cells are involved in
delayed-type hypersensitivity reactions such as contact
dermatitis and other cell-mediated immune responses
in the skin.
Merkel Cells
Merkel cells are epidermal cells that function in cutane-
ous sensation. They are interspersed among the kerati-
nocytes of the stratum germinativum of the epidermis,
usually as single cells, and are particularly abundant
in areas of the skin where sensory perception is acute,
such as the fingertips. Myelinated sensory neurons tra-
verse the basement membrane to approach the Merkel
cell, forming a Merkel cell–neuron complex. These
complexes may function as tactile mechanoreceptors.
Merkel cells display distinctive, dense granules in their
organelles and cytoplasm, suggesting that they possess
neurosecretory function. These functions may include
metabolic support of their associated neurons, neuron
development, and regeneration after injury. These cells
are also involved in a form of skin cancer called Merkel
cell carcinoma.
Dermis
The dermis is the connective tissue layer that sepa-
rates the epidermis from the subcutaneous fat layer
(see Fig. 45-1). It supports the epidermis and serves as
its primary source of nutrition. The main component
of the dermis is collagen, a group of fibrous proteins.
Collagen represents 70% of dry skin weight and serves
as the major stress-resistant material of the skin. The
hair follicles and glandular structures are embedded in
the dermis and continue through the epidermis.
Based on its tissue structure, the dermis can be divided
into a superficial papillary dermis and a deeper reticular
dermis. Both layers contain fibroblasts that synthesize
the tissue’s ground substance and collagen, as well as
immune cells, blood vessels, and nerves.
Papillary Dermis
The papillary layer of the dermis is a thin superficial
layer that interdigitates directly with the epidermis, but
is separated from it by the basement membrane zone. It
consists of loosely arranged collagen fibers and ground
substance. This layer is densely covered with conical
projections called
dermal papillae
(see Fig. 45-1). The
basal cells of the epidermis project into the papillary
dermis, forming
rete ridges.
It is believed that the dense
structure of the dermal papillae serves to maximize the
adhesion of the dermis and the epidermis. This layer of
Antigen
Epidermis
Langerhans
cell
Dermis
Afferent
lymphatic
vessel
T cell zone
Lymph
node
T lymphocyte
FIGURE 45-4.
Langerhans cells, showing a multistep-process
that includes capture and processing of the antigen, movement
into a lymph node, and presentation of the antigen to aT
lymphocyte.
