The skin, the largest organ in the
body, plays many roles as the major interface with the external
environment.1-3 The outermost skin layer, the stratum corneum,
prevents desiccation of a primarily aqueous body in a dry atmosphere.
Extensive burns, drug-induced skin necrosis (toxic epidermal necrolysis),
and other extensive blistering disorders, such as epidermolysis
bullosa, represent situations in which the barrier is breached,
leading to increased morbidity and mortality. In addition to providing
a physical barrier to infection, the skin is an important component
of the body’s immune system. Langerhans cells provide immune
surveillance, presenting antigen that activates lymphocytes. When
the immune function in skin is dysfunctional, as in atopic dermatitis, the
risk of infection is increased.
The skin also serves as the interface with ultraviolet light.
Within the epidermis, ultraviolet B light also provides the impetus
for isomerization of provitamin D to vitamin D3, which
is transported to the liver and then to the kidneys for sequential
hydroxylations to form the active, 1,25-dihydroxyvitamin D3.
Exposure of normal keratinocytes to ultraviolet radiation causes
mutations in tumor suppressor genes, while epidermal melanin impedes
transmission of ultraviolet rays. As a result, patients with albinism
who have a significant decrease in epidermal melanin have an increased
risk of developing ultraviolet-induced malignancies, particularly
basal cell carcinomas and squamous cell carcinomas. In patients
with xeroderma pigmentosum, the repair system after ultraviolet
DNA damage is defective, leading to the dramatically increased risk
of cutaneous sun-induced tumors in these patients as well (see Chapter 360).
The skin’s role in thermoregulation is primarily mediated
by evaporation of sweat, secreted in response to autonomic stimuli.
Thermoregulation is impaired in genetic disorders of eccrine gland
morphogenesis, such as hypohidrotic ectodermal dysplasia, or where
eccrine ducts are obstructed by a thickened stratum corneum, as
in congenital ichthyosiform erythroderma or severe atopic dermatitis.
Premature infants cannot sweat well, and even term infants sweat
less than adults. During heat stress, the failure to sweat can lead
to excessive body temperature, vasodilatation, and resultant hypovolemic
shock. In patients with cystic fibrosis, the normally hypotonic
sweat becomes hypertonic, and thermal stress therefore can induce
The skin provides an important afferent limb to the nervous system
in the interface with the external world through sensory perceptions
of touch, pressure, itch, and pain. Skin, hair, and nails are highly
visible body components, and their appearance is important for self-image and psychosocial
development. Although birthmarks and acquired skin and appendageal
disorders can be disfiguring, the attitude of the patient and the
environment contribute greatly to the perception and resultant effect
The skin consists of epidermis, dermal-epidermal junction, and dermis.
The most superficial skin layer, the epidermis, is
composed predominantly of ectodermally derived keratinocytes and
also contains neural crest–derived melanocytes and bone
marrow–derived Langerhans cells. The epidermis is divided
from the underlying dermis by the dermal-epidermal junction, a
complex structure of particular importance in several acquired ...