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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 dehydration.

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 of disfigurement.

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 ...

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