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Introduction

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Every clinician should be able to identify the primary skin diseases and cutaneous signs of systemic disease. Clinicians should characterize lesions sufficiently to make a diagnosis or make an appropriate referral to the dermatologist. Online resources and the reference listed in the bibliography should be consulted as needed.

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Physiology of the Skin and Nails

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The skin protects the body from injury, infection, heat, and fluid loss and is a major intermediary for sensing the outside world. It is continuous with the mucous membranes at body orifices. The dermis is rich in blood vessels that constrict to conserve heat or dilate to dissipate heat via radiation, conduction, and convection (aided by the production of sweat). The dermal and subcutaneous fat provides insulation to assist heat conservation. The impermeability of the epidermis is maintained by intercellular adhesion molecules forming tight junctions. Integrity of the dermis depends upon interlacing collagen bundles and elastic tissue.

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The skin also is an immunologic organ. When activated by foreign antigens, the Langerhans cells from within the epidermis migrate to regional lymph nodes presenting the antigens to T-lymphocytes initiating an immune response. The skin also contains specialized structures including hair follicles, sebaceous and sweat glands, and special sensory structures often in specific locations.

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Functional Anatomy of the Skin and Nails

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The epidermis, dermis, and subcutaneous tissue are the chief skin layers.

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Epidermis
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The avascular epidermis (Fig. 6-1) is the most superficial layer; it has four strata. The keratinized nonliving cells of the outer keratin layer (stratum corneum) are stratified and overlapping; the outermost regularly slough (desquamation). Underlying the stratum corneum are the granular layer (stratum granulosum), spinous layer (stratum spinosum), and basal layer (stratum basale). The living cells in these layers, mostly keratinocytes, get nourishment from the dermis. Melanocytes in the lower epidermis contain melanin, whose concentration is determined by genetics, sunlight, injury repair, and hormones. The epidermis contains a visible network of furrows, or rhomboid lines, which are exaggerated over joints. The epidermis thickens in areas of high friction, such as the palms and soles. A basement membrane separates the epidermis from the dermis. The epidermis is attached to the basement membrane by hemidesmosomes.

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Dermis and Subcutaneous Tissue
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The superficial papillary dermis forms papillary extensions surrounded by epidermis and containing rich capillary and nerve networks. The deeper reticular layer contains blood vessels, lymphatics, nerves, and fat cells surrounded by collagen bundles mixed with elastic fibers. The dermal appendages, including hair follicles, apocrine glands, eccrine sweat glands, and holocrine sebaceous glands, extend into this layer. The deep reticular dermis merges with the less dense subcutaneous layer. In general, the dermis is thicker over dorsal and lateral ...

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