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Every clinician should be able to characterize skin lesions, identify common conditions, and recognize cutaneous signs of systemic disease.


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 dissipating heat via radiation, conduction, and convection aided by sweating. Dermal and subcutaneous fat provide insulation assisting heat conservation. Impermeability is maintained with tight junctions formed by intercellular adhesion molecules. Integrity of the dermis depends upon interlacing collagen bundles and elastic tissue.

The skin contains specialized structures including hair follicles, sebaceous and sweat glands, and location specific special sensory structures. The skin is also an immunologic organ. Intradermal Langerhans cells, reproducing within the epidermis, are activated by foreign antigens. They then migrate to regional lymph nodes presenting antigens to T-lymphocytes initiating an immune response.


Skin Layers

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


The avascular epidermis (Fig. 6-1) is the most superficial layer; it has four layers. The keratinized nonliving cells of the outer keratin layer (stratum corneum) are stratified and overlapping, the outermost cells sloughing regularly (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 and are held together by proteins, including desmosomes. 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 that are exaggerated over joints. The epidermis thickens in areas of high friction such as palms and soles. A basement membrane separates the epidermis from the dermis. The epidermis attaches to the basement membrane by hemidesmosomes.

FIG. 6-1

Principal Skin Structures.

Dermis and subcutaneous tissue

The superficial papillary dermis forms papillary extensions surrounded by epidermis and containing rich capillary and nerve networks. The deeper reticular dermis 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 subcutaneous layer. In general, the dermis is thicker over dorsal and lateral than over ventral and medial surfaces. It is thickest over the back and extremely thin over ...

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