Info

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Layer

Location

Characteristics

Stratum corneum

Outermost layer

Many layers of keratinized, dead epithelial cells that are flattened and nonnucleated Cells appear clear; nuclei, organelles, and cell membranes are no longer visible Three to five layers of flattened granular cells that contain shrunken fibers of keratin and shriveled nuclei Many layers of cells with centrally located, large, oval nuclei and developing fibers of keratin; cells becoming flattened A single row of cuboidal or columnar cells that divide and grow; this layer also includes melanocytes

Stratum corneum

Stratum lucidum

Stratum granulosum

Stratum spinosum

Stratum basale (basal cell layer)

Outermost layer

Between stratum corneum and stratum granulosum on soles and palms Beneath the stratum corneum

Beneath the stratum granulosum

Deepest layer

Many layers of keratinized, dead epithelial cells that are flattened and nonnucleated Cells appear clear; nuclei, organelles, and cell membranes are no longer visible Three to five layers of flattened granular cells that contain shrunken fibers of keratin and shriveled nuclei Many layers of cells with centrally located, large, oval nuclei and developing fibers of keratin; cells becoming flattened A single row of cuboidal or columnar cells that divide and grow; this layer also includes melanocytes

Contact dermatitis is superficial inflammation (redness and swelling) or irritation of the skin. In allergic contact dermatitis, the immune system reacts to an allergen (an innocuous substance recognized as foreign), causing a red scaliness. The rash resulting from exposure to oils in poison ivy is an example of allergic contact dermatitis; 50 to 70% of people with this allergy also react to poison oak and sumac, mango peel, gingko fruit, and an oil in cashew shells. Metals in jewelry, acids in fruits, and materials in shoes also trigger allergic contact dermatitis. It is also seen among workers in certain fields, such as hairdressers, butchers, furniture makers, shrimp peelers, and bakers.

Irritant contact dermatitis is damage caused by an irritating substance, not an immune system reaction. The skin becomes red and itchy, with small, oozing blisters. Babies are famous for skin irritations — caused by everything from the perpetual drool on their faces to their wet diapers. "Dishpan hands" and reactions to cosmetics are also irritant contact dermatitis. Men with outbreaks on the left sides of their necks may use an aftershave lotion that reacts with sunlight when they drive.

Specialized cells in the epidermis called melanocytes produce the dark pigment melanin (mel ah-nin) that provides skin color, discussed further on page 183 (fig. 6.4). Melanin absorbs ultraviolet radiation in sunlight, preventing mutations in the DNA of skin cells and other damaging effects.

Because blood vessels in the dermis supply nutrients to the epidermis, interference with blood flow may kill epidermal cells. For example, when a person lies in one position for a prolonged period, the weight of the body pressing against the bed blocks the skin's blood supply. If cells die, the tissues begin to break down (necrosis), and a pressure ulcer (also called a decubitus ulcer or bedsore) may appear.

Pressure ulcers usually occur in the skin overlying bony projections, such as on the hip, heel, elbow, or shoulder. Frequently changing body position or massaging the skin to stimulate blood flow in regions associated with bony prominences can prevent pressure ulcers. In the case of a paralyzed person who cannot feel pressure or respond to it by shifting position, caretakers must turn the body often to prevent pressure ulcers.

In body regions other than the palms and soles, the epidermis is usually very thin, averaging 0.070.12 mm. The stratum granulosum may be missing where the epidermis is thin. Table 6.1 describes the characteristics of each layer of the epidermis.

In healthy skin, production of epidermal cells is closely balanced with loss of dead cells from the stratum corneum, so that skin does not wear away completely. In fact, the rate of cell division increases where the skin is rubbed or pressed regularly, causing the growth of thickened areas called calluses on the palms and soles and keratinized conical masses on the toes called corns. Other changes in the skin include the common rashes described in table 6.2.

Melanocytes lie in the deepest portion of the epidermis and in the underlying connective tissue of the dermis. Although melanocytes are the only cells that can produce melanin, the pigment also may be present in nearby epidermal cells. This happens because melanocytes have long, pigment-containing cellular extensions that pass upward between neighboring epidermal cells, and the extensions can transfer granules of melanin into these other cells by a process called cy-tocrine secretion. Nearby epidermal cells may contain more melanin than the melanocytes (fig. 6.5). Clinical Application 6.1 discusses one consequence of excessive sun exposure—skin cancer.

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