Porth's Essentials of Pathophysiology, 4e

1148

Integumentary Function

U N I T 1 3

Like hair, nails are the end product of dead cells that are pushed outward from the nail matrix. Unlike hair, nails grow continuously rather than cyclically, unless perma- nently damaged or diseased. The epithelium of the fold of skin that surrounds the nail consists of the usual layers of skin. The stratum corneum forms the eponychium or cuticle. The nearly transparent nail plate provides a useful window for viewing the amount of oxygen in the blood, and providing a view of the color of the blood in the der- mal vessels. Changes or abnormalities of the nail can also serve to help diagnose skin or systemic diseases. ■■ Skin appendages, including the sweat glands, sebaceous glands, hair follicles, and nails, are derived from outgrowths of the epidermal epithelium during development. ■■ The skin glands are exocrine glands. Eccrine sweat glands secrete sweat directly onto the skin surface, whereas the apocrine sweat glands and sebaceous glands release an oily secretion into hair follicles. ■■ A hair is a keratin filament arising from a hair follicle, whereas a nail is a hardened keratinized plate emerging from a germinal region called the nail matrix. No two skin disorders look exactly alike. Their appear- ance varies according to the causative agent, location, skin color, and many other factors, and may be further influenced by excessive scratching, secondary infection, or the effects of self-treatment. Nevertheless, most skin disorders have some common characteristics that can be used to describe them. This section of the chapter covers lesions and rashes, pigmentary skin disorders, pruritus, dry skin, and variations in dark skin. Lesions and Rashes The term lesion refers to a traumatic or pathologic loss of normal tissue continuity, structure, or function. Skin lesions may occur as primary lesions arising in previ- ously normal skin, or they may develop as secondary lesions resulting from other disease conditions. Figure 45-8 illustrates various types of skin lesions. Rashes are temporary eruptions of the skin, such as those associated with childhood diseases, heat, diaper irritation, or drug-induced reactions. The components of a rash often are referred to as lesions. SUMMARY CONCEPTS Manifestations of Skin Disorders

Lesions and rashes may range in size from a fraction of a millimeter (e.g., the pinpoint spots of petechiae) to many centimeters (e.g., pressure ulcer). They may be blanched (white), erythematous (reddened), hemor- rhagic or purpuric (containing blood), or pigmented (colored). A macule is a small ( ≤ 1.0 cm in diameter) flat spot, such as a freckle; a papule is an elevated ( ≥ 1.0 cm) superficial lesion. Plaques are elevated lesions ( ≥ 1 cm), often formed by the coalescence of papules. Nodules are solid marblelike lesions (>0.5 cm) that are deeper and firmer than a papule. A wheal is a somewhat irregular, relatively transient area of localized skin edema, such as a mosquito bite. Lichenification is a thickened and leathery roughening of the skin with increased visibility of the normal skin furrows and excoriations caused by scratching. Blisters are circumscribed elevations of the skin caused by fluid under or within the epidermis. Pustules are circumscribed pus-filled elevations of the skin. Vesicles are small (<1.0 cm in diameter) and bullae are large (1.0 cm or larger in diameter) fluid-filled blisters (see Fig. 45-8). Friction blisters most commonly occur on the palmar and plantar surfaces of the hands and feet where the skin is constantly exposed to mechanical trauma, such as from shoes and household tools and appliances. An erosion is a loss of the superficial epidermis, the surface of which is moist but does not bleed. An ulcer is a skin defect in which there has been loss of the epider- mis and papillary layer of dermis. It may extend into the subcutaneous tissue, and often occurs in pathologically damaged tissue. A callus is a hyperkeratotic plaque of skin that devel- ops because of chronic pressure or friction. It represents hyperplasia of the dead keratinocytes that make up the stratum corneum. Increased cohesion between cells results in hyperkeratosis and decreased skin shedding. A callus may be filed down but is likely to recur if pressure continues in the localized area. Corns (helomas) are small, well-circumscribed, coni- cal, keratinous thickenings of the skin. They usually appear on the toes from rubbing or ill-fitting shoes. The corn may be either hard (heloma durum) with a central hard, horny core, or soft (heloma molle), as commonly seen between the toes. They may appear on the hands as an occupational hazard. The hard tissue at the center of the corn looks like a funnel with a broad top and a pointed bottom, hence the name “corn.” Corns on the feet often are painful, whereas corns on the hands may be asymptomatic. Corns may be abraded or surgically removed, but they recur if the causative agent is not removed. Pigmentary Skin Disorders Pigmentary skin disorders involve the melanocytes. In some cases, there is an absence of melanin production, as in vitiligo or albinism. In other cases, there is an increase in melanin or some other pigment, as in melasma. In either case, the emotional impact can be devastating.