Porth's Essentials of Pathophysiology, 4e

1146

Integumentary Function

U N I T 1 3

Plasma membrane

SUMMARY CONCEPTS

Tonofilaments

Attachment plaque

Epidermis

Appendages of the Skin Skin appendages are derived from outgrowths of epider- mal epithelium during development. They include the eccrine and apocrine sweat glands, sebaceous glands, hair follicles, and nails. Sweat and Sebaceous Glands Two types of exocrine glands originate in the dermis and release their products onto the skin surface. These are sweat (or sudoriferous) glands and sebaceous glands (commonly referred to as oil glands). There are two types of sweat glands: eccrine and apo- crine. Eccrine sweat glands are simple tubular structures that originate in the dermis and open directly to the skin surface. They are numerous (several million), vary in den- sity, and are located over the entire body surface. Their purpose is to transport sweat to the outer skin surface to regulate body temperature. Apocrine sweat glands are less numerous than eccrine sweat glands. They are larger and located deep in the dermal layer. They open through a hair follicle, even though a hair may not be present, and are found primarily in the axillae and groin. The major difference between the two types of sweat glands is that ■■ The skin is a complex organ that forms the major barrier between the internal and external environments. It consists of two primary layers, the epidermis and the dermis; is richly innervated with pain, temperature, and touch receptors; and plays an essential role in fluid and electrolyte balance. ■■ The outer epidermis, which is avascular, is composed of four to five layers of stratified squamous epithelial cells, predominantly keratinocytes.The keratinocytes are formed in the deepest layer of the epidermis and migrate to the skin surface to replace cells that are lost during normal skin shedding. ■■ The dermis is a connective tissue layer that separates the epidermis from the underlying subcutaneous fat layer. It contains the blood vessels and nerve fibers that supply the epidermis, as well as sensory receptors for pain, temperature, and touch. ■■ The basement membrane zone is a layer of intercellular and extracellular matrices that serves as an interface between the dermis and the epidermis. It not only cements the epidermis to the dermis, but also serves as a selective filter for molecules moving between the two layers.

Hemidesmosome

Lamina lucida Lamina densa

Basement membrane zone

Dermis

Anchoring fibrils

Subdesmosomal dense plate

Microfibrils

Anchoring filaments

moving between the two layers. It is also a major site of immunoglobulin and complement deposition in skin disease. Most of the structures of the basal membrane zone are produced by cells of the epidermis. The basal lam- ina is the zone’s primary organizational structure. Its layers include the lamina lucida, lamina densa, and hemidesmosome (Fig. 45-5). The lamina lucida con- sists of fine anchoring filaments and a cell adhesion glycoprotein, called laminin, which plays a role in the organization of the macromolecules in the basement membrane zone and promotes attachment of cells to the extracellular matrix. The lamina densa contains an adhesive called type IV collagen as well as lam- inin. It is important in dermal–epidermal attachment. The hemidesmosome is similar in structure to half a desmosome, and serves as an attachment site for the dermis and epidermis. Bundles of tonofibrils, epider- mal fibers similar to keratin filaments, converge and terminate in the hemidesmosomes. Because they form a continuous link between the intracellular tonofibrils of the epidermis and the extracellular basement mem- brane, the hemidesmosomes are also thought to be involved in relaying signals between the epidermis and dermis. The basement membrane zone is often involved in skin disorders that cause bullae or blister formation. One of these disorders, bullous pemphigoid, is a blis- tering disease caused by antibodies against basement membrane proteins. The blisters of bullous pemphi- goid are large and tense and may appear on skin that otherwise looks normal. The thighs and flexor ten- dons are most commonly affected. The disease is self- limited but chronic, and the person’s general health is unaffected. FIGURE 45-5. The dermal–epidermal interface and layers of the basement membrane zone. (Adapted from Storm CA, Elder DE.The skin. In: Rubin R, Strayer DS, eds. Rubin’s Pathology: Clinicopathologic Foundations of Medicine. 6th ed. Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams & Wilkins; 2012:1116.)