Our knowledge of the integumentary system (skin) has evolved over the last 20 years. A true biological universe, the skin incorporates all major support systems: blood, muscle, and innervation as well as roles in immunocompetence, psycho-emotional, ultraviolet radiation sensing, and endocrine functions.1
The integumentary system is the external covering of the human body. The name is derived from the Latin integumentum, which means "a covering." The integumentary system includes the skin, hair, nails, and sweat glands and their products. It is often referred to as is the largest organ of the body. It is composed of two main layers: the epidermis, known as the outermost layer, and the dermis, the innermost layer. The dermal-epidermal junction, commonly referred to as the basement membrane zone (BMZ), separates the two layers. Under the dermis lies a layer of loose connective tissue, called subcutis (subcutaneous) tissue, or hypodermis. (Fig. 1.1)
The epidermis is a thin outer layer of skin. It is an avascular layer that regenerates itself every 4 to 6 weeks. It contains melanin, which gives skin its color and allows the skin to tan. Carotene and oxygen-rich hemoglobin also contribute to the color of skin.2 The epidermis is divided into five sublayers, or strata, composed of keratin-producing cells called keratinocytes.1,2,3,4,5 (Fig. 1.2):
Stratum corneum: top layer, consists of dead keratinocyte cells
Stratum lucidum: packed translucent line of cells; not seen in thin skin; found only on the palms and soles
Stratum granulosum: contains keratinocytes and Langerhans cells
Stratum spinosum: "spiny" layer, contains keratinocytes and Langerhans cells
Stratum basale or germinativum: single layer of epidermal cells; contains melanocytes and Merkel cells
The epidermis is composed of hard, horny keratinocyte cells that migrate up from the bottom single layer of cells, the stratum basale, to the stratum corneum. Epidermal cell differentiation usually takes 28 days and is affected by aging and chemotherapy.
The primary functions of the epidermis are protection from epidermal water loss and maintaining skin integrity against bacterial invasion and physical barriers, such as shear, friction, and toxic irritants.6,7,8
The BMZ divides the epidermis from the dermis. It contains fibronectin (an adhesive glycoprotein), type IV collagen (a non-fiber-forming collagen), heparin sulfate proteoglycan, and glycosaminoglycan.8,9 The BMZ has an irregular surface—called rete ridges or pegs—projecting downward from the epidermis that interlocks with the upward projections of the dermis. This structure anchors the epidermis to the dermis, preventing it from sliding back and forth. ...