The skin is the largest single organ of the body, typically accounting for 15%-20% of total body weight and, in adults, presenting 1.5-2 m2 of surface to the external environment. Also known as the integument (L. integumentum, covering) or cutaneous layer, the skin is composed of the epidermis, an epithelial layer of ectodermal origin, and the dermis, a layer of mesodermal connective tissue (Figure 18–1). At the irregular junction between the dermis and epidermis, projections called dermal papillae interdigitate with invaginating epidermal ridges to strengthen adhesion of the two layers. Epidermal derivatives include hairs, nails, and sebaceous and sweat glands. Beneath the dermis lies the subcutaneous tissue or hypodermis (Gr. hypo, under + derma, skin), a loose connective tissue layer usually containing pads of adipocytes. The subcutaneous tissue binds the skin loosely to the underlying tissues and corresponds to the superficial fascia of gross anatomy.
Layers and appendages of skin.
Diagrammatic overview of skin, showing the major layers and epidermal appendages (hair follicles, sweat, and sebaceous glands), the vasculature, and the major sensory receptors. (Reproduced, with permission, from McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York, NY: McGraw-Hill; 2008; McKinley M, O'Loughlin VD. Human Anatomy. 3rd ed. New York, NY: McGraw-Hill; 2012; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. New York, NY: McGraw-Hill; 2013; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. 2nd ed. New York, NY: McGraw-Hill; 2016).
The specific functions of the skin fall into several broad categories.
Protective: It provides a physical barrier against thermal and mechanical insults such as friction and against most potential pathogens and other material. Microorganisms that do penetrate skin alert resident lymphocytes and antigen-presenting cells (APCs) in skin and an immune response is mounted. The dark pigment melanin in the epidermis protects cell nuclei from ultraviolet (UV) radiation. Skin is also a permeability barrier against excessive loss or uptake of water, which has allowed for terrestrial life. Skin’s selective permeability allows some lipophilic drugs such as certain steroid hormones and medications to be administered via skin patches.
Sensory: Many types of sensory receptors allow skin to constantly monitor the environment, and various skin mechanoreceptors help regulate the body’s interactions with physical objects.
Thermoregulatory: A constant body temperature is normally easily maintained thanks to the skin’s insulating components (eg, the fatty layer and hair on the head) and its mechanisms for accelerating heat loss (sweat production and a dense superficial microvasculature).
Metabolic: Cells of skin synthesize vitamin D3, needed in calcium metabolism and proper bone formation, through the local action of UV light on the vitamin’s precursor. Excess electrolytes can be removed in sweat, and the subcutaneous layer stores a significant amount of energy in the form ...