Skin is composed of three layers: the epidermis, the dermis, and the subcutaneous tissue. The epidermis is nonvascular and varies in thickness from 0.04 mm on the eyelid to 1.6 mm on the palms. In decreasing number, the four types of cells in the epidermis are: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. The keratinocytes are the most abundant cells in the epidermis. They change dramatically as they age from the viable, actively dividing basal cells, to the dead, mostly lipid-containing stratum corneum. Keratinocytes provide an efficient barrier against water and electrolyte loss and entrance of foreign materials and agents. Melanocytes, derived from neural crest precursors, contain melanin-containing organelles. The function of melanin is to scatter and absorb ultraviolet light. Langerhans cells, which constitute about 4% of the epidermal cell population, have been shown to possess cell-surface la antigens, and membrane receptors for C3b complement and Fc receptors of IgG, which are thought to facilitate the immune response to foreign antigens through T lymphocytes. The exact function of the Merkel cells is unknown, but they are thought to act as slowly adapting touch receptors. It has also been suggested that these cells regulate proliferation of keratinocytes through cytoplasmic contacts.

Beneath the epidermis is the dermis, which has two layers. The papillary layer is superficial, and the reticular layer is deeper. The dermis is thicker on the dorsal aspect of the body than on the ventral surface. The papillary layer has numerous eminences that may contain vascular structures or sensory tissue, as in the hand or foot. The deeper reticular layer consists of fibroelastic connective tissue. Collagen constitutes approximately 70% of the dry weight of the dermis. Elastic fibers, which give skin its elastic properties, constitute approximately 1%. The reticular layer merges with the subcutaneous fascia at its deepest aspect.

Three other entities in the skin are of particular interest to otolaryngologists. Sebaceous glands, apocrine units, and eccrine sweat units are important in that each can become infected or
undergo malignant change. The sebaceous glands open onto hair follicles. As cells disintegrate within the gland, cellular debris or sebum is discharged from the duct into the hair follicle and skin. The physiologic role of sebum is not known, but it is a participant in the etiology of acne vulgaris. Apocrine units are secretory glands that consist of a lower coiled section and superficial straight portion that empties into the infundibulum of a hair follicle just above the sebaceous gland. Apocrine glands secrete hormones in animals (sexual scent hormones) but have no proven function in humans. Eccrine glands are the true sweat glands. The highest concentrations are on the palms, soles, axillae, and forehead. The glands function as a means of thermal regulation and respond to emotional stimuli.

To communicate effectively about various lesions, it is essential that the otolaryngologist be familiar with dermatologic terms. Table 41-1 consists of the set of basic terms used in describing cutaneous abnormalities, their characteristics, and some examples.

Although the skin of the head and neck region accounts for approximately 10% of the body surface area, most cutaneous tumors occur in this area. Neoplastic disease can be divided into benign and malignant categories.