Skin, Molecular Cell Biology of

Abstract

The skin is the one organ of the body that is constantly exposed to a changing environment. Maintaining its integrity is a complex process, and assaults from surgical incisions, injuries, or burns can lead to life-threatening consequences without appropriate treatment.

Human skin is divided into two primary layers—epidermis (outermost layer) and dermis (innermost layer) (Fig. 1). These two layers are separated by a structure called the basement membrane. Beneath the dermis is a layer of loose connective tissue called the hypodermis or subcutis, and beneath the subcutis is the fat layer. Major functions of the skin are protection, immunity, thermoregulation, sensation, metabolism, and communication. The skin of the average adult covers approximately 3000 square inches, or an area almost equivalent to 2 m². From birth to maturity the skin covering will undergo a sevenfold expansion. It weighs about 6 lbs (or up to 15% of total adult body weight), is the largest organ, and receives one-third of the body's circulating blood volume. The skin forms a protective barrier from the external environment while maintaining a homeostatic internal environment. Epidermal appendages—nails, hair follicles, sweat or sebaceous glands—which are lined with epidermal cells, are also present in the skin. During the healing of partial-thickness wounds, these epidermal cells migrate to resurface the wound. This organ is capable of self-regeneration and can withstand limited mechanical and chemical assaults. The skin varies in thickness from 0.5 mm in the tympanic membrane to 6 mm in the soles of the feet and the palms of the hands. Variations are attributable to differences in the thickness of the skin layers covering underlying organs, bones, muscle, and cartilage. Diseases of the skin can result from genetic causes, so-called genodermatoses, infection, immune dysfunction, and trauma. Skin diseases can involve some or all of the layers of the skin.