Section 18. Skin Disorders

Chapter 201. Biology of the Skin

Topics: Introduction | Structure and Function | Effects of Aging

Structure and Function

The skin has three layers--the epidermis, dermis, and fat layer (also called the subcutaneous layer). Each layer of skin performs specific tasks.

See the figure Getting Under the Skin.

Epidermis: The epidermis is the thin, tough, top layer of skin. The outer portion of the epidermis, the stratum corneum, is waterproof and, when undamaged, prevents most bacteria, viruses, and other foreign substances from entering the body. The epidermis also protects the internal organs, muscles, nerves, and blood vessels against trauma. The dead flat cells on the surface of the epidermis are composed of a tough, fibrous protein called keratin, which is also found in the hair and fingernails. As these dead cells at the surface are worn away, they are continuously replaced by newer cells that are pushed up from below. The outer keratin layer of the epidermis is thicker on skin surfaces that require greater protection, such as the palms of the hands and the soles of the feet.

At the bottom of the epidermis are the melanocytes, the cells that produce melanin--the dark pigment of skin. Melanin filters out ultraviolet radiation from sunlight (see Section 18, Chapter 214) and provides color to the skin.

The epidermis also contains Langerhans' cells, which are part of the skin's immune system. These cells help detect foreign substances and play a role in the development of skin allergies.

Dermis: The dermis, the next layer of skin, is a thick layer of fibrous and elastic tissue (made mostly of the proteins collagen and fibrillin) that gives the skin its flexibility and strength. The dermis contains nerve endings, glands, hair follicles, and blood vessels.

The nerve endings sense pain, touch, pressure, and temperature. Some areas of the skin contain more nerve endings than others. For example, the fingertips and toes contain many nerves and are extremely sensitive to touch.

The sweat glands produce sweat in response to heat. Sweat is composed of water, salt, and other chemicals. As sweat evaporates off the skin, it helps cool the body. Specialized sweat glands of the armpit and genital region (apocrine sweat glands) secrete a thick, oily sweat that produces a characteristic body odor when the sweat is digested by the skin bacteria in those areas. Sebaceous glands produce oil, which keeps the skin moist and soft and acts as a barrier against foreign substances.

The blood vessels of the dermis provide nutrients to the skin and help regulate body temperature. Heat makes the blood vessels widen (dilate), allowing large amounts of blood to circulate near the skin surface, where the heat can be released. Cold makes the blood vessels narrow (constrict), retaining the body's heat.

Over different parts of the body, the number of nerves, sweat glands, sebaceous glands, hair follicles, and blood vessels varies. The top of the head, for example, has many hair follicles; the soles of the feet have none.

Fat Layer: Below the dermis lies a layer of fat that helps insulate the body from heat and cold, provides protective padding, and serves as an energy storage area. The fat is contained in living cells, called fat cells, held together by fibrous tissue. The fat layer varies in thickness, from a fraction of an inch on the eyelids up to several inches on the abdomen and buttocks in some people.