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The Merck Manual of Medical Information--Home Edition
Section 18. Skin Disorders
Chapter 190

Biology of the Skin

The skin isn't just a protective wrapping. It's an organ system that regulates body temperature, senses painful and pleasant stimuli, keeps substances from entering the body, and provides a shield from the sun's harmful effects. Skin color, texture, and folds help mark people as individuals. Anything that goes wrong with skin function or appearance can have important consequences for physical and mental health.

Each layer of skin performs specific tasks. The top layer, the epidermis, is actually thinner over most of the body than plastic wrap. The top portion of the epidermis, the stratum corneum, contains keratin, which is formed from the remains of dead cells and protects the skin from harmful substances. At the bottom of the epidermis are the melanocytes, the cells that produce melanin--the dark-colored pigment of skin.

Below the epidermis lies the dermis, which contains pain and touch receptors, whose tentacles reach up to the skin surface, and many of the functional glands of the skin: sweat glands, which produce sweat; sebaceous glands, which produce oil; and hair follicles, which produce hair. Also within the dermis lie blood vessels that provide nutrition to the skin and make it feel warm and nerves that branch throughout the layers of the skin.

Below the dermis lies a layer of fat that helps insulate the body from heat and cold.

click on thumbnail to see full-size imageOver different parts of the body, the thickness and color of the skin and the number of sweat glands, sebaceous glands, hair follicles, and nerves vary. The top of the head has many hair follicles; the soles of the feet have none. The soles and the palms have much thicker epidermis and keratin layers. The fingertips and toes contain many nerves and are extremely sensitive to touch.

The skin tends to change throughout a person's lifetime. A baby's skin has a much thicker fat layer and a much thinner layer of protective keratin. As people age, they lose much of the underlying fat, the dermis and epidermis become thinner, the elastic fibers in the dermis become fragmented, and the skin becomes more wrinkled. The flow of blood in the skin also decreases with age, so damaged skin heals more slowly in older people. Older skin also makes less protective oil, so the skin dries out more easily.

Diagnosing Skin Conditions

Doctors can identify many skin conditions simply by looking at them. Revealing characteristics include size, shape, color, and location of the abnormality, as well as the presence or absence of other signs or symptoms. Sometimes, a doctor has to remove a small piece of skin for examination under a microscope, a procedure called a biopsy. For this simple procedure, the doctor generally numbs a small area of skin with a local anesthetic and, using a small knife (scalpel) or round cutter (biopsy punch), removes a piece of skin about one-eighth inch in diameter. Often, the doctor uses a stitch to close the site and stop the bleeding.

When doctors think the skin may be infected, they scrape off some material from the skin, send it to a laboratory, and have the specimen placed in a culture medium. If the specimen contains bacteria, fungi, or viruses, they grow in the culture and can be identified.

Other laboratory tests also help doctors diagnose skin infections. In a Wood's light examination, a certain frequency of ultraviolet (black) light makes some fungi visible and may make some pigmentation abnormalities more visible. The Tzanck test helps diagnose viral skin infections such as herpes. With a small scalpel, the doctor scrapes the surface of the inflamed skin and examines it under a microscope. Recognizable enlarged or grouped cells indicate a viral infection. The skin sample also can be sent to a laboratory for a viral culture.

Many problems that appear on the skin are limited to the skin. Sometimes, however, the skin reveals a medical condition of the entire body. For example, people who have systemic lupus erythematosus develop an unusual reddish rash on their cheeks, usually after sun exposure. Consequently, doctors often must consider many possible causes when evaluating skin problems. Examining the entire skin surface and looking for certain patterns of a rash can help them identify any possible medical illness. To check the distribution of a skin problem, the doctor may ask a patient to undress completely, even though the patient only noticed an abnormality on a small area of skin. Doctors may also order blood tests or other laboratory tests even if a person seems to have a problem limited only to the skin.

Medical Names for Marks and Growths on the Skin

Atrophic skin: Paper-thin, wrinkled skin.

Crust (scab): Dried blood, pus, or skin fluids on the surface of the skin. A crust can form wherever the skin has been damaged.

Erosion: Loss of part or all of the top surface of the skin. Erosions occur when infection, pressure, irritation, or temperature has damaged the skin.

Excoriation: A hollowed-out or linear crusted area, caused by scratching, rubbing, or picking at the skin.

Lichenification: Thickened skin that has deep grooves and wrinkles.

Macule: A flat, discolored spot of any shape, less than 0.4 of an inch in diameter. Freckles, flat moles, port-wine stains, and many rashes are macules. A patch is like a macule, but larger.

Nodule: A solid bump, 0.2 to 0.4 of an inch in diameter, that may be raised. A nodule sometimes appears to form below the surface of the skin and press upward.

Papule: A solid bump less than 0.4 of an inch in diameter. Warts, insect bites, skin tags, and some skin cancers are papules. A plaque is a larger papule.

Pustule: A blister containing pus (a collection of white blood cells).

Scales: Areas of heaped-up, dead epidermal cells, producing a flaky, dry patch. Scales occur with psoriasis, seborrheic dermatitis, and many other conditions.

Scar: An area where normal skin has been replaced by fibrous (scar-forming) tissue. Scars form after destruction of some part of the dermis.

Telangiectasia: Dilated blood vessels within the skin that have a tortuous appearance.

Ulcer: Like an erosion, only deeper, penetrating at least part of the dermis. The causes are the same as for erosions.

Vesicle: A small, fluid-filled spot less than 0.2 of an inch in diameter. A blister (bulla) is a larger vesicle. Insect bites, herpes zoster (shingles), chickenpox, burns, and irritations form vesicles and blisters.

Wheal (hive): Swelling in the skin that produces an elevated, soft, spongy area that appears relatively suddenly and then disappears. Wheals are common allergic reactions to drugs, insect bites, or something that touched the skin.

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