The Integumentary System

The integumentary system, formed by the skin, hair, nails, and associated glands, enwraps the body. It is the most visible organ system and one of the most complex. Diverse in both form and function—from delicate eyelashes to the thick skin of the soles—the integumentary system protects the body from the outside world and its many harmful substances. It utilizes the Sun's rays while at the same time shielding the body from their damaging effects. In addition, the system helps to regulate body temperature, serves as a minor excretory organ, and makes the inner body aware of its outer environment through sensory receptors.

DESIGN: PARTS OF THE INTEGUMENTARY SYSTEM

Integument comes from the Latin word integumentum, meaning "cover" or "enclosure." In animals and plants, an integument is any natural outer covering, such as skin, shell, membrane, or husk. The human integumentary system is an external body covering, but also much more. It protects, nourishes, insulates, and cushions. It is absolutely essential to life. Without it, an individual would be attacked immediately by bacteria and die from heat and water loss.

The integumentary system is composed primarily of the skin and accessory structures. Those structures include hair, nails, and certain exocrine glands (glands that have ducts or tubes that carry their secretions to the surface of the skin or into body cavities for elimination).

Skin

Although the skin is not often thought of as an organ, such as the heart or liver, medically it is. An organ is any part of the body formed of two or more tissues that performs a specialized function. As an organ, the skin is the largest and heaviest in the body. In an average adult, the skin covers about 21.5 square feet (2 square meters) and accounts for approximately 7 percent of body weight, or about 11 pounds (5 kilograms) in a 160-pound (73-kilogram) person. It ranges in thickness from 0.04 to 0.08 inches (1 to 2 millimeters), but can measure up to 0.2 inches (6 millimeters) thick on the palms of the hands and the soles of the feet. The skin in these areas is referred to as thick skin (skin elsewhere on the body is called thin skin).

The Integumentary System: Words to Know

Apocrine sweat glands (AP-oh-krin):

Sweat glands located primarily in the armpit and genital areas.

Arrector pili muscle (ah-REK-tor PI-li):

Smooth muscle attached to a hair follicle that, when stimulated, pulls on the follicle, causing the hair shaft to stand upright.

Dermal papillae (DER-mal pah-PILL-ee):

Finger-like projections extending upward from the dermis containing blood capillaries, which provide nutrients for the lower layer of the epidermis; also form the characteristic ridges on the skin surface of the hands (fingerprints) and feet.

Dermis (DER-miss):

Thick, inner layer of the skin.

Eccrine sweat glands (ECK-rin):

Body's most numerous sweat glands, which produce watery sweat to maintain normal body temperature.

Epidermis (ep-i-DER-miss):

Thin, outer layer of the skin.

Epithelial tissue (ep-i-THEE-lee-al):

Tissue that covers the internal and external surfaces of the body and also forms glandular organs.

Integument (in-TEG-ye-ment):

In animals and plants, any natural outer covering, such as skin, shell, membrane, or husk.

Keratin (KER-ah-tin):

Tough, fibrous, water-resistant protein that forms the outer layers of hair, calluses, and nails and coats the surface of the skin.

Lunula (LOO-noo-la):

White, crescent-shaped area of the nail bed near the nail root.

Melanocyte (MEL-ah-no-site):

Cell found in the lower epidermis that produces the protein pigment melanin.

Organ (OR-gan):

Any part of the body formed of two or more tissues that performs a specialized function.

Sebaceous gland (suh-BAY-shus):

Exocrine gland in the dermis that produces sebum.

Sebum (SEE-bum):

Mixture of oily substances and fragmented cells secreted by sebaceous glands.

Squamous cells (SKWA-mus):

Cells that are flat and scalelike.

Subcutaneous (sub-kew-TAY-nee-us):

Tissues between the dermis and the muscles.

The skin has two principal layers: the epidermis and the dermis. The epidermis is the thin, outer layer, and the dermis is the thicker, inner layer. Beneath the dermis lies the subcutaneous layer or hypodermis, which is composed of adipose or fatty tissue. Although not technically part of the skin, it does anchor the skin to the underlying muscles. It also contains the major blood vessels that supply the dermis and houses many white blood cells, which destroy foreign invaders that have entered the body through breaks in the skin.

EPIDERMIS. The epidermis is made of stratified squamous epithelial tissue. Epithelial tissue covers the internal and external surfaces of the body and also forms glandular organs. Squamous cells are thin and flat like fish scales. Stratified simply means having two or more layers. In short, the epidermis is composed of many layers of thin, flattened cells that fit closely together and are able to withstand a good deal of abuse or friction.

The epidermis can be divided into four or five layers. Most important of these are the inner and outer layers. The inner or deepest cell layer is the only layer of the epidermis that receives nutrients (from the underlying dermis). The cells of this layer, called basal cells, are constantly dividing and creating new cells daily, which push the older cells toward the surface. Basal cells produce keratin, an extremely durable and water-resistant fibrous protein.

Another type of cell found in the lower epidermis is the melanocyte. Melanocytes produce melanin, a protein pigment that ranges in color from yellow to brown to black. The amount of melanin produced determines skin color, which is a hereditary characteristic. The melanocytes of dark-skinned individuals continuously produce large amounts of melanin. Those of light-skinned individuals produce less. Freckles are the result of melanin clumping in one spot.

The outermost layer of the epidermis consists of about twenty to thirty rows of tightly joined flat dead cells. All that is left in these cells is their keratin, which makes this outer layer waterproof. It takes roughly fourteen days for cells to move from the inner layer of the epidermis to the outer layer. Once part of the outer layer, the dead cells remain for another fourteen days or so before flaking off slowly and steadily.

DERMIS. The dermis, the second layer of skin, lies between the epidermis and the subcutaneous layer. Much thicker than the epidermis, the dermis contains the accessory skin structures. Hair, sweat glands, and sebaceous (oil) glands are all rooted in the dermis. This layer also contains blood vessels and nerve fibers. Nourished by the blood and oxygen provided by these blood vessels, the cells of the dermis are alive.

Connective tissue forms the dermis. Bundles of elastic and collagen (tough fibrous protein) fibers blend into the connective tissue. These fibers provide the dermis strength and flexibility.

The upper layer of the dermis has fingerlike projections that extend into the epidermis. Called dermal papillae, they contain blood capillaries that provide nutrients for the basal cells in the epidermis. On the skin surface of the hands and feet, especially on the tips of the fingers, thumbs, and toes, the dermal papillae form looped and whorled ridges. These print patterns, known as fingerprints or toeprints, increase the gripping ability of the hands and feet. Genetically determined, the patterns are unique to every individual.

USING FINGERPRINTS TO IDENTIFY PEOPLE

Fingerprints (the pattern of ridges on an individual's fingertips and thumbs formed by dermal papillae) are unique to each individual and the patterns never change. People have long known about the distinctiveness of fingerprints, but their use in identifying people did not arise until the nineteenth century.

It is generally acknowledged that English scientist Francis Galton (1822–1911) was the first person to devise a system of fingerprint identification. In the 1880s, Galton obtained the first extensive collection of fingerprints for his studies on heredity. He also established a bureau for the registration of civilians by means of fingerprints and measurements.

Galton's ideas were further developed by fellow Englishman Edward R. Henry (1850–1931). In the 1890s, Henry developed a more simplified fingerprint classification system. In 1901, he established England's first fingerprint bureau, called the Fingerprint Branch, within the Scotland Yard police force. Henry's system is still used today in Great Britain and the United States.

Within the dermis are sensory receptors for the senses of touch, pressure, heat, cold, and pain. A specific type of receptor exists for each sensation. For pain, the receptors are free nerve endings. For the other sensations, the receptors are encapsulated nerve endings, meaning they have a cellular structure around their endings. The number and type of sensory receptors present in a particular area of skin determines how sensitive that area is to a particular sensation. For example, fingertips have many touch receptors and are quite sensitive. The skin of the upper arm is less sensitive because it has very few touch receptors.

Accessory structures

The accessory structures of the integumentary system include hair, nails, and sweat and sebaceous glands.

HAIR. Roughly 5 million hairs cover the body of an average individual. About 100,000 of those hairs appear on the scalp. Almost every part of the body is covered by hair, except the palms of the hands, the soles of the feet, the sides of the fingers and toes, the lips, and certain parts of the outer genital organs.

Each hair originates from a tiny tubelike structure called a hair follicle that extends deep into the dermis layer. Often, the follicle will project into the subcutaneous layer. Capillaries and nerves attach to the base of the follicle, providing nutrients and sensory information. Inside the base of the follicle, epithelial cells grow and divide, forming the hair bulb or enlarged hair base. Keratin, the primary component in these epithelial cells, coats and stiffens the hair as it grows upward through the follicle. The part of the hair enclosed in the follicle is called the hair root. Once the hair projects from the scalp or skin, it is called a hair shaft.

The older epithelial cells forming the hair root and hair shaft die as they are pushed upward from the nutrient-rich follicle base by newly formed cells. Like the upper layers of the epidermis, the hair shaft is made of dead material, almost entirely protein. The hair shaft is divided into two layers: the cuticle or outer layer consists of a single layer of flat, overlapping cells; the cortex or inner layer is made mostly of keratin.

Hair shafts differ in size, shape, and color. In the eyebrows, they are short and stiff, but on the scalp they are longer and more flexible. Elsewhere on the body they are nearly invisible. Oval-shaped hair shafts produce wavy hair. Flat or ribbonlike hair shafts produce kinky or curly hair. Perfectly round hair shafts produce straight hair. The different types of melanin—yellow, rust, brown, and black—produced by melanocytes at the follicle base combine to create the many varieties of hair color, from the palest blonde to the richest black. With age, the production of melanin decreases, and hair color turns gray.

Attached to each hair follicle is a ribbon of smooth muscle called an arrector pili muscle. When stimulated, the muscle contracts and pulls on the follicle, causing the hair shaft to stand upright.

NAILS. Nails in humans correspond to the hooves of horses and cattle and the claws of birds and reptiles. Found on the ends of fingers and toes, nails are produced by nail follicles just as hair is produced by hair follicles. The nail root is that portion of the nail embedded in the skin, lying very near the bone of the fingertip. Here, cells produce a stronger form of keratin than is found in hair. As new cells are formed, older cells are pushed forward, forming the nail body or the visible attached portion of the nail. The free edge is that portion of the nail that extends over the tip of the finger or toe. Healthy fingernails grow about 0.04 inches (1 millimeter) per week, slightly faster than toenails.

The nail body is made of dead cells, but the nail bed (the tissue underneath the nail body) is alive. The blood vessels running through the nail bed give the otherwise transparent nail body a pink color. Near the nail root, however, these blood vessels are obscured. The resulting white crescent is called the lunula (from the Latin word luna, meaning "moon").

SWEAT GLANDS. More than 2.5 million sweat glands are distributed over most surfaces of the human body. They are divided into two types: eccrine sweat glands and apocrine sweat glands.

Eccrine glands, the more numerous of the two types, are found all over the body. They are especially numerous on the forehead, upper lip, palms, and soles. The glands are simply coiled tubes that originate in the dermis. A duct extends from the gland to the skin's surface, where it opens into a pore. Eccrine glands produce sweat or perspiration, a clear secretion that is 99 percent water. Some salts, traces of waste materials such as urea, and vitamin C form the remainder (the salts give sweat its characteristic salty taste).

Depending on temperature and humidity, an average individual loses 0.6 to 1.7 quarts (0.3 to 0.8 liters) of water every day through sweating. During rigorous physical activity or on a hot day, that amount could rise to 5.3 to 7.4 quarts (5 to 7 liters).

Apocrine glands are found in the armpits, around the nipples, and in the groin. Like eccrine glands, apocrine glands are coiled tubes found in the dermis. However, they are usually larger and their ducts empty into hair follicles. Also, apocrine glands do not function until puberty. At that time, they begin to release an odorless cloudy secretion that contains fatty acids and protein. If the secretion of apocrine glands is allowed to remain on the skin for any length of time, bacteria that lives on the skin breaks down the fatty acids and protein for their growth, creating the unpleasant odor often associated with sweat.

Apocrine glands are activated by nerve fibers during periods of pain and stress, but their function in humans is not well understood. Scientists theorize they may act as sexual attractants.

SEBACEOUS GLANDS. Sebaceous glands, also known as oil glands, are found in the dermis all over the body, except for the palms and soles. They secrete sebum, a mixture of lipids (fats), proteins, and fragments of dead fatproducing cells. The function of sebum is to prevent the drying of skin and hair. It also contains chemicals that kill bacteria present on the skin surface. While most sebaceous glands secrete sebum through ducts into hair follicles, some secrete sebum directly onto the surface of the skin. Arrector pili muscles, which contract to elevate hairs, also squeeze sebaceous glands, forcing out sebum.

WORKINGS: HOW THE INTEGUMENTARY SYSTEM FUNCTIONS

The integumentary system is essential to the body's homeostasis, or ability to maintain the internal balance of its functions regardless of outside conditions. The system works to protect underlying tissues and organs from infections and injury. It also prevents the loss of body fluids.

Receiving about one-third of the blood pumped from the heart every minute, the skin and its glands help maintain normal body temperature. The system also acts as a mini-excretory system, secreting salts, water, and wastes in the form of sweat. Cells in the skin utilize sunlight to create vitamin D, which is necessary for normal bone growth and function. Finally, the skin contains sensory receptors or specialized nerve endings that allow an individual to "feel" sensations such as touch, pain, pressure, and temperature.

Protection

The outermost epidermal layer of the skin is a barrier between the internal environment of the body and the external world. Keratin, in abundance in this outer layer, waterproofs the body. Without it, handling household chemicals, swimming in a pool, or taking a shower (a necessary everyday activity) would be disastrous to the underlying cells of the body. Not only does keratin keep water out, it also keeps water in. Excessive evaporation or loss of body fluids would result in dehydration and eventual death.

WETTERHAHN'S DEADLY RESEARCH

Karen Wetterhahn (1948–1997) was a chemistry professor at Dartmouth College in Hanover, New Hampshire, where she conducted environmental research projects. During an experiment in August 1996, Wetterhahn spilled a tiny drop of dimethyl mercury (a highly toxic chemical) on her hand. Less than a year later, she was dead.

Wetterhahn had been conducting research to determine the effects that heavy metals (metals such as mercury having a high specific gravity) produce on the environment. During her experiment, she was transferring some dimethyl mercury to a tube when she spilled a tiny amount. Although Wetterhahn was wearing latex gloves, the mercury permeated the thin latex and soaked into her skin, passing through its waterproof layers within seconds.

Dimethyl mercury is deadly. Once in the body, it seeps from the bloodstream into brain tissues, causing fatal damage to the central nervous system and the brain. Symptoms of mercury poisoning include loss of motor (movement) control, numbness in the arms and legs, blindness, hearing and speech loss.

Wetterhahn did not feel the effects of the mercury until six months after the accident. Within three months, she was dead. After her death, the U.S. Occupational Safety and Health Administration urged scientists to wear highly resistant laminate gloves (consisting of several bonded layers) under a pair of heavy-duty neoprene gloves when handling compounds such as dimethyl mercury.

The thickness of the outer layer of the epidermis, combined with the toughness provided by keratin, also prevents microorganisms and viruses from entering the body. In addition, sebum secreted by the sebaceous glands helps prevent microorganisms from living and growing on the skin surface. Since it is slightly acidic, sebum creates a condition in which many microorganisms cannot exist. Sebum serves a further protective function by keeping the skin and hair moist; dry skin would crack, allowing viruses and bacteria to enter.

If the protective outer layer of the skin is broken because of an injury and microorganisms enter the body, the many blood vessels in the dermis help prevent the microorganisms from reaching internal tissues. As an immune response, the vessels dilate or expand. This increases the amount of blood flowing to the area, which in turn brings in more white blood cells and other protein factors to battle the infection.

Even though the skin forms a protective barrier, it is still slightly permeable or allows certain substances to pass through it. Vitamins A, D, E, and K all pass through the skin and are absorbed in the capillaries in the dermis. Steroid hormones such as estrogen and chemicals such as nicotine also pass through and are absorbed. With this in mind, medical researchers have developed therapeutic patches that are attached to the skin to deliver chemicals or medication (nicotine patches for those individuals trying to quit smoking are an example).

Nails protect the exposed tips of fingers and toes from physical injury. Fingernails also aid the fingers in picking up small objects.

Hair serves a protective function, although it is limited. On the head, hair protects the scalp from damaging ultraviolet (UV) radiation from the Sun, cushions the head from physical blows, and insulates the scalp to a degree. On the eyelids, eyelashes prevent airborne particles and insects from entering the eyes. Hairs in the nostrils and the external ear canals perform a similar function.

When stimulated by cold or an emotion such as fear, the arrector pili muscles contract, pulling hair follicles upright. In animals (and in our evolutionary ancestors, who had much more body hair), this action adds warmth by adding a layer of insulating air to the fur. In present-day humans, who have very little body hair, this action seems to serve no purpose other than to create dimples or "goose bumps" in the skin.

The body is protected against the Sun's harmful UV radiation by melanin, produced by melanocytes in the epidermis. Melanin accumulates within the cells of the epidermis. It then absorbs UV radiation before that radiation can destroy the cells' DNA or deoxyribonucleic acid (large, complex molecules found in the nuclei of cells that carries genetic or hereditary information for an organism's development). Increased exposure to the Sun causes melanocytes to increase their production of melanin. The temporary result is that the skin becomes darker or tanned and is able to withstand further exposure to UV rays.

The protection afforded by melanin, however, is limited. Prolonged or excessive exposure to UV radiation eventually damages the skin. It causes elastic fibers in the dermis to clump, and the skin takes on a leathery appearance. Overexposure can also result in melanoma, a tumor composed of melanocytes.

Body temperature

Normal internal body temperature averages approximately 98.6°F (37°C). The heat-regulating functions of the body are extremely important. If the internal temperature varies more than a few degrees from normal, life-threatening changes take place in the body.

Eccrine glands play an important part in maintaining normal body temperature. When the temperature of the body rises due to physical exercise or environmental conditions, the hypothalamus (region of the brain containing many control centers for body functions and emotions) sends signals to the eccrine glands to secrete sweat. When sweat evaporates on the skin surface, it carries large amounts of body heat with it and the skin surface cools.

Because blood carries heat (a form of energy), blood flow is another regulator of body temperature. Under warm conditions, the hypothalamus signals blood vessels in the dermis to dilate or expand. This increases blood flow (and carries excess heat) to the body's surface. Like a radiator, the skin then gives off heat to the surrounding environment.

During cold conditions, the hypothalamus signals eccrine glands to stop secreting sweat. It also signals blood vessels in the dermis to constrict or close, which reduces blood flow to the skin surface. As a result, heat is kept within the core of the body.

Excretion and vitamin D formation

Excretion is a very minor function of the skin. Sweat does contain salt and urea (a compound produced when the liver breaks down amino acids), but the amounts of these wastes are slight. The kidneys are mainly responsible for removing waste products from the blood.

SANDBLASTING YOUR FACE

For years, workers have cleaned old stone and concrete structures by blasting their surfaces with a spray of fine sand. In the late 1990s, dermatologists and beauty salon owners in the United States began using a similar technique to remove the signs of aging on people's faces.

The new treatment, already used in Europe since the early 1990s, is called microdermabrasion. A machine blows tiny sterile sand crystals onto the skin of the face, then suctions them off. The crystals rub off the top layer of the skin, helping remove wrinkles.

The procedure is relatively painless and quick. However, its effects are not permanent, and it only removes fine lines. Deep lines around the mouth, crow's feet around the eyes, and deep lines on the forehead remain, although they are softened.

As explained earlier, too much sunlight is harmful to the body. A limited amount, however, is beneficial. In the lower layers of the epidermis, cells contain a form of cholesterol (fatlike substance produced by the liver that is an essential part of cell membranes and body chemicals). When exposed to UV radiation, that cholesterol changes into vitamin D, which the body uses to absorb calcium and phosphorus from food in the small intestine. Those two minerals are then used to build and maintain bones and teeth, among other functions.

Sensory reception

The main function of the sensory receptors in the dermis is to provide the brain with information about the external world and its effect on the skin. Thus, they alert the body to the possible tissue-damaging effects of extreme heat or cold or something that is pressing hard against the skin. They also transmit pleasant sensations, such as a gentle breeze blowing across the face or the soft caress of a loved one.

The receptors differ in their sensitivity. Touch receptors are the most sensitive, responding to the slightest contact. Found mainly in the fingers, tongue, and lips, they number about 500,000. Pain receptors, however, do not react unless the stimulus is strong enough. Located all over the body, pain receptors number between three and four million. Their high numbers indicate their importance to the body.

Receptors send their information to the brain to be interpreted. The brain then directs the body to respond, whether to remove itself from the situation or remain. Sensation, therefore, is a function of the brain and the nervous system.

AILMENTS: WHAT CAN GO WRONG WITH THE INTEGUMENTARY SYSTEM

Unlike some other body systems, the integumentary system quickly shows when it is afflicted by an aliment or malady. Over one thousand different aliments can affect the skin. The most common skin disorders are those caused by allergies or bacterial or fungal infections. Burns and skin cancers, although less common, are more dangerous. In some cases, they can be lethal.

INTEGUMENTARY SYSTEM DISORDERS

Acne (AK-nee): Disorder in which hair follicles of the skin become clogged and infected.

Athlete's foot: Common fungus infection in which the skin between the toes becomes itchy and sore, cracking and peeling away.

Basal cell carcinoma (BAY-sal CELL car-si-NO-ma): Skin cancer that affects the basal cells in the epidermis.

Carcinoma (car-si-NO-ma): Cancerous tumor of the skin, mucous membrane, or similar tissue of the body.

Dermatitis (der-ma-TIE-tis): Any inflammation of the skin.

Malignant melanoma (ma-LIG-nant mel-ah-NO-ma): Cancer of melanocytes; the most serious type of skin cancer.

Psoriasis (so-RYE-ah-sis): Chronic skin disease characterized by reddened lesions covered with dry, silvery scales.

Seborrheic dermatitis (seh-beh-REE-ik der-ma-TIE-tis): Commonly called seborrhea, a disease of the skin characterized by scaly lesions usually on the scalp, hairline, and face.

Squamous cell carcinoma (SKWA-mus CELL carsi-NO-ma): Skin cancer affecting the cells of the second deepest layer of the epidermis.

Vitiligo (vit-i-LIE-go): Skin disorder in which the loss of melanocytes results in patches of smooth, milky white skin.

Warts: Small growths caused by a viral infection of the skin or mucous membrane.

The following are just a few of the many ailments that can target the integumentary system.

Acne

Acne is a skin disease marked by pimples on the face, chest, and back. The most common skin disease, acne affects an estimated 17 to 28 million people in the United States. Although it can strike people at any age, acne usually begins at puberty and worsens during adolescence.

At puberty, increased levels of androgens (male hormones) cause the sebaceous glands to secrete an excessive amount of sebum into hair follicles. The excess sebum combines with dead, sticky skin cells to form a hard plug that blocks the follicle. Bacteria that normally lives on the skin then invades the blocked follicle. Weakened, the follicle bursts open, releasing the sebum, bacteria, skin cells, and white blood cells into the surrounding tissues. A pimple then forms.

Treatment for acne depends on whether the condition is mild, moderate, or severe. The goal is to reduce sebum production, remove dead skin cells, and kill skin bacteria. In very mild cases, keeping the skin clean by washing with a mild soap is recommended. In other cases, medications applied directly to the skin or taken orally may be prescribed in combination with gentle cleansing.

Athlete's foot

Athlete's foot is a common fungus infection in which the skin between the toes becomes itchy and sore, cracking and peeling away. Properly known as tinea pedis, the infection received its common name because the infectioncausing fungi grow well in warm, damp areas such as in and around swimming pools, showers, and locker rooms (areas commonly used by athletes).

The fungi that cause athlete's foot are unusual in that they live exclusively on dead body tissue (hair, the outer layer of skin, and nails). Researchers do not know exactly why some people develop the condition and others do not. It is known that sweaty feet, tight shoes, and the failure to dry feet well after swimming or bathing all contribute to the growth of the fungus.

Symptoms of athlete's foot include itchy, sore skin on the toes, with scaling, cracking, inflammation, and blisters. If the blisters break, raw patches of tissue may be exposed. If the infection spreads, itching and burning may increase.

Athlete's foot usually responds well to treatment. Simple cases are treated with antifungal creams or sprays. In more severe cases, an oral antifungal medication may be prescribed.

Burns

There are few threats more serious to the skin than burns. Burns are injuries to tissues caused by intense heat, electricity, UV radiation (sunburn), or certain chemicals (such as acids). When skin is burned and cells

are destroyed, the body readily loses its precious supply of fluids. Dehydration can follow, leading to a shutdown of the kidneys, a life-threatening condition. Infection of the dead tissue by bacteria and viruses occurs one to two days after skin has been burned. Infection is the leading cause of death in burn victims.

Burns are classified according to their severity or depth: first-, second-, or third-degree burns.

First-degree burns occur when only the epidermis is damaged. The burned area is painful, the outer skin is reddened, and slight swelling may be present. Sunburns are usually first-degree burns. Although they may cause discomfort, these minor burns are usually not serious and heal within a few days.

Second-degree burns occur when the epidermis and the upper region of the dermis are damaged. The burned area is red, painful, and may have a wet, shiny appearance because of exposed tissue. Blisters may form. These moderate burns take longer to heal. If the blisters are not broken and care is taken to prevent infection, the burned skin may regenerate or regrow without permanent scars.

Third-degree burns occur when the entire depth of skin is destroyed. Because nerve endings have been destroyed, the burned area has no sensitivity. The area may be blackened or gray-white in color. Muscle tissue and bone underneath may be damaged. In these serious to critical burns, regeneration of the skin is not possible. Skin grafting—taking a piece of skin from an unburned portion of the burn victim's body and transplanting it to the

burned area—must be done to cover the exposed tissues. Third-degree burns take weeks to heal and will leave permanent scarring.

Dermatitis

Dermatitis is any inflammation of the skin. There are many types of dermatitis and most are characterized by a pink or red rash that itches. Two common types are contact dermatitis and seborrheic dermatitis.

Contact dermatitis is an allergic reaction to something that irritates the skin. It usually appears within forty-eight hours after touching or brushing against a substance to which the skin is sensitive. The resin in poison ivy, poison oak, and poison sumac is the most common source of contact dermatitis. The skin of some people may also be irritated by certain flowers, herbs, and vegetables. Chemical irritants that can cause contact dermatitis include chlorine, cleaners, detergents and soaps, fabric softeners, perfumes, glues, and topical medications (those applied on the skin). Contact dermatitis can be treated with medicated creams or ointments and oral antihistamines and antibiotics.

ARTIFICIAL SKIN

Artificial skin, the synthetic or manmade equivalent of human skin, was first developed in the 1970s. Since then, the lives of many severely burned people have been saved through the use of artificial skin.

In the 1970s, John F. Burke, chief of trauma services at Massachusetts General Hospital in Boston, and Ioannis V. Yannas, chemistry professor at Massachusetts Institute of Technology in Cambridge, teamed up to develop some type of human skin replacement. In their research, the two men found that collagen fibers (protein found in human skin) and a long sugar molecule (called a polymer) could be combined to form a porous material that resembles skin. They then created a kind of artificial skin using polymers from shark cartilage and collagen from cowhide.

Burke and Yannas soon discovered that artificial skin acts like a framework onto which new skin tissue and blood vessels grow. As the new skin grows, the cowhide and shark substances from the artificial skin are broken down and absorbed by the body.

In 1979, Burke and Yannas used their artificial skin on their first patient, a woman who had suffered burns over half her body. After peeling away her burned skin, Burke applied a layer of artificial skin and, where possible, grafted or added on some of her own unburned skin. Three weeks later, the woman's new skin, the same color as her unburned skin, was growing at an amazingly healthy rate.

With continued research and development, synthetic skin may become a more common treatment for burns and other serious skin disorders.

Seborrheic dermatitis, known commonly as seborrhea, appears as red, inflamed skin covered by greasy or dry scales that may be white, yellow, or gray. These scaly lesions appear usually on the scalp, hairline, and face. Dandruff is a mild form of seborrheic dermatitis. Medical researchers do not know the exact cause of this skin disease. They believe that a high-fat diet, alcohol, stress, oily skin, infrequent shampooing, and weather extremes (hot or cold) may play some role. The disease may be treated with special shampoos that help soften and remove the scaly lesions. In more severe cases, medicated creams or shampoos containing coal tar may be prescribed.

Psoriasis

Psoriasis is a chronic (long-term) skin disease characterized by inflamed lesions with silvery-white scabs of dead skin. The disease affects roughly four million people in the United States, women slightly more than men. It is most common in fair-skinned people.

Normal skin cells mature and replace dead skin cells every twenty-eight to thirty days. Psoriasis causes skin cells to mature in less than a week. Because

the body cannot shed old skin as rapidly as new cells are rising to the surface, raised patches of dead skin develop. These patches are seen on the arms, back, chest, elbows, legs, folds between the buttocks, and scalp.

The cause of psoriasis is unknown. In some cases, it may be hereditary or inherited. Attacks of psoriasis can be triggered by injury or infection, stress, hormonal changes, exposure to cold temperature, or steroids and other medications.

The treatment for psoriasis depends on its severity. Steroid creams and ointments are commonly used to treat mild or moderate psoriasis. If the case is more severe, these medications may be used in conjunction with ultraviolet light B (UVB) treatments. Strong medications are reserved for those individuals suffering from extreme cases of psoriasis.

Skin cancer

Skin cancer is the growth of abnormal skin cells capable of invading and destroying other cells. Skin cancer is the single most common type of cancer in humans. The cause of most skin cancers or carcinomas is unknown, but overexposure to ultraviolet radiation in sunlight is a risk factor.

Basal cell carcinoma is the most common form of skin cancer, accounting for about 75 percent of cases. It is also the least malignant or cancerous (tending to grow and spread throughout the body). In this form of skin cancer, basal cells in the epidermis are altered so they no longer produce keratin. They also spread, invading the dermis and subcutaneous layer. Shiny, dome-shaped lesions develop most often on sunexposed areas of the face. The next most common areas affected are the ears, the backs of the hands, the shoulders, and the arms. When the lesion is removed surgically, 99 percent of patients recover fully.

Squamous cell carcinoma affects the cells of the second deepest layer of the epidermis. Like basal cell carcinoma, this type of skin cancer also involves skin exposed to the sun: face, ears, hands, and arms. The cancer presents itself as a small, scaling, raised bump on the skin with a crusting center. It grows rapidly and spreads to adjacent lymph nodes if not removed. If the lesion is caught early and removed surgically or through radiation, the patient has a good chance of recovering completely.

Malignant melanoma accounts for about 5 percent of all skin cancers, but it is the most serious type. It is a cancer of the melanocytes, cells in the lower epidermis that produce melanin. In their early stages, melanomas resemble moles. Soon, they appear as an expanding brown to black patch. In addition to invading surrounding tissues, the cancer spreads aggressively to other parts of the body, especially the lungs and liver. Overexposure to the Sun may be a cause of melanomas, but the greatest risk factor seems to be genetic. Early discovery of the melanoma is key to survival. The primary treatment for this skin cancer is the surgical removal of the tumor or diseased area of skin. When the melanoma has spread to other parts of the body, it is generally considered incurable.

Vitiligo

Vitiligo is a skin disorder in which the loss of melanocytes (cells that produce the color pigment melanin) results in patches of smooth, milky white skin. This often inherited disorder affects about 1 to 2 percent of the world's population. Although it is more easily observed in people with darker skin, it affects all races. It can begin at any age, but in 50 percent of the cases it starts before the age of twenty.

Medical researchers do not know the exact cause of the disorder. Some theorize that nerve endings in the skin may release a chemical that destroys melanocytes. Others believe that the melanocytes simply self-destruct. Still others think that vitiligo is a type of autoimmune disease, in which the body targets and destroys its own cells and tissues.

Vitiligo cannot be cured, but it can be managed. Cosmetics can be applied to blend the white areas with the surrounding normal skin. Sunscreens are useful to prevent the burning of affected areas and to prevent normal skin around the patches from becoming darker.

Warts

Warts are small growths caused by a viral infection of the skin or mucous membrane. The virus infects the surface layer. Warts are contagious. They can easily pass from person to person. They can also pass from one area of the body to another on the same person. Affecting about 7 to 10 percent of the population, warts are particularly common among children, young adults, and women. Common warts include hand warts, foot warts, and flat warts.

Hand warts grow around the nails, on the fingers, and on the backs of the hands. They appear mostly in areas where the skin is broken.

Foot warts (also called plantar warts) usually appear on the ball of the foot, the heel, or the flat part of the toes. Foot warts do not stick up above the surface like hand warts. If left untreated, they can grow in size and spread into clusters of several warts. If located on a pressure point of the foot, these warts can be painful.

Flat warts are smaller and smoother than other warts. They grow in great numbers and can erupt anywhere on the body. In children, they appear especially on the face.

Many nonprescription wart remedies are available that will remove simple warts from hands and fingers. Physicians use stronger chemical medications to treat warts that are larger or do not respond to over-the-counter treatments. Freezing warts with liquid nitrogen or burning them with an electric needle are advanced treatment methods.

TATTOOS: BODY ART OR MUTILATION?

Tattoos are relatively permanent marks or designs made on the skin. Tattoo comes from the Tahitian word tattau, meaning "to mark." The process of tattooing is accomplished by injecting colored pigment into small deep holes made in the skin. The modern method of tattooing employs an electric needle to inject the pigment.

People have been decorating their bodies with pictures of animals, flowers, supernatural creatures, and various designs for thousands of years. Egyptian mummies dating from 3035 b.c. have been discovered with ornate designs of flowers tattooed on their skin. Many ancient cultures believed that a tattoo of an animal could capture the mystical spirit of that animal and magically link the wearer to the animal depicted.

While many cultures have revered tattoos, many others have considered them vulgar and offensive. For as long as people have applied tattoos to their skin, they have sought ways to remove them.

In modern times, tattoos can be removed medically through one of four ways. If the tattoo is small, it can be surgically cut off and the skin sewn back together. In a method called dermabrasion, the tattoo is "sanded" with a rotary abrasive instrument until the layers of skin peel. Another method that uses abrasion is called salabrasion. In this procedure, which is centuries old, salt water is applied to the tattoo and then it is vigorously rubbed with some sort of sanding device until the tattoo pigments are dispersed. All three of these methods leave some sort of scarring, but the last method, laser surgery, does not. Pulses of light from a laser are directed onto the tattoo, breaking up its pigments. The pigments are then removed over the next few weeks by the body's defense cells.

TAKING CARE: KEEPING THE INTEGUMENTARY SYSTEM HEALTHY

As people age, dramatic changes take place in the integumentary system. The epidermis thins as basal cells divide less and less. The dermis also thins and its elastic fibers decrease in size. As a result, the skin becomes weaker and starts to sag, forming wrinkles. Melanocytes decrease production of melanin, and the skin becomes pale and hair turns white. Sebaceous glands also decrease production of sebum, causing the skin to become dry and scaly. Blood supply to the skin is reduced and body temperature cannot be regulated as well. Finally, the skin takes longer and longer to repair itself.

Although there is no way to avoid aging of the skin, there are ways to decrease the effects of aging. The loss of elasticity in the skin is speeded up by sunlight. The skin should be shielded from the Sun through the use of sunscreens, sunblocks, and protective clothing. Sunburns are never healthy and should always be avoided. This will also help reduce the risk of skin cancer.

As in all other body systems, the following play a part in keeping the integumentary system operating at peak efficiency: proper nutrition, healthy amounts of good-quality drinking water, adequate rest, regular exercise, and stress reduction. Hair loss and graying are both genetically controlled, but stress can add to both conditions. Exercise and relaxation techniques are proven ways to reduce stress.

Proper daily cleansing of the skin is highly recommended. However, harsh detergents and scrubbing will not make the skin cleaner. In fact, they can injure the skin and cause excessive drying. Greater benefits can be gained by cleaning the skin with gentle soaps or lotions, then applying an appropriate moisturizer to all areas of the body.

FOR MORE INFORMATION

Books

Balin, Arthur K., Loretta Pratt Balin, and Marietta Whittlesly. The Life of the Skin. New York: Bantam Books, 1997.

Brynie, Faith Hickman. 101 Questions About Your Skin That Got Under Your Skin Until Now. Brookfield, CT: Millbrook Press, 1999.

Kenet, Barney J., and Patricia Lawler. Saving Your Skin: Prevention, Early Detection, and Treatment of Melanoma and Other Skin Cancers. New York: Four Walls Eight Windows, 1998.

Silverstein, Alvin, Robert Silverstein, and Virginia B. Silverstein. Overcoming Acne: The How and Why of Healthy Skin Care. New York: Morrow, 1980.

Turkington, Carol A., and Jeffrey S. Dover. Skin Deep: An A-Z of Skin Disorders, Treatments, and Health. Updated edition. New York: Facts on File, 1998.

WWW Sites

AcneNet
http://www.derm-infonet.com/acnenet
Site developed by Roche Laboratories in association with the American Academy of Dermatology presents a comprehensive online acne information resource, including basic facts, such as how and why acne occurs and various treatments.

American Academy of Dermatology
http://www.aad.org
Homepage of the American Academy of Dermatology.

Integumentary System
http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookINTEGUSYS.html
Site presents a detailed chapter on the integumentary system—including follicles and glands, hair and nails, and skin and sensory reception—from the On-Line Biology textbook.

Integumentary System
http://www.wellweb.com/index/QINTEGUMEN.HTM
Site presents a detailed overview of the main parts of the integumentary system.

Integumentary System Color Images
http://www.udel.edu/Biology/Wags/histopage/colorpage/cin/cin.htm
Site provides links to twenty-six color images of various parts of the integumentary system, such as the a nail bed, hair shaft and follicle, and sebaceous gland.

Introduction to Skin Cancer
http://www.maui.net/~southsky/introto.html
Site contains link to the causes of skin cancer, the effects of heredity and environment, diagnosis and treatment information, and a glossary of terms.

Skin and Connective Tissue Diseases
http://www.mic.ki.se/Diseases/c17.html
Site compiled by the Karolinska Institutet (Sweden) presents an enormous set of links to sites focusing on various skin, nail, and hair diseases.