Skin. I INTRODUCTION Skin, outer body covering of an animal. The term skin is commonly used to describe the body covering of any animal but technically refers only to the body covering of vertebrates (animals that have a backbone). The skin has the same basic structure in all vertebrates, including fish, reptiles, birds, and humans and other mammals. This article focuses primarily on human skin. The skin is essential to a person's survival. It forms a barrier that helps prevent harmful microorganisms and chemicals from entering the body, and it also prevents the loss of life-sustaining body fluids. It protects the vital structures inside the body from injury and from the potentially damaging ultraviolet rays of the sun. The skin also helps regulate body temperature, excretes some waste products, and is an important sensory organ. It contains various types of specialized nerve cells responsible for the sense of touch. The skin is the body's largest organ--that of an average adult male weighs 4.5 to 5 kg (10 to 11 lb) and measures about 2 sq m (22 sq ft) in area. It covers the surface of the body at a thickness of just 1.4 to 4.0 mm (0.06 to 0.16 in). The skin is thickest on areas of the body that regularly rub against objects, such as the palms of the hands and the soles of the feet. Both delicate and resilient, the skin constantly renews itself and has a remarkable ability to repair itself after injury. II STRUCTURE OF THE SKIN The skin is made up of two layers, the epidermis and the dermis. The epidermis, the upper or outer layer of the skin, is a tough, waterproof, protective layer. The dermis, or inner layer, is thicker than the epidermis and gives the skin its strength and elasticity. The two layers of the skin are anchored to one another by a thin but complex layer of tissue, known as the basement membrane. This tissue is composed of a series of elaborately interconnecting molecules that act as ropes and grappling hooks to hold the skin together. Below the dermis is the subcutaneous layer, a layer of tissue composed of protein fibers and adipose tissue (fat). Although not part of the skin itself, the subcutaneous layer contains glands and other skin structures, as well as sensory receptors involved in the sense of touch. A Epidermis About 90 percent of the cells in the epidermis are keratinocytes, named because they produce a tough, fibrous protein called keratin. This protein is the main structural protein of the epidermis, and it provides many of the skin's protective properties. Keratinocytes in the epidermis are arranged in layers, with the youngest cells in the lower layers and the oldest cells in the upper layers. The old keratinocytes at the surface of the skin constantly slough off. Meanwhile, cells in the lower layers of the epidermis divide continually, producing new keratinocytes to replace those that have sloughed off. As keratinocytes push up through the layers of the epidermis, they age and, in the process, produce keratin. By the time the cells reach the uppermost layer of the epidermis, they are dead and completely filled with the tough protein. Healthy epidermis replaces itself in a neatly orchestrated way every month. Scattered among the keratinocytes in the epidermis are melanocytes, cells that produce a dark pigment called melanin. This pigment gives color to the skin and protects it from the sun's ultraviolet rays. After being produced in the melanocytes, packets of melanin called melanosomes transfer to the keratinocytes. There they are arranged to protect the deoxyribonucleic acid (DNA), or genetic material, of the keratinocytes. All people have roughly the same number of melanocytes. Differences in skin color, such as that between light-skinned people of European descent and dark-skinned people of African descent, result from differences in the amount of melanin produced and how melanosomes are arranged in the keratinocytes. Particularly in people with light skin, melanin sometimes accumulates in patches, forming freckles, age spots, or liver spots. In people of almost any skin tone, exposure to the sun causes melanocytes to increase their production of melanin to help protect the skin. This process results in a darkening of the skin tone to form a suntan. The suntan fades when keratinocytes containing the extra melanin are sloughed off. A type of melanin called pheomelanin makes redheaded people more sensitive to the sun. A total lack of melanin, a genetic condition called albinism, makes people extremely sensitive to the sun. People with albinism can be members of any race and have very light skin, hair, and eyes. The epidermis also contains a type of immune cell known as a Langerhans cell. Produced in the bone marrow, Langerhans cells take up sentrylike positions in the epidermis, where they help cells of the immune system recognize potentially dangerous microorganisms and chemicals. Another cell in the epidermis is the Merkel cell, found in sensitive, hairless areas such as the fingertips and lips. Located in the deepest layer of the epidermis, Merkel cells contact nerve endings in the dermis below and function as a type of touch receptor. B Dermis Unlike the epidermis, the dermis or lower layer of the skin is richly supplied with blood vessels and sensory nerve endings. The dermis also contains relatively few cells compared to the epidermis--instead, it is made up mainly of fibrous proteins and other large molecules. The main structural component of the dermis is a protein called collagen. Bundles of collagen molecules pack together throughout the dermis, accounting for threefourths of the dry weight of skin. Collagen is also responsible for the skin's strength. Another protein in the dermis, elastin, is the main component of elastic fibers. These protein bundles give skin its elasticity--the ability to return to its original shape after stretching. Collagen and elastin are produced by cells called fibroblasts, which are found scattered throughout the dermis. The upper part of the dermis is known as the papillary layer. It is characterized by dermal papillae, tiny, fingerlike projections of tissue that indent into the epidermis above. In the thick skin on the palms and soles, the epidermis conforms to the shape of the underlying dermal papillae, forming ridges and valleys that we know as fingerprints. These ridges provide traction that helps people grasp objects and surfaces. Some dermal papillae contain touch receptors called Meissner's corpuscles, and many contain loops of tiny blood vessels. The extensive network of blood vessels in the dermal papillae plays an important role in the regulation of body temperature. The blood vessels dilate in hot environments to help dissipate heat, and they constrict to conserve heat in cold environments. Approximately one-fourth of the body's blood flows through the skin at any given time. The lower layer of the dermis is called the reticular layer. It is made primarily of coarse collagen and elastic fibers. Skin appendages such as glands and hair follicles are often anchored in the reticular layer of the dermis. The reticular layer also contains several different types of sensory receptors, nerve cells specialized to detect various stimuli, including pain, heat, cold, itch, and pressure. For example, Pacinian corpuscles are receptors found in the deep dermis of weight-bearing surfaces, such as the soles of the feet. They are composed of concentric layers of cells, much like an onion, and transmit vibrational stimuli. Sensory receptors are more dense in hairless areas, such as the fingertips and lips, making these areas especially sensitive. III SKIN APPENDAGES In humans, the skin appendages, or structures embedded in the skin, include hair, nails, and several types of glands. Glands are groups of cells that produce and secrete substances needed by other parts of the body. In other vertebrates, the skin appendages include scales (in fish and reptiles) and feathers (in birds). Together, the skin and the skin appendages are known as the integumentary system of the body. A Hair Hair is a distinguishing characteristic of mammals, a group of vertebrates that includes humans. A thick coat of body hair known as fur protects many mammals from the cold and from the sun's ultraviolet rays. In humans, a species whose body hair is relatively sparse, this protective function is probably minimal, limited chiefly to the hair on the scalp. Hair is composed primarily of keratin. The protein is packed into dead keratinocytes, much like those found in the upper layers of the epidermis. The dead keratinocytes fuse together to form the hair. The portion of the hair above the skin is known as the shaft, while that below the surface of the skin is known as the root. Each hair grows from its own follicle, an indentation of the epidermis. At the base of the follicle is the bulb, which contains cells that give rise to the keratinocytes that make up the hair, as well as blood vessels that nourish the growing hair. Hair on the scalp typically grows at a rate of 1 mm (0.04 in) every three days. Each hair follicle also contains the arrector pili, a muscle that contracts in response to cold, fright, and other emotions. When the muscle contracts, it pulls the hair in the follicle into a vertical position. This response may help some mammals keep warm or look bigger to frighten or intimidate their enemies. But in humans, again because of our sparse coat of body hair, it merely produces "goose bumps." The color of hair is due to melanin, produced by melanocytes in the bulb of the hair follicle and then incorporated into the keratinocytes that form the hair. Dark hair contains true melanin like that found in the skin, while blond and red hair result from types of melanin that contain sulfur and iron. Hair goes gray when melanocytes age and lose the enzyme necessary to produce melanin. White hair occurs when air bubbles become incorporated into the growing hair. The texture of hair results from the shape of the hair shaft. Straight hair appears round in cross section, wavy hair has an oval shape in cross section, and the cross section of a curly hair has an elliptical or kidney-shaped appearance. B Nails Nails on the fingers and toes are made of hard, keratin-filled epidermal cells. They protect the ends of the digits from injury, help us grasp small objects, and enable us to scratch. The part of the nail that is visible is called the nail body, and the portion of the nail body that extends past the end of the digit is called the free edge. Most of the nail body appears pink because of blood flowing in the tissue underneath, but at the base of the body is a pale, semicircular area called the lunula. This area appears white due to an underlying thick layer of epidermis that does not contain blood vessels. The part of the nail that is buried under the skin is called the root. Nails grow as epidermal cells below the nail root and transform into hard nail cells that accumulate at the base of the nail, pushing the rest of the nail forward. Fingernails typically grow 1 mm (0.04 in) per week. Toenails generally grow more slowly. C Glands An adult human has between 1.6 million and 4 million sudoriferous glands, or sweat glands. Most are of a type known as eccrine sweat glands, which are found almost all over the surface of the body and are most numerous on the palms and soles. Eccrine sweat glands begin deep in the dermis and connect to the surface of the skin by a coiled duct. Cells at the base of the gland secrete sweat, a mixture of water, salt, and small amounts of metabolic waste products. As the sweat moves along the duct, much of the salt is reabsorbed, preventing excessive loss of this vital substance. When sweat reaches the outer surface of the skin, it evaporates, helping to cool the body in hot environments or during physical exertion. In addition, nerve fibers that encircle the sweat glands stimulate the glands in response to fear, excitement, or anxiety. The sweat glands can secrete up to 10 liters (2.6 gallons) of fluid per day, far more than any other type of gland in the body. Other sweat glands, known as apocrine sweat glands, are much less numerous than eccrine sweat glands. Apocrine sweat glands are located mainly in the armpit, genital area, and around the nipples of the breasts. Apocrine sweat glands are also anchored deep in the dermis, but they open into hair follicles rather than onto the surface of the skin. They secrete a limited amount of a milky fluid--thicker than the secretion of the eccrine sweat glands--that is thought to be involved in sexual attraction in many mammal species. Oil, or sebaceous, glands are found all over the body except on the palms, the soles, and the top of the feet. They are most numerous on the face and scalp. Most sebaceous glands open into hair follicles, but the glands also occur in some hairless areas, such as the lips and inside the mouth. Glands of this type produce an oily substance called sebum, which keeps the skin and hair from drying out and inhibits the growth of certain harmful bacteria. Wax, or ceruminous, glands are located in the ear canal. They secrete a waxy substance that helps prevent foreign particles from entering the ear. Ceruminous glands are modified sweat glands. IV INJURY AND REPAIR The skin's thinness and position on the outside of the body make it vulnerable to injury, but the skin has a remarkable ability to repair itself. The process of wound healing depends on how deep the wound is--whether just the superficial epidermis or both epidermis and dermis are involved. When a wound, such as a scraped knee or elbow, affects only the epidermis, healing can occur very rapidly, often within a day or two. Epidermal cells at the edge of the wound break free from the surrounding cells, enlarge, and migrate into the wound. Meanwhile, the remaining epidermal cells divide to replace those that have left. Cells advance across the wound from all sides and finally meet in the center, covering the wound. The cells then divide to form all the layers of the epidermis, until the skin over the wound reaches normal thickness. A deeper wound affecting both epidermis and dermis requires a longer, more complex process of healing. First, a clot forms to stanch the flow of blood from the wound. White blood cells of the immune system enter the area to consume dead and dying cells, microbes, and any other foreign material that may have entered the wound. During this stage of healing, cells that will develop into fibroblasts also enter the wound. Next, the blood clot hardens into a scab, and epidermal cells migrate beneath the scab to form a continuous sheet of cells over the wound. Fibroblasts begin to deposit collagen and other molecules, repairing the dermis. Finally, when normal skin has been restored, the scab drops off. Often during repair of a deep wound, collagen fibers and other structural molecules of the skin are arranged more densely than in normal tissue, forming a scar. Scar tissue has fewer blood vessels than normal skin and sometimes lacks hair, glands, and nerve cells. Occasionally the dermis alone is injured without injury to the overlying epidermis. When the skin stretches, as sometimes happens during pregnancy or rapid weight gain, small tears may occur in the protein fibers that make up the dermis. This process results in striae, or stretch marks, which are initially red and eventually fade to a silvery white after they heal. A burn is an injury to the skin caused when heat, chemicals, electricity, or radiation destroy tissue. Extensive burns, involving 30 percent or more of the body's surface, can be life-threatening because they disrupt the skin's ability to fight infection, prevent fluid loss, and regulate body temperature. One measure of the severity of a burn is how deep it is. A first-degree burn affects only the epidermis and causes pain and redness in the burned area, but usually heals within a few days and does not cause scarring. A typical sunburn is a first-degree burn. A second-degree burn affects all layers of the epidermis and often part of the dermis, while a third-degree burn affects the full thickness of both epidermis and dermis, and often the skin appendages, such as hair and glands, as well. Secondand third-degree burns require much longer to heal and usually lead to scarring. Deep burns are often treated with skin grafts. In this procedure, healthy skin removed from another area on the body is transferred to the burned area to cover the wound. If the burned area is not too large, the grafted skin will grow in its new location and eventually fuse with the healthy skin at the edges of the burn. But if burns cover a large portion of a patient's body and not enough healthy skin remains to use for a graft, patients may receive grafts from dead bodies or even other animal species, such as cows. Although the patient's immune system eventually rejects and destroys this foreign tissue, these temporary skin grafts help prevent fluid loss and infection while the patient's own skin heals. Recent advances in tissue culture have enabled patients who have extensive burns to receive grafts grown from their own skin. Using these new tissue culture techniques, scientists remove a very small piece of healthy skin from the patient's body. They grow that tissue in the laboratory for several weeks to produce large sheets of epidermal cells. Doctors then transfer these cells to the burned area of the patient's body. If the patient remains free of infection and the loss of body fluids is controlled, these skin grafts mature into functional skin. These techniques have improved survival rates for severely burned patients and have enabled these patients to heal more rapidly, with less pain and scarring. V DISEASES AND DISORDERS OF THE SKIN Any component of the skin can become involved in a variety of different diseases. The skin can often provide doctors with clues about the health of the body, since many diseases that affect other organ systems are evident as telltale clues on the skin. For example, a rash, such as that typical of measles or chicken pox, can indicate an infection that affects the whole body. A blue tinge to the skin, called cyanosis, means that the body is not getting enough oxygen and can indicate heart failure. A yellow tinge to the skin, known as jaundice, signals that the liver is not working properly. Other diseases are specific to the skin itself. These disorders range from the merely annoying or distressing, such as acne, to the potentially life-threatening, such as skin cancer. A Acne Acne occurs when a hair follicle becomes blocked, usually by keratin-containing dead cells, preventing sebum from reaching the surface of the skin. Bacteria that normally live in the hair follicle break down the sebum behind the blockage, producing chemicals that cause inflammation in the surrounding skin. The result of this inflammation is a pimple. Acne tends to occur during the teenage years because hormones that increase at puberty promote both keratin formation and sebum production. Acne can be treated with antibiotics that kill the bacteria in hair follicles, with medicines that decrease sebum production, or with chemical applications that remove the hair follicle blockage. B Eczema and Hives In response to various triggering substances, or allergens, immune cells known as mast cells in the skin may release a chemical called histamine. Other immune cells called T cells may directly damage the epidermis. These events can result in either of two skin disorders, eczema or hives. Eczema is a red, scaly rash that commonly occurs in body folds such as in front of the elbow, behind the knee, and around the groin. It can usually be controlled with oral antihistamines and cortisone creams. Hives, also called urticaria, are red, raised, weltlike lesions on the skin, often occurring on the face and neck. Hives are often triggered by ingestion or inhalation of a substance, such as a medication or food, to which a person is allergic. Therefore, the most effective way of controlling hives is to recognize and avoid future contact with the allergen. Occasionally, hives may be an indication of a more serious, even life-threatening allergic reaction. In such situations, the airways can become constricted, making breathing difficult, and rapid intervention with injections of adrenaline may be required. C Skin Cancer Any type of cell present in the skin can become cancerous. Two types of cancers of keratinocytes, squamous cell carcinoma and basal cell carcinoma, are together the most commonly diagnosed cancers in the United States and Canada. These cancers can usually be cured by surgical removal. A far more dangerous cancer is melanoma, a cancer that develops from melanocytes. When diagnosed early, it can often be surgically cured, but this form of cancer may spread rapidly, or metastasize, to the internal organs and can be fatal within months of diagnosis. Although less common than cancers of the keratinocytes, the number of cases of melanoma is increasing faster than cases of other types of cancer in the United States and Canada (see Skin Cancer). All of these types of skin cancer are related to sun exposure. Scientists believe that the sun's ultraviolet rays damage the DNA of the skin cells, eventually turning them cancerous. Skin cancer develops most commonly on sun-exposed areas, such as the face, hands, arms, and legs. People who have light skin that sunburns easily are at higher risk of skin cancer, as are people who have a history of significant sun exposure, particularly those who regularly sunbathe or those who work outside without protective clothing, such as lifeguards. However, several decades may elapse between sun exposure and the development of skin cancer--someone who was a lifeguard at age 20 may not develop skin cancer until age 50, for example. Moles or other darkly pigmented areas of the skin that change appearance by enlarging, bleeding, or developing irregular borders or coloring may be a sign of cancer. Doctors recommend that people at risk for skin cancer regularly self-examine their body for any skin changes. Whatever their skin tone, people can reduce their risk of developing skin cancer by wearing clothing that covers the body and a hat that shields the face when in the sun and by using a sunscreen with a sun protection factor (SPF) of 15 or greater. Contributed By: Richard L. Edelson Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
