The skin’s outermost portion is the epidermis, consisting of four to five layers (strata) of epithelial cells. The deepest epidermal layer, the stratum basale, or basal layer, produces new cells. As these cells gradually rise toward the surface, they die and become filled with keratin, a protein that thickens and toughens the skin. The outermost epidermal layer, the stratum corneum or horny layer, is composed of flat, dead, protective cells that are constantly being shed and replaced. Some of the cells in the epidermis produce melanin, a pigment that gives the skin color and protects against sunlight. Irregular spots of melanin form freckles.

The dermis is beneath the epidermis. It contains connective tissue, nerves, blood vessels, lymphatics, and sensory receptors. This layer supplies nourishment and support for the skin. The subcutaneous layer beneath the dermis is composed mainly of connective tissue and fat.

Specialized structures within the skin are part of the integumentary system:

• The sweat (sudoriferous) glands act mainly in temperature regulation by releasing a watery fluid that evaporates to cool the body.
• The sebaceous glands release an oily fluid, sebum, that lubricates the hair and skin and prevents drying.
• Hair is widely distributed over the body. Each hair develops within a sheath or hair follicle and grows from its base within the skin’s deep layers. A small muscle
  (arrector pili) attached to the follicle raises the hair to produce “goosebumps” when one is frightened or cold. In animals this is a warning sign and a means of insulation.
• Nails develop from a growing region at the proximal end. The cuticle, technically named the eponychium (ep-o-NIK-e-um), is an extension of the epidermis onto the surface of the nail plate. A lighter region distal to the cuticle is called the lunula because it looks like a half moon. Here the underlying skin is thicker, and blood does not show as much through the nail.
• Hair and nails are composed of nonliving material consisting mainly of keratin. Both function in protection.

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Key Terms: Normal Structure and Function

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Many diseases are manifested by changes in the quality of the skin or by specific lesions. Some types of skin lesions are below. The study of the skin and skin diseases is dermatology, but careful observation of the skin, hair, and nails should be part of every physical examination. The skin should be examined for color, unusual pigmentation, and lesions. It should be palpated to evaluate its texture, temperature, moisture, firmness, and any tenderness.

Types of Skin Lesions

Wounds

Wounds are caused by trauma, as in cases of accidents or attacks, or by surgery and other therapeutic or diagnostic procedures. Wounds may affect not only the injured area but also other body systems. Infection and hemorrhage may complicate wounds, as do dehiscence, disruption of the wound layers, and evisceration, protrusion of internal organs through the lesion.

As a wound heals, fluid and cells drain from the damaged tissue. This drainage, called exudate, may be clear, bloody (sanguinous), or pus-containing (purulent). Tubes may be used to remove exudate from the site of a wound.

Proper wound healing depends on cleanliness and care of the lesion and also on proper circulation, good general health, and good nutrition. The edges of a deep wound should be joined by sutures, either stitches or, for simple cuts in areas that can be kept dry and immobilized, with a tissue adhesive (glue). Healing is accompanied by scar formation or cicatrization (an alternative name for a scar is a cicatrix). Permanent scarring is lessened by appropriate wound care, but some people, especially those of African or Asian descent, may tend to form keloids because of excess collagen formation during healing. Plastic surgery can often improve keloids and other scars.

Various types of dressings are used to protect wounded areas and promote healing. Vacuum-assisted closure (VAC) uses negative pressure to close the tissues and begin the healing process. Healing may be promoted by debridement, the removal of dead or damaged tissue from a wound. Debridement may be accomplished by cutting or scrubbing away the dead tissue or by means of enzymes. A thick, dark crust or scab (eschar) may be removed in an escharotomy.

Deep wounds may require skin grafting for proper healing. Grafts may be a full-thickness skin graft (FTSG), which consists of the epidermis and dermis, or a split-thickness skin graft (STSG), consisting of the epidermis only. Skin is cut for grafting with a dermatome.

Burns

Most burns are caused by hot objects, explosions, or scalding with hot liquids. They may also be caused by electricity, contact with harmful chemicals, or abrasion. Sunlight can also cause severe burns that may result in serious illness. Burns are assessed in terms of the depth of damage and the percentage of body surface area (BSA) involved. Depth of tissue destruction is categorized as follows:

• Superficial—involves the epidermis only. The skin is red and dry; there is minimal pain. Typical causes are mild sunburn and very short heat exposure. This type of burn is also called a first-degree burn. The skin will whiten (blanch) when pressed and then turn bright red again when released.
• Superficial partial thickness—involves the epidermis and a superficial portion of the dermis. The tissue reddens and blisters and is painful, as in cases of severe sunburn or scalding.
• Deep partial thickness—involves the epidermis and both superficial and deeper regions of the dermis. The tissue may be blistered with a weeping surface or dry because of sweat gland damage. These burns may be less painful than superficial burns because of nerve damage. The blisters may fill with blood due to dermal capillary damage. Causes include scalding and exposure to flame or hot grease. Superficial and deep partial thickness burns are also classified as second-degree burns.
• Full thickness—involves the full skin and sometimes subcutaneous tissue and underlying tissues as well. The tissue is broken, dry and pale, or charred. These injuries may require skin grafting and may result in loss of digits or limbs. Full-thickness burns are also classified as third-degree or fourth-degree burns.

The amount of BSA involved in a burn may be estimated by using the rule of nines, in which areas of body surface are assigned percentages in multiples of nine. The more accurate Lund and Browder method divides the body into small areas and estimates the proportion of BSA contributed by each.

Infection is a common complication of burns because a person’s major defense against bacterial invasion is damaged. Respiratory complications and shock may also occur.

Treatment of burns includes respiratory care, administration of fluids, wound care, and pain control. Monitoring for cardiovascular complications, infections, and signs of posttraumatic stress is also important.

Pressure Ulcers

Pressure ulcers are necrotic skin lesions that appear where the body rests on skin that covers bony projections, such as the sacrum, heel, elbow, ischial bone of the pelvis, or greater trochanter of the femur. The pressure interrupts circulation, leading to thrombosis, ulceration, and tissue death (necrosis). Poor general health, malnutrition, age, obesity, and infection contribute to the development of pressure ulcers.

Pressure ulcer lesions first appear as redness of the skin. If ignored, they may penetrate the skin and underlying muscle, extending even to bone, and may require months to heal.

Pads or mattresses to relieve pressure, regular cleansing and drying of the skin, frequent change in position, and good nutrition help to prevent pressure ulcers. Other terms for pressure ulcers are decubitus ulcer and bedsore. Both of these terms refer to lying down in bed, although pressure ulcers may appear in anyone with limited movement, not only those who are confined to bed.

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Dermatitis

Dermatitis is a general term for inflammation of the skin, which may be acute or chronic. Mild forms show erythema (redness) and edema and sometimes pruritus (itching), but the condition may worsen to include deeper lesions and secondary bacterial infections. A chronic allergic form of this disorder that appears early in childhood is called atopic dermatitis or eczema. Although its exact cause is unknown, atopic dermatitis is made worse by allergies, infection, temperature extremes, and skin irritants. Patients have tried a variety of treatments for this disorder over the years. Clinicians are currently studying biologic drugs targeting specific immune system components that are overproduced in cases of eczema and other allergic disorders.

Other forms of dermatitis include contact dermatitis, caused by allergens or chemical irritants; seborrheic dermatitis, which involves areas with many sebaceous glands, such as the scalp and face; and stasis dermatitis, caused by poor circulation.

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Psoriasis

Psoriasis is a chronic overgrowth (hyperplasia) of the epidermis, producing large, erythematous (red) plaques with silvery scales. The cause is unknown, but there is sometimes a hereditary pattern, and autoimmunity may be involved.

Dermatologists treat psoriasis in the following ways depending on severity:

• Topical agents, including corticosteroids, immunosuppressants, vitamins A and D
• Phototherapy—exposure to ultraviolet B (UVB) light; administration of the drug psoralen (P) to increase skin sensitivity to light followed by exposure to ultraviolet A (UVA) light; laser treatment
• Systemic suppression of the immune system

Autoimmune Disorders

The diseases discussed below are caused, at least in part, by autoimmune reactions. They are diagnosed by biopsy of lesions and by antibody studies.

Pemphigus is characterized by the formation of bullae (blisters) in the skin and mucous membranes caused by a separation of epidermal cells from underlying layers. Rupture of these lesions leaves deeper skin areas unprotected from infection and fluid loss, much as in cases of burns. The cause is an autoimmune reaction to epithelial cells. Pemphigus is fatal unless treated by suppressing the immune system.

Lupus erythematosus (LE) is a chronic inflammatory autoimmune disease of connective tissue. The more widespread form of the disease, systemic lupus erythematosus (SLE), involves the skin and other organs. SLE is more prevalent in women than in men and has a higher incidence among Asian and Black patients than among other populations.

Discoid lupus erythematosus (DLE) involves only the skin. It is seen as rough, raised, erythematous papules that are worsened by exposure to the ultraviolet radiation in sunlight. Lupus skin lesions are confined to the face and scalp and may form a typical butterfly-shaped rash across the nose and cheeks.

Scleroderma is a disease of unknown cause that involves thickening and tightening of the skin. There is gradual fibrosis of the dermis because of collagen overproduction. Sweat glands and hair follicles are also involved. A very early sign of scleroderma is Raynaud disease, in which blood vessels in the fingers and toes constrict in the cold, causing numbness, pain, coldness, and tingling. Skin symptoms first appear on the forearms and around the mouth. Internal organs become involved in a diffuse form of scleroderma called progressive systemic sclerosis (PSS).

Skin Cancer

Skin cancer is the most common type of human cancer. Its incidence has been increasing in recent years, mainly because of the mutation-causing effects of sunlight’s ultraviolet rays. Basal cell carcinoma and squamous cell carcinoma are both cancers of epithelial cells. Both appear in areas exposed to sunlight, such as the face and hands. Basal cell carcinoma constitutes more than 75% of all skin cancers. It usually appears as a smooth, pearly papule. Because these cancers are easily seen and do not metastasize, the cure rate after excision is greater than 95%.

Squamous cell carcinoma appears as a painless, firm, red nodule or plaque that may develop surface scales, ulceration, or crusting. This cancer may invade underlying tissue but tends not to metastasize. It is treated by surgical removal and sometimes with x-irradiation or chemotherapy.

Melanoma results from a malignant overgrowth of melanocytes, the pigment-producing cells in the epidermis. It can arise anywhere in the body where melanocytes are found and is the most dangerous form of skin cancer because of its tendency to metastasize. This cancer appears as a lesion that is variable in color with an irregular border. A melanoma shows the so-called ABCDE factors:

• asymmetry
• uneven borders
• different colors
• diameter of more than 6 millimeters
• and evolution (change)

The tumor may spread superficially for up to 1 or 2 years before it begins to invade the deeper skin tissues and to metastasize through blood and lymph. Predisposing factors for melanoma include severe sunburn and frequent tanning. The prognosis for cure is good if the lesion is recognized and removed surgically before it enters this invasive stage.

Kaposi sarcoma, once considered rare, is now seen frequently in association with AIDS. It usually appears as distinct brownish areas on the legs. These plaques become raised and firm as the tumor progresses. In those with weakened immune systems, such as patients with AIDS, the cancer can metastasize.

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Key Terms: Clinical Aspects

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Key Terms: Symptoms and Conditions

Key Terms: Symptoms and Conditions

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Key Terms: Symptoms and Conditions

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Key Terms: Symptoms and Conditions

Key Terms: Diagnosis and Treatment

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Abbreviations