Disorders of the Skin
The skin is subject to many disorders, some of which are more annoying than life-threatening. For example, athlete’s foot is caused by a fungal infection that usually involves the skin of the toes and soles. Impetigo is a highly contagious disease occurring most often in young children. It is caused by a bacterial infection that results in pustules that crust over. Psoriasis is a chronic condition, possibly hereditary, in which the skin develops pink or reddish patches covered by silvery scales due to overactive cell division. Eczema, an inflammation of the skin, is caused by sensitivity to various chemicals (e.g., soaps or detergents), to certain fabrics, or even to heat or dryness. Dandruff is a skin disorder not caused by a dry scalp, as is commonly thought, but by an accelerated rate of keratinization in certain areas of the scalp, producing flaking and itching. Urticaria, or hives, is an allergic reaction characterized by the appearance of reddish, elevated patches and often by itching.
Skin cancer is categorized as either melanoma or nonmelanoma. Nonmelanoma cancers, which include basal cell carcinoma and squamous cell carcinoma, are much less likely to metastasize than melanoma cancer. Basal cell carcinoma (Fig. 5.6a), the most common type of skin cancer, begins when ultraviolet (UV) radiation causes epidermal basal cells to form a tumor, while at the same time suppressing the immune system’s ability to detect the tumor. The signs of a tumor are varied.
They include an open sore that will not heal; a recurring reddish patch; a smooth, circular growth with a raised edge; a shiny bump; or a pale mark. About 95% of patients are easily cured by surgical removal of the tumor, but recurrence is common.
Squamous cell carcinoma (Fig. 5.6b) begins in the epidermis proper. While five times less common than basal cell carcinoma, it is more likely to spread to nearby organs, and death occurs in about 1% of cases. The signs of squamous cell carcinoma are the same as those for basal cell carcinoma, except that it may also show itself as a wart that bleeds and scabs.
Melanoma (Fig. 5.6c), the type that is more likely to be malignant, starts in the melanocytes and has the appearance of an unusual mole. Unlike a normal mole, which is dark, circular, and confined, a melanoma mole looks like a spilled ink spot, and a single melanoma mole may display a variety of shades. A melanoma mole can also itch, hurt, or feel numb. The skin around the mole turns gray, white, or red. Melanoma is most common in fair-skinned persons, particularly if they have suffered occasional severe burns as children. Melanoma risk increases with the number of moles a person has. Most moles appear before the age of 14, and their appearance is linked to sun exposure. Melanoma rates have risen since the turn of the century, but the incidence has doubled in the last decade. In 2002, about 54,000 cases of melanoma were diagnosed in the United States. Raised growths on the skin, such as moles and warts, usually are not cancerous. Moles are due to an overgrowth of melanocytes, and warts are due to a viral infection.
Figure 5.6 Skin cancer. In each of the three types shown, the skin clearly has an abnormal appearance.
A wound that punctures a blood vessel will fill with blood. Chemicals released by damaged tissue cells will cause the blood to clot. The clot prevents pathogens and toxins from spreading to other tissues (Fig. 5.7a). The part of the clot exposed to air will dry and harden, gradually becoming a scab. White blood cells and fibroblasts move into the area. White blood cells help fight infection and fibroblasts are able to pull the margins of the wound together (Fig. 5.7b). Fibroblasts promote tissue regeneration: The basal layer of the epidermis begins to produce new cells at a faster than usual rate. The proliferating fibroblasts bring about scar formation; the scar may or may not be visible from the surface (Fig. 5.7c). A scar is a tissue composed of many collagen fibers arranged to provide maximum strength. A scar does not contain the accessory organs of the skin and is usually devoid of feeling. In any case, epidermis and dermis have now healed (Fig. 5.7d).
Figure 5.7 The process of wound healing. a. A deep wound ruptures blood vessels, and blood flows out and fills the wound. b. After a blood clot forms, a protective scab develops. Fibroblasts and white blood cellsmigrate to the wound site. c. New epidermis forms, and fibroblasts promote tissue regeneration. d. Freshly healed skin.
The epidermal injury known as a burn is usually caused by heat but can also be caused by radioactive, chemical, or electrical agents. Two factors affect burn severity: the depth of the burn and the extent of the burned area. A useful technique for estimating the extent of a burn, called the “rule of nines,” is often employed (Fig. 5.8). In this method, the total body surface is divided into regions as follows: the head and neck, 9% of the total body surface; each upper limb, 9%; each lower limb, 18%; the front and back portions of the trunk, 18% each; and the perineum, which includes the anal and urogenital regions, 1%. One way to classify burns is according to the depth of the burned area. In first-degree burns, only the epidermis is affected. The person experiences redness and pain, but no blisters or swelling. A classic example of a first-degree burn is a moderate sunburn. The pain subsides within 48-72 hours, and the injury heals without further complications or scarring. The damaged skin peels off in about a week. A second-degree burn extends through the entire epidermis and part of the dermis. The person experiences not only redness and pain, but also blistering in the region of the damaged tissue. The deeper the burn, the more prevalent the blisters, which enlarge during the hours after the injury. Unless they become infected, most second-degree burns heal without complications and with little scarring in 10-14 days. If the burn extends deep into the dermis, it heals more slowly over a period of 30-105 days. The healing epidermis is extremely fragile, and scarring is common. First- and second-degree burns are sometimes referred to as partial-thickness burns. Third-degree burns, or full-thickness burns, destroy the entire thickness of the skin. The surface of the wound is leathery and may be brown, tan, black, white, or red. The patient feels no pain because the pain receptors have been destroyed, as have blood vessels, sweat glands, sebaceous glands, and hair follicles. Fourth-degree burns involve tissues down to the bone. Obviously, the chances of a person surviving fourth-degree burns are not good unless a very limited area of the body is affected. The major concerns with severe burns are fluid loss, heat loss, and bacterial infection. Fluid loss is counteracted by intravenous administration of a balanced salt solution. Heat loss is minimized by placing the burn patient in a warm environment. Bacterial infection is treated by isolation and the application of an antibacterial dressing.
As soon as possible, the damaged tissue is removed, and skin grafting is begun. The skin needed for grafting is usually taken from other parts of the patient’s body. This is called autografting, as opposed to heterografting, in which the graft is received from another person. Autografting is preferred because rejection rates are very low. However, if the burned area is quite extensive, it may be difficult to acquire enough skin for autografting. In that case, skin can be grown in the laboratory from only a few cells taken from the patient.
Figure 5.8 The “rule of nines” for estimating the extent of burns.