Dermis

The boundary between the epidermis and dermis is usually uneven. This is because the epidermis has ridges projecting inward and the dermis has conical dermal papillae passing into the spaces between the ridges (see fig. 6.2).

Skin Cancer

Like cigarette smoking, a deep, dark tan was once very desirable. A generation ago, a teenager might have spent hours on a beach, skin glistening with oil, maybe even using a reflecting device to concentrate sun exposure on the face. Today, as they lather on sunblock, many of these people realize that the tans of yesterday may cause cancer tomorrow. However, today many people increase their risk of developing skin cancer by spending time in tanning booths.

Just four hours of unprotected sunbathing has immediate and lasting effects on the skin. The ultraviolet radiation alters collagen and elastin and dilates blood vessels in the dermis. A few days later, the outer skin layer may blister and peel. Cells that peel have undergone programmed cell death (apoptosis), which removes cells that have turned cancerous. Cancer begins when ultraviolet radiation causes mutation in the DNA of a skin cell. People who inherit xero-derma pigmentosum are very prone to developing skin cancer because they lack DNA repair enzymes. They must be completely covered by clothing and sunblock when in the sun to avoid developing skin cancers (fig. 6A).

Skin cancer usually arises from nonpigmented epithelial cells within the deep layer of the epidermis or from pigmented melanocytes. Skin cancers originating from epithelial cells are called cutaneous carcinomas (basal cell carcinoma or squamous cell carcinoma); those arising from melanocytes are cutaneous melanomas (melanocar-cinomas or malignant melanomas) (fig. 6B).

Cutaneous carcinomas are the most common type of skin cancer. They occur most frequently in light-skinned people over forty years of age. These cancers usually appear in persons who are regularly exposed to sunlight, such as farmers, sailors, athletes, and sunbathers.

A cutaneous carcinoma often develops from a hard, dry, scaly growth with a reddish base. The lesion may be flat or raised, and usually firmly adheres to the skin, appearing most often on the neck, face, or scalp. Fortunately, cutaneous carcinomas are typically slow growing and can usually be cured completely by surgical removal or radiation treatment.

A cutaneous melanoma is pigmented with melanin, often with a

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