To better understand skin cancer, it is helpful to have an overview of the anatomy of the skin and its functions. The skin is the largest organ in the body. It covers the internal organs and protects them from injury, serves as a barrier to germs such as bacteria and prevents the loss of too much water and other fluids. The skin also helps control body temperature.

The skin has three layers:

• Epidermis
• Dermis
• Subcutis

The top layer is the epidermis. The two main types of skin cancer, melanomas and non-melanomas, begin in the epidermis. The epidermis is very thin, averaging only 0.2 millimeters thick (about 1/100 of an inch). It protects the deeper layers of skin and the organs of the body from the environment.

Keratinocytes are the main cells in the epidermis. These cells form an important protein called keratin, which contributes to the skin's ability to protect the rest of the body.

The outermost part of the epidermis is called the stratum corneum or horny layer. It is composed of keratinocytes that are no longer living. The cells in this layer are referred to as squamous cells because of their flat shape. These cells are continually shed as new ones form.

Living keratinocytes are found below the stratum corneum. These cells have moved here from the lowest part of the epidermis, the basal layer. The keratinocytes of the basal layer, called basal cells, continually divide to form new keratinocytes. These replace the older keratinocytes that wear off the skin's surface.

Melanocytes, the cells that can become melanoma, are also present in the epidermis. These skin cells make the protective brown pigment called melanin, which makes skin tan or brown. Melanin protects the deeper layers of the skin from the harmful effects of the sun.

The epidermis is separated from the deeper layers of skin by the basement membrane. The basement membrane is an important structure because when a cancer becomes more advanced, it generally grows through this barrier.

Cancer of the skin is the most common of cancers, probably accounting for about half of all cancers. Melanoma accounts for about 3% of skin cancer cases but causes a large majority of skin cancer deaths.

Melanoma is more than 10 times more common in whites than in African Americans. It is slightly more common in males than in females.

Unlike many other common cancers, melanoma has a wide age distribution. It occurs in younger as well as older people. Although rates continue to increase with aging and are highest among those in their 80s, melanoma is not uncommon even among those younger than 30. In fact, it is one of the more common cancers in adolescents and young adults.