Melanoma is a cancer that starts in a certain type of skin cell. To understand melanoma, it helps to know about the normal structure and function of the skin.

About normal skin

The skin is the largest organ in your body. It does several different things:

• covers the internal organs and protects them from injury
• serves as a barrier to germs such as bacteria
• prevents the loss of too much water and other fluids
• helps control body temperature

The skin has 3 layers (see picture below):

• epidermis
• dermis
• subcutis

Epidermis

The top layer of skin is the epidermis. The 2 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 cell type of the epidermis. These cells make an important protein called keratin. Keratin 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 called 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.

Dermis

The middle layer of the skin is called the dermis. The dermis is much thicker than the epidermis. It contains hair follicles, sweat glands, blood vessels, and nerves that are held in place by a protein called collagen. Collagen, made by cells called fibroblasts, gives the skin its resilience and strength.

Subcutis

The last and deepest layer of the skin is called the subcutis. The subcutis and the lowest part of the dermis form a network of collagen and fat cells. The subcutis conserves heat and has a shock-absorbing effect that helps protect the body's organs from injury.

Benign skin tumors

There are many types of benign (non-cancerous) tumors that can develop from different types of skin cells.

Moles (nevi) are benign skin tumors that develop from melanocytes. Nearly all moles are harmless but having some types may raise your risk of melanoma. See the section, "What are the risk factors for melanoma skin cancer?" in this document for more information about moles.

A Spitz nevus is a kind of skin tumor that sometimes looks like melanoma. These tumors are generally benign and don't spread. But sometimes doctors have trouble telling Spitz nevi from true melanomas, even when looking at them under a microscope. Therefore, they are often removed, just to be safe.

Benign tumors that develop from other types of skin cells include:

• seborrheic keratoses: tan, brown, or black raised spots with a "waxy" texture or rough surface

• hemangiomas: benign blood vessel growths often called cherry or strawberry spots, or port wine stains

• lipomas: soft growths of benign fat cells

• warts: rough-surfaced growths caused by a virus

Most of these tumors rarely, if ever, turn into cancers. There are a lot of other kinds of benign skin tumors but most are not very common.

Melanoma skin cancers

Melanoma is a cancer that begins in the melanocytes. Other names for this cancer include malignant melanoma and cutaneous melanoma. Because most melanoma cells still produce melanin, melanoma tumors are usually brown or black. But this is not always true, as melanomas can be non-pigmented (no color). Melanoma is much less common than basal cell and squamous cell skin cancers, but it is far more dangerous.

Melanomas can occur anywhere on the skin, but are more likely to start in certain locations. The trunk is the most common site in men. The legs are the most commonly affected site in women. The neck and face are other common sites.

Having darkly pigmented skin lowers your risk, but it is not a guarantee that you will not get melanoma. Anyone can develop this cancer on the palms of the hands, soles of the feet, and under the nails. Melanomas in these areas represent about half of all melanomas in African Americans but fewer than 10% of melanomas in whites.

Melanomas can also form in other parts of your body such as the eyes, mouth, and vagina, but these are much less common than melanoma of the skin. Melanomas in these organs are discussed in separate American Cancer Society documents.

Melanoma, like basal cell and squamous cell cancers (see below), is almost always curable in its early stages. But it is much more likely than basal or squamous cell cancer to spread to other parts of the body if not caught early.

Other skin cancers

Skin cancers that are not melanoma are sometimes grouped together as non-melanoma skin cancers because they develop from skin cells other than melanocytes. These include basal cell and squamous cell cancers (by far the most common skin cancers, and actually more common than any other form of cancer). Because they rarely metastasize (spread elsewhere in the body), basal cell and squamous cell skin cancers are less worrisome and are treated differently than melanoma. Merkel cell carcinoma is an uncommon type of skin cancer that is sometimes harder to treat. These cancers are discussed in another American Cancer Society document called Skin Cancer: Basal and Squamous Cell.

Still other types of non-melanoma skin cancers are discussed in the American Cancer Society documents, Kaposi Sarcoma and Lymphoma of the Skin.

Last Medical Review: 06/05/2008
Last Revised: 05/14/2009