Melanoma is a cancer that starts in a certain type of skin cell. To understand melanoma, it helps to know some things about normal skin.

Normal skin

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

• covers the organs inside the body and protects them from injury
• helps to keep out germs
• helps keep in water and other fluids
• helps control body temperature

The skin has 3 layers (see picture below):

• epidermis
• dermis
• subcutis

The top layer of the skin, the epidermis, is very thin and protects the deeper layers of skin and the organs. The epidermis itself has 3 layers: an upper, a middle, and a bottom layer made up of basal cells.

These basal cells divide to form keratinocytes, (also called squamous cells) which make a protein called keratin. This protein helps protect the body.

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

A layer called the basement membrane separates the epidermis from the deeper layers of skin. The basement membrane is important because when a cancer becomes more advanced, it may grow through this barrier.

Other skin cancers

Skin cancers that are not melanoma are sometimes grouped together as non-melanoma skin cancers because they start from skin cells other than melanocytes. These cancers include basal cell and squamous cell cancers. They are much more common skin cancers. Because they rarely spread, basal cell and squamous cell skin cancers are less worrisome and are treated differently than melanoma. They are discussed in the American Cancer Society’s document Skin Cancer: Basal and Squamous Cell.

Benign skin tumors

Most tumors of the skin are not cancer and rarely, if ever, turn into cancer. There are many types of non-cancerous (benign) tumors that start in other types of skin cells, such as:

• seborrheic keratoses -- tan, brown, or black raised spots with a "waxy" texture, or rough surface
• hemangiomas -- benign blood vessel growths often called strawberry spots or port wine stains
• lipomas -- soft growths of benign fat cells
• warts -- rough-surfaced growths caused by a virus
• moles (also called nevi) --benign skin tumors that start from melanocytes
• Spitz nevus-- a kind of skin tumor that sometimes looks like melanoma

Melanoma skin cancers

Melanoma is a cancer that begins in the melanocytes. Because most of these cells still make melanin, melanoma tumors are often brown or black. But this is not always the case, and melanomas can also have no color. Melanoma most often starts on the trunk of fair-skinned men and on the lower legs of fair-skinned women, but it can start in other places, too. Having dark skin lowers the risk of melanoma. But it does not mean that a person with dark skin will never get melanoma.

Melanoma can almost always be cured in its early stages. But it is likely to spread to other parts of the body if it is not caught early. Melanoma is much less common than basal cell and squamous cell skin cancers, but it is far more serious.