What is melanoma?

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.

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
• Protects the rest of the body from ultraviolet (UV) rays
• Helps the body make vitamin D
• The skin has 3 layers: the epidermis, the dermis, and the subcutis (see picture)

Epidermis

The top layer of skin is 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 helps the skin protect the rest of the body.

The outermost part of the epidermis is called the stratum corneum, or horny layer. It is composed of dead keratinocytes that are continually shed as new ones form. The cells in this layer are called squamous cells because of their flat shape.

Living squamous cells are found below the stratum corneum. These cells have moved here from the lowest part of the epidermis, the basal layer. The cells 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 brown pigment called melanin, which gives the skin its tan or brown color. Melanin protects the deeper layers of the skin from some of 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 skin 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 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 helps the body conserve heat and has a shock-absorbing effect that helps protect the body's organs from injury.

Benign skin tumors

Many types of benign (non-cancerous) tumors can develop from different types of skin cells.

Melanocytic tumors

A mole (nevus) is a benign skin tumor that develops from melanocytes. Nearly all moles are harmless, but having some types may raise your risk of melanoma. See the section called, "What are the risk factors for melanoma?" 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.

Other benign tumors

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.