What is melanoma skin cancer?
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.
The skin is the largest organ in your body. It does many different things:
- Covers the internal organs and helps protect 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).
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 that helps the skin protect the rest of the body.
The outermost part of the epidermis is called the stratum corneum. 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 found in the epidermis. These skin cells make a 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. This is an important structure because when a skin cancer becomes more advanced, it generally grows through this barrier and into the deeper layers.
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 elasticity and strength.
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.
Tumors that start in melanocytes
A mole (nevus) is a benign skin tumor that develops from melanocytes. Nearly all moles (nevi) are harmless, but having some types may raise your risk of melanoma. See the section, “What are the risk factors for melanoma skin cancers?” 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.
- Hemangiomas: benign blood vessel growths often called cherry or strawberry spots, or port wine stains
- Lipomas: soft growths made up of 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 make melanin, melanoma tumors are usually brown or black. But some melanomas do not make melanin and can appear pink, tan, or even white.
Melanomas can occur anywhere on the skin, but they are more likely to start in certain locations. The trunk (chest and back) is the most common site in men. The legs are the most common site in women. The neck and face are other common sites.
Having darkly pigmented skin lowers your risk of melanoma at these more common sites, but anyone can develop this cancer on the palms of the hands, soles of the feet, and under the nails. Melanomas in these areas account for more than half of all melanomas in African Americans but fewer than 1 in 10 melanomas in whites.
Melanomas can also form in other parts of your body such as the eyes, mouth, and genitals, but these are much less common than melanoma of the skin. Melanomas in these organs are discussed in our other documents.
Melanoma is much less common than basal cell and squamous cell skin cancers, but it is far more dangerous. Like basal cell and squamous cell cancers, melanoma 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 melanomas are sometimes grouped as non-melanoma skin cancers because they develop from skin cells other than melanocytes. They tend to behave very differently from melanomas and are often treated in different ways.
Non-melanoma skin cancers include basal cell and squamous cell cancers. They are by far the most common skin cancers, and actually are more common than any other form of cancer. Because they rarely spread (metastasize) to other parts of the body, basal cell and squamous cell skin cancers are less worrisome and are treated differently from melanoma. Merkel cell carcinoma is an uncommon type of skin cancer that is sometimes harder to treat. These cancers are discussed in our document called Skin Cancer: Basal and Squamous Cell.
Last Medical Review: 09/20/2012
Last Revised: 01/17/2013